A. Partusch, B. Coad, A. Cummins, K. Nelson, R.S. McCulloch

Gonzaga University, Spokane, WA

PURPOSE: Unanticipated jump landings often have worse kinematics than anticipated landings, frequently resulting in knee injury. We hypothesized that a hamstring strength training protocol would decrease the quadriceps to hamstring activation ratio (QHR), which would lead to less variation between anticipated and unanticipated landing and jumping kinematics. METHODS: Nine female Gonzaga students (Height: 171.2 ± 8.7 cm, mass: 70.7 ± 12.2 kg, age: 20.8 ± 1.3 yrs; mean±SD) performed a pre- and post-test, each consisting of 20 two-footed jumps off a 0.3 m tall box onto a force plate. Subjects knew whether to land or jump back up after dropping during anticipated trials and were asked to react real-time to a computer screen with instructions during the unanticipated trials. The 20 jumps consisted of 5 anticipated drop landings (ADL), 5 anticipated drop jumps (ADJ), 5 unanticipated drop landings (UDL), and 5 unanticipated drop jumps (UDJ). A four-week hamstring strengthening protocol was implemented between the pre- and post-tests. EMG was used to record quadriceps and hamstring muscle activation (normalized to max voluntary contractions), a force plate was used to record peak ground reaction forces in the vertical (vertGRF) and anterior (antGRF) directions, and motion capture was used to determine minimum knee angle. QHR and minimum included knee angles were calculated and peak vertGRF and peak antGRF were normalized to body mass. Two-way repeated measures ANOVA was used to determine the individual and synergistic effects of the independent variables. T-tests were used to compare the differences between anticipated and unanticipated movements as separated by time (pre vs post) and action (land vs jump). Correlations were used to examine the relationships between dependent variables. Significance was set at p < 0.05. RESULTS: A significant interaction occurred between time and action for knee flexion angle (p = 0.035). Anticipation significantly affected peak vertGRF (p = 0.046) and antGRF (p = 0.037). There were significant differences (p < 0.05) between anticipated and unanticipated post-test jumps for peak antGRF (9.1±1.6 N/kg, 10.3±2.5 N/kg) and peak vertGRF (36.3 ± 5.2 N/kg, 40.9 ± 7.1 N/kg). Peak antGRF and peak vertGRF were positively, significantly correlated (p < 0.05) for both pre UDL and pre UDJ conditions, as were post UDJ minimum knee angle and QHR. CONCLUSION: No significant changes in QHR were induced by the hamstring strengthening regimen and many of the kinematics remained different between the two jumps. Isolated hamstring strengthening appears to have little-to-no effect on the poor knee kinematics in unanticipated landing situations.

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