Atrophy of the intrinsic foot muscles (IFM) has been associated with pathologies such as diabetic foot ulcers, plantar fasciitis, and instability in the elderly, however the mechanisms by which IFM influence these pathologies remain unclear. Previous studies have illustrated the role of IFM in longitudinal arch support and have suggested a role in balance. We hypothesize that IFM that abduct/adduct the toes (e.g. abductor hallucis, ABDH; abductor digiti minimi, ABDM; dorsal and plantar interossei, PDIO, PPIO) regulate the relative position of the midfoot and forefoot elements during turning/cutting and balance. We predict a) that muscles on the same side of the second-digit pedal axis will act together during balance tasks and turning, b) muscles on the medial side of the axis (ABDH, PDIO1) will be active to facilitate ipsilateral turns while muscles on the lateral side of the axis (ABDM, PDIO3) will be active during contralateral turns, and c) increases in center of pressure (COP) movement and velocity will be associated with increased IFM activity during balance tasks. We used surface electromyography (EMG) to measure activity in 6 IFMs in 12 healthy subjects (age 18-25) during 3 balance tasks, walking, and ipsilateral/contralateral turns on an EMED/E plantar pressure mat. We filtered, smoothed, and normalized the EMG data to the peak for each subject. We quantified peak pressures in 13 regions of the foot, COP movement and COP velocity throughout each step. We used analysis of variance to compare activity in each muscle across walking and balance trial types. We examined the relationship between COP movement/velocity and EMG in muscles on the medial and lateral side of the pedal axis to assess the relationship of COP movement/velocity, measures of instability, with IFM activity. All of the IFM with an abduction component (ABDH, ABDM, PDIO1,3) were more active during turns than walking straight ahead (p<0.01), while flexor digitorum brevis (FDB) was not. ABDH was more active during ipsilateral than contralateral turns or straight walking (p<0.01). PDIO1 was most active during ipsilateral 90° turns (p<0.01). PDIO3 was more active during turning trials compared to straight ahead walking (p<0.01), but it was active during both ipsilateral and contralateral turns, possibly because of crosstalk with PPIO2. FDB and ABDM exhibited no differences among trial types. While few overall differences were exhibited across balance trial types, increases in COP velocity were associated with increased activity in the IFM, and COP shifts were associated with EMG activity on the contralateral side of the pedal axis. These data suggest the IFM, especially those that abduct and adduct, counteract instability during balance and assist stable turning/cutting by moving the midfoot/rearfoot relative to the forefoot while maintaining forefoot contact with the ground. Understanding the role of the intrinsic foot muscles in balance and turning maneuvers is critical to our comprehension of the etiology of foot pathology and instability in individuals with atrophy of these muscles and to the development of methods for strengthening them.
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