1. Myoelectrical signals from a slow (soleus) and fast (lateral gastrocnemius) ankle extensor of six adult female cats were monitored during quadrupedal- and bipedal-standing, treadmill locomotion at speeds varying from 0.5 to 2.5 m/s and vertical jumps to heights of 30-91 cm. Cats, chronically implanted, were trained by food rewards and tested over a period of 4-6 mo.

2. The soleus participated during a postural maintenance, treadmill locomotion at all gaits and jumping. Utilization of soleus units changed little as the dynamics of the ankle action increased. From a quiet quadrupedal stand to a moderately high jump (91 cm), the rectified-average EMG signal increased less than 1.5 times.

3. The lateral gastrocnemius showed only minimal or no indication of recruitment during quiet standing and slow walking. As gait progressed from a slow walk to a slow gallop, peak A-EMG of the lateral gastrocnemius relative to the soleus increased fourfold. During progressive jumps, myoelectrical signals of the LG increased by a factor of 8.

4. The electromyographic data support the long-held, yet largely unsupported, notion (14, 29) that segregation of fast and slow fibers into two different muscles represents functional specializations matched precisely to the demands of motor behavior. The soleus is active during all activities where a stable support is required. The tension it generates probably contributes little to forceful movements typified by jumping. The gastrocnemius, conversely, is anatomically and physiologically suited for more forceful and rapid ankle extensions. Recruitment of the fast-contracting muscle is required during jumping and galloping, dynamic movements which demand greater torques about the ankle joint.