Adaptive response of hypertrophied skeletal muscle to endurance training

Regular endurance exercise of overloaded muscle of young and old male mice does not attenuate hypertrophy and improves fatigue resistance

It has been observed that there is an inverse relationship between fiber size and oxidative capacity due to oxygen, ADP, and ATP diffusion limitations. We aimed to see if regular endurance exercise alongside a hypertrophic stimulus would lead to compromised adaptations to both, particularly in older animals. Here we investigated the effects of combining overload with regular endurance exercise in young (12 months) and old (26 months) male mice. The plantaris muscles of these mice were overloaded through denervation of synergists to induce hypertrophy and the mice ran on a treadmill for 30 min per day for 6 weeks. The hypertrophic response to overload was not blunted by endurance exercise, and the increase in fatigue resistance with endurance exercise was not reduced by overload. Old mice demonstrated less hypertrophy than young mice, which was associated with impaired angiogenesis and a reduction in specific tension. The data of this study suggest that combining endurance exercise and overload induces the benefits of both types of exercise without compromising adaptations to either. Additionally, the attenuated hypertrophic response to overload in old animals may be due to a diminished capacity for capillary growth.

Korean mistletoe (Viscum album coloratum) extract regulates gene expression related to muscle atrophy and muscle hypertrophy

Background

Korean mistletoe (Viscum album coloratum) is a semi-parasitic plant that grows on various trees and has a diverse range of effects on biological functions, being implicated in having anti-tumor, immunostimulatory, anti-diabetic, and anti-obesity properties. Recently, we also reported that Korean mistletoe extract (KME) improves endurance exercise in mice, suggesting its beneficial roles in enhancing the capacity of skeletal muscle.

Methods

We examined the expression pattern of several genes concerned with muscle physiology in C2C12 myotubes cells to identify whether KME inhibits muscle atrophy or promotes muscle hypertrophy. We also investigated these effects of KME in denervated mice model.

Results

Interestingly, KME induced the mRNA expression of SREBP-1c, PGC-1α, and GLUT4, known positive regulators of muscle hypertrophy, in C2C12 cells. On the contrary, KME reduced the expression of Atrogin-1, which is directly involved in the induction of muscle atrophy. In animal models, KME mitigated the decrease of muscle weight in denervated mice. The expression of Atrogin-1 was also diminished in those mice. Moreover, KME enhanced the grip strength and muscle weight in long-term feeding mice.

Conclusions

Our results suggest that KME has beneficial effects on muscle atrophy and muscle hypertrophy.

MicroRNA-23a has minimal effect on endurance exercise-induced adaptation of mouse skeletal muscle

Skeletal muscles contain several subtypes of myofibers that differ in contractile and metabolic properties. Transcriptional control of fiber-type specification and adaptation has been intensively investigated over the past several decades. Recently, microRNA (miRNA)-mediated posttranscriptional gene regulation has attracted increasing attention. MiR-23a targets key molecules regulating contractile and metabolic properties of skeletal muscle, such as myosin heavy-chains and peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1α). In the present study, we analyzed the skeletal muscle phenotype of miR-23a transgenic (miR-23a Tg) mice to explore whether forced expression of miR-23a affects markers of mitochondrial content, muscle fiber composition, and muscle adaptations induced by 4 weeks of voluntary wheel running. When compared with wild-type mice, protein markers of mitochondrial content, including PGC-1α, and cytochrome c oxidase complex IV (COX IV), were significantly decreased in the slow soleus muscle, but not the fast plantaris muscle of miR-23a Tg mice. There was a decrease in type IId/x fibers only in the soleus muscle of the Tg mice. Following 4 weeks of voluntary wheel running, there was no difference in the endurance exercise capacity as well as in several muscle adaptive responses including an increase in muscle mass, capillary density, or the protein content of myosin heavy-chain IIa, PGC-1α, COX IV, and cytochrome c. These results show that miR-23a targets PGC-1α and regulates basal metabolic properties of slow but not fast twitch muscles. Elevated levels of miR-23a did not impact on whole body endurance capacity or exercise-induced muscle adaptations in the fast plantaris muscle.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Resident stem cells are not required for exercise-induced fiber-type switching and angiogenesis but are necessary for activity-dependent muscle growth

Skeletal muscle undergoes active remodeling in response to endurance exercise training, and the underlying mechanisms of this remodeling remain to be defined fully. We have recently obtained evidence that voluntary running induces cell cycle gene expression and cell proliferation in mouse plantaris muscles that undergo fast-to-slow fiber-type switching and angiogenesis after long-term exercise. To ascertain the functional role of cell proliferation in skeletal muscle adaptation, we performed in vivo 5-bromo-2′-deoxyuridine (BrdU) pulse labeling (a single intraperitoneal injection), which demonstrated a phasic increase (5- to 10-fold) in BrdU-positive cells in plantaris muscle between days 3 and 14 during 4 wk of voluntary running. Daily intraperitoneal injection of BrdU for 4 wk labeled 2.0% and 15.4% of the nuclei in plantaris muscle in sedentary and trained mice, respectively, and revealed the myogenic and angiogenic fates of the majority of proliferative cells. Ablation of resident stem cell activity by X-ray irradiation did not prevent voluntary running-induced increases of type IIa myofibers and CD31-positive endothelial cells but completely blocked the increase in muscle mass. These findings suggest that resident stem cell proliferation is not required for exercise-induced type IIb-to-IIa fiber-type switching and angiogenesis but is required for activity-dependent muscle growth. The origin of the angiogenic cells in this physiological exercise model remains to be determined.

Transcriptional profiling in mouse skeletal muscle following a single bout of voluntary running: evidence of increased cell proliferation

Skeletal muscle undergoes adaptation following repetitive bouts of exercise. We hypothesize that transcriptional reprogramming and cellular remodeling start in the early phase of long-term training and play an important role in skeletal muscle adaptation. The aim of this study was to define the global mRNA expression in mouse plantaris muscle during (run for 3 and 12 h) and after (3, 6, 12, and 24 h postexercise) a single bout of voluntary running and compare it with that after long-term training (4 wk of running). Among 15,832 gene elements surveyed in a high-density cDNA microarray analysis, 900 showed more than twofold changes at one or more time points. K-means clustering and cumulative hypergeometric probability distribution analyses revealed a significant enrichment of genes involved in defense, cell cycle, cell adhesion and motility, signal transduction, and apoptosis, with induced expression patterns sharing similar patterns with that of peroxisome proliferator activator receptor-gamma coactivator-1alpha and vascular endothelial growth factor A. We focused on the finding of a delayed (at 24 h postexercise) induction of mRNA expression of cell cycle genes origin recognition complex 1, cyclin A2, and cell division 2 homolog A (Schizoccharomyces pombe) and confirmed increased cell proliferation by in vivo 5-bromo-2'-deoxyuridine labeling following voluntary running. X-ray irradiation of the hindlimb significantly diminished exercise-induced 5-bromo-2'-deoxyuridine incorporation. These findings suggest that a single bout of voluntary running activates the transcriptional network and promotes adaptive processes in skeletal muscle, including cell proliferation.

Effect of endurance training on oestrogen receptor alpha expression in different rat skeletal muscle type

It is well known that oestrogens exert muscle anabolic and metabolic effects. Oestrogens act via specific oestrogen receptor (ER) proteins. The mainly represented oestrogen receptor alpha messenger ribonucleic acid subtype (ER(alpha) mRNA) was described in various tissues including the skeletal muscle. Moreover, it has been shown that endurance training significantly increases ER(alpha) mRNA levels in the female rat gastrocnemius muscle. The aim of this study was to determine if this training programme also modifies ER(alpha) mRNA levels in muscles with different typology, the soleus (slow twitch muscle), extensor digitorum longus (fast twitch muscle) and gastrocnemius (intermediate muscle). So far, two groups of Wistar female rats were set up: untrained (u) (n = 7), and trained (e) (n = 7). The endurance training programme was performed for 7 weeks, 5 days per week and consisted of 1 h of continuous running on an adapted motor-driven treadmill involving progressive intensity and gradient of the treadmill. Three different skeletal muscles, extensor digitorum longus (E), gastrocnemius (G) and soleus (S), were isolated and weighed in the untrained (Eu, Gu and Su) and trained group (Ee, Ge and Se). Semi-quantification of ER(alpha) mRNA levels was performed by the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. In order to attest the efficiency of our endurance training programme, the citrate synthase activity (CS) of each muscle was measured by a fluorimetric method. The CS activity was significantly increased with training in the gastrocnemius [100.00 +/- 4.99% in Gu (n = 6) vs. 138.10 +/- 8.82% in Ge (n = 6), P < 0.01] and in the soleus [100.00 +/- 2.92% in Su (n = 7) vs. 115.90 +/- 3.71% in Se (n = 7), P < 0.01] but not in the extensor digitorum longus [100.00 +/- 1.87% in Eu (n = 7) vs. 96.90 +/- 1.55% in Ee (n = 7)]. Concerning the influence of muscle type on ER(alpha) mRNA level (1) in the untrained group, the ER(alpha) mRNA level was significantly higher in soleus muscle compared with gastrocnemius and extensor digitorum longus muscles [0.43 +/- 0.04 in Su (n = 7) compared with 0.31 +/- 0.03 in Gu (n = 6) and 0.21 +/- 0.03 in Eu (n = 7), P < 0.05; P < 0.05); 2] in the trained group, the ER(alpha) mRNA level was significantly higher insoleus and gastrocnemius muscles compared with extensor digitorum longus muscle [0.43 +/- 0.06 in Se (n = 7) and 0.49 +/- 0.05 in Ge (n = 6) vs. 0.12 +/- 0.01 in Ee (n = 7), P < 0.05; P < 0.05]. Indeed, after training, the ER(alpha) mRNA level significantly increased in gastrocnemius muscle [0.31 +/- 0.03 in Gu(n = 6) vs. 0.49 +/- 0.05 in Ge (n = 6), P < 0.01], significantly decreased in extensor digitorum longus [0.21 +/- 0.03 in Eu (n = 7) vs. 0.12 +/- 0.01 in Ee (n = 7), P < 0.01] and was not significantly modified in soleus [0.43 +/- 0.04 in Su (n = 7) vs. 0.43 +/- 0.06 in Se (n = 7)]. The differences in ER(alpha) mRNA level between trained and untrained animals indicate training-induced effects that are specific to the skeletal muscle type.

Effect of endurance training on oestrogen receptor alpha transcripts in rat skeletal muscle

Endurance training induces, in female rats, alterations of oestrous cycle with decrease in plasma oestradiol levels. Moreover, it is well known that oestradiol concentrations modify oestrogen receptor levels. In order to further explain the effects of oestrogens on skeletal muscles, we hypothesized that endurance training modifies the levels of oestrogen receptor alpha messenger ribonucleic acid (ER alpha mRNA) in rat gastrocnemius muscle. Wistar rats were separated into four groups: male controls (C(m)) (n=7), female controls (C(f)) (n=6), male trained (E(m)) (n=7) and female trained (E(f)) (n=6). The endurance training programme was performed for 7 weeks, 5 days week-1 and consisted of 1 h of continuous running on an adapted motor-driven treadmill. At the end of the training session, the gastrocnemius muscle was isolated, weighed and semiquantification of ER alpha mRNA was performed using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. The citrate synthase (CS) activity of the gastrocnemius muscle was measured by a fluorimetric method. The CS activity of the male and female gastrocnemius muscle, respectively, 100 +/- 7% in C(m) (n=7) vs. 120 +/- 14% in E(m) (n=6, P < 0.01) and 100 +/- 13% in C(f) (n=6) vs. 138 +/- 23% in E(f) (n=6, P < 0.01) was significantly increased after 7 weeks of training. The ER alpha mRNA levels were significantly increased in E(f) compared with C(f) (0.49 +/- 0.15 vs. 0.31 +/- 0.11, P < 0.01) but not in E(m) compared with C(m) (0.37 +/- 0.15 vs. 0.37 +/- 0.13). In conclusion, these results demonstrate that 7 weeks of endurance training increased the level of transcripts encoding ER alpha in rats with the increase restricted to the females.

Metabolic and morphological stability of motoneurons in response to chronically elevated neuromuscular activity

The purpose of this study was to determine the plasticity of spinal motoneuron size and succinate dehydrogenase activity in response to increased levels of neuromuscular activation and/or increased target size. The plantaris muscles of adult rats were functionally overloaded for one or 10 weeks via the removal of the soleus and gastrocnemius muscles bilaterally. In addition, one group of functionally overloaded rats at each time period was trained daily (1 h/day) on a treadmill. The plantaris muscle on one side in each rat was injected with the fluorescent tracer Nuclear Yellow two days prior to the end of the study to retrogradely label the associated motor pool. At one week, the plantaris weight was increased compared to control, whereas there was no change in motoneuron size. Succinate dehydrogenase activity was unaffected in either the muscle or motoneurons. At 10 weeks, the plantaris muscle weight was larger and the succinate dehydrogenase activity lower in the functionally overloaded rats compared to age-matched controls. Training further increased the hypertrophic response, whereas the succinate dehydrogenase activity returned to control levels. In contrast, mean motoneuron size and succinate dehydrogenase activity were similar among the three groups. These data indicate that overload of a specific motor pool, involving both an increase in activation and an increase in target size, had a minimal effect on the size or the oxidative potential of the associated motoneurons. Thus, it appears that the spinal motoneurons, unlike the muscle fibers, are highly stable over a wide range of levels of chronic neuromuscular activity.

Modulation of MHC isoforms in functionally overloaded and exercised rat plantaris fibers

The effects of 1 and 10 wk of functional overload (FO) of the rat plantaris with (FOTr) and without daily endurance treadmill training on its myosin heavy chain (MHC) composition were studied. After 1 and 10 wk of FO, plantaris mass was 22 and 56% greater in FO and 37 and 94% greater, respectively, in FOTr rats compared with age-matched controls. At 1 wk, pure type I and pure type IIa MHC fibers were hypertrophied in FO (39 and 44%) and FOTr (70 and 87%) rats. By 10 wk all fiber types comprising >5% of the fibers sampled showed a hypertrophic response in both FO groups. One week of FO increased the percentage of hybrid (containing both type I and type IIa MHC) fibers and of fibers containing embryonic MHC. By 10 wk, the percentage of pure type I MHC fibers was approximately 40% in both FO groups compared with 15% in controls, and the percentage of fibers containing embryonic MHC was similar to that in controls. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses showed an increase in type I MHC and a decrease in type IIb MHC in both FO groups at 10 wk, whereas little change was observed at 1 wk. These data are consistent with hypertrophy and transformation from faster to slower MHC isoforms in chronically overloaded muscles. The additional overload imposed by daily endurance treadmill training employed in this study (1.6 km/day; 10% incline) results in a larger hypertrophic response but appears to have a minimal effect on the MHC adaptations.

The response of adult and developing rat plantaris muscle to overload

The effect of overload on the rat plantaris muscle was studied in animals of different ages. Overload was induced by removal of gastrocnemius and soleus muscles. As expected, when the operation was carried out in adults, the plantaris muscle became heavier and stronger. These changes occured within 30 days after the operation. In animals in which the operation was carried out 1-12 days after birth and the muscle examined 6-20 weeks later, different results were obtained. In the group operated at 1-9 days of age, the muscles developed a lower maximal twitch and tetanic tension than the contralateral plantaris muscle. There was no difference in the time to peak or muscle weight between the overloaded and the contralateral muscles. Similar changes were observed in animals where the overload was induced at 11 or 12 days of age except for the weight which was significantly higher than that of the control plantaris muscles. The number of slow fibers increased in animals where overload was induced 11-12 days postnatally or in adults, but not when muscles were overloaded at 9 days of age. The possible reasons for the different response of adult and neonatal muscles to overload are discussed.

Response of mouse plantaris muscle to functional overload: comparison with rat and cat

Functional overload (FO) of a muscle by removing its synergists results in a compensatory hypertrophy of the muscle. However, the extent of the response appears to be dependent, at least in part, on the activity and/or loading levels of the muscle following surgery. Thus, differences in the inherent physical activity levels across species may be an important factor to consider. In the present study, the effects of 8 weeks of FO on the isometric mechanical properties of the plantaris of mice (highly active) were determined and the findings compared with the results from previous studies performed on the plantaris of rats (highly active) and cats (less active). FO resulted in approximately a doubling of the mass, the physiological cross-sectional area and the maximum tetanic tension per unit cross-sectional area, was similar in the plantaris of control and FO mice. Isometric twitch speed properties were unaffected, but the tension enhancement in response to an increase in the rate of stimulation showed the pattern of a "faster" muscle following FO. The fatigue resistance of the plantaris in FO mice was significantly higher than in control mice. Although the degree of hypertrophy that occurred in the mouse plantaris was similar to that observed after FO in rats and in cats that are exercised intermittently at high intensities, there were differences in the mechanical properties that may be related to the adaptability of species and/or the behavioral responses to the overload.

Changes in oxidative capacity and fatigue resistance in skeletal muscle

In conclusion, it appears that in general an increase in the fatigue resistance of a muscle is accompanied by an increase in its oxidative capacity. Fatigue resistance of a muscle seems to be partly determined by its oxidative capacity. On the single motor unit (Burke et al, 1973; Hamm et al, 1988; Kugelberg and Lindegren 1979; Larsson et al, 1991) and single fibre level (Nemeth et al, 1981) the relation between fatigue resistance and oxidative capacity seems to be valid. However, this does not appear necessarily to be the case on the level of the whole muscle. Kugelberg and Lindegren (1979) suggested, that the endurance of each link in the chain of events leading to contraction is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity. Therefore, it might be that several tests for endurance capacity are more strenuous than the aerobic capacity of the muscle. Indeed, several studies suggest that the Burke test (Burke et al, 1973) or other fatiguing protocols might primarily test for other endurance-related properties as the excitation-contraction coupling (Kernell et al, 1987; Mayne et al, 1991b). Another explanation for the discrepancy in changes in oxidative capacity and fatigue resistance might be, that the mechanical responses of the motor units (which have different biochemical and contractile properties) during the fatigue test do not summate linearly during whole muscle contraction as was found by Gardiner and Olha (1987).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic capacity, fibre type area and capillarization of rat plantaris muscle. Effects of age, overload and training and relationship with fatigue resistance

1. The influences of age (5, 13 and 25-month-old rats), overload as obtained by denervation of synergists, and training on the metabolic capacity, relative muscle cross-sectional area occupied by each fibre type, capillarization and fatigue resistance of the rat m. plantaris were investigated. 2. Creatine kinase, phosphorylase and citrate synthase activities were lower in muscles of 25 than in those of 13-month-old rats (P < 0.001). 3. Overload resulted in an increased relative area of type I and IIa fibres at all ages (P = 0.001). 4. Capillary density decreased with overload and increasing age (P < 0.001). 5. Fatigue resistance was higher in muscles of 13 than in those of 5-month-old rats (P < 0.05), and increased with overload (P < 0.05) at all ages. 6. Fatigue resistance of the whole muscle was not closely related to its oxidative capacity in contrast to what is generally found for single fibres or motor units.

Effects of exercise training and anabolic steroids on plantaris and soleus phospholipids: a 31P nuclear magnetic resonance study

1. The purpose of this study was to examine the effect of exercise, anabolic steroid treatment, and a combination of both treatments on the phospholipid composition of predominantly fast twitch (plantaris) and slow twitch (soleus) skeletal muscles. The 4 experimental groups analyzed were sedentary control (C), steroid-treated (S), exercise-trained (E), and exercise plus steroid-treated (ES). 2. Among the 11 phospholipids quantitated, for the plantaris muscle, phosphatidylcholine was reduced in ES relative to C, while phosphatidylethanolamine and phosphatidylethanolamine plasmalogen were elevated in E and ES relative to C. For the soleus muscle, phosphatidylserine was reduced in S and E relative to C, and cardiolipin was elevated in E relative to C. 3. Of the 27 metabolic indices calculated for the plantaris, 15 changed significantly among E and ES relative to S and C, while for the soleus, only three indices changed among the four groups, two among E and ES relative to S and C and one between S and C. 4. For the plantaris muscle, the results are consistent with an exercise-induced alteration of membrane phospholipid composition that increases ion translocation activity. For the soleus muscle, this membrane alteration essentially does not take place. 5. Steroid treatment had little to no statistically significant effect on plantaris and soleus muscle phospholipid systems, regardless of the imposed regimen.

Adaptation in synergistic muscles to soleus and plantaris muscle removal in the rat hindlimb

Although the soleus muscle comprises only 6% of the ankle plantar flexor mass in the rat, a major role in stance and walking has been ascribed to it. The purpose of this study was to determine if removal of the soleus muscle would result in adaptations in the remaining gastrocnemius and plantaris muscles due to the new demands for force production imposed on them during stance or walking. A second purpose was to determine whether the mass or the fiber type of the muscle(s) removed was a more important determinant of compensatory adaptations. Male Sprague-Dawley rats underwent bilateral removal of soleus muscle, plantaris muscle, or both muscles. For comparison, compensatory hypertrophy was induced in soleus and plantaris muscles by gastrocnemius muscle ablation. After forty days, synergist muscles remaining intact were removed. Mass, and oxidative, glycolytic, and contractile enzyme activities were determined. Despite its role in stance and slow walking, removal of the soleus muscle did not elicit a measurable alteration in muscle mass, or in citrate synthase, lactate dehydrogenase, or myofibrillar ATPase activity in gastrocnemius or plantaris muscles. Similarly, removal of the plantaris muscle, or soleus and plantaris muscles, had no effect on the gastrocnemius muscle, suggesting that this muscle was able to easily meet the new demands placed on it. These results suggest that amount of muscle mass removed, rather than fiber type, is the most important stimulus for compensatory hypertrophy. They also suggest that slow-twitch motor units in the gastrocnemius muscle play an important role during stance and locomotion in the intact animal.