BACKGROUND: Recent studies have compared countermovement jump (CMJ) performance in high and low performers of the task. These previous investigations have used reactive strength index modified (RSImod) as the outcome variable determining high and low performers. Findings include that both jump height and time to take-off, the two components in the RSImod equation, were different between groups. Limited data currently exist in examining the subphases of the CMJ in these high and low performing groups. Thus, the purpose of this investigation was to examine differences in high and low performers in the CMJ as determined by RSImod in a sample of collegiate female volleyball athletes. METHODS: 28 collegiate volleyball athletes participated in this investigation (age 20.29 ± 2.90 years; height 180.70 ± 7.71 cm; mass 83.00 ± 16.01 kg). Data was pooled from preseason CMJ testing over the course of three seasons as a part of their routine athlete monitoring program. Each participant completed three jump trials with a dowel rod placed on their upper back. All trials were collected using a portable force platform sampling at 1000 Hz. Thirty seconds rest was given between each trial. Determination of group assignment was based on having a RSImod greater or less than the median value of the sample. Independent sample t-tests were used to compare groups. RESULTS: High performing group displayed significantly greater jump height (34.40 ± 4.09 vs 28.11 ± 3.79, cm p < 0.001), and shorter time to take-off (809.05 ± 83.81 vs 916.95 ± 80.69 ms, p = 0.002). The high performing group also demonstrated shorter durations in both the braking (164.79 ± 23.40 vs 206.73 ± 28.06 ms, p < 0.001) and propulsive (292.08 ± 36.48 vs 327.01 ± 27.25 ms, p = 0.008) phases. Mean braking force was significantly greater in the high performing group (525.06 123.30 vs 428.23 ± 103.99 N, p = 0.03) with no statistical difference seen in the propulsive phase (637.54 ± 77.00 vs 584.61 ± 91.20 N, p = 0.11). CONCLUSIONS: This investigation supports previous findings in that both jump height and time to take off are different between high and low performing groups as determined by RSImod. Additionally, this investigation provides insight into the difference between groups residing predominantly in the braking phase with significant differences in both force and durations between groups. Thus, the high performing group utilizes a more effective braking strategy during the countermovement to then create a greater jump performance.

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