The Influence of Countermovement Strategy and External Load on Amortization Forces During Jump Squats

Barker, L, Siedlik, J, and Mercer, J. The influence of countermovement strategy and external load on amortization forces during jump squats. J Strength Cond Res 35(2): 332-339, 2021-The purpose of this study was to investigate the influence of countermovement strategy on amortization forces when performing jump squats. To gather a range of deceleration demands, jump squats were performed across a range of external loads (0, 15, 30, 45, and 60% of repetition maximum [1RM]) and verbally cued countermovement depths (preferred: PREF; quarter: QTR; and full: FULL). Dual force platform ground reaction force data were collected from 12 subjects. Amortization Fz, amortization time, and countermovement depth were analyzed using a 3 × 5 (technique × load) repeated measures analysis of variance. To assess countermovement strategy and amortization Fz, correlations and regression analysis were used to assess relationships among system mass, countermovement kinetic energy and depth, and amortization Fz. Amortization Fz was greater with QTR than PREF (p < 0.05), but similar between PREF and FULL (p > 0.05), and between QTR and FULL (p > 0.05). Amortization Fz was different between 0 and 15% 1RM (p < 0.05) and between 0 and 30% 1RM (p < 0.05). All loaded conditions (15-60% 1-RM) were similar (p > 0.05). As a group, countermovement kinetic energy and system mass were significantly, but weakly, correlated (r = 0.29, p < 0.01). Regression analysis revealed amortization Fz did not change in relation to countermovement kinetic energy but did change in relation to depth. These results suggest that preferred countermovement depths were associated with lesser amortization Fz (strain energy) during maximal effort unloaded and loaded countermovement jumps. Strength and conditioning professionals selecting exercises to maximize amortization Fz should consider the jump squat with light loads (or no load) and short countermovement depths.