Slips during gait are the leading cause of falls and subsequent serious injury in young and older adults. Reactive balance responses generated by the lower limb muscles can restore balance and minimize the chance of experiencing a fall. The purpose of this study was to examine the role of the deep muscles in the lower leg and muscles within the foot during recovery from an unexpected slip perturbation. We hypothesized that lower leg musculature and intrinsic foot muscles would exhibit an earlier onset, longer duration, and increased magnitude during slips compared to level walking and that muscle activity would have a reduced magnitude within individuals with Hallux Valgus.

Young adults with unaffected feet (n= 16, age:​ 24 ± 1.55) and those with Hallux Valgus (n= 4, age:​ 24.75 ± 1.71) completed a series of level walking trials with unexpected slip perturbations elicited with waxed paper adhered to the underside of a sandpaper mat. Participants were equipped with 12 IRED markers to collect kinematics of body segments and were recorded using an OptoTrak Certus camera system. Kinematics were used to calculate whole body centre of mass (COM) velocity, ankle displacement/velocity, and foot angle range/angular velocity in the perturbed/leading limb. Muscle activity was collected from a total of 10 lower limb and foot muscles using a combination of surface and fine-wire intramuscular electromyography (EMG). Force plates were used to record ground reaction forces and identify gait cycle timing during level walking and unexpected slip trials.

Overall, 20 participants experienced at least one slip with the most common type of slip occurring at toe off (93% of all slips that occurred were at toe off). Muscle activation onset, duration, and normalized magnitude across slips (slip 1, 2, and 3) was highly variable. In general, extrinsic and intrinsic foot muscles exhibited a delayed onset, longer duration of muscle activity, and an increase in normalized magnitude in response to slip perturbations compared to level walking. This was observed in both unaffected feet and those with Hallux Valgus. Ankle marker velocity in the anterior-posterior direction revealed the foot travelled backwards when experiencing toe off slips. Ankle angular velocity range increased indicating that the rate in which the ankle moved through plantarflexion and dorsiflexion was faster during slips. Average COM velocity in the anterior-posterior direction increased during slips. Propulsive (anterior-posterior shear) force during the second half of the contact (stance) phase was reduced as a result of a toe off slip perturbation. Additionally, loading rate in the anterior-posterior direction decreased whereas vertical unloading rate increased prior to toe off. Ground reaction forces were not significantly different in the Hallux Valgus sub-group.

These findings suggest that lower leg and foot musculature assist in balance recovery from a toe off slip, ensuring a continuous gait trajectory is maintained. Additionally, observations within muscle activation patterns during gait and slips in individuals with Hallux Valgus provided rationale to further examine the muscle timing and magnitude of the lower leg and foot muscles. Future work may provide further evidence that structural changes in the foot due to Hallux Valgus influences muscle mechanics and thus, dynamic balance.