In the preceding paper (5), we examined the organization of the motor-unit population in the flexor digitorum longus (FDL) muscle of normal cats and compared our findings with data from other studies of various cat hindlimb motor pools. Interpretation of differences in motor-unit organization in various muscles requires information about the behavior of those muscles during normal movements. Using chronically implanted transducers, it is possible to record the electromyographic(EMG) activity, tendon forces, and muscle-length changes of individual muscles during a variety of motor acts in intact cats moving with minimal restraint (18, 19, 36). We applied these techniques to the FDL in order to assess its normal function and to compare this with EMG activities in a variety of other distal hindlimb muscles. We also wished to compare the behavior of normal muscles with FDL muscles cross reinnervated by soleus motoneurons, to be described elsewhere. A preliminary report of some of the present results has appeared in abstract form (2 1).

The results of the present study were surprising in that the functional activity of the FDL muscle during locomotion proved to be complex, including stereotyped flexor behavior and facultative activity that appeared to respond to perturbations in the step cycle. Neither aspect was present in the activity of the FHL, which instead behaved as a stereotyped antigravity extensor. The functional dissociation between two muscles that are strict anatomical synergists, that share a common tendon of insertion, that are interconnected by heteronymous monosynaptic group Ia excitation (5, 1 1), and that exert the same mechanical action was not anticipated.