Drop vertical jumps (DVJ) are often used during training to improve vertical jump performance. Reactive strength index (RSI) may be measured during DVJ to monitor changes in both the acute and/or chronic training status of individuals for whom vertical jump ability is critical for success. RSI is calculated by dividing jump displacement by the elapsed ground contact time (CT) during the rebound phase of the DVJ. Within limits, DVJ displacement appears to be negatively correlated with CT and positively correlated with RSI. However, limited data are available concerning the utility of RSI and CT. PURPOSE: To determine the stability reliability and precision of DVJ displacement and both RSI and CT during DVJ's, and to ascertain their inter-associations. METHODS: Fifty-two subjects (27 women and 25 men) with squat training experience were recruited from a university population. All were able to perform a back squat to 90[degrees] of knee flexion with an external load >= body weight. One week subsequent to two practice sessions, subjects performed duplicate DVJ's from 40 cm while holding a 1-in diameter PVC pipe across their shoulders. A second testing session was performed 48 hrs later. CT data were collected using a uniaxial force plate channeled through a signal conditioner/amplifier interfaced to a PC via a 12-bit analog-digital converter, and sampled at 500 Hz. Datapac 5(TM) was utilized for data extraction. Data were low-pass filtered (4th order, zero-lag Butterworth) with a cutoff frequency of 30 Hz. DVJ displacement was calculated by flight time. Stability reliability was assessed using intraclass correlation (ICC), precision by coefficient of variation (CV), and association via bivariate correlation (r). RESULTS: Stability reliability and precision were acceptable for reactive strength index (ICC = 0.94; CV = 11.3%), ground contact time (ICC = 0.83; CV = 8.2%), and drop vertical jump displacement (ICC = 0.94; CV = 6.1%). Therefore, inter-associations were determined separately for each session. For DVJ displacement, correlations with RSI were moderate to high (men, rs1 = 0.66, rs2 = 0.68; women, rs1 = 0.86, rs2 = 0.81), but relatively low with CT (men, rs1 = -0.21, rs2 = -0.23; women, rs1 = -0.33, rs2 = -0.26). The correlation of RSI with CT was relatively high (men, rs1 = -0.84, rs2 = -0.85; women, rs1 = -0.75, rs2 = -0.76). CONCLUSIONS: DVJ displacement and both RSI and CT during DVJ from 40 cm were reliable and precise for young men and women with a squat training background. DVJ displacement was strongly associated with RSI, more so in women than men, but was not associated with CT (low inverse association). RSI was inversely associated with CT. PRACTICAL APPLICATIONS: It appears that RSI accounts for a substantial portion of the variability in DVJ displacement while CT does not. However, since RSI is calculated by dividing DVJ displacement by CT, it also appears that RSI is not independent of DVJ displacement; and therefore, has negligible utility for explaining 40 cm drop jump performance.
(C) 2011 National Strength and Conditioning Association