Effects of Knee Flexion Angle on Gluteus Medius Activation and Activation Ratios During the Modified Clamshell Exercise

Background and Objectives: The gluteus medius plays a key role in hip stabilization and lower extremity alignment. However, the influence of knee flexion angle during the modified clamshell exercise on muscle activation patterns remains unclear. This study aimed to examine the effects of different knee flexion angles on gluteus medius activity and relative muscle activation patterns during the modified clamshell exercise. Materials and Methods: Thirty healthy women aged 20–30 years performed the modified clamshell exercise at three knee flexion angles (60°, 90°, and 110°). Muscle activity was recorded using surface electromyography and normalized to maximal voluntary isometric contraction (%MVIC). Relative muscle activation was assessed using muscle activation ratios (GMed/TFL, GMed/QL, GMed/Sar, and GMed/GMax). Differences among conditions were analyzed using repeated-measures ANOVA or the Friedman test, as appropriate. Results: Gluteus medius activation (p < 0.001, Kendall’s W = 0.57) and superior gluteus maximus activation (p < 0.001, ηp2 = 0.34) increased significantly with greater knee flexion angle, whereas tensor fasciae latae and quadratus lumborum activation decreased. The GMed/TFL (p < 0.001, ηp2 = 0.45) and GMed/QL (p < 0.001, Kendall’s W = 0.47) ratios also increased significantly, while the GMed/GMax ratio did not differ significantly. No meaningful differences were observed between 90° and 110°. Conclusions: Knee flexion angle influenced muscle activation patterns during the modified clamshell exercise. Compared with 60°, knee flexion angles of 90° and 110° were associated with greater gluteus medius activity and more favorable relative activation patterns; however, no additional benefit was observed beyond 90°. Given the low absolute activation levels, these findings reflect low-load motor control characteristics rather than strengthening effects. The results may be relevant for early-phase rehabilitation under controlled conditions, but their clinical applicability remains limited.