Phil. Trans. R. Soc. A (2010) 368, 2783–2798 doi:10.1098/rsta.2010.0091 Effect of fatigue on force fluctuations in knee extensors in young adults BY NAVRAG B. SINGH 1 ,ADAMANTIOS ARAMPATZIS 2 ,GEORG DUDA 1 , MARKUS O. HELLER 1 AND WILLIAM R. TAYLOR 1, * 1 Julius Wolff Institute, Charité—Universitätsmedizin Berlin, and 2 Department of Training and Movement Science, Philosophical Faculty IV, Humboldt-Universität zu Berlin, Centre of Sport Science and Sport Medicine Berlin, Philippstr. 13, Haus 11, 10115 Berlin, Germany This study investigated the hypothesis that fatiguing exercises led to increased force fluctuations during submaximal isometric knee extensions and to decreased accuracy and steadiness in the time and frequency domains. Sixteen young adults (eight males, eight females) were tested, in a seated posture with 90° knee flexion, to assess their ability to reproduce target extensor torques set at 15 per cent and 20 per cent of their maximum voluntary isometric contraction, both before and after fatiguing exercises. Normalized mean (NMAE) and peak (NPAE) of the absolute error were both used to quantify accuracy, whereas normalized standard deviation of the absolute error (NSAE) was used to quantify steadiness of the torque trials in the time domain. Mean and median power frequencies (MnPF, MdPF) and normalized peak power (NPkP) were used to assess the spectral structure of the torque signals. NMAE, NSAE and NPAE all showed excellent intra- as well as intersession reliabilities (intraclass correlation values greater than 0.75 and low standard error of measurement values), demonstrating repeatability of the test set-up. NMAE, NSAE and NPAE increased significantly post- fatigue (greater than 42%), together with a shift towards higher frequency (MnPF and MdPF) components, indicating that the set-up was sensitive enough to detect the decreased force accuracy and steadiness of the musculature after fatigue. Increased force variability in both the time and frequency domains could therefore explain decreased steadiness after fatigue. Keywords: muscle fatigue; force fluctuation; power spectrum; steadiness; knee extensors; physiological tremor 1. Introduction Muscle weakness, particularly in the quadriceps and ankle dorsiflexors (Tinetti 1986; Moreland et al. 2004), has been identified as one of the major risk factors for falls (Berg et al. 1997; Masud & Morris 2001; Moreland et al. 2004). *Author for correspondence (bill.taylor@charite.de). One contribution of 13 to a Theme Issue ‘The virtual physiological human: computer simulation for integrative biomedicine I’. This journal is © 2010 The Royal Society 2783