Load-dependence of fatigue related changes in tremor around 10 Hz

Objectives: The aim of the study was to investigate the effects of differen t loads on tremor around 10 Hz during fatiguing contractions. Methods: Eighteen healthy volunteers performed sustained isometric knee ext ensions at 30%, 50% and 70% maximum voluntary contraction (MVC). During the fatiguing contractions, mechanical recordings were made with a high-resolu tion force sensor. Tremor-power was calculated for the 6-20 Hz frequency wi ndow as a function of time normalized to endurance time. Results: Initial tremor power was different between the high and low load t asks. Changes of tremor with contraction time differed between the three ta sks, in that tremor of the 30% MVC contraction showed the least decrease th roughout the sustained contraction, whilst that of the 50% and 70% MVC show ed progressively higher decreases. At failure, all 3 contractions merged to the same tremor level. Conclusion: Load-dependent, fatigue-related 6-20 Hz tremor changes during s ustained submaximum voluntary contractions seem mainly the consequence of r ecruitment of new units and fatigue-related properties of the high threshol d motor units of muscles. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.