Neural influence on slow muscle properties: inactivity with and without cross-reinnervation

The effects of altering (cross-reinnervation, CR) and/or eliminating (spinal isolation, SI) the activation-dependent neural input to the cat soleus for 8 months on its mechanical and biochemical properties were determined. The percent fast fibers was 0, 27, 38, and 54% for normal control (NC), NC-CR, SI, and SI-CR cats. Calcium-activated whole muscle myofibrillar adenosine triphosphatase activity was higher in SI-CR and NC cats. Isometric twitch speed properties and frequency of stimulation:tension response were faster in all experimental groups compared to NC. Maximum twitch and tetanic (Po) force, physiological cross-sectional area (PCSA), and specific tension (Po/PCSA) were unaffected in NC-CR cats, but reduced in SI and SI-CR cats. The soleus was more fatigable in SI-CR compared to NC cats. Alpha-glycerophosphate dehydrogenase activity was higher in all experimental groups compared to NC. All eight parameters reflecting the type of muscle were shifted toward those observed in a "faster" muscle in SI-CR vs. SI cats. These data suggest that electrically silent motoneurons can influence type-related skeletal muscle properties.

Theoretical basis for patterning EMG amplitudes to assess muscle dysfunction

A theoretical basis for assessing muscle dysfunction due to sprain/strain injuries is presented. We propose that muscle tissue trauma results in an alteration in the patterns of neural recruitment, a reduction in the force-generating capability of the injured muscle, and/or pain sensations. Furthermore, a lower than normal recruitment of motoneuron pools in the injured area can result in elevated recruitment levels from compensating motoneuron pools for a given motor task. It is proposed that these changes in motoneuron recruitment can be readily apparent in the ratios of EMG amplitudes among multiple pairs of muscles associated kinoffologically with the affected muscle. Chronic compensating actions, such as those resulting from faulty neural feedback of the force-length-velocity relationships for a stretched tendon or muscle unit, could cause further injuries. It is proposed that consistent and valid measures of ratios of EMG amplitudes between many muscle pairs acquired for well-defined motor tasks can be used to facilitate diagnoses and direct treatment strategies for sprain/strain injuries and pain.

Comparison of succinate dehydrogenase activity and soma size relationships among neurons in dorsal root ganglia of rats and cats

Compared to dorsal root ganglion (DRG) neurons at L5 in rats, DRG neurons at L7 in cat have a larger mean soma size, a bimodal rather than unimodal distribution of sizes and lower succinate dehydrogenase (SDH) activities for neurons of all sizes. In contrast to spinal motoneurons in both cats and rats, the larger DRG neurons have the higher SDH activities. The 10-20% higher SDH activity of DRG cells in rats than in cats may reflect, in part, a species difference of about 40% in metabolic rates.

Recruitment of the Rhesus soleus and medial gastrocnemius before, during and after spaceflight

Electromyograms were recorded from the soleus and medial gastrocnemius muscles and tendon force from the medial gastrocnemius muscle of 2 juvenile Rhesus monkeys before, during and after Cosmos flight 2229 and of ground control animals. Recording sessions were made while the Rhesus were performing a foot pedal motor task. Preflight testing indicated normal patterns of recruitment between the soleus and medial gastrocnemius, i.e. a higher level of recruitment of the soleus compared to the medial gastrocnemius during the task. Recording began two days into the spaceflight and showed that the media gastrocnemius was recruited preferentially over the soleus. This observation persisted throughout the flight and for the 2 week period of postflight testing. These data indicate a significant change in the relative recruitment of slow and fast extensor muscles under microgravity conditions. The appearance of clonic-like activity in one muscle of each Rhesus during flight further suggests a reorganization in the neuromotor system in a microgravity environment.

MHC and sarcoplasmic reticulum protein isoforms in functionally overloaded cat plantaris muscle fibers

To determine whether the adaptations in myosin heavy chain (MHC) isoform expression after functional overload (FO) are accompanied by commensurate adaptations in protein isoforms responsible for relaxation [sarco(endo)plasmic reticulum (SR) Ca(2+)-adenosinetriphosphatase (SERCA) and phospholamban (PHL)] in single muscle fibers, the isoforms of MHC and SERCA and the presence or absence of PHL were determined for cat plantaris fibers 3 mo after FO. In control plantaris the relative MHC isoform composition was 23% type I, 21% type IIa, and 56% type IIb. FO resulted in a shift toward slower isoforms (33% type I, 44% type IIa, and 23% type IIb). In the deep region of the plantaris the proportions of type I MHC and hybrid MHC fibers (containing type I and II MHCs) were 40 and 200% greater in FO cats, respectively. FO resulted in a 47% increase in the proportion of fibers containing only the slow SERCA isoform (SERCA2) and a 41% increase in the proportion of fibers containing PHL. The proportions of fibers containing type I MHC, SERCA2, and PHL in control and FO plantaris were linearly correlated. These data show that adaptations in MHC isoform expression are accompanied by commensurate adaptations in sarcoplasmic reticulum protein isoforms in single muscle fibers after FO.

Specific tension of human plantar flexors and dorsiflexors

Reported specific tension measurements for human skeletal muscle vary widely. This variability could be due, at least in part, to the determination of the physiological cross-sectional area (PCSA) of the muscles. In the present study, serial magnetic resonance images were taken every 10 mm along the lower leg of 8 male subjects to calculate the volume and subsequently the PCSAs of the individual muscles producing plantar flexor and dorsiflexor torques. Maximum plantar flexor and dorsiflexor voluntary isometric torques were determined at ankle joint angles of 90, 100, 110, and 120 degrees. Peak tendon force estimated from torque and moment arm measurements was more than fourfold higher in the plantar flexors (3,623 +/- 136 N) than in the dorsiflexors (832 +/- 19 N). PCSAs were about eight- and threefold higher than the anatomic cross-sectional areas at the level of maximum girth of the calf for the plantar flexor and dorsiflexor groups, respectively. Mean muscle volume and PCSA were 4.6 and 12 times larger in the plantar flexors compared with the dorsiflexors, respectively. The PCSAs of both plantar flexors (r = 0.92) and dorsiflexors (r = 0.80) were highly correlated with the tendon tension of the respective muscle groups. The maximum specific tension was more than twofold higher in dorsiflexors than in plantar flexors. These data suggest that factors other than PCSA contribute to the force output potential of ankle plantar flexors and dorsiflexors in humans.

Skeletal muscle fiber cross-sectional area: effects of freezing procedures

The cross-sectional area (CSA) of individual fibers is an important measure of skeletal muscle plasticity. To investigate the effects of different freezing procedures on CSA measurements, the CSA of type-identified fibers in the cat tibialis anterior were determined following quick-freezing the muscle at a fixed physiological length (ipsilateral, frozen at length) and compared to the fiber CSA following quick-freezing a mid-portion of the muscle where the fibers were allowed to freely shorten during tissue preparation (contralateral, frozen as a block). The mean CSA of each fiber type was significantly smaller in the muscles frozen at length vs. frozen as a block in both a deep (close to the bone) and a superficial (away from the bone) region of the muscle, except for the slow oxidative (SO) fibers in the superficial region. The percent difference in mean fiber CSA was smaller for the SO compared to the fast oxidative glycolytic and fast glycolytic fibers in both the deep (41, 47 and 56%, respectively) and superficial (20, 36 and 48%, respectively) regions. In addition, the differences in the mean CSAs of the fast fibers between the two freezing procedures were approximately 10% larger in the deep compared to the superficial region. The fiber-type differential responses may be related, at least in part, to the architectural features of the fibers. These data indicate that the freezing procedures used to prepare the muscle tissue are an important consideration when determining the CSA of individual skeletal muscle fibers and consequently the specific tension, tension per unit CSA, of the muscle units.

Influence of spaceflight on succinate dehydrogenase activity and soma size of rat ventral horn neurons

Succinate dehydrogenase (SDH) activities and soma cross-sectional areas (CSA) of neurons in the dorsolateral region of the ventral horn at the L5 segmental level of the spinal cord in the rat were determined after 14 days of spaceflight and after 9 days of recovery on earth. The results were compared to those in age-matched ground-based control rats. Spinal cords were quick-frozen, and the SDH activity and CSA of a sample of neurons with a visible nucleus were determined using a digitizer and a computer-assisted image analysis system. An inverse relationship between CSA and SDH activity of neurons was observed in all groups of rats. No change in mean CSA or mean SDH activity or in the size distribution of neurons was observed following spaceflight or recovery. However, there was a selective decrease in the SDH activity of neurons with soma CSA between 500 and 800 microns2 in the flight rats, and this effect persisted for at least 9 days following return to 1 g. It remains to be determined whether the selected population of motoneurons or the specific motor pools affected by spaceflight may be restricted to specific muscles.

Three-dimensional structure of cat tibialis anterior motor units

The motor unit is the basic unit for force production in a muscle. However, the position and shape of the territory of a motor unit within the muscle have not been defined precisely. The territories of five motor units in the cat tibialis anterior muscle were reconstructed three-dimensionally (3-D) from tracings of the glycogen-depleted fibers belonging to each unit. The motor unit territories did not span the entire length of the muscle and their cross-sectional areas tapered along the proximodistal axis producing a conical shape. In addition, the position of the territory of each unit shifted in an anterior-posterior plane along the longitudinal axis of the muscle, presumably as a consequence of the pinnation of the fibers. The area of the motor unit territory at any given level along the proximodistal axis was highly correlated with the number of fibers within the territory at that level. Connective tissue boundaries (outlining fascicles) appeared to have a strong influence on the shape of the territory, territories showed abrupt changes at connective tissue boundaries as groups of motor unit fibers within a fascicle often terminated together while motor unit fibers in neighboring fascicles did not terminate. It is likely that the mechanical impact of the recruitment of a motor unit is affected by the location and shape of motor units within the same muscle area.(ABSTRACT TRUNCATED AT 250 WORDS)

Succinate dehydrogenase activity and soma size of motoneurons innervating different portions of the rat tibialis anterior

The spatial distribution, soma size and oxidative enzyme activity of gamma and alpha motoneurons innervating muscle fibres in the deep (away from the surface of the muscle) and superficial (close to the surface of the muscle) portions of the tibialis anterior in normal rats were determined. The deep portion had a higher percentage of high oxidative fibres than the superficial portion of the muscle. Motoneurons were labelled by retrograde neuronal transport of fluorescent tracers: Fast Blue and Nuclear Yellow were injected into the deep portion and Nuclear Yellow into the superficial portion of the muscle. Therefore, motoneurons innervating the deep portion were identified by both a blue fluorescent cytoplasm and a golden-yellow fluorescent nucleus, while motoneurons innervating the superficial portion were identified by only a golden-yellow fluorescent nucleus. After staining for succinate dehydrogenase activity on the same section used for the identification of the motoneurons, soma size and succinate dehydrogenase activity of the motoneurons were measured. The gamma and alpha motoneurons innervating both the deep and superficial portions were located primarily at L4 and were intermingled within the same region of the dorsolateral portion of the ventral horn in the spinal cord. Mean soma size was similar for either gamma or alpha motoneurons in the two portions of the muscle. The alpha motoneurons innervating the superficial portion had a lower mean succinate dehydrogenase activity than those innervating the deep portion of the muscle. An inverse relationship between soma size and succinate dehydrogenase activity of alpha, but not gamma, motoneurons innervating both the deep and superficial portions was observed. Based on three-dimensional reconstructions within the spinal cord, there were no apparent differences in the spatial distribution of the motoneurons, either gamma or alpha, associated with the deep and superficial compartments of the muscle. The data provide evidence for an interdependence in the oxidative capacity between a motoneuron and its target muscle fibres in two subpopulations of motoneurons from the same motor pool, i.e. the same muscle.

Myofibrillar ATPase activity of feline muscle fibers expressing slow and fast myosin heavy chains

The interrelationships among myofibrillar ATPase activity (Quant-mATPase), qualitative myofibrillar ATPase staining after acid (Acid-mATPase) and alkaline (Alk-mATPase) preincubations, and myosin heavy chain (MHC) composition were determined in frozen sections of soleus (Sol) and medial gastrocnemius (MG) muscle fibers from adult control cats and cats 6 months after complete spinal cord transection (Sp). Fibers were categorized as either fast, slow, or fast and slow (Fast-Slow) based on monoclonal antibody labeling. Slow fibers had low Quant-mATPase activity and stained lightly with Alk-mATPase and darkly with Acid-mATPase. Fast fibers had high Quant-mATPase activity (approximately twice that of slow fibers) and stained darkly with Alk-mATPase and lightly with Acid-mATPase. Fast-Slow fibers had intermediate Quant-mATPase activity and stained intermediately for Acid-mATPase and darkly for Alk-mATPase. There was a positive linear relationship between Alk-mATPase and Quant-mATPase for all fibers of Sol and MG from control and Sp cats. There was a negative linear relationship between Acid-mATPase and Quant-mATPase for all fibers of Sol and MG. However, within the fast fiber population of the MG there was a positive relationship between these two measures of mATPase. In summary, quantitative and qualitative measures of mATPase are highly correlated with the types of MHC expressed by single fibers from control and Sp cat muscles.

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.

Myosin heavy chain isoforms of human muscle after short-term spaceflight

The influence of microgravity on the myosin phenotype of skeletal muscle fibers in the vastus lateralis of eight crew members was studied before and after 5-day (n = 3) and 11-day (n = 5) spaceflights (space shuttle flights: STS-32, -33 and -34). Single-fiber electrophoresis analyses showed that the proportion of fibers expressing only slow (type I) myosin heavy chain (MHC) in the vastus lateralis was significantly lower after than before 11 days of spaceflight. Although the family of type II MHC isoforms was elevated post- compared with preflight, the distribution among the isoforms of type II MHC was not statistically different. Based on monoclonal and polyclonal antibodies specific for three adult MHC isoforms and single-fiber electrophoresis, approximately 3% of the fibers analyzed coexpressed all three adult MHC isoforms. The results from immunohistochemical staining with two different sets of antibodies indicate a reduction in the percentage of fibers expressing type I MHC as a result of spaceflight. The mean difference, however, was significant only when the fibers were categorized simply as type I or II. These changes appeared to be highly individualized among the astronauts. These results suggest that a rapid change in MHC isoform expression can occur in some muscle fibers after a relatively brief exposure to spaceflight.

Plasticity of myonuclear number in hypertrophied and atrophied mammalian skeletal muscle fibers

Although a mammalian skeletal muscle fiber may contain thousands of myonuclei, the importance of this number or the potential to modulate it in adult muscle has not been clearly demonstrated. Using immunohistochemistry and confocal microscopy, we examined the plasticity of myonuclear number and fiber size in isolated fast and slow fiber segments from adult cat hindlimb muscles in response to chronic alterations in neuromuscular activity and loading. Compared with slow fibers in the soleus of control cats, myonuclear number in presumably transformed fast fibers was 32% lower and fiber size was decreased 73% after elimination of neuromuscular activation for 6 mo by spinal isolation. Slow fibers in the soleus of spinal-isolated cats had smaller cross-sectional areas, whereas myonuclear number was not significantly different than that in the control cats. Myonuclear number in fast plantaris fibers was more than threefold higher and fiber size was 2.8-fold higher after 3 mo of functional overload compared with the plantaris of control cats. Compared with control slow plantaris fibers, myonuclear number and fiber size also increased in overloaded slow plantaris fibers. These results demonstrate that changes in myonuclear number are associated with changes in myosin type and suggest that modulations in the amount of available DNA may be a factor in regulating cytoplasmic volume of muscle fibers in response to chronic changes in neuromuscular activity.

Human fiber size and enzymatic properties after 5 and 11 days of spaceflight

Biopsies from the vastus lateralis muscle were obtained from three astronauts before and after two 5-day flights and from five astronauts before and after one 11-day flight (space shuttle flights: STS-32, -33, and -34). Muscle fibers from two separate samples from each biopsy were classified as type I and II or as type I, IIA, and IIB by using qualitative myofibrillar adenosinetriphosphatase (ATPase) staining. Cross-sectional area (CSA), number of capillaries per fiber, and the activities of succinate dehydrogenase (SDH), alpha-glycerophosphate dehydrogenase (GPD), and myofibrillar ATPase were determined from one sample of fibers of each myofibrillar ATPase type. Postflight biopsies had 6-8% fewer type I fibers than preflight. Mean fiber CSAs were 16-36% smaller after the 11-day flight with the relative effect being type IIB > IIA > I. Mean fiber CSAs were 11 and 24% smaller in type I and II fibers after 5 days of flight. Myofibrillar ATPase activities increased in type II but not in type I fibers after flight, whereas SDH activity was unaffected in either fast or slow fibers. GPD activity in type I fibers was approximately 80% higher (P > 0.05) postflight compared with preflight. Myofibrillar ATPase/SDH ratios in type II fibers were higher after than before flight, suggesting that some fast fibers were more susceptible to fatigue after flight. The GPD/SDH ratios were elevated in some type I fibers after spaceflight. The number of capillaries per fiber was 24% lower after than before flight, whereas the number of capillaries per unit CSA of muscle tissue was unchanged. These data suggest that adaptations in the size, metabolic properties, and vascularity of muscle fibers can occur rapidly in the space environment. These adaptations were qualitatively similar to those observed in animals after actual or simulated spaceflight conditions for short periods.

Succinate dehydrogenase activity and soma size relationships among cat dorsal root ganglion neurons

A large range in succinate dehydrogenase (SDH) activity and soma size among neurons in the dorsal root ganglion (DRG) and the dorsolateral region of the ventral horn (DLVH) at spinal cord level L7 was observed. Mean soma sizes were similar for the two populations. DLVH, but not DRG, neurons showed an inverse relationship between SDH activity and soma size. DRG neurons had a higher mean SDH activity than DLVH neurons, reflecting the observation that there was a population of DRG neurons with a higher oxidative capacity than DLVH neurons.

Prominence of myosin heavy chain hybrid fibers in soleus muscle of spinal cord-transected rats

The effect of a midthoracic spinal cord transection (ST) on myosin heavy chain (MHC) isoform expression in the rat soleus muscle was studied. Electrophoretic analyses demonstrated that 15 days after ST there were significant proportional increases in type IIx, decreases in type IIa, and no change in type I MHC composition. Thirty days after ST, some type IIb MHC was expressed, there were further proportional increases in type IIx, an increase in type IIa (compared with 15-day ST), and a decrease in type I MHC. At both time periods after ST, many fibers expressed multiple MHCs, as demonstrated by immunohistochemistry where a battery of monoclonal antibodies specific to MHC isoforms was used. Fibers were observed containing types I and II together or multiple type II MHC isoforms. These data suggest that the expression of the normal complement of MHC isoforms in the adult rat soleus muscle is dependent, in part, on normal neuromuscular activation.

Further evidence of incomplete neural control of muscle properties in cat tibialis anterior motor units

To examine the influence of a motoneuron in maintaining the phenotype of the muscle fibers it innervates, myosin heavy chain (MHC) expression, succinate dehydrogenase (SDH) activity, and cross-sectional area (CSA) of a sample of fibers belonging to a motor unit were studied in the cat tibialis anterior 6 mo after the nerve branches innervating the anterior compartment were cut and sutured near the point of entry into the muscle. The mean, range, and coefficient of variation for the SDH activity and the CSA for both motor unit and non-motor unit fibers for each MHC profile and from each control and each self-reinnervated muscle studied was obtained. Eight motor units were isolated from self-reinnervated muscles using standard ventral root filament testing techniques, tested physiologically, and compared with four motor units from control muscles. Motor units from self-reinnervated muscles could be classified into the same physiological types as those found in control tibialis anterior muscles. The muscle fibers belonging to a unit were depleted of glycogen via repetitive stimulation and identified in periodic acid-Schiff-stained frozen sections. Whereas muscle fibers in control units expressed similar MHCs, each motor unit from self-reinnervated muscles contained a mixture of fiber types. In each motor unit, however, there was a predominance of fibers with the same MHC profile. The relative differences in the mean SDH activities found among fibers of different MHC profiles within a unit after self-reinnervation and those found among fibers in control muscles were similar, i.e., fast-2 < fast-1 < or = slow MHC fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptations of human skeletal muscle fibers to spaceflight

Human skeletal muscle fibers seem to share most of the same interrelationships among myosin ATPase activity, myosin heavy chain (MHC) phenotype, mitochondrial enzyme activities, glycolytic enzyme activities and cross-sectional area (CSA) as found in rat, cat and other species. One difference seems to be that fast fibers with high mitochondrial content occur less frequently in humans than in the rat or cat. Recently we have reported that the type of MHC expressed and the size of the muscle fibers in humans that have spent 11 days in space change significantly. Specifically, about 8% more fibers express fast MHCs and all phenotypes atrophy in the vastus lateralis (VL) post compared to preflight. In the present paper we examine the relationships among the population of myonuclei, MHC type and CSA of single human muscle fibers before and after spaceflight. These are the first data that define the relationship among the types of MHC expressed, myonuclei number and myonuclei domain of single fibers in human muscle. We then compare these data to similar measures in the cat. In addition, the maximal torque that can be generated by the knee extensors and their fatigability before and after spaceflight are examined. These data provide some indication of the potential physiological consequences of the muscle adaptations that occur in humans in response to spaceflight.

Can the mammalian lumbar spinal cord learn a motor task?

Progress toward restoring locomotor function in low thoracic spinal transected cats and the application of similar techniques to patients with spinal cord injury is reviewed. Complete spinal cord transection (T12-T13) in adult cats results in an immediate loss of locomotor function in the hindlimbs. Limited locomotor function returns after several months in cats that have not received specific therapies designed to restore hindlimb stepping. Training transected cats to step on a treadmill for 30 min.d-1 and 5 d.wk-1 greatly improves their stepping ability. The most successful outcome was in cats where training began early, i.e., 1 wk after spinal transection. Cats trained to stand instead of stepping had great difficulty using the hindlimbs for locomotion. These effects were reversible over a 20-month period such that cats unable to step as a result of standing training could be trained to step and, conversely, locomotion in stepping-trained cats could be abolished by standing training. These results indicate that the spinal cord is capable of learning specific motor tasks. It has not been possible to elicit locomotion in patients with clinically complete spinal injuries, but appropriately coordinated EMG activity has been demonstrated in musculature of the legs during assisted locomotion on a treadmill.

Level of independence of motor unit properties from neuromuscular activity

Neuromuscular activity was eliminated in the tibialis anterior muscle of adult cats for 6 months by spinal isolation (SI), i.e., complete spinal cord transections at T-12-13 and at L-7-S-1, plus bilateral dorsal rhizotomy between the two transection sites. One motor unit from each muscle was isolated using ventral root teasing procedures and physiologically tested. The fibers belonging to each motor unit were visualized in PAS-stained sections by the loss of glycogen following prolonged repetitive stimulation. Qualitatively, the normal enzymatic interrelationships among fibers identified by myosin heavy chain composition were unchanged by SI. Generally, each motor unit from SI cats were of a single myosin immunohistochemical type. The same physiological motor unit types that typify control muscles were found in SI cats. In SI compared to control cats, there was approximately a 10% increase in the number of muscle fibers expressing fast myosin. Mean fiber activity levels of ATPase and SDH for a given fiber type (based on MHC antibody reactions) decreased by approximately 10% and 25%, whereas GPD activity increased approximately 35%. It is concluded that differential levels or patterns of activity are not essential to maintain the range of histochemical and physiological motor unit types found in the tibialis anterior of normal adult cats.

Extensor- and flexor-like modulation within motor pools of the rat hindlimb during treadmill locomotion and swimming

EMG activity was recorded from the vastus lateralis (VL, knee extensor), rectus femoris (RF, hip flexor and knee extensor), tibialis anterior (TA, ankle flexor and digit extensor) and either the lateral or medial gastrocnemius (LG, MG, knee flexors and ankle extensors) muscles of 7 adult rats during treadmill locomotion and swimming. Most flexors and extensors are activated as a single burst but each is known to be modulated differently during locomotion. For example, the extensor EMG bursts are shortened and amplitude elevated as speed increases, whereas little change occurs in the EMG duration and amplitude in flexors. The RF and VL displayed a double burst of EMG activity per cycle during treadmill locomotion and a single burst during swimming. Kinematic and EMG analyses showed that during running, one of these EMG bursts occurred primarily during swing while the other burst occurred primarily during stance. Modulation of the burst occurring during swing approximated a flexor pattern, while the second burst was modulated like a typical extensor when running over a range of speeds and grades on a treadmill. These data suggest that motoneurons within a motor pool of a uniarticular (VL) as well as a biarticular (RF) muscle can be modulated by more than one cyclical input, probably of central origin, and that under some conditions several motor pools may share the same central commands.

Metabolic adaptation of skeletal muscles to gravitational unloading

Responses of high-energy phosphates and metabolic properties to hindlimb suspension were studied in adult rats. The relative content of phosphocreatine (PCr) in the calf muscles was significantly higher in rats suspended for 10 days than in age-matched cage controls. The Pi/PCr ratio, where Pi is inorganic phosphate, in suspended muscles was less than controls. The absolute weights of soleus and medial gastrocnemius (MG) were approximately 40% less than controls. Although the % fiber distribution in MG was unchanged, the % slow fibers decreased and the % fibers which were classified as both slow and fast was increased in soleus. The activities (per unit weight or protein) of succinate dehydrogenase and lactate dehydrogenase in soleus were unchanged but those of cytochrome oxidase, beta-hydroxyacyl CoA dehydrogenase, and citrate synthase were decreased following unloading. None of these enzyme activities in MG changed. However, the total levels of all enzymes in whole muscles decreased by suspension. It is suggested that shift of slow muscle toward fast type by unloading is associated with a decrease in mitochondrial biogenesis. Further, gravitational unloading affected the levels of muscle proteins differently even in the same mitochondrial enzymes.

Interactive effects of growth hormone and exercise on muscle mass in suspended rats

Measures to attenuate muscle atrophy in rats in response to stimulated microgravity [hindlimb suspension (HS)] have been only partially successful. In the present study, hypophysectomized rats were in HS for 7 days, and the effects of recombinant human growth hormone (GH), exercise (Ex), or GH+Ex on the weights, protein concentrations, and fiber cross-sectional areas (CSAs) of hindlimb muscles were determined. The weights of four extensor muscles, i.e., the soleus (Sol), medial (MG) and lateral (LG) gastrocnemius, and plantaris (Plt), and one adductor, i.e., the adductor longus (AL), were decreased by 10-22% after HS. Fiber CSAs were decreased by 34% in the Sol and by 17% in the MG after HS. In contrast, two flexors, i.e., the tibialis anterior (TA) and extensor digitorum longus (EDL), did not atrophy. In HS rats, GH treatment alone maintained the weights of the fast extensors (MG, LG, Plt) and flexors (TA, EDL) at or above those of control rats. This effect was not observed in the slow extensor (Sol) or AL. Exercise had no significant effect on the weight of any muscle in HS rats. A combination of GH and Ex treatments yielded a significant increase in the weights of the fast extensors and in the CSA of both fast and slow fibers of the MG and significantly increased Sol weight and CSA of the slow fibers of the Sol. The AL was not responsive to either GH or Ex treatments. Protein concentrations of the Sol and MG were higher only in the Sol of Ex and GH + Ex rats.(ABSTRACT TRUNCATED AT 250 WORDS)