Differential activation of the vastus medialis and the vastus medialis oblique in individuals with and without patellofemoral pain syndrome
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An imbalance of medial and lateral quadriceps control can lead to patellofemoral pain syndrome (PFPS). The incidence rate of PFPS is twice as high in females than in males. The aims of this research were to investigate sex differences in quadriceps neuromuscular control and motor unit recruitment properties in healthy individuals comparing to those with PFPS. We also investigated optimal leg position and levels of force production to target the vastus medialis oblique (VMO). In Study One, quadriceps surface EMG onset time and amplitude were examined at different submaximal force levels in asymptomatic males and females with and without PFPS. Females showed a 320 ± 70 ms delay in average quadriceps onset time relative to males. The vastus lateralis (VL) and VMO activated together and prior to the vastus medialis (VM) and rectus femoris. A low force (25%MVC) generated the lower VMO:VL ratio compared to 50% and 75% MVC. In order to determine if the VMO was controlled independently of the VM, Study Two examined motor unit recruitment patterns in the VM and VMO in healthy males and females when performing straight leg tasks in two different hip positions. VMO motor units were recruited 2.92 ± 1.28% MVC earlier than VM motor units. Females fired motor units in the vastus medialis complex faster than the males. We found that hip position is a crucial factor influencing motor unit recruitment properties in the vastus medialis complex. In a neutral hip position, motor units in the VM were activated at lower recruitment thresholds and at faster firing rates compared to in lateral hip rotation. Study Three evaluated the effect of PFPS on VM and VMO motor unit recruitment during two different hip positions of straight leg raise in females with and without PFPS. Individuals with PFPS showed an altered control strategy of VMO motor units between the two hip positions compared to healthy individuals. A traditional straight leg raise without hip rotation generated greater VMO motor unit initial firing rates in healthy individuals. However, VMO motor units were recruited at faster rates in lateral hip rotation for individuals with PFPS. The findings from this series of studies indicate that the neuromuscular control strategies of the quadriceps muscle subsections were affected by sex, PFPS, hip position as well as force level. These factors should be taken into consideration when designing rehabilitation protocols.