Laser Doppler vibrometry measurement of the mechanical myogram John W. Rohrbaugh, Erik J. Sirevaag, and Edward J. Richter Citation: Review of Scientific Instruments 84, 121706 (2013); doi: 10.1063/1.4845435 View online: http://dx.doi.org/10.1063/1.4845435 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vocal fold vibration measurements using laser Doppler vibrometry J. Acoust. Soc. Am. 133, 1667 (2013); 10.1121/1.4789937 Surface wave measurements using a single continuously scanning laser Doppler vibrometer: Application to elastography J. Acoust. Soc. Am. 133, 1245 (2013); 10.1121/1.4789929 Broadband measurement of translational and angular vibrations using a single continuously scanning laser Doppler vibrometer J. Acoust. Soc. Am. 132, 1384 (2012); 10.1121/1.4740473 Laser Doppler Vibrometry measurement of the mechanical myogram AIP Conf. Proc. 1457, 266 (2012); 10.1063/1.4730566 Teeth mobility measurement by laser Doppler vibrometer Rev. Sci. Instrum. 70, 2850 (1999); 10.1063/1.1149806
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 121706 (2013)
Laser Doppler vibrometry measurement of the mechanical myogram John W. Rohrbaugh,1,a) Erik J. Sirevaag,1 and Edward J. Richter2 1 2
Washington University School of Medicine, 4625 Lindell Blvd., Suite 200, Saint Louis, Missouri 63108, USA Washington University School of Engineering, 1 Brookings Ave., Saint Louis, Missouri 63110, USA
(Received 31 May 2013; accepted 29 July 2013; published online 30 December 2013) Contracting muscles show complex dimensional changes that include lateral expansion. Because this expansion process is intrinsically vibrational, driven by repetitive actions of multiple motor units, it can be sensed and quantified using the method of Laser Doppler Vibrometry (LDV). LDV has a number of advantages over more traditional mechanical methods based on microphones and accelerometers. The LDV mechanical myogram from a small hand muscle (the first dorsal interosseous) was studied under conditions of elastic loading applied to the tip of the abducted index finger. The LDV signal was shown to be related systematically to the level of force production, and to compare favorably with conventional methods for sensing the mechanical and electrical aspects of muscle contraction. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4845435] I. INTRODUCTION: THE MECHANICAL MYOGRAM
Although the mechanical assessment of muscle activity has generally focused on contraction and force production along the longitudinal axis, there are additional dimensional changes during contraction. These dimensional changes are closely related to force production, and because they entail mechanical displacement can be sensed by the Laser Doppler Vibrometry (LDV) method. The most useful signal in this context is the vibration activity that is known to accompany contraction (see review by Orizio1 ). Although not as generally appreciated, or widely applied, as the electromyogram (EMG), the study of muscle vibrations has a long history dating to the early published description of Grimaldi in 1665. Within the past three decades, perhaps sparked by the provocative review of Oster,2 muscle vibrations have received extensive study. Following recommendations of Orizio et al.,3 we adopt here the nomenclature “mechanical myogram (MMG)” to describe these muscle vibrations. Although the published descriptions of the MMG vary considerably among studies, there is general agreement that the spectrum of the signal is low, with peaks in the range of 10–25 Hz (sometimes one at 40 Hz), and little discernible energy beyond about 50–60 Hz. Much of the variability among studies can be related systematically to properties of the muscles studied (including location, fiber composition, level and rate of force production, joint angle, and fatigue). However, a major share of the variability can be traced to the absence of standardized transduction and recording methods. In human studies, the transducers have usually been microphones, piezo electric pressure sensors, or accelerometers.4 Each of these methods has recognized limitations. The microphone-recorded MMG signal (often called the Acoustic Myogram, AMG), for example, shows low repeatability even across microphones of the same manufacturer, is generally insensitive to extremely low frequencies, is highly dependent a) Author to whom correspondence should be addressed. Electronic mail:
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on the dimensions of the cavity separating the microphone diaphragm from the skin, and the coupling media, and is difficult to calibrate with respect to physical vibration units.5–7 In a similar vein, the MMG signals recorded with mechanical transducers have been found to be dependent on such factors as sensor contact pressure,6, 8 to have intrinsic resonant frequencies, and to add a small but non-negligible mass loading to the skin. In this context, the LDV method would appear to offer significant technical advantages, with respect to the broad signal bandwidth (DC to at least 40 kHz), complete absence of mass loading, and lack of intrinsic resonances. It has been convincingly established that the MMG signal arises from contractile properties of the engaged muscle.9, 10 The signal is generally largest over the belly, and diminishes in the directions of insertion and attachment,11 or laterally toward the border.6 There are several non-exclusive candidate mechanisms for the MMG signal. All relate the vibration to the firing patterns of individual motor units, and the resultant mechanical pressure waves. A major contribution appears to derive from the lateral dimensional change of the muscle during contraction.1 Since the muscle bundle remains isovolumic, the decrease in length during contraction is accompanied by lateral expansion. Up to a certain level of force production, increases in force are achieved principally by recruiting additional motor units. The associated MMG grows monotonically in amplitude. Beyond some force level at which all available motor units are recruited, there is a strategy shift whereby additional force is achieved by increased firing rates in the engaged motor units. The MMG amplitude typically becomes asymptotic or reduces in amplitude beyond this point, reflecting a mechanical fusion of the individual motor unit activity at high firing rates. The point of this inflection depends on the muscle fiber composition (proportion of Type I and Type II fibers, and intramuscular distribution with respect to the surface), ranging from 60%–80% maximum voluntary contraction (MVC) for large muscles such as the quadriceps or biceps brachii, to 50% for small hand muscles.12 A number of investigators have observed concomitant increases in frequency, thought to reflect both the increased firing rates of
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individual motor units as well as changes in the types of units recruited.13 Consistent with the mechanical origin of the MMG, there is evidence that its relationship with force production is more veridical than is the case for the EMG—in the face of such factors as fatigue,5, 14–18 temperature,19 and torque angle.20 This has led to the suggestion that the MMG would be particularly useful in the study of muscles for which it is difficult to ascertain loading, e.g., paraspinal or facial muscles. The MMG signal also appears to provide a useful measure of muscle fiber composition,21 and to be useful in the assessment of neuromuscular diseases22–24 and aging.12 In sum, the MMG signal has a number of desirable features as a measure of muscle contraction. Among the most important in the present context is that it is present in the form of a mechanical vibration that can be detected at the skin, and, as such, can be sensed remotely using the LDV method. II. VALIDATION STUDY USING A SMALL HAND MUSCLE: RATIONALE
As an initial study to investigate the effectiveness of the LDV method in this context, we investigated the activity of a small muscle in the hand (the first dorsal interosseous, 1DI). There were two principal advantages to this approach: (i) the muscle loadings could be systematically controlled, allowing us to establish the relationships between signal characteristics and contractile force; (ii) our findings could be anchored in the prior literature based on detailed study of the small hand muscles, using other recording methods. An additional benefit is that, by controlling hand and finger position, the method offered a standardized situation in which to assess physiological tremor. Tremor is a potentially useful measure of stress and emotion as well as neuromuscular function, but is regulated by numerous factors including body position and muscle tension levels that ideally would be controlled at this initial stage of investigation. Our procedures were adopted from those used by McAuley et al.,25 who studied 1DI and finger tremor activity under conditions of controlled elastic loading applied to the fingertip. Unlike mass loading, elastic loading does not affect the intrinsic resonance of the body segment, nor does it damp high-frequency components of tremor. The tremor, EMG, and MMG activities were found to share several spectral peaks, at about 10, 20, and 40 Hz. The amplitude but not frequency of these peaks was affected by muscle loading. Within each frequency band, the tremor, EMG, and MMG signals showed high coherence, indicating that they were different manifestations of the same oscillatory mechanism. The authors concluded that the signals in the different frequency bands were not harmonics but rather derived from independent neural oscillators, each with different functional properties. III. METHODS
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session. Data shown here are based on 14 participants (5 female) from whom technically satisfactory records were obtained in all conditions. Participants were paid at the rate of $15 per h for their participation. Participants provided informed consent, according to procedures approved by the Human Research Protection Office at Washington University School of Medicine. Exclusion criteria included self-reported skeletal, movement or neuromuscular disorders, hypertension or other cardiovascular disease, use of tobacco, and use of tremorolytic or tremorogenic medications. Participants were instructed to avoid caffeine for at least 3 h prior to the test. B. Recording methods
During recording, the hand was rotated to a position midway between pronation and supination, and supported on the padded surface of the chair arm. Participants were instructed to extend the index finger, and to curl the remaining fingers into a loose fist. During contraction, the index finger was lifted (abducted) against the elastic loading (or mass loading in some conditions) applied to the fingertip. Participants were instructed, and frequently reminded, about the need to restrict contraction to the 1DI muscle and to avoid coactivation of other hand or arm muscles. This recording position differs somewhat from that of McAuley et al.25 who used a position in which the hand was rested in a pronated position, with all of the fingers outstretched. Our intent in using the mid position was to allow mass loading to be applied in the abduction/adduction plane. The recording jig is illustrated in Figure 1. Loading was applied to a light-weight plastic ring, machined from PVC tubing (21 mm ID, 5 mm long), into which the index finger was inserted to a depth of approximately 1 cm (“B” in Figure 1). This ring was attached to either an elastic link that was connected to the beam of a load cell force transducer (World Precision Instruments FORT 5000) (“B” in Figure 1) or to mass loadings hung from the ring. The elastic link was a #19 rubber band (1/16 in. × 3 12 in.) which was stretched by varying the distance to the force transducer beam so as to control the applied loading. During resting conditions, the
C
D
E
F G
B
A
A. Participants
Twenty participants (10 female), all right handed and ranging in age from 18 to 31 served in a single 2–3 h
FIG. 1. Recording setup. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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ring rested in a cradle. Participants were trained to lift the ring slightly—just enough barely to clear the cradle—during contraction and to ease it back into position at the end of contraction. Recordings were taken from several places on the fingertip and over the 1DI muscle: Finger tremor was measured with an Entran EGA-25/R miniature single axis accelerometer (with the active axis in the abduction/adduction plane; “D” in Figure 1), taped to the surface of the index finger just proximal to the distal joint. EMG was measured with a Biopac MP150 physiological recording system, amplified using a TEL100 remote recording unit. The data were amplified 5 k, and analog bandpass filtered 0.5–500 Hz before sampling. The active electrode was placed over the proximal 1DI muscle with a reference electrode over the trapezium (“G” in Figure 1). AMG was measured using a small electret microphone (Optimus 33-0013) that was cemented to the plastic housing of a 9 mm diameter electrode, in which the metal element had been removed (“E” in Figure 1). The interior of the shell was machined to a conical shape, terminating in a 3 mm aperture which matched the opening of the microphone. The microphone was positioned over the distal end of the 1DI muscle, and attached to the skin using a double-sided adhesive collar with care taken to assure that the seal was air-tight. The microphone signals were amplified 50 times with a bandpass of 1–500 Hz. Test recordings showed that the MMG signal was abolished when the microphone cavity was filled with modeler’s clay, indicating that the recorded signals originated from the skin surface (rather than from gross movements of the microphone itself). LDV signals were recorded, in different runs, from either the fingertip (“C” in Figure 1) or over the belly of the 1DI muscle (“F”), with an offset distance of 275 mm. To enhance the optical quality of the laser signal, a small piece of reflective tape (1 cm square) was attached to the target sites. Data were acquired with a Polytec OFV-303 laser head and OFV 3001 controller, with a 90◦ attachment so as to direct the beam vertically (radial to the measurement sites). LDV data were measured with a sensitivity of 5 mm/s/V. Data were sampled at a rate of 1600 Hz, using a Biopac MP150 system. Additional channels were devoted to cardiorespiratory variables, which are not reported here.
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conditions were obtained at the end of the session, in balanced order. Each run included a 15 s baseline (pre-contraction) segment followed by a lift of 20 s. Instructions emphasized the importance of maintaining a steady lift and avoiding cocontraction of other muscles. Individual lifts were separated by at least 2 min, to minimize possible fatigue effects. IV. RESULTS
For the present purposes, we restrict our presentation of results to the findings bearing on the most relevant questions of (i) the general form of the LDV MMG signal, (ii) the relationships with level of force production, (iii) comparisons with the conventional EMG, and (iv) comparisons between the LDV-and accelerometer-recorded tremor signals.
A. General appearance of LDV-recorded MMG signal
An example of typical myographic signals is presented in Figure 2. These data were obtained from a single participant under 30% MVC elastic loading. The second trace illustrates the microphone-recorded MMG, and the third is the velocity record from the LDV. For comparison purposes, the simultaneous EMG is shown in the top trace. These records all have been digitally high-pass filtered at 5 Hz. The microphoneand LDV-recorded signals show vibration activity throughout the lift. As has been noted by other investigators (see review above), there tends to be a burst of activity at the onset of contraction, in both the microphone- and LDV-recorded signals. Also clearly seen is another burst at the end of contraction. These bursts are thought to accompany the large dimensional changes that are seen at the beginning and end of contraction.1 The onsets of the MMG signals occur simultaneously, and precede slightly the attainment of maximum EMG amplitude (which appears to have a more ramp-like onset). The reverse pattern is seen at the end of contraction, with the MMG signals persisting beyond the cessation of the EMG signal. In the bottom panel, the LDV velocity signal has been integrated to produce a displacement signal, for purposes of illustrating the absolute magnitude of the movements present in vibration signal. In this typical example, the peak-to-peak vibration
EMG
C. Procedure
Participants lifted, and relaxed, their finger according to instructions displayed on a projection screen (on which fixation was maintained throughout the contractions). In addition to resting (no load) conditions, data were obtained at 5%, 15%, and 30% MVC elastic loading, and at 15% and 30% mass loading. MVC was individually determined for each participant by inserting a solid link between the ring and the beam of the force transducer. MVC was defined as the maximum value of three attempts. The 5% MVC elastic loading data were obtained during baseline recording conditions, in a phase of the experiment aimed at assessing the effects of mental stress on tremor (not described here). The remaining
1 mV
Microphone LDV Velocity 1 mm/s
LDV Displacement 25 µm
“Lift”
“Relax”
FIG. 2. General appearance of LDV-recorded MMG signal. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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FIG. 3. Spectra of LDV-recorded MMG signals, at rest and three levels of elastic loading. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
displacement during the sustained portion of the contraction is on the order of 30–50 μm.
B. Effects of elastic loading on muscle signals
The mean spectra (averaged over the 14 participants) for the LDV-recorded MMG signals are shown in Figure 3. These spectra are based on uniform data segments of 19 s, starting 1 s after the instruction to lift (abduct) the finger, which were tapered using a Welch window. As illustrated in Figure 2, the data are fairly stable during this period. Spectra were computed using the method described by Welch,26 as implemented in Matlab. The four overlaid spectra correspond with the baseline (zero loading), 5%, 15%, and 30% MVC elastic loading. The spectra reveal two clearly defined peaks (plus a third peak that is sharply tuned at 30 Hz which is artifactual, deriving from vibration in the vertical axis produced by the building’s mechanical equipment). The major early peak is close to 10 Hz, and the second is a broader peak with maximum power in the range of 20–30 Hz. There is a weak suggestion of an abundance of power at about 35–40 Hz, but this was not a consistent feature in the records from individual participants. As is clearly evident in the figure, the amplitude of the signal grows as a function of increasing load, for both the 10 Hz and 25 Hz components. This systematic relationship with elastic load is illustrated in Figure 4, in which the mean peak power within a window of 23–27 Hz has been plotted for baseline (preabduction) and abduction periods. Analysis of variance confirmed that the existence of significant effects, which were decomposed using tests of simple effects to confirm that power was significantly increased during all three loading conditions (p < 0.05). The loading also varied significantly from one another (p < 0.05 for all comparisons except the comparison between the 15% and 30% conditions, which was marginally significant, p = 0.056). Similar results (not
illustrated) were obtained for the earlier peak at about 10 Hz, measured as the peak within the window of 9–13 Hz. The corresponding EMG values, expressed in terms of RMS amplitude during the identical data segments, are shown in Figure 5. As was the case for the LDV-recorded MMG measures, the EMG amplitudes were significantly increased with respect to baseline under all loading conditions, and the amplitude significantly increased in proportion to the loading (p < 0.05 for all comparisons). The possibility that there might be systematic fatiguerelated effects over the 20 s contraction was assessed by splitting the 19 s analysis segment in half. As illustrated in Figure 6, there was a weak suggestion of decreased power late in the abduction for the 5% and 30% MVC conditions, but this apparent change did not approach statistical significance. For comparison purposes, the spectra associated with the acoustic microphone-recorded MMG are shown in Figure 7.
FIG. 4. Mean peak power of LDV signal, within 23 Hz band, at rest and three levels of force production. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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The spectra disclose a broad peak with maximum power at about 10 Hz. Although the power is increased during the loading conditions, the spectra in general show less definition than seen in the LDV-recorded MMG signals. Also, the relationship with loading is less regular, with the largest values obtained under the 5% loading. There are two likely reasons for the discrepancies, one being the different (and quite variable) recording properties of microphone sensors, and the second being a difference in transducer placement (with the microphone being placed more distal than the LDV recording site). C. Effects of elastic loading on finger tremor
FIG. 5. EMG amplitude (RMS) at rest and three levels of force production. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
FIG. 6. LDV-recorded MMG power during early and late contraction, at three levels of force. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
FIG. 7. Acoustic microphone-recorded mechanical myogram, at rest and three levels of force. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
Tremor at the fingertip was assessed using both the LDV, and the miniature Entran accelerometer. The associated LDV tremor spectra, assessed during the same data segments as defined above, are illustrated in Figure 8. These spectra are very similar in waveform, and responsiveness to loading conditions, to those associated with the LDV-MMG obtained from the 1DI muscle (Figure 3) (except that the 30 Hz artifact component is damped). Statistical analyses of the peak power within the 23–27 Hz window showed that the 5% loading condition was significantly different from both the 15% and 30% loading conditions (p > 0.05), although these latter two were not statistically distinguishable. The LDV tremor spectra were also very similar to the spectra of the signals obtained simultaneously from the Entran accelerometer (not illustrated). The degree of similarity between the LDV- and accelerometer-recorded finger tremor was assessed by computing the coherence between the two measures. The LDV signal was converted by differentiation to an acceleration signal prior to this analysis. Coherence overall was high, in the range of 95% across the band of about 5–40 Hz that encompassed the majority of tremor power. D. Effects of mass loading
Figure 9 illustrates the spectra associated with the LDVrecorded MMG signal during the application of 15% and 30%
FIG. 8. Spectra of LDV-recorded fingertip tremor, at rest and three levels of elastic loading. Reprinted with permission from AIP Conf. Proc. 1457, 266– 274 (2012). Copyright 2012 American Institute of Physics.36
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V. DISCUSSION
FIG. 9. LDV1DI spectra at rest and under two conditions of mass loading. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
mass loading. The spectra contain a well defined peak at about 10 Hz, as was seen in the MMG spectra associated with elastic loading. The higher-frequency (∼25 Hz) peak is absent. In addition, the spectra include a sharply tuned component at lower frequencies, peaking at about 4.5 Hz for the 30% MVC loading, and 5.5 Hz for the 15% MVC loading. The peak frequency was significantly lower (p < 0.05) for the 30% MVC loading. The associated LDV-recorded finger tremor data are shown in Figure 10. The overall amplitude was increased by approximately 7-fold in comparison to the data from the elastic loading conditions (shown in Figure 8), with the majority of power concentrated in the narrow, low frequency bands corresponding to those seen in the LDV-recorded MMG signal. As with the LDV-recorded MMG signal, the peak frequency was significantly lower with the 30% MVC loading.
The primary purpose of this study was to investigate the basic properties of the LDV-recorded MMG and tremor signals, with a goal of establishing whether meaningful and useful features of muscle activity could be sensed remotely. The long-range goals are to utilize the method to assess muscle (especially facial muscle) and tremor signs of emotion and mental stress; however, for this initial study, the emphasis was on a much simpler and more rigorously controlled recording situation involving the fingertip and small hand muscle (1DI). As noted in the Introduction, this model recording situation has the advantages of allowing for the precision application of elastic and mass loading, and there is a substantial existing literature regarding the form of the electrical and mechanical muscle signals from the 1DI, to provide a basis for interpreting the LDV-based signals. The findings indicate clearly that the LDV method is capable of recording high-quality MMG and tremor signals. This conclusion is supported by multiple lines of evidence: (1) The LDV-recorded MMG signal was observed to be of greater amplitude during contraction, and the duration of the increase was coextensive with the period of force production (as illustrated in Figure 2). (2) The nature of the effects associated with elastic and mass loading were consistent with anticipated effects, insofar as the signal amplitude was monotonically related to the level of force production, and the mass loading produced predictable inertial-related effects, particularly in finger tremor. (3) The LDV-recorded MMG and tremor signals showed high overall levels of agreement with signals recorded using conventional EMG, microphone, and accelerometer methods. (4) The signals agreed well with published descriptions of tremor and MMG activity in the 1DI muscle, as assessed by other investigators using a variety of methods (e.g., Refs. 5, 25, and 27). A. Effects of elastic loading on the MMG
FIG. 10. LDV Fingertip tremor spectra at rest and under two conditions of mass loading.
The MMG spectra associated with elastic loading showed multiple peaks, with the predominant peak at about 10 Hz and a broader peak at about 25 Hz. As noted by McAuley et al.,25 the application of elastic loading generally enhances and sharpens the higher frequency components. Even in the absence of elastic loading, a number of investigators have observed multi-peaked spectra for the MMG signal.12, 22, 28, 29 Previous descriptions of the spectral composition of the MMG signal have been remarkably variable, probably for several reasons. The measured frequency of the MMG signal generally depends on the force applied, increasing at high forces, but is stable below 30% MVC1 —consistent with our findings that the peak frequencies were not affected by force level within that range. Another major source of variability almost certainly lies in the transducer type used. Microphonebased recorders, in particular, are known to be quite variable, depending on cavity size, conducting media, and recording bandwidth.4, 7 Although the microphone-based MMG records
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in the present study were similar in overall form to the LDVbased MMG records, they showed less spectral resolution and a relative attenuation of activity at frequencies above the dominant ∼10 Hz component. Despite this variability among studies, however, there is some consistency in pointing to two principal bands, at ∼10 Hz and 25 Hz,1 as were observed in the present study. Although there was a slight abundance of energy at about 35–40 Hz, there was no clear peak in the averaged MMG and tremor spectra at ∼40 Hz (Piper rhythm), as has been observed by other investigators.22, 25 Other investigators have commented on the marked individual differences in this peak, with respect to both amplitude and frequency, and it is quite possible that its absence in the averaged spectra reflects this variability. The magnitude of the spectral power in both the ∼10 Hz and 25 Hz MMG bands was progressively and significantly increased as the elastic loading was increased from 0% to 5%, 15%, and 30% MVC. Unlike the case for the EMG, however, the LDV-recorded MMG increase was nonlinear, with a relatively small increment as the loading was increased from 15% to 30%. This is consistent with a well established property of MMG signals, which typically are observed to become asymptotically large in the range of 60%–80% MVC, with stable or decreasing values beyond that point.1 The point of inflection depends on the specific muscle tested, and the associated fiber composition.12 Although several factors apparently contribute to this nonlinearity,30 the principal reason for the inflection in the force/MMG amplitude function is related to strategy engaged in producing increasing force. Below the inflection point, increased force production is achieved principally by recruiting larger numbers of motor units, which produce correspondingly larger MMG amplitudes. At some point, all available units are recruited, and additional force is produced by increasing the firing rates of individual motor units. This results in a mechanical fusion of the vibration signal, and attendant reduction of overall surface vibration activity. B. Effects of elastic loading on finger tremor
The original signals, and accompanying spectra, for tremor at the fingertip (in the elastic loading conditions) closely resembled those obtained from the 1DI. This is to be expected, given the close mechanical coupling between the finger and 1DI muscle, and the low mass of the finger in the elastic loading conditions. As with the 1DI signal, the finger tremor spectra showed two principal peaks, at ∼10 and 25 Hz, both of which were monotonically related to elastic loading. Comparison of the simultaneous LDV-and accelerometer-recorded tremor records disclosed a high coherence across the band encompassing the majority of tremor power.
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especially, finger tremor. The added mass served to damp the higher frequency tremor activity, producing a large, narrowband, low frequency tremor signal that was nearly sinusoidal in appearance. The peak frequency was slightly (∼1 Hz) but significantly lower for the 30% mass loading than for the 15% mass loading. These effects associated with the mass loading are consistent with well-established principles relating to inertial effects in resonant systems, and are consistent with prior reports (e.g., Ref. 31). Tremor under these conditions is thought to derive principally from phase-preserved stimulation of stretch receptors, and accompanying activation of monosegmental stretch reflexes.32, 33 The clear preservation of the ∼10 Hz peak in the accompanying LDV-recorded MMG record, which was not apparent in the finger tremor record, indicates that the rhythmic driving signal (of apparent central origin25 ) was not abolished in the presence of the reflex loop activity.
D. Additional considerations
The application of the LDV method to MMG recording has several identifiable constraints and limitations. A largely self-evident consideration is that the signal will be best recorded from the skin (or, as in the case here, from a reflective surface attached directly to the skin). Even though mechanophysiological energy, if large enough, can be transmitted to clothing if the contact is intimate, the small size of the MMG signal suggests that access to unclothed skin would be required. We elected to treat the target skin site by applying a small (and low mass) patch of retro-reflective tape. This served two purposes: (1) to mark the laser target zone and thus support consistent pointing, and (2) to improve the reflectivity of the surface and thus the quality of the LDV signal. The primary artifact when recording from optically noncooperative surfaces appears in the form of speckle dropouts, manifest here as aperiodic and aphysiologically brief, large transients. There are a number of linear and nonlinear methods that have been developed for suppressing this artifact (e.g., Ref. 34), including those integrated in the signal processing software of some commercial LDV systems. We have confirmed the effectiveness of such methods, particularly for the relatively low band that encompasses MMG signals, and now routinely record from untreated skin. We note in this context that there are additional mechanophysiological signals in this band—including cardiorespiratory activities, particularly from the thorax but broadly distributed—which must be distinguished and separated from the MMG signal. Additional considerations include the requirement for dynamic tracking if the laser target site is kinetically active, as well as the general considerations concerning eye and skin safety that apply to the use of lasers.35
E. Future directions C. Effects of mass loading on the MMG and finger tremor
The addition of mass loading to the fingertip produced substantial qualitative changes to the MMG signal and,
In follow-up studies, we have recorded from facial muscles and have confirmed that the method is indeed effective in that application. Our findings indicate that highly focal patterns of MMG activity can be obtained selectively,
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ACKNOWLEDGMENTS
This work was supported by grant from the Department of Defense Polygraph Institute (now the National Center for Credibility Assessment). EMG
Frown 1 C.
LDV Neutral
Smile
Weak Smile
FIG. 11. Illustration of LDV MMG signals from facial muscles under conditions of selective activation, and accompanying EMG.
over the facial muscles involved in the production of common emotion- and stress-related facial gestures. One of our early recordings, which illustrate the general pattern of results, is shown in Figure 11. Records were obtained from two facial sites (overlying the corrugator and zygomatic muscles), as the participant frowned (top traces) and smiled (middle traces). In each case, there are two overlaid traces, representing the EMG record (black) and simultaneous LDV MMG signal (red). As the subject frowns, there is activity from the corrugator muscle, but the zygomatic muscle remains silent. Conversely, during a smile the zygomatic is active, whereas there is little activity over the corrugator muscle. Shown at the bottom are records from a weak smile, in which case both the EMG and LDV signals are reduced in amplitude. These findings indicate that differential patterns of facial muscle activation can be recorded with the LDV method, that the signals are graded in amplitude in proportion to the strength of the expression, and that the signals agree well with conventional EMG records. We have since conducted much more detailed studies of the character and spatial distribution of the LDVrecorded facial MMG signals. We have also determined that MMG activity can be consistently detected, even at levels of tension that are too low to produce visible deformations of the face. Overall, the sensitivity of the method is comparable to the EMG method, but, unlike that method, can be obtained remotely, on a non-contact basis. VI. CONCLUSIONS
The present study was conducted as an initial validation study under carefully controlled conditions. The principal goal was to establish the extent to which mechanical myographic activity and tremor could be recorded on a non-contact basis, using the LDV. The experiment showed that high quality signals could indeed be recorded. The signals appear to be valid measures of muscle and tremor activity, on a number of counts. These include the systematic relationships with the nature of the elastic and mass loadings and the similarity of these relationships with those observed using conventional measures based on EMG, accelerometry, and microphone recording.
Orizio, “Muscle sound: Bases for the introduction of a mechanomyographic signal in muscle studies,” Crit. Rev. Biomed. Eng. 21, 201–243 (1993). 2 G. Oster, “Muscle sounds,” Sci. Am. 250, 108–114 (1984). 3 C. Orizio, M. Gobbo et al., “Transients of the force and surface mechanomyogram during cat gastrocnemius tetanic stimulation,” Eur. J. Appl. Physiol. 88, 601–606 (2003). 4 A. Courteville, T. Gharbi, and J.-Y. Cornu, “MMG measurement: A highsensitivity microphone-based sensor for clinical use,” IEEE Trans. Biomed. Eng. 45, 145–150 (1998). 5 D. T. Barry, “Vibrations and sounds from evoked muscle twitches,” Electromyogr. Clin. Neurophysiol. 32, 35–40 (1992). 6 C. F. Bolton, A. Parkes et al., “Recording sound from human skeletal muscle: Technical and physiological aspects,” Muscle Nerve 12, 126–134 (1989). 7 M. Watakabe, K. Mita et al., “Mechanical behaviour of condenser microphone in mechanomyography,” Med. Biomed. Eng. Comput. 39, 195–201 (2001). 8 T. G. Smith and M. J. Stokes, “Technical aspects of acoustic myography (AMG) of human skeletal muscle: Contact pressure and force/AMG relationships,” J. Neurosci. Methods 47, 85–92 (1993). 9 C. Orizio, D. Liberati et al., “Surface mechanomyogram reflects muscle fibres twitches summation,” J. Biomech. 29, 475–481 (1996). 10 Y. Yoshitake, M. Shinohara et al., “Characteristics of surface mechanomyogram are dependent on development of fusion motor units in humans,” J. Appl. Physiol. 93, 1744–1752 (2002). 11 C. Cescon, D. Farina et al., “Effect of accelerometer location on mechanomyogram variables during voluntary, constant-force contractions in thee human muscles,” Med. Biol. Eng. Comput. 42, 121–127 (2004). 12 F. Esposito, D. Malgrati et al., “Time and frequency domain analysis of electromyogram and sound myogram in the elderly,” Eur. J. Appl. Physiol. 73, 503–510 (1996). 13 K. Akataki, K. Mita et al., “Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle,” Eur. J. Appl. Physiol. 84, 19–25 (2001). 14 C. Orizio, R. Perini, and A. Veicsteinas, “Changes of muscular sound during sustained isometric contraction up to exhaustion,” J. Appl. Physiol. 66, 1593–1598 (1989). 15 C. Orizio, B. Dietmont et al., “Surface mechanomyogram reflects the changes in the mechanical properties of muscle at fatigue,” Eur. J. Appl. Physiol. 80, 276–284 (1999). 16 C. Orizio, M. Gobbo et al., “The surface mechanomyogram as a tool to describe the influence of fatigue on biceps brachii motor unit activation strategy. Historical basis and novel evidence,” Eur. J. Appl. Physiol. 90, 326–336 (2003). 17 M. J. Stokes and P. A. Dalton, “Acoustic myography for investigating human skeletal muscle fatigue,” J. Appl. Physiol. 71, 1422–1426 (1991). 18 M. J. Zwarts and M. Keidel, “Relationship between electrical and vibratory output of muscle during voluntary contraction and fatigue,” Muscle Nerve 14, 756–761 (1991). 19 B. Maton, M. Petitjean, and J. C. Cnockaert, “Phonomyogram and electromyogram relationships with isometric force reinvestigated in man,” Eur. J. Appl. Physiol. 60, 194–201 (1990). 20 N. Miyamoto and S. Oda, “Mechanomyographic and electromyographic responses of the triceps surae during maximal voluntary contractions,” J. Electromyogr. Kinesiol. 13, 451–459 (2003). 21 M. Marchetti, F. Felici et al., “Can evoked phonomyography be used to recognize fast and slow muscle in man?,” Int. J. Sports Med. 13, 65–68 (1992). 22 P. Brown, “Muscle sounds in Parkinson’s disease,” Lancet 349, 533–535 (1997). 23 C. Orizio, F. Esposito et al., “Muscle surface mechanical and electrical activities in myotonic dystrophy,” Electromyogr. Clin. Neurophysiol. 37, 231–239 (1997). 24 B. A. Rhatigan, K. C. Mylrea, and L. Z. Stern, “Investigation of sounds produced by healthy and diseased human muscular contraction,” IEEE Trans. Biomed. Eng. 33, 967–971 (1986).
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H. McAuley, J. C. Rothwell, and C. D. Marsden, “Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing,” Exp. Brain Res. 114, 525–541 (1997). 26 P. D. Welch, “The use of fast Fourier transforms for the estimation of power spectra: A method based on time averaging over short, modified periodograms,” IEEE Trans. Audio Electroacoust. 15, 170–173 (1967). 27 K. Akataki, K. Mita et al., “Mechanomyographic responses during voluntary ramp contractions of the human first dorsal interosseous muscle,” Eur. J. Appl. Physiol. 89, 520–525 (2003). 28 A. S. Wee and R. A. Ashley, “Vibrations and sounds produced during sustained voluntary muscle contraction,” Electromyogr. Clin. Neurophysiol. 29, 333–337 (1989). 29 A. S. Wee and R. A. Ashley, “Transmission of acoustic or vibratory signals from a contracting muscle to relatively distant tissues,” Electromyogr. Clin. Neurophysiol. 30, 303–306 (1990).
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and force during voluntary contractions reinvestigated using spectral decomposition,” Eur. J. Appl. Physiol. 80, 173–179 (1999). 31 T. I. H. Brown, P. H. M. Rack, and H. F. Ross, “Different types of tremor in the human thumb,” J. Physiol. 332, 113–123 (1982). 32 K.-E. Hagbarth and R. R. Young, “Participation of the stretch reflex in human physiological tremor,” Brain 102, 509–526 (1979). 33 O. C. J. Lippold, “Oscillation in the stretch reflex arc and the origin of the rhythmical, 8-12 c/s component of physiological tremor,” J. Physiol. 206, 359–382 (1970). 34 J. Vass, R. Smid et al., “Avoidance of speckle noise in laser vibrometry by the use of kurtosis ratio: Application to mechanical fault diagnosis,” Mech. Syst. Signal Process. 22, 647–671 (2008). 35 R. Henderson and K. Schuymeister, Laser Safety (Institute of Physics, Philadelphia, 2004). 36 J. W. Rohrbaugh, E. J. Sirevaag, and E. J. Richter, AIP Conf. Proc. 1457, 266–274 (2012).
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 121706 (2013)
Laser Doppler vibrometry measurement of the mechanical myogram John W. Rohrbaugh,1,a) Erik J. Sirevaag,1 and Edward J. Richter2 1 2
Washington University School of Medicine, 4625 Lindell Blvd., Suite 200, Saint Louis, Missouri 63108, USA Washington University School of Engineering, 1 Brookings Ave., Saint Louis, Missouri 63110, USA
(Received 31 May 2013; accepted 29 July 2013; published online 30 December 2013) Contracting muscles show complex dimensional changes that include lateral expansion. Because this expansion process is intrinsically vibrational, driven by repetitive actions of multiple motor units, it can be sensed and quantified using the method of Laser Doppler Vibrometry (LDV). LDV has a number of advantages over more traditional mechanical methods based on microphones and accelerometers. The LDV mechanical myogram from a small hand muscle (the first dorsal interosseous) was studied under conditions of elastic loading applied to the tip of the abducted index finger. The LDV signal was shown to be related systematically to the level of force production, and to compare favorably with conventional methods for sensing the mechanical and electrical aspects of muscle contraction. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4845435] I. INTRODUCTION: THE MECHANICAL MYOGRAM
Although the mechanical assessment of muscle activity has generally focused on contraction and force production along the longitudinal axis, there are additional dimensional changes during contraction. These dimensional changes are closely related to force production, and because they entail mechanical displacement can be sensed by the Laser Doppler Vibrometry (LDV) method. The most useful signal in this context is the vibration activity that is known to accompany contraction (see review by Orizio1 ). Although not as generally appreciated, or widely applied, as the electromyogram (EMG), the study of muscle vibrations has a long history dating to the early published description of Grimaldi in 1665. Within the past three decades, perhaps sparked by the provocative review of Oster,2 muscle vibrations have received extensive study. Following recommendations of Orizio et al.,3 we adopt here the nomenclature “mechanical myogram (MMG)” to describe these muscle vibrations. Although the published descriptions of the MMG vary considerably among studies, there is general agreement that the spectrum of the signal is low, with peaks in the range of 10–25 Hz (sometimes one at 40 Hz), and little discernible energy beyond about 50–60 Hz. Much of the variability among studies can be related systematically to properties of the muscles studied (including location, fiber composition, level and rate of force production, joint angle, and fatigue). However, a major share of the variability can be traced to the absence of standardized transduction and recording methods. In human studies, the transducers have usually been microphones, piezo electric pressure sensors, or accelerometers.4 Each of these methods has recognized limitations. The microphone-recorded MMG signal (often called the Acoustic Myogram, AMG), for example, shows low repeatability even across microphones of the same manufacturer, is generally insensitive to extremely low frequencies, is highly dependent a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2013/84(12)/121706/9/$30.00
on the dimensions of the cavity separating the microphone diaphragm from the skin, and the coupling media, and is difficult to calibrate with respect to physical vibration units.5–7 In a similar vein, the MMG signals recorded with mechanical transducers have been found to be dependent on such factors as sensor contact pressure,6, 8 to have intrinsic resonant frequencies, and to add a small but non-negligible mass loading to the skin. In this context, the LDV method would appear to offer significant technical advantages, with respect to the broad signal bandwidth (DC to at least 40 kHz), complete absence of mass loading, and lack of intrinsic resonances. It has been convincingly established that the MMG signal arises from contractile properties of the engaged muscle.9, 10 The signal is generally largest over the belly, and diminishes in the directions of insertion and attachment,11 or laterally toward the border.6 There are several non-exclusive candidate mechanisms for the MMG signal. All relate the vibration to the firing patterns of individual motor units, and the resultant mechanical pressure waves. A major contribution appears to derive from the lateral dimensional change of the muscle during contraction.1 Since the muscle bundle remains isovolumic, the decrease in length during contraction is accompanied by lateral expansion. Up to a certain level of force production, increases in force are achieved principally by recruiting additional motor units. The associated MMG grows monotonically in amplitude. Beyond some force level at which all available motor units are recruited, there is a strategy shift whereby additional force is achieved by increased firing rates in the engaged motor units. The MMG amplitude typically becomes asymptotic or reduces in amplitude beyond this point, reflecting a mechanical fusion of the individual motor unit activity at high firing rates. The point of this inflection depends on the muscle fiber composition (proportion of Type I and Type II fibers, and intramuscular distribution with respect to the surface), ranging from 60%–80% maximum voluntary contraction (MVC) for large muscles such as the quadriceps or biceps brachii, to 50% for small hand muscles.12 A number of investigators have observed concomitant increases in frequency, thought to reflect both the increased firing rates of
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individual motor units as well as changes in the types of units recruited.13 Consistent with the mechanical origin of the MMG, there is evidence that its relationship with force production is more veridical than is the case for the EMG—in the face of such factors as fatigue,5, 14–18 temperature,19 and torque angle.20 This has led to the suggestion that the MMG would be particularly useful in the study of muscles for which it is difficult to ascertain loading, e.g., paraspinal or facial muscles. The MMG signal also appears to provide a useful measure of muscle fiber composition,21 and to be useful in the assessment of neuromuscular diseases22–24 and aging.12 In sum, the MMG signal has a number of desirable features as a measure of muscle contraction. Among the most important in the present context is that it is present in the form of a mechanical vibration that can be detected at the skin, and, as such, can be sensed remotely using the LDV method. II. VALIDATION STUDY USING A SMALL HAND MUSCLE: RATIONALE
As an initial study to investigate the effectiveness of the LDV method in this context, we investigated the activity of a small muscle in the hand (the first dorsal interosseous, 1DI). There were two principal advantages to this approach: (i) the muscle loadings could be systematically controlled, allowing us to establish the relationships between signal characteristics and contractile force; (ii) our findings could be anchored in the prior literature based on detailed study of the small hand muscles, using other recording methods. An additional benefit is that, by controlling hand and finger position, the method offered a standardized situation in which to assess physiological tremor. Tremor is a potentially useful measure of stress and emotion as well as neuromuscular function, but is regulated by numerous factors including body position and muscle tension levels that ideally would be controlled at this initial stage of investigation. Our procedures were adopted from those used by McAuley et al.,25 who studied 1DI and finger tremor activity under conditions of controlled elastic loading applied to the fingertip. Unlike mass loading, elastic loading does not affect the intrinsic resonance of the body segment, nor does it damp high-frequency components of tremor. The tremor, EMG, and MMG activities were found to share several spectral peaks, at about 10, 20, and 40 Hz. The amplitude but not frequency of these peaks was affected by muscle loading. Within each frequency band, the tremor, EMG, and MMG signals showed high coherence, indicating that they were different manifestations of the same oscillatory mechanism. The authors concluded that the signals in the different frequency bands were not harmonics but rather derived from independent neural oscillators, each with different functional properties. III. METHODS
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session. Data shown here are based on 14 participants (5 female) from whom technically satisfactory records were obtained in all conditions. Participants were paid at the rate of $15 per h for their participation. Participants provided informed consent, according to procedures approved by the Human Research Protection Office at Washington University School of Medicine. Exclusion criteria included self-reported skeletal, movement or neuromuscular disorders, hypertension or other cardiovascular disease, use of tobacco, and use of tremorolytic or tremorogenic medications. Participants were instructed to avoid caffeine for at least 3 h prior to the test. B. Recording methods
During recording, the hand was rotated to a position midway between pronation and supination, and supported on the padded surface of the chair arm. Participants were instructed to extend the index finger, and to curl the remaining fingers into a loose fist. During contraction, the index finger was lifted (abducted) against the elastic loading (or mass loading in some conditions) applied to the fingertip. Participants were instructed, and frequently reminded, about the need to restrict contraction to the 1DI muscle and to avoid coactivation of other hand or arm muscles. This recording position differs somewhat from that of McAuley et al.25 who used a position in which the hand was rested in a pronated position, with all of the fingers outstretched. Our intent in using the mid position was to allow mass loading to be applied in the abduction/adduction plane. The recording jig is illustrated in Figure 1. Loading was applied to a light-weight plastic ring, machined from PVC tubing (21 mm ID, 5 mm long), into which the index finger was inserted to a depth of approximately 1 cm (“B” in Figure 1). This ring was attached to either an elastic link that was connected to the beam of a load cell force transducer (World Precision Instruments FORT 5000) (“B” in Figure 1) or to mass loadings hung from the ring. The elastic link was a #19 rubber band (1/16 in. × 3 12 in.) which was stretched by varying the distance to the force transducer beam so as to control the applied loading. During resting conditions, the
C
D
E
F G
B
A
A. Participants
Twenty participants (10 female), all right handed and ranging in age from 18 to 31 served in a single 2–3 h
FIG. 1. Recording setup. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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ring rested in a cradle. Participants were trained to lift the ring slightly—just enough barely to clear the cradle—during contraction and to ease it back into position at the end of contraction. Recordings were taken from several places on the fingertip and over the 1DI muscle: Finger tremor was measured with an Entran EGA-25/R miniature single axis accelerometer (with the active axis in the abduction/adduction plane; “D” in Figure 1), taped to the surface of the index finger just proximal to the distal joint. EMG was measured with a Biopac MP150 physiological recording system, amplified using a TEL100 remote recording unit. The data were amplified 5 k, and analog bandpass filtered 0.5–500 Hz before sampling. The active electrode was placed over the proximal 1DI muscle with a reference electrode over the trapezium (“G” in Figure 1). AMG was measured using a small electret microphone (Optimus 33-0013) that was cemented to the plastic housing of a 9 mm diameter electrode, in which the metal element had been removed (“E” in Figure 1). The interior of the shell was machined to a conical shape, terminating in a 3 mm aperture which matched the opening of the microphone. The microphone was positioned over the distal end of the 1DI muscle, and attached to the skin using a double-sided adhesive collar with care taken to assure that the seal was air-tight. The microphone signals were amplified 50 times with a bandpass of 1–500 Hz. Test recordings showed that the MMG signal was abolished when the microphone cavity was filled with modeler’s clay, indicating that the recorded signals originated from the skin surface (rather than from gross movements of the microphone itself). LDV signals were recorded, in different runs, from either the fingertip (“C” in Figure 1) or over the belly of the 1DI muscle (“F”), with an offset distance of 275 mm. To enhance the optical quality of the laser signal, a small piece of reflective tape (1 cm square) was attached to the target sites. Data were acquired with a Polytec OFV-303 laser head and OFV 3001 controller, with a 90◦ attachment so as to direct the beam vertically (radial to the measurement sites). LDV data were measured with a sensitivity of 5 mm/s/V. Data were sampled at a rate of 1600 Hz, using a Biopac MP150 system. Additional channels were devoted to cardiorespiratory variables, which are not reported here.
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conditions were obtained at the end of the session, in balanced order. Each run included a 15 s baseline (pre-contraction) segment followed by a lift of 20 s. Instructions emphasized the importance of maintaining a steady lift and avoiding cocontraction of other muscles. Individual lifts were separated by at least 2 min, to minimize possible fatigue effects. IV. RESULTS
For the present purposes, we restrict our presentation of results to the findings bearing on the most relevant questions of (i) the general form of the LDV MMG signal, (ii) the relationships with level of force production, (iii) comparisons with the conventional EMG, and (iv) comparisons between the LDV-and accelerometer-recorded tremor signals.
A. General appearance of LDV-recorded MMG signal
An example of typical myographic signals is presented in Figure 2. These data were obtained from a single participant under 30% MVC elastic loading. The second trace illustrates the microphone-recorded MMG, and the third is the velocity record from the LDV. For comparison purposes, the simultaneous EMG is shown in the top trace. These records all have been digitally high-pass filtered at 5 Hz. The microphoneand LDV-recorded signals show vibration activity throughout the lift. As has been noted by other investigators (see review above), there tends to be a burst of activity at the onset of contraction, in both the microphone- and LDV-recorded signals. Also clearly seen is another burst at the end of contraction. These bursts are thought to accompany the large dimensional changes that are seen at the beginning and end of contraction.1 The onsets of the MMG signals occur simultaneously, and precede slightly the attainment of maximum EMG amplitude (which appears to have a more ramp-like onset). The reverse pattern is seen at the end of contraction, with the MMG signals persisting beyond the cessation of the EMG signal. In the bottom panel, the LDV velocity signal has been integrated to produce a displacement signal, for purposes of illustrating the absolute magnitude of the movements present in vibration signal. In this typical example, the peak-to-peak vibration
EMG
C. Procedure
Participants lifted, and relaxed, their finger according to instructions displayed on a projection screen (on which fixation was maintained throughout the contractions). In addition to resting (no load) conditions, data were obtained at 5%, 15%, and 30% MVC elastic loading, and at 15% and 30% mass loading. MVC was individually determined for each participant by inserting a solid link between the ring and the beam of the force transducer. MVC was defined as the maximum value of three attempts. The 5% MVC elastic loading data were obtained during baseline recording conditions, in a phase of the experiment aimed at assessing the effects of mental stress on tremor (not described here). The remaining
1 mV
Microphone LDV Velocity 1 mm/s
LDV Displacement 25 µm
“Lift”
“Relax”
FIG. 2. General appearance of LDV-recorded MMG signal. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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FIG. 3. Spectra of LDV-recorded MMG signals, at rest and three levels of elastic loading. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
displacement during the sustained portion of the contraction is on the order of 30–50 μm.
B. Effects of elastic loading on muscle signals
The mean spectra (averaged over the 14 participants) for the LDV-recorded MMG signals are shown in Figure 3. These spectra are based on uniform data segments of 19 s, starting 1 s after the instruction to lift (abduct) the finger, which were tapered using a Welch window. As illustrated in Figure 2, the data are fairly stable during this period. Spectra were computed using the method described by Welch,26 as implemented in Matlab. The four overlaid spectra correspond with the baseline (zero loading), 5%, 15%, and 30% MVC elastic loading. The spectra reveal two clearly defined peaks (plus a third peak that is sharply tuned at 30 Hz which is artifactual, deriving from vibration in the vertical axis produced by the building’s mechanical equipment). The major early peak is close to 10 Hz, and the second is a broader peak with maximum power in the range of 20–30 Hz. There is a weak suggestion of an abundance of power at about 35–40 Hz, but this was not a consistent feature in the records from individual participants. As is clearly evident in the figure, the amplitude of the signal grows as a function of increasing load, for both the 10 Hz and 25 Hz components. This systematic relationship with elastic load is illustrated in Figure 4, in which the mean peak power within a window of 23–27 Hz has been plotted for baseline (preabduction) and abduction periods. Analysis of variance confirmed that the existence of significant effects, which were decomposed using tests of simple effects to confirm that power was significantly increased during all three loading conditions (p < 0.05). The loading also varied significantly from one another (p < 0.05 for all comparisons except the comparison between the 15% and 30% conditions, which was marginally significant, p = 0.056). Similar results (not
illustrated) were obtained for the earlier peak at about 10 Hz, measured as the peak within the window of 9–13 Hz. The corresponding EMG values, expressed in terms of RMS amplitude during the identical data segments, are shown in Figure 5. As was the case for the LDV-recorded MMG measures, the EMG amplitudes were significantly increased with respect to baseline under all loading conditions, and the amplitude significantly increased in proportion to the loading (p < 0.05 for all comparisons). The possibility that there might be systematic fatiguerelated effects over the 20 s contraction was assessed by splitting the 19 s analysis segment in half. As illustrated in Figure 6, there was a weak suggestion of decreased power late in the abduction for the 5% and 30% MVC conditions, but this apparent change did not approach statistical significance. For comparison purposes, the spectra associated with the acoustic microphone-recorded MMG are shown in Figure 7.
FIG. 4. Mean peak power of LDV signal, within 23 Hz band, at rest and three levels of force production. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
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The spectra disclose a broad peak with maximum power at about 10 Hz. Although the power is increased during the loading conditions, the spectra in general show less definition than seen in the LDV-recorded MMG signals. Also, the relationship with loading is less regular, with the largest values obtained under the 5% loading. There are two likely reasons for the discrepancies, one being the different (and quite variable) recording properties of microphone sensors, and the second being a difference in transducer placement (with the microphone being placed more distal than the LDV recording site). C. Effects of elastic loading on finger tremor
FIG. 5. EMG amplitude (RMS) at rest and three levels of force production. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
FIG. 6. LDV-recorded MMG power during early and late contraction, at three levels of force. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
FIG. 7. Acoustic microphone-recorded mechanical myogram, at rest and three levels of force. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
Tremor at the fingertip was assessed using both the LDV, and the miniature Entran accelerometer. The associated LDV tremor spectra, assessed during the same data segments as defined above, are illustrated in Figure 8. These spectra are very similar in waveform, and responsiveness to loading conditions, to those associated with the LDV-MMG obtained from the 1DI muscle (Figure 3) (except that the 30 Hz artifact component is damped). Statistical analyses of the peak power within the 23–27 Hz window showed that the 5% loading condition was significantly different from both the 15% and 30% loading conditions (p > 0.05), although these latter two were not statistically distinguishable. The LDV tremor spectra were also very similar to the spectra of the signals obtained simultaneously from the Entran accelerometer (not illustrated). The degree of similarity between the LDV- and accelerometer-recorded finger tremor was assessed by computing the coherence between the two measures. The LDV signal was converted by differentiation to an acceleration signal prior to this analysis. Coherence overall was high, in the range of 95% across the band of about 5–40 Hz that encompassed the majority of tremor power. D. Effects of mass loading
Figure 9 illustrates the spectra associated with the LDVrecorded MMG signal during the application of 15% and 30%
FIG. 8. Spectra of LDV-recorded fingertip tremor, at rest and three levels of elastic loading. Reprinted with permission from AIP Conf. Proc. 1457, 266– 274 (2012). Copyright 2012 American Institute of Physics.36
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V. DISCUSSION
FIG. 9. LDV1DI spectra at rest and under two conditions of mass loading. Reprinted with permission from AIP Conf. Proc. 1457, 266–274 (2012). Copyright 2012 American Institute of Physics.36
mass loading. The spectra contain a well defined peak at about 10 Hz, as was seen in the MMG spectra associated with elastic loading. The higher-frequency (∼25 Hz) peak is absent. In addition, the spectra include a sharply tuned component at lower frequencies, peaking at about 4.5 Hz for the 30% MVC loading, and 5.5 Hz for the 15% MVC loading. The peak frequency was significantly lower (p < 0.05) for the 30% MVC loading. The associated LDV-recorded finger tremor data are shown in Figure 10. The overall amplitude was increased by approximately 7-fold in comparison to the data from the elastic loading conditions (shown in Figure 8), with the majority of power concentrated in the narrow, low frequency bands corresponding to those seen in the LDV-recorded MMG signal. As with the LDV-recorded MMG signal, the peak frequency was significantly lower with the 30% MVC loading.
The primary purpose of this study was to investigate the basic properties of the LDV-recorded MMG and tremor signals, with a goal of establishing whether meaningful and useful features of muscle activity could be sensed remotely. The long-range goals are to utilize the method to assess muscle (especially facial muscle) and tremor signs of emotion and mental stress; however, for this initial study, the emphasis was on a much simpler and more rigorously controlled recording situation involving the fingertip and small hand muscle (1DI). As noted in the Introduction, this model recording situation has the advantages of allowing for the precision application of elastic and mass loading, and there is a substantial existing literature regarding the form of the electrical and mechanical muscle signals from the 1DI, to provide a basis for interpreting the LDV-based signals. The findings indicate clearly that the LDV method is capable of recording high-quality MMG and tremor signals. This conclusion is supported by multiple lines of evidence: (1) The LDV-recorded MMG signal was observed to be of greater amplitude during contraction, and the duration of the increase was coextensive with the period of force production (as illustrated in Figure 2). (2) The nature of the effects associated with elastic and mass loading were consistent with anticipated effects, insofar as the signal amplitude was monotonically related to the level of force production, and the mass loading produced predictable inertial-related effects, particularly in finger tremor. (3) The LDV-recorded MMG and tremor signals showed high overall levels of agreement with signals recorded using conventional EMG, microphone, and accelerometer methods. (4) The signals agreed well with published descriptions of tremor and MMG activity in the 1DI muscle, as assessed by other investigators using a variety of methods (e.g., Refs. 5, 25, and 27). A. Effects of elastic loading on the MMG
FIG. 10. LDV Fingertip tremor spectra at rest and under two conditions of mass loading.
The MMG spectra associated with elastic loading showed multiple peaks, with the predominant peak at about 10 Hz and a broader peak at about 25 Hz. As noted by McAuley et al.,25 the application of elastic loading generally enhances and sharpens the higher frequency components. Even in the absence of elastic loading, a number of investigators have observed multi-peaked spectra for the MMG signal.12, 22, 28, 29 Previous descriptions of the spectral composition of the MMG signal have been remarkably variable, probably for several reasons. The measured frequency of the MMG signal generally depends on the force applied, increasing at high forces, but is stable below 30% MVC1 —consistent with our findings that the peak frequencies were not affected by force level within that range. Another major source of variability almost certainly lies in the transducer type used. Microphonebased recorders, in particular, are known to be quite variable, depending on cavity size, conducting media, and recording bandwidth.4, 7 Although the microphone-based MMG records
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in the present study were similar in overall form to the LDVbased MMG records, they showed less spectral resolution and a relative attenuation of activity at frequencies above the dominant ∼10 Hz component. Despite this variability among studies, however, there is some consistency in pointing to two principal bands, at ∼10 Hz and 25 Hz,1 as were observed in the present study. Although there was a slight abundance of energy at about 35–40 Hz, there was no clear peak in the averaged MMG and tremor spectra at ∼40 Hz (Piper rhythm), as has been observed by other investigators.22, 25 Other investigators have commented on the marked individual differences in this peak, with respect to both amplitude and frequency, and it is quite possible that its absence in the averaged spectra reflects this variability. The magnitude of the spectral power in both the ∼10 Hz and 25 Hz MMG bands was progressively and significantly increased as the elastic loading was increased from 0% to 5%, 15%, and 30% MVC. Unlike the case for the EMG, however, the LDV-recorded MMG increase was nonlinear, with a relatively small increment as the loading was increased from 15% to 30%. This is consistent with a well established property of MMG signals, which typically are observed to become asymptotically large in the range of 60%–80% MVC, with stable or decreasing values beyond that point.1 The point of inflection depends on the specific muscle tested, and the associated fiber composition.12 Although several factors apparently contribute to this nonlinearity,30 the principal reason for the inflection in the force/MMG amplitude function is related to strategy engaged in producing increasing force. Below the inflection point, increased force production is achieved principally by recruiting larger numbers of motor units, which produce correspondingly larger MMG amplitudes. At some point, all available units are recruited, and additional force is produced by increasing the firing rates of individual motor units. This results in a mechanical fusion of the vibration signal, and attendant reduction of overall surface vibration activity. B. Effects of elastic loading on finger tremor
The original signals, and accompanying spectra, for tremor at the fingertip (in the elastic loading conditions) closely resembled those obtained from the 1DI. This is to be expected, given the close mechanical coupling between the finger and 1DI muscle, and the low mass of the finger in the elastic loading conditions. As with the 1DI signal, the finger tremor spectra showed two principal peaks, at ∼10 and 25 Hz, both of which were monotonically related to elastic loading. Comparison of the simultaneous LDV-and accelerometer-recorded tremor records disclosed a high coherence across the band encompassing the majority of tremor power.
Rev. Sci. Instrum. 84, 121706 (2013)
especially, finger tremor. The added mass served to damp the higher frequency tremor activity, producing a large, narrowband, low frequency tremor signal that was nearly sinusoidal in appearance. The peak frequency was slightly (∼1 Hz) but significantly lower for the 30% mass loading than for the 15% mass loading. These effects associated with the mass loading are consistent with well-established principles relating to inertial effects in resonant systems, and are consistent with prior reports (e.g., Ref. 31). Tremor under these conditions is thought to derive principally from phase-preserved stimulation of stretch receptors, and accompanying activation of monosegmental stretch reflexes.32, 33 The clear preservation of the ∼10 Hz peak in the accompanying LDV-recorded MMG record, which was not apparent in the finger tremor record, indicates that the rhythmic driving signal (of apparent central origin25 ) was not abolished in the presence of the reflex loop activity.
D. Additional considerations
The application of the LDV method to MMG recording has several identifiable constraints and limitations. A largely self-evident consideration is that the signal will be best recorded from the skin (or, as in the case here, from a reflective surface attached directly to the skin). Even though mechanophysiological energy, if large enough, can be transmitted to clothing if the contact is intimate, the small size of the MMG signal suggests that access to unclothed skin would be required. We elected to treat the target skin site by applying a small (and low mass) patch of retro-reflective tape. This served two purposes: (1) to mark the laser target zone and thus support consistent pointing, and (2) to improve the reflectivity of the surface and thus the quality of the LDV signal. The primary artifact when recording from optically noncooperative surfaces appears in the form of speckle dropouts, manifest here as aperiodic and aphysiologically brief, large transients. There are a number of linear and nonlinear methods that have been developed for suppressing this artifact (e.g., Ref. 34), including those integrated in the signal processing software of some commercial LDV systems. We have confirmed the effectiveness of such methods, particularly for the relatively low band that encompasses MMG signals, and now routinely record from untreated skin. We note in this context that there are additional mechanophysiological signals in this band—including cardiorespiratory activities, particularly from the thorax but broadly distributed—which must be distinguished and separated from the MMG signal. Additional considerations include the requirement for dynamic tracking if the laser target site is kinetically active, as well as the general considerations concerning eye and skin safety that apply to the use of lasers.35
E. Future directions C. Effects of mass loading on the MMG and finger tremor
The addition of mass loading to the fingertip produced substantial qualitative changes to the MMG signal and,
In follow-up studies, we have recorded from facial muscles and have confirmed that the method is indeed effective in that application. Our findings indicate that highly focal patterns of MMG activity can be obtained selectively,
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ACKNOWLEDGMENTS
This work was supported by grant from the Department of Defense Polygraph Institute (now the National Center for Credibility Assessment). EMG
Frown 1 C.
LDV Neutral
Smile
Weak Smile
FIG. 11. Illustration of LDV MMG signals from facial muscles under conditions of selective activation, and accompanying EMG.
over the facial muscles involved in the production of common emotion- and stress-related facial gestures. One of our early recordings, which illustrate the general pattern of results, is shown in Figure 11. Records were obtained from two facial sites (overlying the corrugator and zygomatic muscles), as the participant frowned (top traces) and smiled (middle traces). In each case, there are two overlaid traces, representing the EMG record (black) and simultaneous LDV MMG signal (red). As the subject frowns, there is activity from the corrugator muscle, but the zygomatic muscle remains silent. Conversely, during a smile the zygomatic is active, whereas there is little activity over the corrugator muscle. Shown at the bottom are records from a weak smile, in which case both the EMG and LDV signals are reduced in amplitude. These findings indicate that differential patterns of facial muscle activation can be recorded with the LDV method, that the signals are graded in amplitude in proportion to the strength of the expression, and that the signals agree well with conventional EMG records. We have since conducted much more detailed studies of the character and spatial distribution of the LDVrecorded facial MMG signals. We have also determined that MMG activity can be consistently detected, even at levels of tension that are too low to produce visible deformations of the face. Overall, the sensitivity of the method is comparable to the EMG method, but, unlike that method, can be obtained remotely, on a non-contact basis. VI. CONCLUSIONS
The present study was conducted as an initial validation study under carefully controlled conditions. The principal goal was to establish the extent to which mechanical myographic activity and tremor could be recorded on a non-contact basis, using the LDV. The experiment showed that high quality signals could indeed be recorded. The signals appear to be valid measures of muscle and tremor activity, on a number of counts. These include the systematic relationships with the nature of the elastic and mass loadings and the similarity of these relationships with those observed using conventional measures based on EMG, accelerometry, and microphone recording.
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