Knee and ankle joint moments and muscle activation during stair ascent and descent in young adults with flexible pes planus: A comparison with normal foot posture.

This study compared knee and ankle joint moments, muscle activation, and co-contraction between young adults with flexible pes planus and normal foot posture. Forty young adults (20 per group) performed barefoot stair ascent and descent. Three-dimensional motion analysis, ground reaction force, and surface electromyography (EMG) data were collected. Peak joint moments were analyzed using two-way mixed-design analyses of variance. Peak normalized electromyographic amplitudes and co-contraction indices (CCIs) were compared between groups for each task using Mann-Whitney U tests with false discovery rate adjustment. A group × task interaction was observed for the knee frontal-plane moment. The flexible pes planus group demonstrated a greater peak knee varus moment during descent but not ascent. The normal foot group demonstrated greater peak ankle dorsiflexion moments across both tasks. During stair ascent, the flexible pes planus group showed lower peak normalized gluteus medius and tibialis anterior activity relative to task-specific reference values. No between-group differences were observed in peak normalized muscle activity during stair descent or in knee and ankle CCIs. These findings highlight knee varus moment differences during descent and peak normalized EMG differences during ascent. Generalizability to older adults or individuals with symptomatic flexible pes planus, pain, or functional limitations is limited.