Predictors of Vertical Ground Reaction Forces during Maximal Single-leg Vertical Hopping in Young Athletes

Abstract

PURPOSE: Athletes who return to sports following ACL injury are at increased risk of reinjury relative to previously uninjured athletes. Late-phase ACL rehabilitation incorporating dynamic neuromuscular training has been recommended to improve outcomes and reduce reinjury risk. Objective guidelines for staged progressions to introduce high-joint load maneuvers in training are obscure. The purpose of this investigation was to determine the relationship of isokinetic measures at the knee and hip to vGRF in young athletes. We hypothesized that increased lower extremity strength would be related to increased force attenuation indicated by decreased landing vGRF. METHODS: Forty-nine female and sixty male high-school athletes were given verbal instructions to hop as high as they could, land under control, then repeat the task for 15 seconds on a portable forceplate (Accupower; AMTI, Watertown, MA) Vertical ground reaction force was analyzed at initial contact. Raw force data was filtered with a generalized cross-validation spline using a 50 Hz cut-off frequency, then normalized to potential energy. To assess knee flexor and extensor torques, athletes were seated on a dynamometer (Biodex Medical Systems, System 2, Shirley, NY) with trunk perpendicular to floor and hip and knee flexed 90°. Athletes performed 10 max repetitions at 300°/sec. To assess hip abductor torque, athletes were secured in a standing position facing the dynamometer (Biodex Medical Systems, System 3, Shirley, NY). Athletes performed 5 max repetitions at 120°/sec. Raw torque data was normalized to body weight. Forward step-wise regression analysis and Pearson correlation assessed the relationship between vGRF and knee and hip torques. RESULTS: The linear regression model that included knee extensor, knee flexor, and hip abductor torques significantly predicted vGRF (p<0.0001, R = 0.668). Knee flexor and extensor torques demonstrated a moderate, negative correlation with vGRF (r = −0.48 and −0.62 respectively). Hip abductor torque demonstrated a low, negative correlation with vGRF (−0.37). CONCLUSIONS: Increased lower extremity muscle strength may be linked to decreased landing force during maximum effort vertical hopping in young athletes. Normative knee and hip strength should be achieved prior to initiating high-joint load training.