Effects of trunk lean on lower-limb muscle activity ratios during single-leg squat in individuals with patellofemoral pain syndrome

Background: Patellofemoral pain syndrome (PFPS) is associated with altered activation of the lower-limb muscles during functional tasks such as the single-leg squat (SLS). Although trunk position can influence the recruitment of the hip and knee muscles, the effects of different trunk lean angles on muscle activation ratios in individuals with PFPS remain unclear.

Objective: This study aimed to examine how trunk lean angles of 0°, 15°, and 30° affect lower-limb muscle activation ratios during SLS in individuals with PFPS.

Methods: Thirty-two adults with PFPS performed SLS at 60° of knee flexion under three trunk lean conditions (0°, 15°, and 30°) while surface electromyography was used to record the activation of the vastus medialis oblique (VMO), vastus lateralis (VL), gluteus medius (Gmed), gluteus maximus (Gmax), tensor fascia latae (TFL), semitendinosus (ST), and biceps femoris (BF). Muscle activity ratios, including VMO: VL, Gmed:TFL, quadriceps-to-hamstrings (Q:H), and Gmax:H were calculated, and a repeated-measures analysis of variance with Bonferroni correction was used to compare differences across trunk lean angles.

Results: The Gmed:TFL ratio was significantly higher at 30° of trunk lean than at 0° and 15°, and the Q:H ratio decreased progressively as trunk lean increased ([Formula: see text]). No significant differences were observed in the VMO:VL or Gmax:H ratios across conditions.

Conclusion: These findings suggest that increasing trunk lean during SLS may enhance Gmed activation relative to the TFL and may contribute to a decrease in the Q:H ratio, whereas VMO:VL and Gmax:H ratios appeared relatively unaffected by trunk lean angle. Performing SLS with greater trunk inclination may optimize lower-limb muscle coordination and may offer clinical benefits for individuals with PFPS.