European Journal of
Eur. J. Appl. Physiol. 42, 159-163 (1979)
and Occupational Physiology 9 Springer-Verlag 1979
Electromechanieal Delay in Human Skeletal Muscle Under Concentric and Eccentric Contractions P. R. Cavanagh I and P. V. Komi 2 113iomechanics Laboratory, Penn State University, University Park, PA 16802, USA 2Kinesiology Laboratory, Department of Biology of Physical Activity, University of Jyv~iskyl~i, SF-40100 Jyv/iskyl~i 10, Finland
Summary. In contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The rigth forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of ,,~ 0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface E M G (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p < 0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development. Key words: Muscle activation -- Muscle mechanics - Muscle elasticity
It is well known that there is a delay between the onset of active state in skeletal muscle and the development of tension. This delay, called here electromechanical delay (EMD), was important in the formulation of the two component model of muscle by Hill (1950) in which he postulated that the slow development of tension was due to the presence of elastic elements in series with the contractile element. Inman et al. (1952) observed a delay of 80 4- 20 ms between peak muscle force and Offprint requests to: Prof. Dr. Paavo V. Komi (address see above)
P.R. Cavanagh and P. V. Komi
the ' a p p a r e n t ' peak in a rectified and smoothed E M G from three muscles of cineplastic amputees performing isometric contractions. Corser (1974) recorded surface E M G ' s from the flexors and extensors o f the forearm during various forearm movements. H e reported delays of 2 5 - 7 5 ms between the onset of E M G and the onset of movement. Since Corser chose to measure displacement and not force or acceleration, it is likely that the true E M D was shorter than his measurements indicated. N o r m a n and K o m i (1979), measuring E M D to the point of acceleration or deceleration of the forearm, observed values in the order of 2 5 - 4 5 ms. Hill (1970) has shown that the rate o f rise o f tension in isolated muscle is a function o f the rate o f change o f muscle length at the time o f stimulation. The present study was therefore designed to investigate the dependence o f E M D in the h u m a n elbow flexor group upon selected initial conditions at the time o f muscle activation. A preliminary report o f the paper has been given ( K o m i and Cavanagh, 1977).
Fourteen male physical education students (mean height: 179.4 cm; weight: 73.4 kg) were seated in a recumbent position with their right forearms attached in a semi-pronated position to a motorized dynamometer previously described by KomJ (1973). This device enabled the forearm to be flexed or extended through a 135-deg range at a constant rate regardless of the force exerted on a wrist cuff by the subject. The wrist cuff was strain-gauged so that any force output whether due to gravity or muscular action could be recorded. The dynamometer could also be fLxed at any location so that isometric force could be measured. During passive oscillation of the limb at approximately 0.5 rad/s a stimulus light was presented to the subject as the arm passed through the test angle of approximately 110 deg (measured between the ventral surfaces of the limb segments). Upon presentation of this stimulus the subject was instructed to
.~, 30~V ...
srl.uLu l E.,
FORCE : COMPARATOR
i IELECTROMECHANICAL i
i >; i
Fig. 1. Schematic representation of the experimental record showing the threshold levels required to trigger the EMG and force comparators
Electromechanical Delay in Human Skeletal Muscle
maximally activate his relaxed elbow flexors for a brief period of time. Visual feedback of EMG activity was given to aid subjects who found relaxation during oscillation difficult and, to avoid anticipatory responses a random generator was incorporated so that the stimulus light did not appear every time the test angle was encountered. When the subjects were fully accustomed to the experimental conditions, 12 trials were recorded during flexion (concentric, CONC), 12 during extension (eccentric, ECC) and a further 12 during a brief maximal isometric effort (ISOM) at the test angle. Silver-silver Chloride surface electrodes 1 cm in diameter were placed in a bipolar configuration over the biceps brachii and brachioradialis. Low noise preamplifiers (10 ~V p to p) with a gain of 60 dB between I0 Hz to 5 kHz were used for the electromyograms in conjunction with analog comparators. The comparators, which incorporated full wave rectifiers, changed their logical output from 0 to 1, when the amplitude referred to input was greater than 30 pN. A similar comparator with the force signal as input changed state when forces above 20 N were applied to the wrist cuff. This threshold was needed to ensure that vibration in the dynamometer did not result in spurious events being recorded. The comparator outputs and a pulse coincident with the stimulus light (see schematic diagram in Fig. 1) were monitored on-line by a Hewlett Packard 2116C digital computer. Software calculations of the delays between stimulus and the various comparator pulses were performed with a precision of 0.1 ms. The value selected as the EMD during any trial was the time between the onset of the first active muscle and the change of state of the force comparator.
The data given in Table 1 and shown graphically in Fig. 2, were subjected to an analysis of variance with repeated measures (Winer, 1971). This procedure indicated the mean EMD under eccentric conditions (49.5 ms) was significantly shorter (t7 < 0.01) than the delay in both the isometric (59.3 ms) and concentric (55.5 ms) conditions. The difference between the ISOM and CONC conditions was not significant (p > 0.01).
Table 1. Mean values of 12 trials (in ms) for EMD in all subjects Subject
1 2 3 4 5 6 7 8 9 10 11 12 13 14
57.0 69.7 44.7 45.0 35.6 56.8 58.2 66.2 42.6 35.7 47.0 38.2 43.0 52.5
68.0 75.1 60.5 44.4 45.9 54.7 48.7 62.2 46.8 46.4 48.4 44.8 48.8 60.0
67.3 77.0 49.6 43.8 42.9 49.0 59.5 68.2 59.6 46.6 52.1 41.9 49.0 69.8
P.R. Cavanagh and P. V. Komi
/./ */*/~ 50
50 ANGULAR VELOCITY (Rod.=$"1)
I I 0
Fig. 2. Mean (+ S.D.) values of electromechanical delay in eccentric, isometric, and concentric conditions
The electromechanical delay (EMD) calculated in the present study contains several components, which all are linked to the generation of force in the skeletal muscle. These are (1) conduction of the action potential along the T-tubule system; (2) release of calcium by the sarcoplasmic reticulum; (3) cross-bridge formation between actin and myosin filaments, and the subsequent tension development in the contractile component (CC); (4) stretching of the series elastic component (SEC) by CC. From these the time spent for the events in 1 to 3 normally occupy only a small portion of EMD. Therefore, the possible explanation to account for the observation reported here should result from the differences between CONC and ECC conditions in stretching of SEC. Experiments reported by Hill (1970) have shown that a faster rate of tension development is possible in isolated muscle which is being stretched at the onset of stimulation. Thus, in the present study ECC condition was likely to cause a faster stretch of SEC and consequently to result in exceeding of the force threshold level earlier than in ISOM or CONC conditions. It should be pointed out that the velocities used in this experiment represent only a small portion of the range which is observed in normal human movement. Angular velocities almost 50 and 25 times greater than the 0.5 rad/s used here have been observed for elbow extension and flexion, respectively (Cavanagh and Landa, 1976; Nelson and Fahrney, 1965). It is likely that increased rates of stretch and shortening at the onset of activation would tend to accentuate the small time differences between th three conditions which have been demonstrated in this experiment. We can thus anticipate EMD durations considerably shorter than 50 ms for activation of the elbow flexors during rapid extension and considerably longer than 55 ms for activation during rapid flexion.
Electromechanical Delay in Human Skeletal Muscle
The shorter E M D during the E C C condition is of particular relevance in situations where there is a 'counter movement' preceding the main movement - a phen o m e n o n which can be called the 'stretch-shorten cycle'. In this cycle the active muscle is first stretched during which period it is assumed to store elastic energy, which then can be recovered at least partially in the subsequent concentric condition ( C a v a g n a et al., 1965). The more rapid development o f force observed here under the E C C condition is certainly a contributing factor to the higher power outputs which have been calculated during vertical j u m p i n g following a counter movement (Asmussen and Bonde-Petersen, 1974; K o m i and Bosco, 1978), and 'elastic storage' should not be used exclusively to explain this increment in power output.
References Asmussen, E., Bonde-Petersen, F.: Apparent efficiency and storage of elastic energy in human muscles during exercise. Acta Physiol. Scand. 92, 537-545 (1974) Cavagna, G. A., Salbene, F. P., Margaria, R.: Effect of negative work on the amount of positive work performed by an isolated muscle. J. Appl. Physiol. 20, 157-158 (1965) Cavanagh, P. R., Landa, J.: A biomechanical analysis of the karate chop. Res. Quart. 47, 610-618 (1976) Corser, T.: Temporal discrepancies in the electromyographic study of rapid movement. Ergonomics 17, 389--400 (1974) Hill, A. V.: The series elastic component of muscle. Proc. R. Soc. [Biol.] Lond. 137, 273-280 (1950) Hill, A. V.: First and last experiments in muscle mechanics, p. 71. Cambridge: University Press 1970 Inman, V. T., Ralston, H. J.: Saunders, C. M., Feistein, B., Wright, E. W.: Relation of human electromyogram to muscular tension. Electroeneephalogr. Clin. Neurophysiol. 4, 187--194 (1952) Komi, P. V.: Measurement of the force-velocity relationship in human muscle under concentric and eccentric contractions. In." Medicine and sport series, Vol. 8, Biomechanics III, Cerquiclini, S., Venerando, A., Wartenweiler, J. (eds.), pp. 224-229. Basel: Karger 1973 Komi, P. V., Cavanagh, P. R.: Electromechanical delay in human skeletal muscle. Med. Sci. Sports 9, 49 (1977) Komi, P. V., Bosco, C.: Utilization of stored elastic energy in leg extensor muscles by men and women. Med. Sci. Sports 10, 261--265 (1978) Nelson, R. C., Fahrney, R. A.: Relationship between strength and speed of elbow flexion. Res. Quart. 36, 455-463 (1965) Norman, R. W., Komi, P. V.: Electromechanical delay in skeletal muscle under normal movement conditions. Acta Physiol. Stand. (in press) (1979) Winer, B. J.: Statistical principles in experimental design, 2nd ed. New York: McGraw-HiU 1971 Accepted July 22, 1979