The purpose was to examine the ability to control knee-extension force during discrete isometric (IC), concentric (CC) and eccentric contractions (EC), in 24 young (mean ± SD, 25.3 ± 2.8 years) and 24 old (mean ± SD, 73.3 ± 5.5 years) healthy and active individuals. Subjects were to match a parabola with time-to-peak force of 200 ms during isometric, concentric and eccentric contractions at 6 target-levels of force (20, 35, 50, 65, 80, and 90% of the maximum voluntary contraction, MVC). Isometric contractions were performed at 90° of knee flexion, whereas concentric and eccentric contractions ranged from 90° to 80° of knee flexion (0° is full extension) at a slow velocity (25°/s). The results showed that subjects produced similar MVC forces for the three types of contractions. Young subjects produced greater MVC forces than old subjects, and within each age group, males produced greater force than females. The variability (Standard Deviation, SD) of peak force and impulse in absolute values was greater for young compared with the old. When variability was normalized to the force produced (Coefficient of Variation, CV), however, old subjects exhibited greater CV than young subjects for peak force and impulse. Both the SD and CV of time-to-peak force and impulse duration were greater for the old adults. In general, eccentric contractions were more variable than isometric and concentric contractions, and the old adults exhibited greater CV compared with the young adults during rapid discrete isometric, concentric and particularly eccentric contractions of the quadriceps.