Sex differences in neuromuscular control of quadriceps.

Purpose: Patellofemoral pain syndrome (PFPS) is twice as prevalent in females as males, yet a few studies have evaluated differences in quadriceps muscle control between sexes or across force levels. This study investigated sex differences in quadriceps EMG onset times and amplitude at different force levels during isometric knee extension in asymptomatic males and females and in females with PFPS.

Methods: Thirteen healthy males, 12 healthy females, and 10 females with PFPS performed isometric knee extension ramp contractions at 25%, 50%, and 75% of maximal voluntary contraction (MVC). Surface EMG was recorded from the vastus lateralis (VL), vastus medialis oblique (VMO), vastus medialis (VM), and rectus femoris (RF).

Results: Healthy females showed delayed VL (222 ± 67 ms, p = 0.002), VMO (357 ± 101 ms, p = 0.001), and VM (258 ± 62 ms, p < 0.001) recruitment in comparison with healthy males. Healthy males activated the VL earlier than the VM (156 ± 51 ms, p = 0.02) and RF (379 ± 74 ms, p < 0.001), and at a similar time as the VMO; healthy females activated the VL earlier than the VM (192 ± 53 ms, p = 0.004) and VMO (239 ± 73 ms, p = 0.01). A lower VMO:VL activation ratio was found at 25% MVC (p < 0.001) than at higher force levels.

Conclusions: Delayed activation of the VMO relative to the VL has been proposed as a risk factor for PFPS. This study confirms a delay in VMO onset time in females.