Abstract
During multijoint limb movements such as reaching, rotational forces
arise at one joint due to the motions of limb segments about other
joints. We report the results of three experiments in which we assessed
the extent to which control signals to muscles are adjusted to counteract
these ?interaction torques.? Human subjects performed single- and
multijoint pointing movements involving shoulder and elbow motion,
and movement parameters related to the magnitude and direction of
interaction torques were manipulated systematically. We examined
electromyographic (EMG) activity of shoulder and elbow muscles and,
specifically, the relationship between EMG activity and joint interaction
torque. A first set of experiments examined single-joint movements.
During both singlejoint elbow (experiment 1) and shoulder (experiment
2) movements, phasic EMG activity was observed in muscles spanning
the stationary joint (shoulder muscles in experiment 1 and elbow
muscles in experiment 2). This muscle activity preceded movement
and varied in amplitude with the magnitude of upcoming interaction
torque (the load resulting from motion of the nonstationary limb
segment). In a third experiment, subjects performed multijoint movements
involving simultaneous motion at the shoulder and elbow. Movement
amplitude and velocity at one joint were held constant, while the
direction of movement about the other joint was varied. When the
direction of elbow motion was varied (flexion vs. extension) and
shoulder kinematics were held constant, EMG activity in shoulder
muscles varied depending on the direction of elbow motion (and hence
the sign of the interaction torque arising at the shoulder). Similarly,
EMG activity in elbow muscles varied depending on the direction of
shoulder motion for movements in which elbow kinematics were held
constant. The results from all three experiments support the idea
that central control signals to muscles are adjusted, in a predictive
manner, to compensate for interaction torques?loads arising at one
joint that depend on motion about other joints.
Users
Please
log in to take part in the discussion (add own reviews or comments).