(35) THE IMPACT OF LOAD ON THE FORCE-TIME CHARACTERISTICS OF THE SQUAT JUMP IN RESISTANCE-TRAINED MEN

Purpose: To examine the impact that load has on the force-time characteristics of the static squat jump (SJ).

Methods: 20 resistance-trained men (age = 23.5 ± 3.0 years, height = 175.1 ± 8.7 cm, body mass 79.6 ± 11.8 kg, relative one repetition maximum (1RM) back squat = 1.90 ± 0.28 kg/kg) participated in two separate testing sessions. The first session was used to assess the relative strength of the subjects during a 1RM back squat test and to familiarize them with the SJ exercise. The second session required the subjects to perform unloaded SJ repetitions as well as repetitions with loads corresponding to 20, 40, 60, 80, and 100% of their body mass (BM). For each SJ repetition, subjects squatted under the barbell to assume a knee angle of approximately 90° ± 5° and held a quiet standing period of at least one second before receiving a countdown prior to each jump trial. The added load rested on boxes that were adjusted to each subject’s height. Two jumps were performed at each load on a force platform and the force-time data were used to determine propulsion duration net relative mean force (NetRelMF), duration (Dur), and net impulse (NetIMP). The average performance between repetitions at each load were used for statistical comparison. Each variable was compared between the SJ loads using a series of one-way repeated measures ANOVA with Bonferroni post hoc tests. In addition, Hedge’s g effect sizes were calculated to describe the magnitude of the differences between loads.

Results: There were significant differences in SJ propulsion NetRelMF, Dur, and NetIMP (all p< 0.001). Descriptive data and post hoc comparisons are shown in Table 1. The greatest NetRelMF was produced at 40% BM and trivial-moderate (g=0.01-1.00) differences existed between loads. The shortest propulsion Dur occurred at the lightest loads (0-40% BM) and there were small-very large (g=0.23-2.28) differences across the loading spectrum. Finally, the greatest NetIMP occurred at 100% BM and trivial-large (g=0.13-1.77) differences existed between loads.

Conclusions: The external load used during SJ significantly alters NetRelMF, Dur, and NetIMP. Greater NetRelMF magnitudes were produced during the SJ with light-moderate loads (20-60% BM) while shorter propulsion Dur and larger NetIMP magnitudes were produced using lighter and heavier loads, respectively. PRACTICAL APPLICATIONS: The Dur is specific to the load being moved, but when training for faster propulsive actions, 20 and 40% BM can provide similar results to 0% BM. Training with 0% BM can produce similar NetRelMF to 80 and 100% BM loads, so 0% BM may be used when the goal of training is to reduce stress on the body or improve SJ Dur. However, if the goal is to maximize NetRelMF and NetIMP, heavier loads during the SJ should be prescribed. Practitioners should recognize however that individuals may respond differently to the prescribed BM loads as their relative strength may have a significant impact on their performance.

Acknowledgements: None