Ankle positioning and knee perturbation affect temporal recruitment of the vasti muscles in people with patellofemoral pain

Objectives: To compare the temporal recruitment of the vastus medialis obliquus (VMO) and vastus lateralis (VL) during voluntary ankle movements and perturbed standing in people with patellofemoral pain, and to determine the effects of different reflex and voluntary postural exercise tasks on VMO facilitation. Design: Repeated-measures design. Participants: Twenty-three subjects with patellofemoral pain. Interventions: Quadriceps reflex contraction in response to postero-anterior knee perturbations was measured with three crural muscle contraction conditions and three postural exercises (semi-squatting, tip-toeing and heel standing). Main outcome measures: The electromyographic (EMG) onset time of the VMO and VL during each task was measured and compared across the different tasks. Results: The mean EMG onset time of the VMO was later than that of the VL in the voluntary tasks such as tip-toeing (VMO 95.3. ms vs VL 36.4. ms, mean difference 58.9. ms, 95% confidence interval -33.7 to 151.5. ms), whereas earlier VMO activation was found in the perturbation tests such as toe standing (VMO 17.6. ms vs VL 22.9. ms, mean difference -5.3. ms, 95% confidence interval -25.3 to 14.7. ms). Conclusion: These findings suggest the potential benefits of unexpected perturbation activities for facilitating VMO activation. The clinical applications of perturbation tasks in rehabilitation exercise programmes and the underlying mechanisms warrant further investigation.