The firing pattern of single motor units from the anterior tibial muscle was studied during constant isometric contraction. Recordings were made with bipolar, fine-wire electrodes from 4 normal subjects and from 10 patients with spasticity. A microcomputer was used to classify the motor unit potentials automatically and to measure the interspikeintervals. The patients had reduced firing frequencies and reduced variability between neighbor intervals. Seven patients with spasticity due to supraspinal lesions had positive correlation between neighbor intervals. MUSCLE & NERVE
1:416-418
1978
lMPAlRED REGULATION OF THE FIRING PATTERN OF SINGLE MOTOR UNITS STEEN ANDREASSEN, PhD, and ANNELISE ROSENFALCK, PhD
Investigations of the firing pattern of motor units are hampered by the difficulties in obtaining recordings from single units at moderate and high levels of voluntary contraction. Except for a study on patients with Parkinson's d i ~ e a s eonly , ~ few patients with central motor lesions have been ~tudied.~.~.' In this study, the firing pattern of single motor units in the anterior tibial muscle was investigated in 4 normal subjects and in 10 patients with spasticity caused by cerebral or cerebellar hemorrhage or by multiple sclerosis. Differences were found between normal and spastic subjects in the statistical parameters employed to analyze the firing pattern.'ag The results also indicate that these changes in the statistical parameters may be characteristic of the site of the lesion. MATERIALS AND METHODS Rocording. The single-unit electromyogram (EMG) is
recorded from the anterior tibial muscle using a bipolar, fine-wire e l e ~ t r o d e The . ~ force developed by the anterior tibial muscle is measured by means of a strain-gauge arrangement as torque around the ankle joint. The force is displayed on a large meter suspended over the patient,
From the Institute of Electronic Systems, Aalborg University Center, Aalborg, Denmark. Acknowledgments: This study was supported by the Danish Council for Scientific and Industrial Research and by the Danish Medical Research Council. Address reprint requests to Dr. Andreassen at the Institute of Electronic Systems, Aalborg University Center, Badehusvej 1A, Postbox 159. DK9100 Aalborg, Denmark. 0148-639X/0105/0416 $OO.OO/O 1978 Houghton Miff in Professional Publishers
@
416
Regulation of Firing Pattern
and the patient is asked to keep the force constant d u r ing the recording. Both the EMG and the force signal are recorded on an FM tape recorder for later analysis. Moaruromont of IntorspikeIntervals. At least 200 con-
secutive intervals between action potentials from each motor unit are measured in order to determine the statistical parameters with sufficient accuracy. In each patient, 10 to 20 motor units are sampled. A microcomputer is used to classify action potentials and to measure intervals from a maximum of three different motor units.lg In most cases, superposition potentials are classified correctly, and noise is rejected (fig. 1). The output from the computer is used for a visual check of the classification, and the microcomputer is programmed to allow easy correction of classification errors. RESULTS AND DISCUSSION
The differences in firing pattern between normal subjects and patients are exemplified by a plot of instantaneous frequency from a normal subject and from a patient with a cerebellar hemorrhage (fig. 2). For the normal subject, the mean firing rate is about 15 Hz, while the mean firing rate for the patient is about 8 Hz. This is a typical finding: the firing rate was reduced in all 10 spastic patients. The firing rate ranged from 7 Hz to 20 Hz for the normal subjects, and from 5 Hz to 13 Hz for the patients. The appearance of the instantaneous frequency curve is quite different for the normal subject and for the patient (fig. 2). We have used two statistical measures to describe this difference: VAR and FRHO. VAR is a measure of the variability between neighboring intervals.* Nine spastic patients showed reduced variability between
MUSCLE & NERVE
Sept/Oct 1978
kgxcm
I
100 m s
I
Figure 1 . Automatic classification of action potentials from two motor units (second trace). The classification of each action potential is indicated by the height of the pulse (top trace). Traces three and four are, respectively, the force recording and a timing trace generated by the microcomputer. This recording contains two superposition potentials; both were classified correctly, as indicated by the pair of high-low pulses in the classification trace (top trace). The small-amplitude potential was rejected by the computer as noise, as indicated by the negative pulse.
spiky appearance of the instantaneous frequency curve for normals (fig. 2). Although negative values of FRHO were found in the normal subjects, FRHO was less negative or even positive in spastic patients. In the three patients with multiple sclerosis and primarily spinal lesions, a few significantly positive values of FRHO were found, but the majority of the values were within the normal range. In seven patients with supraspinal lesions, the values of FRHO were significantly increased. If this difference between patients with spinal and supraspinal lesions can he confirmed on a larger number of patients, it may prove helpful as a diagnostic tool.
neighboring intervals; one patient with a cerebral hemorrhage showed increased variability. This reduced variability is a surprising finding, as most of the patients had difficulty maintaining a constant force level during recording. FRHO is a serial correlation coefficient corrected for trends. A negative FRHO indicates that long and short intervals tend to alternate. If FRHO is positive, a long interval is usually followed by a long interval; correspondingly, a short interval is usually followed by another short interval. We generally found negative values of FRHO in normal subjects, and this tendency for alternating long and short intervals contributes to the
NORMAL
I
INSTANTANEOUS FREQUENCY
a*.
0
2 0
a*.
0
1 5 1 0
4 4 . 0
I * .
5
0
FRHO-0.33
. 0 0
2 4 .
1 0
er
2 0
5
n
7 2 6
3 5 . a
1 5
18. 0
1 0
5.
5
Q
0
. a 0
Regulation of Firing Pattern
1 4
2 0
Figure 2. Instantaneous frequency and force, measured as torque at the ankle joint for a normal 26-year-old subject and a 59-year-old patient with cerebellar hemorrhage. Both the normal subject and the patient exert a force corresponding to 20% of maximum voluntary contraction.
MUSCLE & NERVE
Sept/Oct 1978
417
REFERENCES 1 . Andreassen S: Interval pattern of single motor units. PhD thesis, Technical University of Denmark and University of Copenhagen, 1977, pp 1-197. 2. Andreassen S, Rosenfalck A: Single motor unit recording. Proceedings of the IVth Nordic Meeting on Medical and Biological Engzneering. Copenhagen, Lyngby, 19:1-3, 1977. 3. Andreassen S, Rosenfalck A: Recording from a single motor unit during strong effort. IEEE Trans Biomed Eng (in press). 4. Dietz V, Hillesheimer W, Freund HJ: Correlation between tremor, voluntary contraction, and firing pattern of motor units in Parkinson’s disease. J Neurol Neurosurg Psychiatly 371927-937, 1974. 5. Freund HJ, Dietz V, Wita CW, Kapp H: Discharge characteristics of single motor units in normal subjects and patients with supraspinal motor disturbances. In Desmedt J (Editor): New Developments in Electromyography and Clinical Neurophysiology. Basel, Karger, 1973, 3:242-250.
418
Regulation of Firing Pattern
6. Freund HJ, Wita CW: Computeranalyse des Intervallmusters einzelner motorischer Einheiten bei Gesunden und Patienten mit supraspinalen motorischen Storungen. Arch Psychiatr Neruenkr 214:56-71, 1971. 7. Kranz H: Dynamic aspects of motoneurone control and its disturbance. In Desmedt J (Editor): Progress in Clinical Neurophysiology, Vol 9 (in press). 8. Prochazka VJ, Conrad B, Sindermann F: Computerized single-unit interval analysis and its clinical application. In Desmedt J (Editor): New Developments in Electromyography and Clinical Neurophysiology, Vol 2. Basel, Karger, 1973, pp 462468. 9. Rosenfalck A, Andreassen S: Firing pattern of individual motor unit potentials during voluntary effort. Biomechanics IV, Int. SOC.of Biomechanics. Baltimore, University Park Press, 1978.
MUSCLE & NERVE
SepffOct 1978
1:416-418
1978
lMPAlRED REGULATION OF THE FIRING PATTERN OF SINGLE MOTOR UNITS STEEN ANDREASSEN, PhD, and ANNELISE ROSENFALCK, PhD
Investigations of the firing pattern of motor units are hampered by the difficulties in obtaining recordings from single units at moderate and high levels of voluntary contraction. Except for a study on patients with Parkinson's d i ~ e a s eonly , ~ few patients with central motor lesions have been ~tudied.~.~.' In this study, the firing pattern of single motor units in the anterior tibial muscle was investigated in 4 normal subjects and in 10 patients with spasticity caused by cerebral or cerebellar hemorrhage or by multiple sclerosis. Differences were found between normal and spastic subjects in the statistical parameters employed to analyze the firing pattern.'ag The results also indicate that these changes in the statistical parameters may be characteristic of the site of the lesion. MATERIALS AND METHODS Rocording. The single-unit electromyogram (EMG) is
recorded from the anterior tibial muscle using a bipolar, fine-wire e l e ~ t r o d e The . ~ force developed by the anterior tibial muscle is measured by means of a strain-gauge arrangement as torque around the ankle joint. The force is displayed on a large meter suspended over the patient,
From the Institute of Electronic Systems, Aalborg University Center, Aalborg, Denmark. Acknowledgments: This study was supported by the Danish Council for Scientific and Industrial Research and by the Danish Medical Research Council. Address reprint requests to Dr. Andreassen at the Institute of Electronic Systems, Aalborg University Center, Badehusvej 1A, Postbox 159. DK9100 Aalborg, Denmark. 0148-639X/0105/0416 $OO.OO/O 1978 Houghton Miff in Professional Publishers
@
416
Regulation of Firing Pattern
and the patient is asked to keep the force constant d u r ing the recording. Both the EMG and the force signal are recorded on an FM tape recorder for later analysis. Moaruromont of IntorspikeIntervals. At least 200 con-
secutive intervals between action potentials from each motor unit are measured in order to determine the statistical parameters with sufficient accuracy. In each patient, 10 to 20 motor units are sampled. A microcomputer is used to classify action potentials and to measure intervals from a maximum of three different motor units.lg In most cases, superposition potentials are classified correctly, and noise is rejected (fig. 1). The output from the computer is used for a visual check of the classification, and the microcomputer is programmed to allow easy correction of classification errors. RESULTS AND DISCUSSION
The differences in firing pattern between normal subjects and patients are exemplified by a plot of instantaneous frequency from a normal subject and from a patient with a cerebellar hemorrhage (fig. 2). For the normal subject, the mean firing rate is about 15 Hz, while the mean firing rate for the patient is about 8 Hz. This is a typical finding: the firing rate was reduced in all 10 spastic patients. The firing rate ranged from 7 Hz to 20 Hz for the normal subjects, and from 5 Hz to 13 Hz for the patients. The appearance of the instantaneous frequency curve is quite different for the normal subject and for the patient (fig. 2). We have used two statistical measures to describe this difference: VAR and FRHO. VAR is a measure of the variability between neighboring intervals.* Nine spastic patients showed reduced variability between
MUSCLE & NERVE
Sept/Oct 1978
kgxcm
I
100 m s
I
Figure 1 . Automatic classification of action potentials from two motor units (second trace). The classification of each action potential is indicated by the height of the pulse (top trace). Traces three and four are, respectively, the force recording and a timing trace generated by the microcomputer. This recording contains two superposition potentials; both were classified correctly, as indicated by the pair of high-low pulses in the classification trace (top trace). The small-amplitude potential was rejected by the computer as noise, as indicated by the negative pulse.
spiky appearance of the instantaneous frequency curve for normals (fig. 2). Although negative values of FRHO were found in the normal subjects, FRHO was less negative or even positive in spastic patients. In the three patients with multiple sclerosis and primarily spinal lesions, a few significantly positive values of FRHO were found, but the majority of the values were within the normal range. In seven patients with supraspinal lesions, the values of FRHO were significantly increased. If this difference between patients with spinal and supraspinal lesions can he confirmed on a larger number of patients, it may prove helpful as a diagnostic tool.
neighboring intervals; one patient with a cerebral hemorrhage showed increased variability. This reduced variability is a surprising finding, as most of the patients had difficulty maintaining a constant force level during recording. FRHO is a serial correlation coefficient corrected for trends. A negative FRHO indicates that long and short intervals tend to alternate. If FRHO is positive, a long interval is usually followed by a long interval; correspondingly, a short interval is usually followed by another short interval. We generally found negative values of FRHO in normal subjects, and this tendency for alternating long and short intervals contributes to the
NORMAL
I
INSTANTANEOUS FREQUENCY
a*.
0
2 0
a*.
0
1 5 1 0
4 4 . 0
I * .
5
0
FRHO-0.33
. 0 0
2 4 .
1 0
er
2 0
5
n
7 2 6
3 5 . a
1 5
18. 0
1 0
5.
5
Q
0
. a 0
Regulation of Firing Pattern
1 4
2 0
Figure 2. Instantaneous frequency and force, measured as torque at the ankle joint for a normal 26-year-old subject and a 59-year-old patient with cerebellar hemorrhage. Both the normal subject and the patient exert a force corresponding to 20% of maximum voluntary contraction.
MUSCLE & NERVE
Sept/Oct 1978
417
REFERENCES 1 . Andreassen S: Interval pattern of single motor units. PhD thesis, Technical University of Denmark and University of Copenhagen, 1977, pp 1-197. 2. Andreassen S, Rosenfalck A: Single motor unit recording. Proceedings of the IVth Nordic Meeting on Medical and Biological Engzneering. Copenhagen, Lyngby, 19:1-3, 1977. 3. Andreassen S, Rosenfalck A: Recording from a single motor unit during strong effort. IEEE Trans Biomed Eng (in press). 4. Dietz V, Hillesheimer W, Freund HJ: Correlation between tremor, voluntary contraction, and firing pattern of motor units in Parkinson’s disease. J Neurol Neurosurg Psychiatly 371927-937, 1974. 5. Freund HJ, Dietz V, Wita CW, Kapp H: Discharge characteristics of single motor units in normal subjects and patients with supraspinal motor disturbances. In Desmedt J (Editor): New Developments in Electromyography and Clinical Neurophysiology. Basel, Karger, 1973, 3:242-250.
418
Regulation of Firing Pattern
6. Freund HJ, Wita CW: Computeranalyse des Intervallmusters einzelner motorischer Einheiten bei Gesunden und Patienten mit supraspinalen motorischen Storungen. Arch Psychiatr Neruenkr 214:56-71, 1971. 7. Kranz H: Dynamic aspects of motoneurone control and its disturbance. In Desmedt J (Editor): Progress in Clinical Neurophysiology, Vol 9 (in press). 8. Prochazka VJ, Conrad B, Sindermann F: Computerized single-unit interval analysis and its clinical application. In Desmedt J (Editor): New Developments in Electromyography and Clinical Neurophysiology, Vol 2. Basel, Karger, 1973, pp 462468. 9. Rosenfalck A, Andreassen S: Firing pattern of individual motor unit potentials during voluntary effort. Biomechanics IV, Int. SOC.of Biomechanics. Baltimore, University Park Press, 1978.
MUSCLE & NERVE
SepffOct 1978
