Effects of different foot orthoses on muscular frequency spectrum during the swing phase of gait following ACL reconstruction: a cross-sectional, mixed-model within-subject study

This study aimed to investigate the acute effects of various foot orthoses on the frequency-domain characteristics of lower-limb electromyographic (EMG) activity during the swing phase of gait in individuals with pronated feet following anterior cruciate ligament reconstruction (ACLR). A cross-sectional experimental study with a within-subject repeated-measures design was conducted. Forty male participants were recruited, including 30 participants post-ACLR with pronated feet and 10 healthy controls. EMG activity was recorded from eight key lower-limb muscles during the gait cycle of the dominant limb under four orthotic/footwear conditions: control shoe, placebo foot orthoses, medial arch-support foot orthoses, and double-density foot orthoses. Acceleration and deceleration sub-phases of swing phase were defined using in-shoe footswitch sensors. EMG frequency analysis focused on median frequency (MF) as an indicator of changes in neuromuscular activation. During the acceleration sub-phase, the group assessed 6 months post-ACLR exhibited significantly lower MF in the tibialis anterior ( p = 0.003, η²=0.152), gastrocnemius ( p < 0.001, η²=0.324), and gluteus medius ( p = 0.008, η²=0.124) compared with participants at later postoperative stages and healthy controls. The double-density foot orthoses reduced biceps femoris MF across all ACLR with pronated feet groups ( p = 0.012, η²=0.095). In the deceleration sub-phase, significant main effects of group were observed for tibialis anterior ( p = 0.001, η²=0.218), gastrocnemius ( p < 0.001, η²=0.376), and semitendinosus ( p = 0.005, η²=0.135). A significant effect of foot orthoses on gastrocnemius MF was also identified ( p = 0.038, η²=0.102), with a significant group × foot orthoses interaction ( p = 0.022, η²=0.118). Medial arch-support and double-density foot orthoses reduced gastrocnemius MF compared with the control shoe, with the greatest reduction (8–10%) occurring in participants in the early post-ACLR stage using the double-density foot orthoses. Foot orthoses, particularly medial arch-support and double-density designs, are associated with acute changes in lower-limb neuromuscular recruitment, especially during the high-demand deceleration sub-phase. These findings suggest potential alterations in motor unit recruitment patterns and muscle coordination during gait in individuals with pronated feet following ACLR. However, given the acute experimental design, the results should be interpreted as associations rather than causal effects, and further longitudinal studies are required to determine potential clinical implications.