Quadriceps femoris torque and EMG activity in seated versus supine position

To compare voluntary and electrically evoked knee extensor torque, surface electromyography (EMG), and activation level obtained under seated versus supine position, i.e., with shortened versus lengthened rectus femoris (RF) muscle.

The knee extensor torque obtained during maximal voluntary contractions (MVC) and after single and paired stimuli applied at rest was measured under seated (i.e., 90 degrees hip angle) and supine (i.e., 180 degrees hip angle) conditions. The associated EMG activity from biarticular RF and monoarticular vasti was also recorded, and activation level was estimated by means of the twitch interpolation technique.

Knee extensor MVC was 10.6% higher in the seated compared with the supine position (P < 0.01). EMG normalized to the M-wave amplitude for respective muscles and for respective positions was significantly lower under supine conditions, and deficits averaged approximately 20% for vasti and approximately 41% for RF (P < 0.05). Similarly, activation level estimated in the supine position was approximately 4% lower than the seated counterpart. Relative MVC losses observed supinely were significantly correlated with the corresponding activation level deficits (P < 0.05). On the other hand, both single and paired stimuli resulted in higher torque amplitudes in the supine with respect to the seated position, and mean differences were comprised between 10% (single twitch, P < 0.05) and 20% (potentiated doublet, P < 0.001). RF M-wave amplitude recorded supinely was 19% higher than the seated counterpart (P < 0.01).

The higher neural activation observed for the knee extensor muscles in the seated versus supine position, likely attributable to improved motor unit recruitment, may reflect a neurophysiological mechanism partly compensating the neuromuscular transmission-propagation impairment and/or mechanical disadvantage of shortened RF muscle.