Running is the most popular form of exercise in the United States, but nearly 50% of runners sustain an annual lower-extremity injury. Reports of a lower prevalence of injuries in habitual barefoot runners have encouraged researchers to assess the influence of this running style on injury potential. Barefoot running is promoted based on its association with a forefoot-strike (FFS) contact pattern and a reduction in the vertical ground reaction force loading rate. However, recent investigations indicate that novice barefoot runners may not adopt a FFS. Yet, despite these findings, most studies have not differentiated lower-extremity dynamics of novice barefoot runners by foot-strike pattern and/or have explicitly instructed forefoot-strikes. Therefore, it is unknown whether novice barefoot runners innately adopt a FFS and the loading characteristics consistent with lower injury risk. It is also unknown whether novice barefoot runners who adopt a mid-foot strike (MFS) or FFS can maintain this pattern following a bout of barefoot running, or whether these runners can adapt this pattern in-response to a long term transition. Therefore, the specific aims of this dissertation were developed in order to better understand the lower-extremity dynamics of habitually shod runners who transition to barefoot running.

Chapter III reviews the first study, the purpose of which was to identify the acute changes in strike patterns and biomechanical characteristics of habitually shod runners who acutely transition to barefoot running. To do this, 22 recreational runners performed shod and novice barefoot running. Of these 22 RFS shod runners, 8 maintained a RFS, 9 adopted a MFS, and 5 adopted a FFS during novice barefoot running. Foot-strike patterns, joint mechanical demands and loading rates were examined using a 3x2 (strike-pattern x running condition) ANOVA. When compared to shod running, all groups of novice barefoot runners demonstrated a shift in mechanical demand from the knee to the ankle. However, only FFS runners demonstrated a reduction in loading rate. These findings highlight that in order to glean the biomechanical characteristics consistent with lower injury risk, there is a need to explicitly instruct patients/athletes to FFS when performing novice barefoot running.

Chapter IV examines how habitually shod RFS runners respond to an exertion bout of novice barefoot running. To do this, 21 recreational runners performed novice barefoot running, before and after an exertion bout of barefoot running. Of these 21 RFS shod runners, 6 maintained a RFS, 10 adopted a MFS, and 5 adopted a FFS during novice barefoot running. Foot-strike patterns, joint mechanical demands, loading rates and triceps surae EMG median frequency and peak torque were examined using a 3x2 (strike-pattern x exertion) ANOVA. Following exertion, novice barefoot MFS and FFS runners demonstrated fatigue of the triceps surae as indicated by reductions in median frequency and peak torque. The fatigue effects contributed to shifts in mechanical demand, from the ankle to the knee, and associated increases in loading rates, among these runners. RFS runners demonstrated a slight reduction in loading rate following exertion, however, values were still above traditional shoe running. The findings demonstrate a foot-strike dependent response to barefoot funning and indicate the need for a slow, and possibly instructed progression to FFS barefoot running in order to minimize fatigue effects to the triceps surae.

Chapter V examines whether habitually shod runners innately adapt the habituated barefoot lower-extremity dynamic pattern, in-response to a long-term (8-10 weeks) transition to barefoot running. To do this, 18 recreational RFS shoe runners underwent an 8-10-week transition to barefoot running. Of these 18 RFS shod runners, 7 maintained a RFS, 8 adopted a MFS, and 3 adopted a FFS during novice barefoot running. Foot-strike patterns, joint energetics, loading rates and arch indices were examined using a 3x2 (strike-pattern x transition) ANOVA. Despite increases in arch indices, following the transition, MFS and FFS runners demonstrated reduced ankle energy absorption and similar loading rates to novice barefoot running. RFS runners similarly demonstrated increases in arch indices, but non-significant changes in energetics and loading rates following the transition. These findings indicate that habitual shoe runners do no innately adapt the biomechanical characteristics consistent with lower injury risk in-response to an 8-10-week transition to barefoot running, but rather, likely require an instructed FFS progression.

Taken altogether, the results of this dissertation demonstrate variability in foot-strike patterns and lower-extremity dynamics of habitually shod runners who transition to barefoot running. Likely as a result of these variable dynamics, runners respond differently to an exertion bout of barefoot running and a long-term transition to barefoot. It appears that in order to glean the biomechanical characteristics consistent with lower injury risk, individuals must employ a FFS pattern when performing barefoot running. However, runners often do not innately adopt this pattern, cannot maintain this pattern in-response to exertion, nor innately adapt this pattern in-response to a longterm transition. Therefore, if a transition to barefoot is desired, the findings highlight the need for a slow, and possibly instructed progression to FFS barefoot running.