Kinesiological studies of self- and cross-reinnervated FDL and soleus muscles in freely moving cats.

The activity patterns in self- and cross-reinnervated flexor digitorum longus (FDL) and soleus (SOL) muscles were examined during natural movements in awake, unrestrained cats in which electromyographic (EMG) electrodes, tendon-force gauges, and muscle-length gauges had been chronically implanted under anesthesia and aseptic conditions. Kinesiological data were recorded between 13 and 22 mo after nerve surgery. Self-reinnervated FDL and SOL muscles (i.e., FDL----FDL and SOL----SOL, respectively) exhibited locomotor activity patterns that were the same as observed in normal, unoperated FDL and SOL muscles (26). FDL----FDL muscles exhibited primarily brief bursts of activity in early swing, just after the toes had left the ground, whereas SOL----SOL muscles showed bursts of activity just before and during stance. In contrast, the cross-reinnervated muscles (both SOL----FDL and FDL----SOL) that had little or no unwanted self-reinnervation showed the patterns of activity that are associated with the innervating foreign motoneurons. That is, cross-reinnervated SOL----FDL muscles were intensely active in quadrupedal standing and, during the stance phase of stepping, producing large force transients while actively lengthening. Conversely, cross-reinnervated FDL----SOL muscles were active mainly in short bursts at the onset of the swing phase of stepping, just after the foot had left the ground. There was considerable modulation of EMG and peak force output in FDL----SOL muscles with changing speed of locomotion, whereas little modulation was evident in SOL----FDL muscles. The activity patterns in self- and cross-reinnervated FDL and SOL muscles were also recorded during scratch and paw-shaking reflexes. As in locomotion, the observed patterns were in all cases consistent with those expected for the innervating motor pool rather than the innervated muscle. Muscles that had been dually reinnervated by both the original and foreign motor pools displayed activity patterns that were a mixture of the FDL and SOL activity patterns described above. The present results demonstrate that motoneuron activation patterns remain qualitatively unaltered when their motor axons reinnervate foreign muscles. In addition, the observations permit some quantitative estimates of the degree to which cross-reinnervated muscles are subjected to patterns of motoneuron activity and to conditions of mechanical loading that are markedly different from those in the self-reinnervated or normal conditions.