Role of limb movement in the modulation of motor unit discharge rate during fatiguing contractions

Motor unit firing rates of the triceps brachii muscle have been shown to de cline during sustained isometric contractions, but not if the fatiguing con traction incorporates arm movements. The purpose of this study was to deter mine the impact of the actual physical displacement of the limb on the main tenance of motor unit discharge rate during dynamic muscle fatigue. An isom etric force pulse paradigm was used to recreate the motor unit activity pat terns that occur during a dynamic contraction. With this paradigm, the vari able force output that would occur during a dynamic contraction remained in tact, but the movement of the limb was eliminated. Motor unit firing rates declined in the isometric force pulse protocol. Thus, factors related to th e actual movement of the limb appear to enable the maintenance of motor uni t discharge rates during fatigue.