Abnormal biomechanics increase the risk of sustaining noncontact and overuse lower extremity (LE) injuries. Acute bouts of fatiguing exercise alter LE biomechanics and can increase the risk of injury. Currently, the cumulative effects of repetitive fatiguing exercise bouts on biomechanics are unknown. PURPOSE: To determine the effects of repetitive fatiguing exercise (endurance running) on LE biomechanics. METHODS: Movement compensations (errors) during an overhead (OHS) and single leg squat (SLS) were assessed in thirty-five NCAA division I cross country athletes (males = 19, females = 16). Testing was completed prior to and midway through the competitive cross country season. The athletes were not acutely fatigued during testing. Movement errors were identified by a certified athletic trainer who has experience evaluating the OHS and SLS. Errors included: feet flattening/turning out, heels lifting off the ground, knee valgus/varus motion, knee flexion angle <60°, asymmetrical hip/weight shift, hip hike/drop, low back arching/rounding, uncontrolled trunk motion, arms falling forward, and loss of balance. Differences in total errors and the proportions of errors at pre- and mid-season were assessed using Wilcoxon signed rank and McNemar’s exact tests. RESULTS: Differences for total errors during the SLS (pre = 4.71±1.45, mid = 3.34±0.94; p<0.01) and the proportion of errors during the SLS for asymmetrical hip shift (pre = 0.97, mid = 0.66; p<0.01) and uncontrolled trunk motion (pre = 0.74, mid = 0.43; p=0.02) from pre- to mid-season were observed. The proportions of SLS hip hike/drop (pre = 0.71, mid = 0.46; p=0.06), OHS asymmetrical weight shift (pre = 0.74, mid = 0.51; p=0.06), and OHS knee varus motion (pre = 0.23, mid = 0.09; p=0.06) were near significance. No other differences existed between pre- and mid-season. CONCLUSIONS: The total number and proportions of movement errors between pre- and mid-season assessments improved; especially errors specific to hip motion. The SLS is more physically demanding than the OHS, and may be more sensitive to detecting alterations in movement errors over a sport season. Repetitive fatiguing exercise alters LE biomechanics; these alterations may be different than those caused by acute exercise fatigue. Also, the effects of these alterations on injury risk remain unknown.