(21) COMPARISON OF SIX DIFFERENT TAKE-OFF THRESHOLDS WHEN ASSESSING COUNTERMOVEMENT VERTICAL JUMP PERFORMANCE

Being able to reliably identify take-off from a force-time (F-T) curve to assess vertical jump (VJ) metrics is essential when analyzing VJs performed on a force plate.

Purpose: The purpose of this study was to compare six take-off thresholds when assessing jump height (JH), movement time (MT), reactive strength index modified (RSImod), net impulse (netIMP), and propulsive impulse (prIMP)) from VJs.

Methods: College-aged participants (N=21; 9 female, age, 21.4+1.2 yrs; height, 165.4+4.8 cm; body mass, 68.3+19.5 kg; 12 male, age, 22.5+3.3 yrs, height, 179.1+5.5 cm, body mass, 84.7+10.9 kg) performed five VJs on a force plate. Take-off was determined using following methods: when force fell below 50 N, 20 N, 10 N, 5 N, < 1 N, and within five standard deviations of a 300 ms window during the flight phase (5SD). Absolute and relative reliability was determined using coefficient of variation (CV) and intraclass correlation coefficients (ICC), respectively. Separate repeated measures ANOVAs (RMANOVA) were used to assess differences among the thresholds in terms of JH, MT, RSImod, netIMP, and prIMP. Pairwise comparisons using Bonferroni correction was used when the RMANOVA demonstrated statistical significance. Hedge’s g was used to determine effect sizes.

Results: All thresholds demonstrated acceptable absolute reliability (CV=2.3–9.7%) and good-to-excellent relative reliability (ICC=0.835–0.993). RMANOVA results were statistically significant for each metric (p< 0.05). Table 1 shows post hoc results for each metric. Effect sizes were classified as trivial (0–0.2) for statistically significant comparisons.

Conclusions: Each take-off method demonstrated acceptable reliability. While there were several statistically significant differences among these thresholds, these differences may not be practically meaningful. PRACTICAL APPLICATION: These data suggest that any of these take-off thresholds can be used reliably to assess JH, MT, RSImod, netIMP, and prIMP. When deciding upon method, the practitioner should consider the noise contained within the force signal while selecting the threshold that preserves more of the F-T curve.

 

Acknowledgements: None