Effects of Trunk Lean on Lower Limb Muscle Activity during Single-Leg Squat Hold in Individuals with Patellofemoral Pain Syndrome

Background Patellofemoral pain syndrome (PFPS) is a musculoskeletal disorder characterized by retropatellar or peripatellar pain that worsens during functional loading activities. Trunk lean during weight-bearing has been suggested to influence hip and knee muscle activation in individuals with PFPS. Purpose This study investigated the effects of single-leg squat hold performed with varying trunk-lean angles (0°, 15°, and 30°) on lower limb muscle activity in individuals with PFPS. Study design Cross-sectional study Methods Thirty-two adults with PFPS performed single-leg squat hold under three trunk-lean conditions. Surface electromyography assessed activity of the vastus medialis oblique, vastus lateralis, gluteus maximus, gluteus medius, tensor fasciae latae, semitendinosus, and biceps femoris. Muscle activity was normalized to maximal voluntary isometric contraction and analyzed using one-way repeated-measures ANOVA with Bonferroni post hoc correction. Results Significant differences in muscle activity were observed for the vastus medialis oblique, vastus lateralis, gluteus maximus, and tensor fasciae latae (p<0.05). Greater trunk lean was associated with increased gluteus maximus activation and decreased vastus medialis oblique, vastus lateralis, and tensor fasciae latae activity. In contrast, no significant differences were observed for the gluteus medius, semitendinosus, or biceps femoris (p>0.05). Conclusions Increasing trunk lean during single-leg squat hold shifted muscle activation toward a hip-dominant strategy, with increased gluteus maximus activity and reduced quadriceps and tensor fasciae latae activity. These findings suggest that incorporating 15° to 30° of forward trunk lean during single-leg squat hold may be considered for individuals with PFPS, particularly those who experience anterior knee pain during upright squatting. However, patellofemoral joint stress was not directly measured in the present study, and the observed decrease in vastus medialis oblique activity warrants careful consideration. This trunk lean strategy should therefore be complemented with exercises targeting vastus medialis oblique activation to ensure adequate patellar stabilization.