Forces produced by medial gastrocnemius and soleus muscles during locomotion in freely moving cats.

I. Force transducers designed for chronic implantation were placed on the individual tendons of the medial gastrocnemius (MG) and soleus (SOL) muscles in the same hindlimb of adult cats. Electromyographic (EMG) and kinesiological data were also recorded. Force, EMG, and movement data obtained from intact, freely moving cats illustrate the division of labor between the mixed MG and its slow-twitch synergist SOL during posture, treadmill locomotion, and jumping. 2. Throughout a wide range of speeds, from slow walk (0.6 m/s) to fast run (3 m/s), the SOL develops approximately the same peak force (between 1.6 and 2.0 kg wt, depending on the animal) while the average MG force varies over a threefold range (from about 0.6 to 2.0 kg wt). However, even during fast running, the SOL can produce as much, or more, peak force as the MG. 3. In quadrupedal standing, the SOL can at times produce as much force as during locomotion, but on the average, SOL output is somewhat less (about 1.21.4 kg wt). In contrast, the MG usually produces no more than 0.5 kg wt of force during quiet standing. 4. Vertical jumping, up to a maximum height of 120 cm, involves peak MG forces up to about 9 kg wt (average approximately 8.0 kg wt), which can be lo-15% greater than the maximum isometric force developed bv the same muscle during fused tetanic contraction (stimulus rate X00 Hz) at optimum length.