Weight-bearing hindlimb stepping in treadmill-exercised adult spinal cats.

Hindlimb locomotion on a motor-driven treadmill was studied in 5 cats spinalized at a low thoracic level adults. Six months after surgery, the cats were anesthetized and implanted for electromyographic (EMG) and force recordings in hindlimb muscles. For the last 5 months of the spinalization period, the hindlimbs of each cat were exercised daily for 30 minutes on a treadmill. Data were collected during hindlimb locomotion on a treadmill across the entire range of speeds each cat could accommodate. All trials were filmed (100 frames/s) for kinematic analysis. EMG data were recorded from the soleus (Sol), medial gastrocnemius (MG), tibialis anterior (TA) and extensor digitorum longus (EDL). Forces were recorded in vivo from the Sol and MG tendons. All cats could sustain full weight-bearing stepping without the need for mechanical stimulation of the tail. Although the general stepping pattern of the spinal cats was remarkably similar to that of normal cats, several key differences were identified. Compared to normal cats, the adult spinal cats walked at a lower range of speeds and exhibited a longer swing phase duration. The Sol produced forces and displayed activation periods comparable to those observed in normal cats. The MG of adult spinal cats, however, produced lower forces and had a later onset of activation in comparison to normal cats. Each of the muscles in all spinal cats exhibited tremor during stepping. These results suggest that there were limitations in the activation levels of some hindlimb flexor and extensor muscles during treadmill locomotion. These data further suggest that, in normal cats, accommodation to treadmill speed is accomplished by modulating supraspinal input to the lumbar spinal cord while leaving many of the timing details to be regulated by lumbar spinal networks.

Mechanical output of the cat soleus during treadmill locomotion: in vivo vs in situ characteristics.

To study the mechanical output of skeletal muscle, four adult cats were trained to run on a treadmill and then implanted under sterile conditions and anesthesia with a force transducer on the soleus tendon and EMG electrodes in the muscle belly. After a two-week recovery period, five consecutive step cycles were filmed at treadmill speeds of 0.8, 1.3 and 2.2 m s-1. Locomotion data in vivo included individual muscle force, length and velocity changes and EMG during each step cycle. Data for an average step cycle at each speed were compared to the force-velocity properties obtained on the same muscle under maximal nerve stimulation and isotonic loading conditions in situ. Results indicate that the force and power generated at a given velocity of shortening during late stance in vivo were greater at the higher speeds of locomotion than the force and power generated at the same shortening velocity in situ. Strain energy stored in the muscle-tendon unit during the yield phase in early stance is felt to be a major contributor to the muscle's enhanced mechanical output during muscle shortening in late stance.

Effects of training on the recovery of full-weight-bearing stepping in the adult spinal cat.

The effects of ambulatory training on the extent and time course of recovery of weight-bearing-stepping in cats spinalized (T12-T13) as adults were investigated. One month after spinal cord transection, 14 of 16 cats were capable of bearing the full weight of their hindquarters with their hind limbs during stepping on a motor-driven treadmill if the tail was pinched or crimped. Of those 14 cats 8 were assigned to a trained and 6 to an untrained group. Trained cats were subjected to 30 min/day of treadmill exercise, 5 days/week. Training was initiated 1 month posttransection and continued until 5 to 7 months posttransection. Daily records were kept on the treadmill speeds used, the time at each speed, and the number of steps that were not full weight bearing. The number of full-weight-bearing steps times treadmill speed was used as a measure of performance. The tail was crimped whenever necessary, but was required less and less as training progressed. Performance plateaus were reached between 25 and 85 days after initiating training (mean = 48 +/- 22 days). Maximum treadmill speeds increased in untrained cats from 0.075 +/- 0.042 m/s 1 month posttransection to 0.240 +/- 0.042 m/s 5 to 7 months posttransection and those of trained cats increased from 0.079 +/- 0.045 m/s to 0.619 +/- 0.133 m/s during this same period. We conclude that a much larger proportion of adult spinal cats are capable of full-weight-bearing stepping than reported, and that training which emphasizes early tail crimping and complete weight bearing at all times results in marked improvements in the locomotor capacity of the hind limbs.