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

Objectives: The aim of the study was to investigate the effects of different loads on tremor around 10 Hz during fatiguing contractions. Methods: Eighteen healthy volunteers performed sustained isometric knee extensions at 30%, 50% and 70% maximum voluntary contraction (MVC). During the fatiguing contractions, mechanical recordings were made with a high-resolution force sensor. Tremor-power was calculated for the 6–20 Hz frequency window as a function of time normalized to endurance time. Results: Initial tremor power was different between the high and low load tasks. Changes of tremor with contraction time differed between the three tasks, in that tremor of the 30% MVC contraction showed the least decrease throughout the sustained contraction, whilst that of the 50% and 70% MVC showed 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 sustained submaximum voluntary contractions seem mainly the consequence of recruitment of new units and fatigue-related properties of the high threshold motor units of muscles.