Fiber type and fiber size changes in selected thigh muscles six months after low thoracic spinal cord transection in adult cats: exercise effects.

The effects of low thoracic spinal cord transection on muscle weights, fiber type compositions and, fiber cross-sectional areas of selected hind limb muscles were studied. Adult cats were spinalized at T12 and maintained for approximately 6 months. Some spinalized cats were exercised on a treadmill 30 min/day, 5 days/week to determine the role of weight support in maintaining the muscle properties. Spinalization resulted in a significant decrease in the weights of most extensors, whereas the flexors or those that act as both flexors and extensors were maintained near control values. All muscles showed a significant increase in the percentage of fast-twitch fibers and decrease in slow oxidative fibers following spinalization. In addition, the predominant fast fiber type in each muscle tended to have the largest decrease in cross-sectional area in the spinalized cats. The atrophic and fiber type adaptations were less pronounced in the exercised cats. In contrast, the relative cross-sectional area of high oxidative fibers generally was similar among the three groups. These results demonstrated that the muscles below the level of the lesion became more "fast-like" histochemically following spinal cord transection, whereas the oxidative properties were relatively unaffected. Further, daily exercise involving weight support appeared to be an important deterrent to these atrophic responses, particularly in the muscles that normally have a postural function (i.e., the slow extensors).