Thigh muscle activity during maximum-height jumps by cats

Cats were trained to jump from a force plate and touch a cotton ball suspended as high as 1.6 m. Force-plate reaction forces and double-joint hamstring muscle activity observed early in propulsion varied from one maximal jump to another. This variability is consistent with theory (31, 32, 42); that is, different coordination strategies can be implemented prior to the heels losing contact with the force plate (heel-off). Single-joint hip extensor and double-joint posterior thigh (hip extensor-knee flexor) muscles were coactivated prior to heel-off. This coactivation is probably partially responsible for the observed backward rotation of the trunk. Forepaws, observed to contact the force plate prior to heel-off, probably assist the hindlimbs in generating trunk rotation. Both single-joint knee extensor and hip extensor muscles exhibited greatest activation between heel-off and body lift-off. Single-joint flexor muscles were inactive throughout propulsion. Double-joint posterior thigh muscles were deactivated at heel-off and remained inactivated until lift-off. These observations agree with the theoretical notion that muscles should be either fully activated, inactivated, or switched from one extreme to the other (i.e., bang-bang control) between heel-off and body lift-off (31, 32, 42, 44). All seven muscles studied shortened while activated. Using computations based on muscle geometry, fiber architecture, and joint angle trajectories, I propose that sarcomeres shorten along the flat and ascending regions of the force-length curve. De- and inactivation of double-joint posterior thigh muscles between heel-off and lift-off coincided with muscle stretch. The reason for inactivation of these muscles is that the negative work that would have been generated had these muscles stayed activated would have hindered propulsion. Contractions preceded by active stretch were not observed. Enhancement of positive work by previous storage of energy in elastic musculotendinous structures is thus not used by cat thigh musculature in jumps starting from the squat. Adductor femoris, semimembranosus anterior, and biceps femoris anterior muscles were activated synergistically as one group yet differently from the synergistic activation of gracilis, semitendinosus, and biceps femoris posterior muscles. The separation of these muscles into two groups based on their activation patterns during jumping is compatible with the classification of these muscles into hip extensor and knee flexor muscle groups, respectively, based on their reflex patterns (37), spinal cord reflex connectivity (18, 30), and firing patterns during locomotion (20).(ABSTRACT TRUNCATED AT 400 WORDS)