COMMON PRINCIPLES UNDERLYING THE CONTROL OF VAPID, SINGLE DEGREE-OF-FREEDOM MOVEMENTS AT DIFFERENT JOINTS

Studies of rapid, single degree-of-freedom movements have shown differ ent changes in electromyographic patterns for movement tasks that appe ar very similar (e.g., movements over different ranges of distance). H owever, it is not clear whether these differences are a result of join t-specific control schemes or whether they are instead due to the limi ted range of task parameters studied relative to the mechanical constr aints of each joint (e.g., short compared with long movements relative to the range of motion of a particular joint). In this study, we meas ured and compared the kinematic trajectories and electromyograms recor ded during various movement tasks at the wrist, elbow, and ankle. Subj ects performed movements over a wide range of distances ''as fast as p ossible,'' ''at a comfortable speed,'' and against two inertial loads (at the elbow only), and they performed movements over a fixed distanc e at three different speeds at the wrist and ankle. For fast movements we show that, in spite of some joint-specific differences, the basic pattern of electromyographic (EMG) modulation is similar at all three joints; for example, the agonist EMG burst transitions from a fixed du ration to an increasing duration with increasing movement distance at all three joints. Moreover, the distance at which this transition occu rs in one joint relative to the distance at which this transition occu rs in the other two joints is consistent across subjects. The transiti on occurs at the shortest distance at the ankle and the longest distan ce at the wrist. In general we suggest that the data are consistent wi th a single set of control rules applied at all three joints, with the biomechanical constraints at each joint accounting for the difference s in the EMG and kinematic patterns observed across joints.