Fast Six-minute Walking Induced Abductor Hallucis Muscle Recruitment In Individuals With Flatfoot

Abstract

PURPOSE: This study examined lower extremity muscle activities before and after six-minute treadmill walking at highest speeds in individuals with flatfoot. METHODS: Eight individuals with flatfoot (4 women, 4 men, age= 24.8±2.4 years) with no other history of lower extremity injury in six months and no neurological disease, participated in this study. Subjects were first screened using subtalar static navicular drop tests. Surface electromyography (BIOPAC system, USA) of the abductor hallucis muscle (AbdH), tibia anterior, gastrocnemius, and peroneus longus muscles were recorded during maximum voluntary isometric contractions (MVIC). Next, three-dimensional motion capture system (Vicon motion system, UK) was used to assess dynamic navicular mobility during walking. Subjects were then asked to walk on the treadmill at their maximal pace for 6 minutes. After walking, the MVIC and dynamic navicular mobility were assessed again. Median frequency (MedF) of EMG during MVIC was calculated for both before and after 6-minute walking. RESULTS: Our preliminary results show substantial increase in the MedF of the AbdH (10.2Hz ± 29.9Hz), suggesting an increase in motor unit recruitment in AbdH with 6-minute fast treadmill walking. MedF of tibia anterior was significantly decreased 16.7 ± 11.5Hz after fast treadmill walking, but no significant change was observed in peroneus longus and gastrocnemius. Subjects exhibited similar navicular drop before (17.3 ± 5.4 mm) and after (16.0 ± 5.2 mm) fast treadmill walking. CONCLUSION: Six-minute fast treadmill walking induced greater muscle recruitment of the AbdH in individuals with flatfoot. We infer that the AbdH of flat foot doesn’t function as a dynamic stabilizer during walking. However, how increased work loads in other lower extremity muscles results in greater activity of AbdH remains to be investigated. The observation that muscle activity of AbdH increased when the other extrinsic muscle fatigue indicates that 6 minutes walking can serve as functional training of the intrinsic muscles to support the medial longitudinal arch.