Purpose: Fatigue-related muscle activity in the superficial quadriceps femoris (QF) muscles has been widely examined; however, there is no information on the activity of the deep vastus intermedius (VI) muscle during fatiguing dynamic knee extensions. The purpose of this study was to investigate neuromuscular activation patterns of the QF synergists, including the VI, during fatiguing dynamic knee extensions at two submaximal loads. Methods: Nine healthy men performed dynamic knee extensions with loads of 50 and 70 % of one-repetition maximum (1RM) until failure. Muscle activation of the VI, vastus lateralis, vastus medialis (VM) and rectus femoris was recorded using surface electrodes. Root mean square (RMS) amplitude was calculated during the concentric (CON) and eccentric (ECC) phases of each repetition, and normalized to the RMS amplitude during the CON and ECC phases of the 1RM. Each CON and ECC phase was further divided into three subphases according to knee joint angle. Results: The normalized RMS amplitude of the four individual QF muscles during the CON phase linearly increased with fatigue with contractions at both 50 and 70 % 1RM. The highest RMS amplitude was found in VI at flexed knee joint angles until fatigue. This activation pattern was more prominent at 70 % 1RM than 50 % 1RM. The RMS amplitude of VM at extended knee joint angles was selectively higher at 70 % 1RM than 50 % 1RM. Conclusions: These results suggest that the contribution of the four individual QF muscles to fatiguing dynamic knee extensions differs according to knee joint angle and intensity of load.
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