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 accele
ration magnitude was kept constant, to determine the relationship betw
een motor unit activity and the acceleration and deceleration characte
ristics of single joint movements. 2. The number of motor unit action
potentials was found to vary with the magnitude of the movement decele
ration. Ln addition, the duration of the discharge of a motor unit was
found to parallel the duration of the acceleration phase of the movem
ent, when the acceleration duration was varied while acceleration magn
itude was kept constant. 3. Approximately half of the recorded motor u
nits in triceps brachii were active both in the initiation and in the
termination of the extension movements. However, motor units were iden
tified that participated in specific phases of the movement (i.e., eit
her during the Ist agonist or 2nd agonist burst of muscle activity) de
pending on the magnitude of the acceleration or deceleration. 4. Durin
g flexion movements. when the triceps muscle served as an antagonist,
approximately half of the motor units were recruited only when the mag
nitude of the flexion deceleration was large. Further, this decelerati
on magnitude was larger than that evident during the extension movemen
ts in which the motor unit discharged. 5. The findings of this study d
emonstrate that the nervous system activates the same motor units whet
her the muscle is functioning as an agonist or antagonist so as to con
trol the characteristics of acceleration and deceleration of single jo
int movements.