TRANSFER OF MECHANICAL ENERGY BETWEEN ANKLE AND KNEE JOINTS BY GASTROCNEMIUS AND PLANTARIS MUSCLES DURING CAT LOCOMOTION

The purposes of this study were (1) to define and estimate the directi on and amount of the energy transfer between the knee and ankle throug h gastrocnemius (GA) and plantaris (PL) muscles during cat locomotion, and (2) to test the assumption that the force and activity patterns o f soleus (SO), GA, and PL are mechanically and physiologically advanta geous for providing the transfer of energy between these joints. The d irection, amount and rate of the energy transfer through a two-joint m uscle were defined using a theoretical analysis of movements in two ad jacent joints spanned by the two-joint muscle. The energy transferred between the ankle and the knee was calculated using the time integrati on of the difference between the power developed by the moments of SO, GA, and PL at the ankle joint and the total power of these muscles. T he total power of SO, GA, and PL muscles, and the power of their movem ents about the ankle and knee, were obtained using the experimentally determined muscle forces, the rates of change in muscle length, and th e angular velocities at the knee and ankle which were calculated from the kinematics and the geometry of the cat hindlimb. Muscular forces a nd hindlimb kinematics of the cats were recorded during normal walking and trotting on a treadmill at speeds of 0.4, 0.8, 1.2, 1.5, and 1.8 m s(-1) using 'E'-shaped tendon transducers and high-speed video, resp ectively. It was found that during the early phase of support, there w as a transfer of mechanical energy from the ankle to the knee through GA and PL. During the late phase of support, mechanical energy was tra nsferred from the knee to the ankle. The amount of energy transferred increased with increasing speeds of locomotion. The energy transferred from the ankle to the knee was 3-60 mJ (7-22% of the negative work do ne by the moments of SO, GA, and PL at the ankle), and the energy tran sferred from the knee to the ankle was 10-67 mJ (7-14% of the positive work done by the moments of SO, GA, and PL at the ankle). The results of this study suggest that the activation and the forces of one-joint SO and multi-joint GA and PL are organized in such a way as to fit th e features of the design of these ankle extensor muscles in order to p rovide locomotion efficiently. For example, the decrease in the contra ctile abilities of SO during the late phase of support at fast speeds of locomotion may be compensated for by the transfer of energy from th e knee to the ankle through GA and PL. The design of GA and PL (a high percentage of fast-twitch muscle fibers, large angles of pinnation an d short length of the fibers, long tendons, and the location about the ankle and knee joints) seems to be well suited for transferring mecha nical energy between the ankle and knee at fast speeds of locomotion. Because of the design of GA and PL, their contractile abilities remain close to the maximum at fast speeds of locomotion. The design of GA a nd PL allows for extension of the ankle joint through the action of th e knee extensor muscles during knee extension with a relatively small change in length of GA and PL.