Changes in inter-joint relationships of muscle moments and powers accompanying the acquisition of a multi-articular kicking task

Abstract

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 socalled degrees-of-freedom problem. One strategy which the nervous system could use to simplify the selection of a unique set of motor patterns (i.e. net muscle moments) for movement production would be to constrain the relationships between the net muscle moments of the various joints. Therefore, we addressed the following question: Do the inter-joint relationships of muscle moments and powers become more phaselocked during the acquisition of a novel multi-joint movement. Subjects performed 16 blocks (16 trials per block) of a discrete kicking movement, which involved motion of the hip, knee, and ankle—a weight (1.674 kg) attached to the subject's foot increased movement novelty. Subjects attempted to perform all movements as close as possible to a goal movement time of 400 ms, while maintaining spatial accuracy. An optoelectric imaging system was used to record movement kinematics for the hip, knee, ankle, and toe of the kicking leg, and inverse dynamics were then used to obtain net-muscle-moment profiles. Following learning, inter-joint moments became more closely phase-locked, with hip-flexor and knee-extensor moments being produced simultaneously, as were hip-extensor and knee-flexor moments. This increased phase-locking of inter-joint moment relationships with learning resulted in increased phase-locking of inter-joint power relationships. Therefore, subjects learned to use a motor control strategy for movement production that constrained the relationships of inter-joint net muscle moments and inter-joint muscle powers, thus simplifying the selection of a unique set of motor patterns.