Normal human subjects made discrete flexions of the elbow over a fixed distance in the horizontal plane from a stationary initial position to a visually defined target. We measured joint angle, acceleration, and electromyograms (EMGs) from two agonist and two antagonist muscles. Changes in movement speed were elicited either by explicit instruction to the subject or by adjusting the target width. Instructions always required accurately stopping in the target zone. Peak inertial torques and accelerations, movement times, and integrated EMGs were all highly correlated with speed. We show that inertial torque can be used as a linking variable that is almost sufficient to explain all correlations between the task, the EMG, and movement kinematics. When subjects perform tasks that require control of movement speed, they adjust the rate at which torque is developed by the muscles. This rate is modulated by the way in which the muscles are activated. The rate at which joint torque develops is correlated with the rate at which the agonist EMG rises as well as with integrated EMG. The antagonist EMG shows two components. The latency of the first is 30-50 ms and independent of movement dynamics. The latency of the second component is proportional to movement time. The rate of rise and area of both components scale with torque. We propose organizing principles for the control of single-joint movements in which tasks are performed by one of two strategies. These are called speed-insensitive and speed-sensitive strategies. A model is proposed in which movements made under a speed-sensitive strategy are executed by controlling the intensity of an excitation pulse delivered to the motoneuron pool. The effect is to regulate the rate at which joint torque, and consequently acceleration, increases. Movements of variable distance, speed, accuracy, and load are shown to be controlled by one of two consistent sets of rules for muscle activation. These rules apply to the control of both the agonist and antagonist muscles. Rules of activation lead to distinguishable patterns of EMG and torque development. All observable changes in movement kinematics are explained as deterministic consequences of these effects.
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