Differential response of fast hindlimb extensor and flexor muscles to exercise in adult spinalized cats

Adult cats were spinal transected (T12-13) and maintained for similar to 6 months. Spinal cats were either not trained (N-T) or trained for 30 min/day to either step on a treadmill (Stp-T) or stand (Std-T), Spinalization resu lted in a decrease in the mass and maximum tension potential of the medial gastrocnemius (MG), a fast ankle extensor. These adaptations were ameliorat ed in Std-T but not Stp-T cats. The maximum rate of shortening was elevated by 18 (ns), 34, and 19 (ns)% in the N-T, Std-T, and Stp-T cats, respective ly, a finding consistent with a shift in the percentage of fast fibers, a d ecrease in the percentage of fibers expressing only type I myosin heavy cha in, and an increase in myofibrillar adenosine triphosphatase activity. The shirt toward a faster fiber type profile in the tibialis anterior (TA), a f ast ankle flexor, was of a lesser magnitude than in the MG. There were no s ignificant effects on the contractile properties of the TA in any group of spinal cats. The greater preservation of muscle mass, shift toward faster p hysiological and biochemical properties, and fatigability in the MG of Std- T than Stp-T cats suggest that factors other than the level of activation a nd force generation must play a role in muscle homeostasis. From a clinical perspective, the results indicate that muscles innervated by motor neurons below the level of a complete spinal cord lesion are affected differential ly by specific neuromuscular activity patterns. (C) 1999 John Wiley & Sons, Inc.