Contraction mode shift in quadriceps femoris muscle activation during dynamic knee extensor exercise with increasing loads.

The objective of the present study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during dynamic sub-maximal knee extension exercise between young adult men and women. Thirty subjects completed, in a random order, 2 sub-maximal repetitions of single-leg knee extensions at 20–90% of their one-repetition maximum (1RM). Vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscle integrated EMG (IEMG) during each sub-maximal lift was normalized to the respective 1RM for concentric, isometric and eccentric modes. The EMG median frequency (fmed) was determined over the isometric mode. Men attained a significantly (p<0.05) greater knee angular velocity than the women during the concentric mode (83.6±19.1°/s and 67.4±19.8°/s, respectively). RF IEMG was significantly lesser than the VM (p=0.014) and VL (p<0.001) muscles, when collapsed across all contraction modes, loads, and sex. Overall IEMG was significantly greater during the concentric (p<0.001) and isometric (p<0.001) modes, than the eccentric mode. Men generated significantly (p=0.03) greater VL muscle IEMG than the women, while the opposite pattern emerged for the RF muscle. VM fmed (105.1±11.1Hz) was significantly lesser than the VL (180.3±19.5Hz) and RF (127.7±13.9Hz) muscles across all lifting intensities, while the men (137.7±10.7Hz) generated greater values than the women (129.0±11.4Hz). The findings demonstrate a reduction in QF muscle activation across the concentric to eccentric transition, which may be related to the mode-specific velocity pattern.