Success in recreational and competitive running is influenced by factors that increase performance and minimize the risk of injury. The mechanics of runners have been shown to become less efficient as energy costs rise (Candau et al., 1998) and as fatigue occurs (Elliot & Roberts, 1980). Specifically, stride rate decreases and knee angles become more flexed at the end of an exhaustive run (Derrick, Dereu, & McLean, 2002). While these researchers have primarily examined the role of fatigue on biomechanical changes, none have assessed the influence of running intensity on biomechanical parameters. Therefore, the purpose of this study was to investigate the effects of intensity on step frequency (SF), and trunk, hip, and knee angles during treadmill running. Thirteen participants from a southeastern university volunteered for the study. After written consent was obtained, resting heart rate (RHR) data were collected using standard measures. A single camera (120 Hz) positioned sagittal to the treadmill filmed the participants. Treadmill speed was adjusted so that data could be collected below (BTHR), in (ITHR), and above (ATHR) the individual’s target heart rate (THR). Biomechanical data were analyzed using Peak Motion Measurement System, and heart rate monitored using the Polar® Heart Rate Monitor System. No significant differences were found in the trunk (F(2,36) = 1.22, p > .05) or hip (F(2,36) = .313 p > .05) during the three heart rate intensity levels. However, significant differences were found at the knee (F(2,36) = 11.8, p < .05). A Tukey HSD revealed that BTHR (145.7 ± 8.4°) was significantly greater than ITHR (134.9 ±6.8°) and ATHR (130.8 ± 8.4°). Differences in SF were also found (F(2, 36) = 10.3, p < .05). A Tukey HSD showed that BTHR (1.45 ± .18 sps) was significantly lower than ITHR (1.60 ±.10 sps) and ATHR (1.69 ± .09). These results indicate an inverse relationship between SF and knee angle. As heart rate intensity increased, SF increased while knee angles decreased. This indicates that the knee was more flexed during the higher heart rate intensity levels possibly because the increased SF did not allow for enough time for the knee to go through its entire range of motion, and as a mechanism to aid in force absorption. Application for this study is for coaches to concentrate on knee movement during higher heart rate intensity as it may play a role in more efficient force absorption at higher heart rate intensities. Keyword(s): athletics/sports, performance, research