0836 Force fluctuations during contractions of hand and leg muscles are often greater in old compared with young adults, but this variability has not been examined across a range of activation levels in muscles surrounding the ankle. PURPOSE: To compare the fluctuations in force during isometric contractions performed with the ankle dorsiflexor (AD) muscles in young and old adults. METHODS: Young (n = 10; 23 ± 3 yrs, 5 men, 5 women) and old (n = 11; 73 ± 5 yrs, 5 men, 6 women) healthy adults performed isometric maximum voluntary contractions (MVC's) and submaximal constant-force isometric contractions with the AD muscles at target forces of 2.5, 5, 10, 30, 50, and 80% of MVC. Subjects exerted the force as steadily as possible with visual feedback. The standard deviation (SD) and coefficient of variation (CV) of force were measured during constant-force segments of the submaximal trials. RESULTS: MVC force (N) was reduced in the old compared with the young subjects (196 ± 72 vs. 265 ± 94 N, P = 0.07). The SD of force increased with target force, as expected (from 0.35 N to 4.4 N for 2.5 – 80% MVC target forces). The CV of force was not significantly different between young and old adults for any target force level (P>0.05). The CV of force was not different between men and women in either age group (P>0.05). The CV of force was highest at 2.5% MVC (CV = 6.0%) and decreased to a minimum (CV = 2.0%) from 10–80% MVC (P<0.001), agreeing with previous findings on the effect of target force in hand and leg muscles. The dominant frequency of the force fluctuations was below 2 Hz. CONCLUSION: This data suggests that the neural factors responsible for the effect of target force on force fluctuations are similar between hand muscles, knee extensors, and AD muscles. Across a range of voluntary activation levels of the AD motor unit pool, the magnitude of fluctuations in motor output during isometric contractions is similar between young and old adults. Supported by AG19171 to BLT.