Slips are considered one of the most common causes of major accidental injuries. The objective of this thesis is two-fold. The first objective is to determine the role of the subtalar joint during a slipping perturbation. The second is to determine if certain footwear characteristics, that may restrict the normal function of the subtalar joint (i.e., insole stiffness and heel counter stiffness), will change the response to unexpected heel contact slipping perturbations.

Forty-two participants (30 females, 12 males) were recruited from a university aged population (21.19 years ± 2.7 years). Trials were performed over a 10 m walkway with rectangular sheets of sandpaper placed at each foot contact. Ten participants performed walking trials barefoot while the other 32 participants were randomly assigned to one of four footwear conditions (n=8) (condition 1:​ flexible insole, soft heel counter;​ condition 2:​ flexible insole, stiff heel counter;​ condition 3:​ rigid insole, soft heel counter;​ condition 4:​ rigid insole, stiff heel counter). Electromyography (EMG) signals were collected from eight lower limb muscles (tibialis anterior, peroneus longus, medial gastrocnemius, rectus femoris and medial hamstring). Kinematic data was collected using a 20 marker set-up. Marker triads were placed on the tibia, calcaneous and mid-foot to determine subtalar joint motion. Kinetic data was collected using forces plates embedded in the walkway. Unexpected slips were presented after a predetermined number of normal walking trials. Wax paper adhered to the underside of a sandpaper sheet was exchanged on the second force plate to cause an unexpected heel contact slip perturbation.

Overall, 20 participants experienced a slip. Within the barefoot condition, 80% of the participants experienced an unexpected slip perturbation. The prevalence of slips was not as great within all of the footwear conditions (25% - 50%). During slip trials the average onset of eversion occurred slightly later than in normal walking trials, but was not statistically significant. The tibialis anterior elicited a burst of activity during the middle phase of stance that is typically not seen during normal walking. The average onset of tibialis anterior activity was earlier with the similar durations and relatively higher magnitudes than normal walking. During slip trials, the peroneus longus did not have significantly different onsets or durations and the magnitudes were slightly higher compared to normal walking trials. During slip trials, the medial gastrocnemius onset was not found to be significantly different when compared to normal walking trails, but the magnitudes were significantly lower.

A higher rate of vertical loading was the only significant finding that would have indicated an increased risk of slipping within the barefoot condition;​ while lower stance durations, gait velocities, heel velocities, and smaller shank and foot-floor angles indicated an increased risk of slipping within the shod conditions. These finding would suggest that individuals who were in the shod conditions would have been at a higher risk of slipping than the barefoot condition, which should have resulted in higher incidences and severities;​ when in fact, the severity and incidences of slips was much lower. Therefore, the footwear, along with decreasing loading rate, must offer a level of stability to the foot and ankle during heel contact that controls foot motion. In particular, decreasing the rate of pronation or eversion at the time the slip was detected, which would likely decrease the severity of the slip;​ evident due to diminished recovery times.The peroneus longus does contribute to controlling subtalar motion alongside the tibialis anterior and finally, footwear characteristics that restrict normal subtalar joint motion seen in barefoot individuals will help decrease the risk of slipping and decrease the severity, improving chances for recovery.