Background: Anterior cruciate ligament (ACL) injuries commonly reduce knee stability and increase joint loading, often leading to compensatory gait alterations that may increase injury risk. Functional electrical stimulation (FES) of the biceps femoris long head (BFLH) during the gait stance may improve knee stability by reducing harmful joint loading, but the effects on voluntary muscle control remain unclear.

Research question: This study examined whether FES of the BFLH during the stance phase of the gait reduces ACL-relevant knee joint loading in healthy adults and whether it alters voluntary muscle control. Additionally, the use of gluteus maximus (GLMAX) sEMG as a proxy for BFLH activation was assessed.

Method: Nine healthy participants walked on a treadmill under control and FES-assisted conditions. Kinematic, kinetic, and sEMG data were analyzed using statistical parametric mapping and linear mixed-effects models.

Results: FES of the BFLH significantly reduced internal knee rotation moment (KRM) with 9.37% during 42–48% of the gait cycle (p = 0.0002; d = 0.42). Knee adduction moment (KAM) showed non-significant reductions in both legs (non-stimulated: p = 0.0317, d = 0.18; stimulated: p = 0.0492, d = 0.37). Knee abduction angle (KAA) and knee rotation angle (KRA) showed no significant changes (p > 0.05). In sEMG analysis, inconsistent timing between GLMAX and BFLH activation indicated GLMAX is not a reliable surrogate for estimating BFLH activity. Regarding voluntary control, only peak KAM increased slightly over strides during FES-assisted walking (p = 0.006), possibly due to muscle fatigue. No significant retention or after-effects were observed.

Conclusion: Targeted FES of the BFLH can reduce ACL-relevant knee loading without impairing voluntary motor control. sEMG results highlight the need for direct BFLH monitoring, as GLMAX is an unreliable proxy. These findings support further exploration of FES strategies for ACL injury prevention and rehabilitation.