Background Appropriate timing of activity of the vastus medialis obliqus (VMO) and vastus lateralis (VL) muscles is a key factor for proper tracking of the patella in the trochlear groove during knee extension. This study investigates the relative timing of activation of the VMO and VL muscles during unexpected perturbations performed before and after eccentric exercise.

Methods Surface electromyography signals were recorded from the VMO and VL muscles of the right leg in 11 healthy men during rapid postural perturbations performed at baseline, immediately after eccentric exercise of the quadriceps, and at 24 and 48 h after exercise. Participants stood on a moveable platform during which eight randomised postural perturbations were performed (4 repetitions of 2 perturbation types: 8 cm forward slides, 8 cm backward slides).

Results Before the eccentric exercise, the onset of VMO activity was significantly earlier than the VL muscle (average for both forward and backward perturbations: VMO 39.0±7.1 ms; VL 43.7±7.9 ms). However, the onset of VMO activity was significantly later compared with VL muscle immediately after eccentric exercise and this remained 24 and 48 h after eccentric exercise (average across all postexercise sessions and perturbation directions: VMO 72.3±11.1 ms; VL 56.0±8.2 ms; p<0.05).

Conclusions The onset of VMO–VL activity in response to rapid destabilising perturbations is altered immediately after eccentric exercise and during eccentric exercise-induced muscle soreness up to 48 h later. These observations may help explain the high prevalence of knee disorders after high intensity eccentric exercise.