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INFLUENCE OF ONSET MOVEMENT THRESHOLD ON PHASE DURATION DURING THE COUNTERMOVEMENT JUMP

Abstract

BACKGROUND: A primary concern in the vertical jump literature involves the methods used in the analysis of force-time data obtained during the jumping tasks. When analyzing this data, the first decision that needs to be made is the onset movement threshold that is used. Previous investigations have examined the use of different onset movement thresholds in the squat jump and isometric mid-thigh pull tasks, finding that the use of 5 times the standard deviation of the weighting phase provided the highest level of reliability and maintained a large portion of the force-time data. Thus, this investigation sought to compare different onset movement thresholds on temporal variables of the countermovement jump (CMJ). METHODS: 11 female collegiate volleyball athletes participated in this investigation. Participants completed two maximal effort trials. Each trial was separated by thirty seconds. All trials were performed on a portable force platform sampling at 1000 Hz. Each trial was analyzed using each onset movement threshold. A within-subject repeated measures analysis of variance was used to compare four thresholds reported in the literature. RESULTS: Statistically significant differences between thresholds for time to take off were seen (f(3,30) = 9.76, p < 0.001). The use of 10 N threshold produced the greatest time to take off (991.95 ± 192.12 ms) which was statistically higher than all other thresholds. A 5% threshold had the lowest time to take off (834.14 ± 90.50 ms) and was statistically lower than all other thresholds. Similarly, statistical differences were present in unweighting phase duration (f(3,30) = 8.97, p < 0.001). 10 N threshold produced the greatest unweighting phase duration (468.05 ± 165.42 ms) which was statistically higher than all other thresholds. 5% threshold had the lowest unweighting phase duration (313.86 ± 44.45 ms) which was statistically lower than all other thresholds. No statistical differences were present in braking duration (f(3,30) = 0.48, p = 0.70) and propulsive duration (f(3,30) = 1.37, p = 0.27). CONCLUSIONS: This investigation found supporting evidence that the use of low absolute threshold values produced the greatest durations, while the use of a relative threshold produced the shortest durations. Thus, researchers and practitioners should use caution selecting onset movement thresholds as this can greatly impact interpretation of temporal variables derived from data in the CMJ.

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