EXPERIMENTAL
46, 115-131
NEUROLOGY
Properties
V. R.
of Immobilized of the Galago
EDGERTON,
R. J. AND
Brain
Research of Medicine,
(1975)
Institute,
Hind-Limb senegalensis
BARNARD,
D. R.
SIMPSON
Neuromuscular
University of California, Received
J.
July
B. PETER,
Muscles
A.
MAIER
1
Research Laboratory, Los Angeles, Califorrzia
and School 90024
26,1974
Morphological, biochemical, and physiolagical properties of longterm (6 mo) immobilized skeletal muscles of a nonhuman primate were studied in seven Galago senegalensis which were immobilized at the ankle and knee of one hind limb with an external brace. Electromyographic activity of the ipsilateral and contralateral quadriceps and muscles of the calf were assessed after 5 and 6 mo of immobilization. The EMG was markedly reduced in the immobilized muscles compared to the contralateral control when the brace was intact and the animal moving freely. Without exception, extensor muscles atrophied more than flexors. The soleus, a slow-twitch muscle atrophied more than any other muscle of the lower leg but the same was not true of ‘the vastus intermedius the analogous muscle of the thigh. Slow-twitch oxidative fibers (SO) were atrophied more than fast-twitch oxidative glycolytic fibers (FOG), and FOG tended to atrophy more than fast-twitch glycolytic fibers (FG). The immobilized soleus and vastus intermedius had a smaller percentage of SO fibers than their controls, suggesting that they had faster contraction times. With respect to alterations in reduced nicotinamide adenine dinucleotide diaphorase activity, no consistent pattern was observed except for a greater coarseness of staining granules and more homogeneous dispersion of the granules throughout the crosssection of the fibers. No changes were found in phosphorylase, lactate dehydrogenase, or succinate dehydrogenase specific activity or in myoglobin concentration in homogenates of ankle flexors or the vastus lateralis. Myosin ATPase, but not actomyosin ATPase activity was significantly less in the immobilized ,gastrocnemius-plantaris muscles. No change in contractile properties related to speed were observed in the plantaris. This muscle did exert more twitch and tetanic tension per gram of muscle in the immobilized leg. 1 Supported by USPHS NS-08509, Biomedical Research Support Grant. Health Sciences Computing Facility, Resources Grant PR-3.
NS-10403 and NS-10497 Computing assistance was UCLA. Sponsored by NIH
11.5 Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
and the UCLA obtained from the Special Research
116
EDGERTON
ET
AL.
0.1 I
mv
/
L 1 set FIG. 1. EMG of the control (top tracing) and immobilized (bottom tracing) leg recorded from bipolar electrodes inserted into the gastrocnemius. Recordings represent periods when the bushbaby was (A) sitting quietly in an enclosed chamber; (B) changing postural positions, and; (C) moving vigorously while in the chamber. Note the overall marked difference in EMG amplitude and frequency but that occasionally high activity is evident in the immobilized limb. The calibration bar represents the approximate voltage (mv) and time period (set).
Various kinds of skeletal muscle hyperactivity have been studied in a number of mammals including primates (2, 3, 4, 12, 14, 27, 34, 39), but the prolonged hypoactivity to which a primate muscle might be subjected in clinical conditions has received only limited attention. For example, surgical procedures, whereby muscles are immobilized for long periods of time, have been common in patients with arthritic hips. More recently, procedures have been developed which permit mobilization of the previously stabilized joints, The response of the immobilization on skeletal muscle in these procedures is largely unknown. Most studies of disuse of skeletal muscle have been limited in scope in that either morphological, physiological, or chemical parameters were studied. This study, however, deals with the combined morphological, physiological, and biochemical characteristics of several nonhuman primate (G&go
senegalensis)
skeletal
muscles
of a hindlimb
which
had been im-
mobilized with a metal brace for 6 mo. This study emphasizes the perspective needed if one is to analyze critically the response of skeletal muscle to immobilization for varying periods.
IMMOBILIZED
117
MUSCLES
Selective muscle fiber atrophy was assessed in terms of muscle weight, fiber size, contractile speed, metabolic patterns and the muscle’s function with respect to flexion or extension and isometric or isotonic contractions. Information is presented which suggests that each of the variables is important in determining the susceptibility of a muscle fiber to atrophy during limb immobilization. METHODS
AND
MATERIALS
Aniwzal Treatment and Limb Iwamobilization. Environmental condition, diet, anesthetic and methods for morphological, histochemical, biochemical and physiological determinations were described previously (14). The right hind limbs of seven male Galago senegazensis (approximately 240 g) were externally immobilized for 6 mo with a lightweight aluminum brace (27-43 g) extending from the upper thigh to the toes with the knee and ankle fixed at right angles. The brace consisted of flat lateral and medial aluminum bars (3.5, 5.5 and 5.0 cm for thigh, calf and toe length, re5 Control .3170
Immobilnec!
I.2310
f
.0522
0.01
I.1253
.7866
0.08
2.2958
3.6474 11.6435
.562313623 .398713969
muscle
weight
0.56 0.22
I I I
1 1 1
0.03 0.02
(grams) lOO%Of Control
SEMD
002
.I296
29129
seneqalensis
OVERLOADED
1
”
f
PL
7
4.97
*
SOL
7
5.50
*
G
7
4.20 X
TA
7
213
X X
VL
7
766
VM
7
7.66%
VI
7
4.89
RF
7
0.10
SM
7
1.49
ST
5
0.25
*
FIG. 2. Actual mean wet muscle weights are placed in the column to the left with the standard error of the mean difference between the immobilized and contralateral control muscle. The bars represent the muscle’s wet wt as a percent of control. The bars with the cross-hatching represents flexor muscles. The rectus femoris (RF) is marked “overloaded” because it is in an anatomical position to support the weight of the brace, in that it crosses the hip joint. The t-ratios represent the results of a correlated t-test and are marked (*) if there is a significant difference (P < 0.05) ; plantaris (Pl), soleus (SOL), gastrocnemius (G), tibialis anterior (TA), vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), semimembranosus (SM), and semitendinosus (ST).
118
EDGERTON
2 Control
lmmobf
40 42 46 46
32 33 40 45
SEMD
senegalensls
ET
FIBER
AL.
DIAMETER
:onlrol
PL
(y
1 t 659% 6.73% 760%
TA
1.25 1.04 0.98 I.38
SOL
3.60% 716%
VI
37
327X 2.56* 2.77% 1.00
32 33 39 41
RF
2.05* 23436 2.16% 6.3934
46 42
42 36. 42
37 43 42
37 42 42 42
42 43 .?a 45
LG 2.28 1.86 1.35 9.00)(
FIG. 3. Actual means and standard error of the mean differences are given on the left of the figure. The length of the bars represent fiber diameter relative to the contralateral control leg. The types of fibers are separated according to the patterns shown below; slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG),
IMMOBILIZED
MUSCLES
119
specively), joined by a 2.3 cm brass toe grip rod with immobilization facilitated by six adjustable plastic tie wraps or slotted aluminum bands padded with orthopedic adhesive flannel. Morphology and Histochenkstry. Fiber size was determined by measuring the shortest diameter of a muscle fiber from photographic prints enlarged by a factor of 600. Muscle fiber types (13, 30) were determined by a combination of myofibrillar adenosine triphosphatase ( ATPase) (20, 32) reduced nicotinamide adenine dinucleotide diaphorase (NADHD) (31) , and mitochondrial a-glycerophosphate dehydrogenase activity (wGPD) (43). On the basis of these three enzyme activities, skeletal muscle fibers were classified as slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG) and fast-twitch glycolytic (FG) as described previously (33). Also, all fast-twitch fibers (high myofibrillar ATPase activity when incubated in an alkaline medium) were arbitrarily divided into three categories on the basis of NADH-D activity (highest, FOG ; moderate, M ; and lowest, FG). Contractile properties were determined in situ on both legs with the sequence of testing of control and experimental leg being alternated with each succeeding animal. In the last two animals a temperature-controlled chamber was used in which both muscles could be tested simultaneously. Unlike the plantaris-gastrocnemius complex of rodents (12)) the plantaris of Galago senegalensiscan be isolated from the surrounding musculature without any apparent trauma to this muscle (40,41). On the first five animals tested, muscle temperature was maintained at 37 f 1.0 C with radiant heat and by continuously bathing the muscle in warmed mineral oil. Muscle temperature was kept 37 -F-0.5 C for the last two animals with a controlled mineral oil bath. The lower leg and femur were fixed at a right angle. The compliance of the two different isometric force transducers was 1.3 mm/kg and 1.9 mm/kg. The plantaris was stimulated with 0.2 msec duration square wave monophasic impulses (Grass Instruments S4) via the distal end of the tibia1 nerve which was ligated and severed at mid-femur level. The stimulating bipolar electrode was made of stainless steel. The muscle was stimulated at about 2 v greater than that used to elicit a maximum twitch tension. Muscle length was adjusted so that a maximal twitch tension was obtained. The transducer output was amplified and recorded with an Oscillo/ fast-twitch moderate (I‘M”) and fast-twitch glycolytic fiber (FG). “M” represents fibers with an NADH-D staining that is intermediate to that in the FOG and FG fibers and high myofibrillar ATPase activity (fast-twitch). Correlated t-ratios and significance (P < 0.05) as shown by * are shown in the right column. Abbreviations for the muscle are the same as indicated in Fig. 2. In addition, the medial and lateral gastrocnemius (MG and LG) are shown.
120
EDGERTON
ET
AL.
16 MUSCLE
WElGHT
%OF
CONTROL .L
I
,
FIG. 4. The relationship of the actual muscle weight and overall muscle fiber diameter is shown with individual points. The line drawn is the theoretical relationship between muscle weight and fiber diameter assuming that each muscle fiber is a cylinder. The letters by each point represent the muscle as shown in the legend for Fig, 2.
riter (Texas Instruments Inc.) or Grass 7P Polygraph (Grass Instruments). Twitch contraction time from initiation of rise in tension to maximum tension, half relation time and maximum twitch tension was determined from three to five single twitch responses. The maximum rate of tetanic tension development and maximum tetanic tension was determined by stimulating at 300 Hz, 0.2 msec duration per pulse, for 2-4 sec. EMG activity was recorded from subdermal platinum alloy 30 gauge bipolar needle electrodes (25 mm apart at base and 1.5 cm long, Grass E2) and passed through an EMG time averaging integrator (Grass Instrument) and then into a two-channel Tektronix oscilloscope linked to a Grass Polygraph. The needle electrodes were placed subcutaneously and parallel to the femur for testing the quadriceps and parallel to the tibia for testing the gastrocnemius muscle to determine the relative effectiveness of the immobilization of the knee and ankle joints. Recordings were made on the immobilized and contralateral control leg simultaneously over a 2 hr period approximately 1 mo prior to the end of the
IMMOBILIZED
MUSCLES
121
experiment and again within 3 days of the measurement of contractile properties. During the final recording session, EMG activity was determined on the immobilized leg with and without the brace. Recordings were also made while the Galago was moving about within an enclosed chamber, while being held by the experimenter with the legs hanging freely, and while pressure was being applied to the sole of the foot. With the brace removed, EMG activity was compared in the control and experimental legs as the tested extremity grasped objects weighing 3-5 g. Statistical comparisons of all parameters were made between the immobilized limb and the contralateral leg with a paired t-test. RESULTS Electrovnyography. EMG activity (amplitude and frequency) of the gastrocnemius of the immobilized leg was markedly less than in the contralateral leg (Fig. 1). This was true whether the animal was at rest or was moving about in a small chamber. Less EMG activity was also evident in the immobilized gastrocnemius and vastus medialis while the legs hung freely and while pressure was being applied to the bottom of the foot with the brace in place. Immediately after removal of the brace, but during relatively inactive periods, the gastrocnemius and vastus medialis of the immobilized limb had less than or the same EMG activity as the control as reflected by amplitude and frequency of electrical potentials. When cylindrical objects of a similar size and weight were grasped by the unrestricted right and left legs, the overall EMG potential of the gastrocnemius was variable, being considerably greater in the control of some animals, while the opposite was true of others. To summarize the EMG findings, there was obviously less activity in the immobilized leg when the brace was on the hind limb but immediately upon removal of the bate, EMG activity approximated that of the contralateral control. Muscle Weight and Fiber Size. The effect of immobilization on the muscle weights of several hind limb muscles are shown in Fig. 2. Atrophy in the various muscles of the hind limb, as reflected by muscle weight, ranged from about O-60% weight loss compared to controls. In general, changes in fiber diameters reflected changes in wet muscle weight (Figs. 3 and 4). Figure 4 illustrates the relationship between degree of atrophy, as determined by fiber size, and as determined by muscle weight. The points on the graph represent actual data points, whereas the line drawn represents a theoretical curve which was calculated on the basis of fiber diameter changes to predict muscle weight changes. Assuming a muscle fiber to be a cylinder and that all other variables remain constant except fiber diameter, fiber volume change varies with the square of the radius change.
122
EDGERTON
5 Colltr
46, 115-131
NEUROLOGY
Properties
V. R.
of Immobilized of the Galago
EDGERTON,
R. J. AND
Brain
Research of Medicine,
(1975)
Institute,
Hind-Limb senegalensis
BARNARD,
D. R.
SIMPSON
Neuromuscular
University of California, Received
J.
July
B. PETER,
Muscles
A.
MAIER
1
Research Laboratory, Los Angeles, Califorrzia
and School 90024
26,1974
Morphological, biochemical, and physiolagical properties of longterm (6 mo) immobilized skeletal muscles of a nonhuman primate were studied in seven Galago senegalensis which were immobilized at the ankle and knee of one hind limb with an external brace. Electromyographic activity of the ipsilateral and contralateral quadriceps and muscles of the calf were assessed after 5 and 6 mo of immobilization. The EMG was markedly reduced in the immobilized muscles compared to the contralateral control when the brace was intact and the animal moving freely. Without exception, extensor muscles atrophied more than flexors. The soleus, a slow-twitch muscle atrophied more than any other muscle of the lower leg but the same was not true of ‘the vastus intermedius the analogous muscle of the thigh. Slow-twitch oxidative fibers (SO) were atrophied more than fast-twitch oxidative glycolytic fibers (FOG), and FOG tended to atrophy more than fast-twitch glycolytic fibers (FG). The immobilized soleus and vastus intermedius had a smaller percentage of SO fibers than their controls, suggesting that they had faster contraction times. With respect to alterations in reduced nicotinamide adenine dinucleotide diaphorase activity, no consistent pattern was observed except for a greater coarseness of staining granules and more homogeneous dispersion of the granules throughout the crosssection of the fibers. No changes were found in phosphorylase, lactate dehydrogenase, or succinate dehydrogenase specific activity or in myoglobin concentration in homogenates of ankle flexors or the vastus lateralis. Myosin ATPase, but not actomyosin ATPase activity was significantly less in the immobilized ,gastrocnemius-plantaris muscles. No change in contractile properties related to speed were observed in the plantaris. This muscle did exert more twitch and tetanic tension per gram of muscle in the immobilized leg. 1 Supported by USPHS NS-08509, Biomedical Research Support Grant. Health Sciences Computing Facility, Resources Grant PR-3.
NS-10403 and NS-10497 Computing assistance was UCLA. Sponsored by NIH
11.5 Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
and the UCLA obtained from the Special Research
116
EDGERTON
ET
AL.
0.1 I
mv
/
L 1 set FIG. 1. EMG of the control (top tracing) and immobilized (bottom tracing) leg recorded from bipolar electrodes inserted into the gastrocnemius. Recordings represent periods when the bushbaby was (A) sitting quietly in an enclosed chamber; (B) changing postural positions, and; (C) moving vigorously while in the chamber. Note the overall marked difference in EMG amplitude and frequency but that occasionally high activity is evident in the immobilized limb. The calibration bar represents the approximate voltage (mv) and time period (set).
Various kinds of skeletal muscle hyperactivity have been studied in a number of mammals including primates (2, 3, 4, 12, 14, 27, 34, 39), but the prolonged hypoactivity to which a primate muscle might be subjected in clinical conditions has received only limited attention. For example, surgical procedures, whereby muscles are immobilized for long periods of time, have been common in patients with arthritic hips. More recently, procedures have been developed which permit mobilization of the previously stabilized joints, The response of the immobilization on skeletal muscle in these procedures is largely unknown. Most studies of disuse of skeletal muscle have been limited in scope in that either morphological, physiological, or chemical parameters were studied. This study, however, deals with the combined morphological, physiological, and biochemical characteristics of several nonhuman primate (G&go
senegalensis)
skeletal
muscles
of a hindlimb
which
had been im-
mobilized with a metal brace for 6 mo. This study emphasizes the perspective needed if one is to analyze critically the response of skeletal muscle to immobilization for varying periods.
IMMOBILIZED
117
MUSCLES
Selective muscle fiber atrophy was assessed in terms of muscle weight, fiber size, contractile speed, metabolic patterns and the muscle’s function with respect to flexion or extension and isometric or isotonic contractions. Information is presented which suggests that each of the variables is important in determining the susceptibility of a muscle fiber to atrophy during limb immobilization. METHODS
AND
MATERIALS
Aniwzal Treatment and Limb Iwamobilization. Environmental condition, diet, anesthetic and methods for morphological, histochemical, biochemical and physiological determinations were described previously (14). The right hind limbs of seven male Galago senegazensis (approximately 240 g) were externally immobilized for 6 mo with a lightweight aluminum brace (27-43 g) extending from the upper thigh to the toes with the knee and ankle fixed at right angles. The brace consisted of flat lateral and medial aluminum bars (3.5, 5.5 and 5.0 cm for thigh, calf and toe length, re5 Control .3170
Immobilnec!
I.2310
f
.0522
0.01
I.1253
.7866
0.08
2.2958
3.6474 11.6435
.562313623 .398713969
muscle
weight
0.56 0.22
I I I
1 1 1
0.03 0.02
(grams) lOO%Of Control
SEMD
002
.I296
29129
seneqalensis
OVERLOADED
1
”
f
PL
7
4.97
*
SOL
7
5.50
*
G
7
4.20 X
TA
7
213
X X
VL
7
766
VM
7
7.66%
VI
7
4.89
RF
7
0.10
SM
7
1.49
ST
5
0.25
*
FIG. 2. Actual mean wet muscle weights are placed in the column to the left with the standard error of the mean difference between the immobilized and contralateral control muscle. The bars represent the muscle’s wet wt as a percent of control. The bars with the cross-hatching represents flexor muscles. The rectus femoris (RF) is marked “overloaded” because it is in an anatomical position to support the weight of the brace, in that it crosses the hip joint. The t-ratios represent the results of a correlated t-test and are marked (*) if there is a significant difference (P < 0.05) ; plantaris (Pl), soleus (SOL), gastrocnemius (G), tibialis anterior (TA), vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), semimembranosus (SM), and semitendinosus (ST).
118
EDGERTON
2 Control
lmmobf
40 42 46 46
32 33 40 45
SEMD
senegalensls
ET
FIBER
AL.
DIAMETER
:onlrol
PL
(y
1 t 659% 6.73% 760%
TA
1.25 1.04 0.98 I.38
SOL
3.60% 716%
VI
37
327X 2.56* 2.77% 1.00
32 33 39 41
RF
2.05* 23436 2.16% 6.3934
46 42
42 36. 42
37 43 42
37 42 42 42
42 43 .?a 45
LG 2.28 1.86 1.35 9.00)(
FIG. 3. Actual means and standard error of the mean differences are given on the left of the figure. The length of the bars represent fiber diameter relative to the contralateral control leg. The types of fibers are separated according to the patterns shown below; slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG),
IMMOBILIZED
MUSCLES
119
specively), joined by a 2.3 cm brass toe grip rod with immobilization facilitated by six adjustable plastic tie wraps or slotted aluminum bands padded with orthopedic adhesive flannel. Morphology and Histochenkstry. Fiber size was determined by measuring the shortest diameter of a muscle fiber from photographic prints enlarged by a factor of 600. Muscle fiber types (13, 30) were determined by a combination of myofibrillar adenosine triphosphatase ( ATPase) (20, 32) reduced nicotinamide adenine dinucleotide diaphorase (NADHD) (31) , and mitochondrial a-glycerophosphate dehydrogenase activity (wGPD) (43). On the basis of these three enzyme activities, skeletal muscle fibers were classified as slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG) and fast-twitch glycolytic (FG) as described previously (33). Also, all fast-twitch fibers (high myofibrillar ATPase activity when incubated in an alkaline medium) were arbitrarily divided into three categories on the basis of NADH-D activity (highest, FOG ; moderate, M ; and lowest, FG). Contractile properties were determined in situ on both legs with the sequence of testing of control and experimental leg being alternated with each succeeding animal. In the last two animals a temperature-controlled chamber was used in which both muscles could be tested simultaneously. Unlike the plantaris-gastrocnemius complex of rodents (12)) the plantaris of Galago senegalensiscan be isolated from the surrounding musculature without any apparent trauma to this muscle (40,41). On the first five animals tested, muscle temperature was maintained at 37 f 1.0 C with radiant heat and by continuously bathing the muscle in warmed mineral oil. Muscle temperature was kept 37 -F-0.5 C for the last two animals with a controlled mineral oil bath. The lower leg and femur were fixed at a right angle. The compliance of the two different isometric force transducers was 1.3 mm/kg and 1.9 mm/kg. The plantaris was stimulated with 0.2 msec duration square wave monophasic impulses (Grass Instruments S4) via the distal end of the tibia1 nerve which was ligated and severed at mid-femur level. The stimulating bipolar electrode was made of stainless steel. The muscle was stimulated at about 2 v greater than that used to elicit a maximum twitch tension. Muscle length was adjusted so that a maximal twitch tension was obtained. The transducer output was amplified and recorded with an Oscillo/ fast-twitch moderate (I‘M”) and fast-twitch glycolytic fiber (FG). “M” represents fibers with an NADH-D staining that is intermediate to that in the FOG and FG fibers and high myofibrillar ATPase activity (fast-twitch). Correlated t-ratios and significance (P < 0.05) as shown by * are shown in the right column. Abbreviations for the muscle are the same as indicated in Fig. 2. In addition, the medial and lateral gastrocnemius (MG and LG) are shown.
120
EDGERTON
ET
AL.
16 MUSCLE
WElGHT
%OF
CONTROL .L
I
,
FIG. 4. The relationship of the actual muscle weight and overall muscle fiber diameter is shown with individual points. The line drawn is the theoretical relationship between muscle weight and fiber diameter assuming that each muscle fiber is a cylinder. The letters by each point represent the muscle as shown in the legend for Fig, 2.
riter (Texas Instruments Inc.) or Grass 7P Polygraph (Grass Instruments). Twitch contraction time from initiation of rise in tension to maximum tension, half relation time and maximum twitch tension was determined from three to five single twitch responses. The maximum rate of tetanic tension development and maximum tetanic tension was determined by stimulating at 300 Hz, 0.2 msec duration per pulse, for 2-4 sec. EMG activity was recorded from subdermal platinum alloy 30 gauge bipolar needle electrodes (25 mm apart at base and 1.5 cm long, Grass E2) and passed through an EMG time averaging integrator (Grass Instrument) and then into a two-channel Tektronix oscilloscope linked to a Grass Polygraph. The needle electrodes were placed subcutaneously and parallel to the femur for testing the quadriceps and parallel to the tibia for testing the gastrocnemius muscle to determine the relative effectiveness of the immobilization of the knee and ankle joints. Recordings were made on the immobilized and contralateral control leg simultaneously over a 2 hr period approximately 1 mo prior to the end of the
IMMOBILIZED
MUSCLES
121
experiment and again within 3 days of the measurement of contractile properties. During the final recording session, EMG activity was determined on the immobilized leg with and without the brace. Recordings were also made while the Galago was moving about within an enclosed chamber, while being held by the experimenter with the legs hanging freely, and while pressure was being applied to the sole of the foot. With the brace removed, EMG activity was compared in the control and experimental legs as the tested extremity grasped objects weighing 3-5 g. Statistical comparisons of all parameters were made between the immobilized limb and the contralateral leg with a paired t-test. RESULTS Electrovnyography. EMG activity (amplitude and frequency) of the gastrocnemius of the immobilized leg was markedly less than in the contralateral leg (Fig. 1). This was true whether the animal was at rest or was moving about in a small chamber. Less EMG activity was also evident in the immobilized gastrocnemius and vastus medialis while the legs hung freely and while pressure was being applied to the bottom of the foot with the brace in place. Immediately after removal of the brace, but during relatively inactive periods, the gastrocnemius and vastus medialis of the immobilized limb had less than or the same EMG activity as the control as reflected by amplitude and frequency of electrical potentials. When cylindrical objects of a similar size and weight were grasped by the unrestricted right and left legs, the overall EMG potential of the gastrocnemius was variable, being considerably greater in the control of some animals, while the opposite was true of others. To summarize the EMG findings, there was obviously less activity in the immobilized leg when the brace was on the hind limb but immediately upon removal of the bate, EMG activity approximated that of the contralateral control. Muscle Weight and Fiber Size. The effect of immobilization on the muscle weights of several hind limb muscles are shown in Fig. 2. Atrophy in the various muscles of the hind limb, as reflected by muscle weight, ranged from about O-60% weight loss compared to controls. In general, changes in fiber diameters reflected changes in wet muscle weight (Figs. 3 and 4). Figure 4 illustrates the relationship between degree of atrophy, as determined by fiber size, and as determined by muscle weight. The points on the graph represent actual data points, whereas the line drawn represents a theoretical curve which was calculated on the basis of fiber diameter changes to predict muscle weight changes. Assuming a muscle fiber to be a cylinder and that all other variables remain constant except fiber diameter, fiber volume change varies with the square of the radius change.
122
EDGERTON
5 Colltr
