Introduction: Over-pronated feet (OPF) are associated with altered lower-limb mechanics and elevated injury risk during running. Long-term training on different sport surfaces may modulate neuromuscular control, including knee muscular co-contraction; however, surface-specific adaptations in individuals with OPF remain insufficiently characterized. To determine the long-term effects of training on natural grass, artificial turf, and synthetic surfaces on knee muscular co-contraction during running in individuals with OPF.
Method: In this randomized controlled trial (IRCT20170806035517N5), thirty-two adults (aged 18–30 years) with clinically diagnosed OPF were randomly allocated to one of three intervention groups (natural grass, artificial turf, or synthetic surface) or a no-training control group. The intervention groups completed a supervised, eight-week running program with standardized frequency and progression. Surface electromyography (sEMG) was used to quantify knee muscular co-contraction during treadmill running at a controlled speed. General co-contraction and directed flexor-extensor co-contraction indices were computed over discrete stance sub-phases (heel-contact, mid-stance, and push-off) using established processing pipelines. Assessments were performed pre- and post-intervention by the same blinded operators. Group-by-time effects were examined for primary outcomes, with alpha set at 0.05.
Result: No significant changes emerged for general knee co-contraction across time or between groups. In contrast, significant group-by-time interactions were observed for directed flexor-extensor co-contraction during mid-stance (p = 0.035) and push-off (p = 0.020), indicating that training induced surface-specific neuromuscular adaptations rather than uniform joint stiffening. A main effect of time was also noted at heel-contact (p = 0.050), consistent with a generalized training-related modulation early in the stance phase. Collectively, these patterns suggest that extended exposure to distinct surfaces selectively re-tunes knee muscle coordination strategies in individuals with OPF without globally increasing co-contraction.
Conclusion: Eight weeks of running training on different surfaces prompted targeted adjustments in knee flexor-extensor coordination in runners with OPF, while general co-contraction remained unchanged. Although such adaptations may be beneficial for control, they might not fully mitigate injury risk in this at-risk population. Individualized surface selection alongside integrative neuromuscular conditioning should be considered in rehabilitation and return-to-running planning for individuals with OPF.