An analysis of frequency response of motor units during voluntary isometric contractions at various speeds.

To ascertain the differences in discharge properties of a hand muscle between ramp and ballistic contractions, firing frequencies of 48 motor units of adductor pollicis muscle were analyzed during 3-kg isometric voluntary contractions at various force speeds. The force exertions were classified into 3 degrees: slow ramp, fast ramp, and ballistic contractions. The instantaneous frequencies (IFs) of motor units increased gradually when the force exertions were progressed to the target in slow and fast ramp contractions. The slopes of the linear regression equations between IFs and force levels were almost all constant during slow ramp contractions and were scattered slightly during fast ramp contractions. However, the slopes during ballistic contractions scattered widely. The y-intercepts of the same regression lines increased slightly with respect to the increment of force speed when slow and fast ramp contractions were performed. However, a remarkably wide variation of the y-intercepts was observed during ballistic contractions. Motor units could be fired at high frequencies at the beginning of discharge when ballistic contractions were performed as compared with slow and fast ramp contractions.