CONTEXT: The effects of fatigue on impact loading during running are unclear, with some authors reporting increased impact forces and others reporting decreased forces. OBJECTIVE: To examine the effects of isokinetic fatigue on muscle cocontraction ratios about the knee and ankle during running. DESIGN: Cross-sectional study. SETTING: Neuromechanics laboratory. PATIENTS OR OTHER PARTICIPANTS: Female middle-distance runners (age = 21.3 +/- 1.93 years) with at least 5 years of training experience. INTERVENTION(S): Participants ran on the treadmill at 3.61 m/s before and immediately after the fatigue protocol, which consisted of consecutive, concentric knee extension-flexion at 120 degrees /s until they could no longer produce 30% of the maximum knee-extension moment achieved in the familiarization session for 3 consecutive repetitions. MAIN OUTCOME MEASURE(S): Electromyographic (EMG) amplitude of the vastus medialis (VM), biceps femoris (BF), gastrocnemius (GAS), and tibialis anterior (TA) was recorded using surface electrodes. Agonistratioantagonist EMG ratios for the knee (VMratioBF) and ankle (GASratioTA) were calculated for the preactivation (PR), initial loading response (LR(1)), and late loading response (LR(2)) phases of running. Hip-, knee-, and ankle-joint angular displacements at initial foot contact were obtained from 3-dimensional kinematic tracings. RESULTS: Fatigue did not alter the VMratioBF EMG ratio during the PR phase (P > .05), but it increased the ratio during the LR(1) phase (P < .05). The GASratioTA EMG ratio increased during the LR(1) phase after fatigue (P < .05) but remained unchanged during the PR and LR(2) phrases (P > .05). CONCLUSIONS: The increased agonist EMG activation, coupled with reduced antagonist EMG activation after impact, indicates that the acute decrease in muscle strength capacity of the knee extensors and flexors results in altered muscle-activation patterns about the knee and ankle before and after foot impact.
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