STEREOTYPIC MUSCLE-TORQUE PATTERNS ARE SYSTEMATICALLY ADOPTED DURING ACQUISITION OF A MULTIARTICULAR KICKING TASK

Motor-control mechanisms used to learn multi-joint (kinematically inde terminate) movements, which involve the control of intersegmental dyna mics, are poorly understood, because the few kinetic studies which exa mined them studied only a few trials performed early and late in learn ing. Therefore, we examined changes in movement kinematics and kinetic s accompanying multi-joint movement acquisition to address the followi ng questions: Once subjects can produce accurate movements, do motor p atterns (i.e. net muscle torques) change with further learning? Are mo tor patterns learned using a systematic strategy? Following learning, are the same motor patterns consistently used for movement production? Subjects performed 16 blocks of 16 trials of a discrete weighted (mas s = 1.674 kg) kicking movement, involving hip, knee, and ankle motion. They attempted to perform 400 ms spatially accurate movements. Kinema tics were recorded for the hip, knee, ankle, and toe of the kicking le g, and inverse dynamics were used to obtain net-muscle-torque profiles . Subjects did not adopt the motor patterns initially used to produce accurate movements. With further learning, net muscle torques became l ess variable both within and between blocks, inter-joint dependency of muscle torques increased, as evidenced by decreased variability in th e pair of muscle torques which directly affect a segment's motion (i.e . hip-knee and knee-ankle muscle torques); and inter-joint relationshi ps of muscle torques became more phase-locked, with hip and knee torqu es being produced simultaneously, as were knee and ankle torques. As t here was a progression across blocks until the preferred motor pattern s were adopted, the learned stereotypic motor patterns were systematic ally selected. (C) 1998 Elsevier Science Ltd. All rights reserved.