Examination of the number of spikes (turns) per 5 sec and the mean amplitude of electrical activity, measured both at a force adjusted relative to maximum force (Po)and at a standard force, made it possible to distinguish between (1) a decrease in force arising from loss or inactivity of motor units, and (2) loss of muscle fibers or diminution in their cross-sectional area. The initial decrease in force of disused quadriceps muscle (following immobilization of the knee joint) resulted mainly from inactivity of motor units. In patients with motoneuron disease or a lesion of the brachial plexus, the number of turns at a force adjusted relative to Powas diminished because of increased potential duration and loss of motor units. Turns at a standard force of 2 kg were normal. In myopathy, the ratio of turns to amplitude was increased in two-thirds of the patients. MUSCLE & NERVE
1:419-420
1978
PATTERN OF ELECTRICAL ACTIVITY IN NORMAL AND DISEASED MUSCLE A. FUGLSANG-FREDERIKSEN, MD
Wdlison' developed a procedure with which to quantify the pattern of electrical activity of muscle during moderate effort. In patients with myopathy, he found an increased number of spikes per time unit. We have adopted Willison's method and have modified it in one respect: in addition to obtaining measurements at a suitable standard force, we measured electrophysiologic activity at a force adjusted relative to maximum force (Po).' The pattern of electrical activity was characterized according to the following criteria: (1) the number of spikes (turns) per unit time with a voltage-shift threshold of 100 P V ; ~ (2) the mean amplitude between turns (the number of counts of amplitude per unit time divided by the number of turns);2 (3) the ratio of the number of turns to mean amplitude between turns;' (4)the distribution of time intervals between turns.' A STANDARD FORCE COMPARED WITH A FORCE ADJUSTED RELATIVE TO Po
At a standard force of 2 kg, the number of turns decreased by 30%-40% with increasing Po in adult controls.' When the force was adjusted relative to P,, however, the number of turns was the same in subjects having differing muscle strength. This suggests that the
From the Laboratory of Clinical Neurophysiology, Rigshospital, University of Copenhagen, Denmark. Acknowledgment: The author wishes to thank Mrs. U. Scheel for granting permission to refer to unpublished material. This work was supported by a grant from the Muscular Dystrophy Association of America to Professor Fritz Buchthal. Address reprint requests to Dr. Fuglsang-Frederiksen at the Laboratory of Clinical Neurophysiology, Rigshospital, University of Copenhagen, Denmark. 0148-639X/0105/0419 $OO.OO/O 1978 Houghton Miflin Professional Publishers
Electrical Activity in Muscle
total number of motor units is the same regardless of the subject's muscle strength: a weak muscle merely activates a greater number of motor units than does a strong muscle to enable it to exert a force of 2 kg. An increase in the number of turns would thus indicate a decrease in the diameter of muscle fibers, in the number of fibers per motor unit, or in both. For diagnostic purposes, a standard force of 2 kg was disadvantageous in that when it was exerted by a "weak" control, the number of turns rose above the upper limit of normal, as in myopathy. The number of turns and the mean amplitude increased with an increase in force of up to 30%-50% of P, (fig. 1). This resulted from an increase in firing frequency and in the number of motor units activated.' Hence, in a muscle that lost motor units, 30% of the maximum force was associated with a lower value of turns, in the plot of turns as a function of relative force, in comparison with controls.' Above 30%-50% of Po, the number of turns remained unaltered or it decreased; the mean amplitude continued to rise. This is probably an outcome of increased summation of component potentials. DISUSED MUSCLE
It follows from these findings that turns and mean amplitude, measured both at a given relative force and at a standard force, allow one to distinguish between a decrease in force arising from inactivity of motor units and that resulting from a diminution in the cross-sectional area of muscle fibers. This is illustrated by findings in the femoral quadriceps muscle after immobilization of the knee for four weeks (Fuglsang-Frederiksen and Scheel, unpublished observations). At a force adjusted relative to Po, the number of turns and the mean amplitude in the disused muscle were diminished the day after removal of the cast, when the patient could just
MUSCLE & NERVE
SeptiOct 1978
419
TURNS PER 5 s
MEAN AMPL.
TURNS
I
1.q-
No
3000
1
0%
0%
om
or)
2000
0
0
i' I
0
25
54
75
100 0 25 5a 75 FORCE, P E R CENT OF M A X I M U M
IW
1000
0 :
: Figure 1 . Number of turns per 5 sec and mean amplitude as a function of force in the brachial biceps and brachialis muscles. The broken line represents the average for muscles of five subjects.
bend his knee (fig. 2 ) . Eight days later, the number of turns and the mean amplitude were normal, and half of the initial decrease in maximum force had been regained. This transient decrease in the number of turns and in amplitude indicates a reduction in the number of motor units activated. In contrast, the electrical activity at a standard force of 5 kg did not differ from that in the contralateral leg; the compensatory increase in number of turns expected from the decreased crosssectional area of muscle fibers must have been within the margin of error of the method. The finding that turns and amplitude were unchanged at a standard force is consistent with the assumption that most of the initial diminution in force resulted from transient inactivation of motor units. DIAGNOSTIC YIELD
In the brachial biceps and brachialis muscles of patients with motoneuron disease or with a lesion of the brachial plexus, the number of turns at a relative force of 30% of Po was diminished in 70% of 30 patients.' At a standard force of 2 kg, the number of turns was normal. The cumulative diagnostic yield of the four criteria (see introduction) was 80%. The decrease in turns had two main causes: (1) loss of motor units, and/or ( 2 ) an increase in the duration of individual motor unit potentials that often occurs in neurogenic disease. T h e relation of an increase in duration of motor-unit potentials to a decrease in turns at a force relative to Po was demonstrated in previous studies.' When the muscle was cooled, the duration of motor-unit potentials was increased, the number of turns at 30% of Po was diminished, and Po was unaltered. T h e decrease in the number of turns was probably the result of cancellation of potential compo-
420
Electrical Activity in Muscle
I
.
I
Figure 2. Number of turns per 5 sec at 10% of maximum force (Po). 0 = normal, 0 = disused quadriceps muscle. Each point represents a recording in a given site in the muscle. Above the columns: day(s) after the patient could bend his knee to 90". Below the columns: Po of the disused muscle, as a percentage of Po of the contralateral muscle. (Two patients investigated with the knee in an angle of 45" are excluded from the figure.)
nents by summation. An increased incidence of polyphasic potentials can be expected to increase the number of turns. This was counteracted by the decrease in turns associated with the prolonged duration of the motorunit potentials and with loss of motor units.' In 40% of 41 patients with myopathy, the number of turns tested at a force adjusted relative to Po was increased.l On the basis of the ratio between number of turns and mean amplitude, 70% of the patients were classified as having myopathy. Six of these patients were not identified by the diminished duration of the motorunit potentials of simple shape sampled at weak effort, and the muscle of five patients with diminished duration showed a normal ratio between the number of turns and mean amplitude. The cumulative diagnostic yield of the adopted criteria was 90%.
REFERENCES 1 . Fuglsang-Frederiksen A, Scheel U, Buchthal F: Diagnostic yield of the analysis of the pattern of electrical activity of muscle and of individual motor unit potentials in neurogenic involvement. J Neurol Neurosurg Psychiatry 40:544-554, 1977. 2. Willison RG: Analysis of electrical activity in healthy and dystrophic muscle in man. J Neurol Nmrosurg PsychiatT 27:386-394, 1964.
MUSCLE & NERVE
SepUOct 1978
1:419-420
1978
PATTERN OF ELECTRICAL ACTIVITY IN NORMAL AND DISEASED MUSCLE A. FUGLSANG-FREDERIKSEN, MD
Wdlison' developed a procedure with which to quantify the pattern of electrical activity of muscle during moderate effort. In patients with myopathy, he found an increased number of spikes per time unit. We have adopted Willison's method and have modified it in one respect: in addition to obtaining measurements at a suitable standard force, we measured electrophysiologic activity at a force adjusted relative to maximum force (Po).' The pattern of electrical activity was characterized according to the following criteria: (1) the number of spikes (turns) per unit time with a voltage-shift threshold of 100 P V ; ~ (2) the mean amplitude between turns (the number of counts of amplitude per unit time divided by the number of turns);2 (3) the ratio of the number of turns to mean amplitude between turns;' (4)the distribution of time intervals between turns.' A STANDARD FORCE COMPARED WITH A FORCE ADJUSTED RELATIVE TO Po
At a standard force of 2 kg, the number of turns decreased by 30%-40% with increasing Po in adult controls.' When the force was adjusted relative to P,, however, the number of turns was the same in subjects having differing muscle strength. This suggests that the
From the Laboratory of Clinical Neurophysiology, Rigshospital, University of Copenhagen, Denmark. Acknowledgment: The author wishes to thank Mrs. U. Scheel for granting permission to refer to unpublished material. This work was supported by a grant from the Muscular Dystrophy Association of America to Professor Fritz Buchthal. Address reprint requests to Dr. Fuglsang-Frederiksen at the Laboratory of Clinical Neurophysiology, Rigshospital, University of Copenhagen, Denmark. 0148-639X/0105/0419 $OO.OO/O 1978 Houghton Miflin Professional Publishers
Electrical Activity in Muscle
total number of motor units is the same regardless of the subject's muscle strength: a weak muscle merely activates a greater number of motor units than does a strong muscle to enable it to exert a force of 2 kg. An increase in the number of turns would thus indicate a decrease in the diameter of muscle fibers, in the number of fibers per motor unit, or in both. For diagnostic purposes, a standard force of 2 kg was disadvantageous in that when it was exerted by a "weak" control, the number of turns rose above the upper limit of normal, as in myopathy. The number of turns and the mean amplitude increased with an increase in force of up to 30%-50% of P, (fig. 1). This resulted from an increase in firing frequency and in the number of motor units activated.' Hence, in a muscle that lost motor units, 30% of the maximum force was associated with a lower value of turns, in the plot of turns as a function of relative force, in comparison with controls.' Above 30%-50% of Po, the number of turns remained unaltered or it decreased; the mean amplitude continued to rise. This is probably an outcome of increased summation of component potentials. DISUSED MUSCLE
It follows from these findings that turns and mean amplitude, measured both at a given relative force and at a standard force, allow one to distinguish between a decrease in force arising from inactivity of motor units and that resulting from a diminution in the cross-sectional area of muscle fibers. This is illustrated by findings in the femoral quadriceps muscle after immobilization of the knee for four weeks (Fuglsang-Frederiksen and Scheel, unpublished observations). At a force adjusted relative to Po, the number of turns and the mean amplitude in the disused muscle were diminished the day after removal of the cast, when the patient could just
MUSCLE & NERVE
SeptiOct 1978
419
TURNS PER 5 s
MEAN AMPL.
TURNS
I
1.q-
No
3000
1
0%
0%
om
or)
2000
0
0
i' I
0
25
54
75
100 0 25 5a 75 FORCE, P E R CENT OF M A X I M U M
IW
1000
0 :
: Figure 1 . Number of turns per 5 sec and mean amplitude as a function of force in the brachial biceps and brachialis muscles. The broken line represents the average for muscles of five subjects.
bend his knee (fig. 2 ) . Eight days later, the number of turns and the mean amplitude were normal, and half of the initial decrease in maximum force had been regained. This transient decrease in the number of turns and in amplitude indicates a reduction in the number of motor units activated. In contrast, the electrical activity at a standard force of 5 kg did not differ from that in the contralateral leg; the compensatory increase in number of turns expected from the decreased crosssectional area of muscle fibers must have been within the margin of error of the method. The finding that turns and amplitude were unchanged at a standard force is consistent with the assumption that most of the initial diminution in force resulted from transient inactivation of motor units. DIAGNOSTIC YIELD
In the brachial biceps and brachialis muscles of patients with motoneuron disease or with a lesion of the brachial plexus, the number of turns at a relative force of 30% of Po was diminished in 70% of 30 patients.' At a standard force of 2 kg, the number of turns was normal. The cumulative diagnostic yield of the four criteria (see introduction) was 80%. The decrease in turns had two main causes: (1) loss of motor units, and/or ( 2 ) an increase in the duration of individual motor unit potentials that often occurs in neurogenic disease. T h e relation of an increase in duration of motor-unit potentials to a decrease in turns at a force relative to Po was demonstrated in previous studies.' When the muscle was cooled, the duration of motor-unit potentials was increased, the number of turns at 30% of Po was diminished, and Po was unaltered. T h e decrease in the number of turns was probably the result of cancellation of potential compo-
420
Electrical Activity in Muscle
I
.
I
Figure 2. Number of turns per 5 sec at 10% of maximum force (Po). 0 = normal, 0 = disused quadriceps muscle. Each point represents a recording in a given site in the muscle. Above the columns: day(s) after the patient could bend his knee to 90". Below the columns: Po of the disused muscle, as a percentage of Po of the contralateral muscle. (Two patients investigated with the knee in an angle of 45" are excluded from the figure.)
nents by summation. An increased incidence of polyphasic potentials can be expected to increase the number of turns. This was counteracted by the decrease in turns associated with the prolonged duration of the motorunit potentials and with loss of motor units.' In 40% of 41 patients with myopathy, the number of turns tested at a force adjusted relative to Po was increased.l On the basis of the ratio between number of turns and mean amplitude, 70% of the patients were classified as having myopathy. Six of these patients were not identified by the diminished duration of the motorunit potentials of simple shape sampled at weak effort, and the muscle of five patients with diminished duration showed a normal ratio between the number of turns and mean amplitude. The cumulative diagnostic yield of the adopted criteria was 90%.
REFERENCES 1 . Fuglsang-Frederiksen A, Scheel U, Buchthal F: Diagnostic yield of the analysis of the pattern of electrical activity of muscle and of individual motor unit potentials in neurogenic involvement. J Neurol Neurosurg Psychiatry 40:544-554, 1977. 2. Willison RG: Analysis of electrical activity in healthy and dystrophic muscle in man. J Neurol Nmrosurg PsychiatT 27:386-394, 1964.
MUSCLE & NERVE
SepUOct 1978
