The recruitment of motor units during voluntary isometric contraction was studied in the abductor digiti minimi muscle of 13 human subjects trained to increase tension in this muscle almost linearly from zero to maximum. The subjects matched the output of a transducer, measuring the tension in this muscle, to a linear target ramp voltage. The target ramps varied in duration from 2 to 10 sec. When speed of contraction was kept constant there was considerable degree of constancy in the tension at which individual motor units were recruited. The recruitment order of motor units, therefore, appeared to be determined at repeated contractions. However, the order was not rigidly fixed among units whose tension range of recruitment is overlapped, especially among those units recruited at medium to strong contraction. When the contraction was performed more quickly the motor units were recruited at lower tension. Larger population of units were recruited at lower tension range: 45.8% of units were recruited before the tension attained 20% of maximum tension. There is a tendency for motor units with larger action potentials to be recruited at higher tension.