Journal of Science and Medicine in Sport
October 2019 Volume 22, Supplement 2, Pages S17–S18
N. Collinsz
Introduction: Sports medicine practitioners frequently use foot orthoses to manage lower limb pain and injury. Individual variability in biomechanical response to foot orthoses questions the conventional kinematic paradigm, whereby orthoses effects occur via passive control of foot pronation. It is plausible that foot orthoses exert their therapeutic effects by modulating activity of the intrinsic foot muscles, acting as the foot’s own internal stabilising mechanism. The aim of this study was to evaluate the immediate effects of foot orthoses on patterns of EMG activity of the intrinsic foot muscles during walking.
Methods: Seven pain-free individuals (4 males; mean[SD] age 27[10] years; BMI 24.3[6] kg/m2) participated. Intramuscular electrodes were inserted into three intrinsic foot muscles (adductor hallucis [AddH], first dorsal interosseous [FDI], abductor hallucis [AbdH]) and three extrinsic foot muscles (tibialis posterior [TP], flexor digitorum longus [FDL], peroneus longus [PL]) under ultrasound guidance. Participants walked on a treadmill at a standardised speed (4 km/h) wearing shoes alone (Dunlop Volley) and wearing shoes with prefabricated foot orthoses (Vasyli Custom Red). Three trials of 20 strides were recorded for each condition. EMG data were sampled at 2000 Hz, and filtered (band-pass, 8th order Butterworth, 70-1000 Hz). Individual muscles patterns were compared between conditions using a wavelet-based mixed effects model.
Results: Foot orthoses resulted in altered EMG activity of the intrinsic foot muscles compared to shoes alone. AddH and AbdH demonstrated reduced activity around heel strike, while AddH and FDI demonstrated reduced activity at toe off. This was accompanied by reduced AbdH activity during resupination, followed by greater AbdH activity at toe off. Small increases in TP activity were observed after heel strike and toe off when walking with foot orthoses, while PL activity was reduced after foot flat.
Discussion: Immediate neuromuscular adaptations when walking with foot orthoses appear to reduce neural drive to the deep intrinsic muscles in the forefoot (AddH, FDI), particularly during key periods of the gait cycle where they typically demonstrate peaks of activity. This may affect the capacity of these muscles to contribute to forefoot stiffness and aid propulsion in terminal stance. Greater AbdH activity at toe-off may reflect muscle length changes when walking with foot orthoses and subsequent need to increase neural drive for the same force generation or may represent a compensatory strategy to assist first metatarsophalangeal joint stability for propulsion. Further research is required to determine whether the same patterns are observed in people with lower limb pain or injury.