The relationship of electromyographic activity and force of the vastus medialis oblique and vastus lateralis muscles during maximal isometric knee extension contractions

We examined the relationship between the vastus medialis oblique (VMO) and the vastus lateralis (VL) during maximal isometric knee extension contractions by measuring force and electromyographic (EMG) activity of the two muscles at 45 and 90 deg of knee flexion. Testing involved three maximal isometric contractions at the two positions for 27 subjects (50 legs). Recordings of average EMG and force output were measured by the Bio-Prompt 3000 and the FARO Axis Muscle Tester, respectively. The results showed these statistically significant differences pertaining to EMG activity: (1) greater VL at 45 deg for the whole sample; (2) greater VMO and VL in males at 45 deg; (3) greater VMO in females at 90 deg; (4) greater VMO and VL in males than in females at 45 deg; and (5) a correlation of VMO and VL in females at 45 deg. These results demonstrate that sex differences exist in EMG activity between the VMO and VL at two angles of knee extension.