CHANGES IN INTER-JOINT RELATIONSHIPS OF MUSCLE MOMENTS AND POWERS ACCOMPANYING THE ACQUISITION OF A MULTIARTICULAR KICKING TASK

When the nervous system produces a multi-joint movement it must select a unique set of motor patterns for movement production from a variety of appropriate motor pattern solutions: this is the so-called degrees -of-freedom problem. One strategy which the nervous system could use t o simplify the selection of a unique set of motor patterns (i.e. net m uscle moments) for movement production would be to constrain the relat ionships between the net muscle moments of the various joints. Therefo re, we addressed the following question: Do the inter-joint relationsh ips of muscle moments and powers become more phase-locked during the a cquisition of a novel multi-joint movement. Subjects performed 16 bloc ks (16 trials per block) of a discrete kicking movement, which involve d motion of the hip, knee, and ankle-a weight (1.674 kg) attached to t he subject's foot increased movement novelty. Subjects attempted to pe rform all movements as close as possible to a goal movement time of 40 0 ms, while maintaining spatial accuracy. An optoelectric imaging syst em was used to record movement kinematics for the hip, knee, ankle, an d toe of the kicking leg, and inverse dynamics were then used to obtai n net-muscle-moment profiles. Following learning, inter-joint moments became more closely phase-locked, with hip-flexor and knee-extensor mo ments being produced simultaneously, as were hip-extensor and knee-fle xor moments. This increased phase-locking of inter-joint moment relati onships with learning resulted in increased phase-locking of inter-joi nt power relationships. Therefore, subjects learned to use a motor con trol strategy for movement production that constrained the relationshi ps of inter-joint net muscle moments and inter-joint muscle powers, th us simplifying the selection of a unique set of motor patterns.