THE CONTROL OF MONOARTICULAR MUSCLES IN MULTIJOINT LEG EXTENSIONS IN MAN

1. Movements often require control of direction and a magnitude of for ce exerted externally on the environment. Bi-articular upper leg muscl es appear to play a unique role in the regulation of the net torques a bout the hip and knee joints, necessary for the control of this extern al force. 2. The aim of this study was to test the hypothesis that the mono-articular muscles act as work generators in powerful dynamic leg extensions, which means that they should be activated primarily in th e phases during which they can contribute to work, irrespective of the net joint torques required to control the external force. 3. Cycling movements of six trained subjects were analysed by means of inverse dy namics, yielding net joint torques as well as activity patterns and sh ortening velocities of four mono- and four bi-articular leg muscles. 4 . The results show that the mono-articular muscles exert force only in the phase in which these muscles shorten, whereas this appears not to be the case for the bi-articular muscles. 5. Reciprocal patterns of a ctivation of the rectus femoris and hamstring muscles appear to tune t he distribution of net joint torques about the hip and knee joints, ne cessary to control the (changing) direction of the force on the pedal. 6. An analysis of running in man and additional related literature ba sed on animal studies appears to provide further support for the hypot hesis that mono- and bi-articular muscles have essentially different r oles in these powerful multijoint leg extension tasks.