1. To explore the neural control of single joint movements in humans, the activity of 47 motor units in triceps brachii muscle was recorded during elbow flexion and extension movements. Movements were performed with different but changing deceleration magnitudes, while the acceleration magnitude was kept constant, to determine the relationship between motor unit activity and the acceleration and deceleration characteristics of single joint movements. 2. The number of motor unit action potentials was found to vary with the magnitude of the movement deceleration. In addition the duration of the discharge of a motor unit was found to parallel the duration of the acceleration phase of the movement, when the acceleration duration was varied while acceleration magnitude was kept constant. 3. Approximately half of the recorded motor units in triceps brachii were active both in the initiation and in the termination of the extension movements. However, motor units were identified that participated in specific phases of the movement (i.e., either during the 1st agonist or 2nd agonist burst of muscle activity) depending on the magnitude of the acceleration or deceleration. 4. During flexion movements, when the triceps muscle served as an antagonist, approximately half of the motor units were recruited only when the magnitude of the flexion deceleration was large. Further, this deceleration magnitude was larger than that evident during the extension movements in which the motor unit discharged. 5. The findings of this study demonstrate that the nervous system activates the same motor units whether the muscle is functioning as an agonist or antagonist so as to control the characteristics of acceleration and deceleration of single joint movements.