Vastus lateralis surface and single motor unit EMG during shortening, lengthening and isometric contractions corrected for mode dependent strength differences

Knee extensor neuromuscular activity was investigated during isometric (60° knee angle), shortening and lengthening contractions (50-70°, 10° ·s-1) corrected for force-velocity related differences in intrinsic muscle strength. However, during dynamic contractions additional factors such as shortening induced force losses and lengthening induced force gains may affect muscle strength and thereby neuromuscular activity. Therefore, even after correction for force-velocity related differences in muscle strength we expected neuromuscular activity to be higher and lower during respectively shortening and lengthening compared to isometric contractions. Neuromuscular activity was determined using rectified surface electromyography (rsEMG) of the three superficial muscle heads in a first session (10 and 50 % MVC) and additionally with EMG of (46) vastus lateralis (VL) motor units recorded during a second session (4-76 % MVC). RsEMG behaved similar (P > 0.05) during both sessions. Using superimposed electrical stimulation, muscle strength independent of voluntary activation for shortening and lengthening contractions was found to be respectively 0.96 and 1.16 times the isometric (Iso) strength. Therefore, neuromuscular activity during submaximal shortening and lengthening was compared muscle strength were taken into account, neuromuscular activity was not lower during lengthening but was still higher during shortening compared to isometric contractions.