The Effect of Fatigue on the Biomechanics of Recreational Runners with Patellofemoral Pain

Abstract

Patellofemoral pain syndrome (PFPS) is one of the most common injuries experienced by recreational runners. Pain is often absent at the beginning of activity, but tends to worsen with fatigue during a run. This may be due to the effect of fatigue on the magnitude of activation of muscles responsible for controlling hip and knee biomechanics, thus altering the loading environment of the patellofemoral joint and predisposing to PFPS pathology. PURPOSE: To compare the effects of fatigue on strength and muscle activation during an exhaustive run in female runners with and without PFPS. Kinematic and spatiotemporal variables were also examined as a means to understand the strength and muscle activation differences between groups. METHODS: Five females (33.5 +/-8.8 years of age) with a history of PFPS for > 2 months and five age-matched, healthy control females (32.8 +/-7.4 years of age) ran on a treadmill at a self-selected speed to a pre-determined level of fatigue (Rating of Perceived Exertion=17). Strength of the major hip muscles was measured pre- and post-run using handheld dynamometry. Surface EMG was recorded wirelessly from the following muscles: gluteus medius, tensor fascia lata, rectus femoris, and biceps femoris. EMG as well as kinematic and spatiotemporal data were collected at 3-minute intervals until termination. Comparisons were made between the 0%, 50%, and 100% intervals of the run using separate 2-way ANOVAs, with fatigue as the repeated measure for each variable of interest and post hoc Tukey HSD tests for significant main effects. RESULTS: Hip external rotation (ER) strength was significantly decreased in the PFPS group from pre- to post-run (mean difference = 0.09 Nm/kg, p = 0.045). Females with PFPS also demonstrated increased peak activation of gluteus medius. Peak hip internal rotation (IR) (mean difference = 3.44°, p = 0.291) and adduction (mean difference = 0.84°, p = 0.294) both trended toward an increase in the PFPS group from the beginning to the end of the exhaustive run. CONCLUSIONS: Females with PFPS exhibited decreased strength of hip external rotators following an exhaustive run compared to healthy, age-matched controls, which may be a factor in the increased hip IR angle in this group. EMG analysis revealed increased activation of gluteus medius. Greater activation levels may result in earlier fatigue to hip external rotators, leading to altered hip kinematics and a predisposition to PFPS.