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The clarity of categorizing skeletal muscle fibers in individual motor units into phenotypes based on quantitative single fiber enzyme activities and as a function of neuromuscular activity level was examined. Neuromuscular activity was eliminated in adult cat hindlimb muscles by spinal cord isolation (SI), i.e. complete spinal cord transection at a low thoracic and a high sacral level with bilateral dorsal rhizotomy between the transection sites. One motor unit was isolated via ventral root teasing procedures from the tibialis anterior (TA) muscle of each hindlimb in control and SI cats, and physiologically tested and glycogen depleted through repetitive stimulation; fibers comprising each motor unit were visualized through glycogen staining. Each motor unit was composed of fibers of the same myosin immunohistochemical type. Myofibrillar adenosine triphosphatase, succinate dehydrogenase and alpha-glycerophosphate dehydrogenase activities were determined for a sample of motor unit and non-motor unit fibers, providing a measure of three enzyme activities often used to characterize fiber phenotype within a single unit. Although normal enzyme activities were altered after 6 months of inactivity, the relationships among the three enzymes were largely maintained. These data demonstrate that it is not the diversity in any single enzyme property but the profile of several metabolic pathways that underlies the significance of fiber phenotypes. These profiles must reflect a high level of coordination of expression of selected combinations of genes. Although neuromuscular activity level influences fiber phenotype, the present results demonstrate that activity-independent mechanisms remain important sources of the control of phenotype establishment in the near absence of activity.

Ballistic tongue projection in toads is a remarkably fast and powerful movement. The goals of this study were to: (1) quantify in vivo power output and activity of the depressor mandibulae muscles that are responsible for ballistic mouth opening, which powers tongue projection; (2) quantify the elastic properties of the depressor mandibulae muscles and their series connective tissues using in situ muscle stimulation and force-lever studies; and (3) develop and test an elastic recoil model, based on the observed elastic properties of the depressor mandibulae muscles and series connective tissues, that accounts for displacement, velocity, acceleration and power output during ballistic mouth opening in toads. The results demonstrate that the depressor mandibulae muscles of toads are active for up to 250 ms prior to mouth opening. During this time, strains of up to 21.4% muscle resting length (ML) develop in the muscles and series connective tissues. At maximum isometric force, series connective tissues develop strains up to 14% ML, and the muscle itself develops strains up to 17.5% ML. When the mouth opens rapidly, the peak instantaneous power output of the depressor mandibulae muscles and series connective tissues can reach 9600 W kg(-1). The results suggest that: (1) elastic recoil of muscle itself can contribute significantly to the power of ballistic movements; (2) strain in series elastic elements of the depressor mandibulae muscle is too large to be borne entirely by the cross bridges and the actin-myosin filament lattice; and (3) central nervous control of ballistic tongue projection in toads likely requires the specification of relatively few parameters.

Monodelphis domestica (Didelphidae: Marsupialia) lacks brown adipose tissue and thus relies on skeletal muscle as its primary thermogenic organ. Following cold exposure, the aerobic capacity of skeletal muscle in these animals is greatly increased. We investigated the effects of this plastic response to thermogenesis on locomotion and muscle mechanics. In cold-exposed animals, cost of transport was 15% higher than in controls but was unaffected by exercise training. Twitch kinetics in isolated semitendinosus muscles of cold-exposed animals were characteristic of slow-oxidative fiber types. Both time-to-peak tension and half-relaxation time were longer and maximal shortening velocity was slower following cold exposure compared to either thermoneutral controls or exercise-trained animals. Further, muscles from the cold-exposed animals had greater fatigue resistance than either control or exercise-trained animals, indicating greater oxidative capacity. Finally, we identified an uncoupling protein 3 homologue, whose gene expression was upregulated in skeletal muscle of cold-exposed Monodelphis domestica. Cold exposure provided a potent stimulus for muscle plasticity, driving a fast-to-slow transition more effectively than exercise training. However, linked to the dramatic shift in muscle properties is an equally dramatic increase in whole animal muscle energetics during locomotion, suggesting an uncoupled state, or 'training for inefficiency'.