In this study, the training effects of static and dynamic exercise programs on the firing patterns of 450 single motor units (SMU) in the human tibialis anterior muscle were investigated. In a six week program, the static group (N=5) participated in daily high intensity, short duration, isometric exercises while the dynamic group (N=5) participated three times weekly in low intensity, moderate duration, isotonic exercises. The control group (N=4) did not participate in an exercise program. Pre- and post-test SMU firing patterns were recorded by means of 26 gauge tygon coated monopolar needle electrodes. In order to assess the degree of impersistence or periodicity of motor unit firing, an interspike interval (ISI) longer than 260 milliseconds was defined as a lapse in motor firing. An individual ratio of normal ISIs to lapses for each SMU three minute recording run and the mean ratio for each subject (MLR) were calculated. Significant differences were found between MLR pre- and post-test values for both groups (p less than 0.05). These results indicate that changes in motor unit firing can be produced by specific exercise programs. High intensity, short duration exercises produce more variable than normal firing rates, and low intensity, long duration exercises produce firing rates less variable than normal. (Author/BD)