An electromyographic analysis of compartments in cat lateral gastrocnemius muscle during unrestrained locomotion.

Cat lateral gastrocnemius (LG) muscle is composed of four compartments, each of which is supplied by a primary branch of the LG muscle nerve and each of which contains a discrete population of motor units/muscle fiber types. The activation patterns of these different motor-unit populations were examined from recordings made using selective electromyographic (EMG) electrodes, which were carefully placed into the different LG compartments. Recordings were made as the cats stepped overground along a wooden walkway at a variety of different stepping speeds. Electrical activity was correlated with stepping movements using high-speed cinematography. A number of different activation patterns of the different compartments were noted during stepping. In one such pattern, more intense EMG activity was found in distal than proximal compartments at slow stepping speeds, but at moderate to fast stepping speeds the intensity of activity in proximal compartments was equal to or greater than in distal compartments. In other patterns, a less consistent relationship between compartment EMG intensity and stepping speed was observed. These results are generally consistent with an orderly pattern of activation of motor units in the LG during stepping but also indicate that more than one activation pattern exists.