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Jumping performance has traditionally been measured by jump height alone. In recent years, the reactive strength index (RSI = Jump height / jump time)) has been used as another measure of jump performance. According to RSI, which was developed to assess eccentric force production, jump performance can improve by increasing jump height, decreasing jump time, or both simultaneously. However, it is not clear how force production correlates to RSI variables. If RSI is meant to be a practical measure of eccentric force production, it should correlate strongly to eccentric and amortization force production during jumping. Thus, the purpose of the first study was to determine the relationship between ground reaction force (GRF) variables to jump height, jump time, and the Reactive Strength Index (RSI). Twenty-six Division I male soccer players performed three maximum effort CMJs on a dual-force platform system that measured three-dimensional kinetic data. Vertical GRF (Fz) variables were divided into unloading, eccentric, amortization, and concentric phases and correlated to jump height, RSI (RSI= Jump height/jump time), and jump time (ground contact time from start to takeoff). Significant correlations were observed between jump height and RSI, concentric kinetic energy, peak power, concentric work, and concentric displacement. Significant correlations were observed between RSI and jump time, peak power, unload Fz, eccentric work, eccentric rate of force development (RFD), amortization Fz, amortization time, 2nd Fz peak, average concentric Fz, and concentric displacement. Significant correlations were observed between jump time and unload Fz, eccentric work, eccentric RFD, amortization Fz, amortization time, average concentric Fz, and concentric work. In conclusion, jump height correlated to variables derived from the concentric phase only, while Fz variables from the unloading, eccentric, amortization, and concentric phases correlated highly to RSI and jump time. These observations demonstrate the importance of countermovement Fz characteristics for time-sensitive CMJ performance measures. Further, RSI correlated strongly to Fz variables during eccentric and amortization phases. Researchers and practitioners should include RSI to improve their assessment of jump performance. The first study observed a strong relationship between jump performance and force production during the eccentric and amortization phases. But, there is limited research on force production during eccentric and amortization phases of the jump squat (JS), which is a countermovement jump performed with external load via barbell. Further, limited research has investigated the influence of countermovement technique on these variables. Therefore, the second and third studies investigated the effect of load and countermovement technique on kinetics during the eccentric and amortization phases of the jump squat. The second and third studies used the same protocol: On day one, participants performed a 3-repetition maximum (RM) back squat. On day two, participants performed JS with 0%, 15%, 30%, 45%, and 60% of estimated 1-RM using three countermovement techniques: preferred (PREF), quarter (QTR), and full (FULL) depths. Participants wore flat athletic shoes, and were outfitted with reflective markers on the lower extremity to collect 3D kinematics. JS were performed on dual force platforms synchronized with the 3D data. The purpose of the second study was to compare vertical ground reaction forces (Fz) from the eccentric and amortization phases of the JS across loads and countermovement techniques. A convenience sample of 12 healthy, resistance-trained men (24.8 ± 4.04 yrs, 86.71 ± 15.59 kg, 1.78 ± 0.79 m, 3-RM Back Squat: 123.2 ± 23.79 kg) were recruited from the university kinesiology department. Dependent variables included: (1) eccentric rate of force development (RFD1 and RFD2); (2) first Fz peak (Fz1); (3) amortization Fz and time; (4) jump height; (5) RSI; (6) peak and average concentric power; (7) and countermovement depth. Eccentric RFD1 did not change with increasing loads (p>0.05), but eccentric RFD2 decreased with increasing loads (p0.05), but was greater with load (15%-60% of 1-RM) than without (0% of 1-RM). Jump height and RSI declined with increasing loads (p0.05). Performance variables followed similar results of the second study. The QTR JS elicited greater RSI, peak and average concentric power, and less countermovement kinetic energy. Countermovement kinetic energy peaked using 15% of 1-RM with a FULL JS, indicating that increasing loads does not ensure an increase in downward kinetic energy despite verbal instruction to lower the weight as quickly as possible. In conclusion, the eccentric and amortization phases may have been previously undervalued for jump performance because they do not correlate to jump height. However, RSI has a strong relationship with eccentric and amortization force production, as intended. The presented studies further our understanding of force production in these phases when load and countermovement depth changes. The QTR JS elicits greater power output and RSI values, and is knee dominant during deceleration. The FULL JS elicits peak deceleration demands using 15% and 30% of 1-RM, accompanied by increasing contributions from the hip. It appears advisable to view the QTR and FULL JS as separate exercises with complementary stresses that could be combined in a comprehensive jump training program. Further, because the highest load did not result in greater deceleration demands (i.e. peak countermovement kinetic energy), coaches should consider defining, monitoring, and cueing specific countermovement strategies when organizing training programs. |
