Different strike patterns result in differences in kinematics and kinetics during running. The objective of this study was to examine differences in muscle activation in rearfoot stike (RFS) and nonrearfoot strike (nRFS) running. It was hypothesized that the activation of the medial grastrocnemius would be earlier and of greater magnitude in runners using a nonrearfoot strike pattern. Furthermore, it was hypothesized that the medial gastrocnemius would show two distinct peaks in activity in nonrearfoot striking, the first resulting from preparation for, and support of, initial contact and the second from propulsion during the stance phase. Finally, it was hypothesized that the activation of the tibialis anterior would be earlier and of greater magnitude in runners using a rearfoot strike pattern. This study included 9 female runners, five habitual rearfoot strikers and four habitual nonrearfoot strikers, who each completed 3-dimensional running analysis under two conditions:​ their habitual strike pattern and a converted strike pattern during which habitual nonrearfoot runners converted to rearfoot and habitual rearfoot runners converted to nonrearfoot. Variables of interest were activation of the medial gastrocnemius and tibialis anterior, lower body kinematics, body segment positions and velocities at foot strike, and tibial acceleration. This study found no significant differences in the activation time or magnitude of the tibialis anterior or medial gastrocnemius muscles during RFS and nRFS running. Visual inspection of the medial gastrocnemius electromyography (EMG) data showed the hypothesized pre-foot strike activation peak in the nRFS condition, however because of the small number of subjects and high variability of the EMG data, this difference was not statistically significant and should be investigated further. Resultant tibial acceleration was greater in the nRFS condition, indicating a potential factor for increased risk of tibial stress fracture in individuals running with a nRFS pattern.