Impulse is the product of a force and the time during which the force is imparted. Impulse is also the product of an object's mass and a change in its velocity. Simply, it is equivalent to a change in momentum, elucidating the connection between impulse and the resulting acceleration experienced in sports (both speeding up and slowing down). A recent study examined the effect of different squat depths on relative net vertical impulse, jump height, peak force, and peak power, during the concentric phase of body weight jumping (1). Collegiate males, experienced in weight training and sports requiring jumping ability, performed countermovement jumps and static jumps (squat jumps). The authors determined squat depth as the absolute vertical distance between the standing position and the bottom of the squat, measured with a weightless bar held across the shoulders.
In both jumps, greater squat depth produced greater relative net vertical impulse (vertical impulse applied above body weight and expressed per kilogram of body mass), greater peak velocity, and greater jump height. Even self-selected depth did not generate higher values than the deepest setting (0.75 m). Correlations showed that relative net vertical impulse was a strong predictor of jump height in both types of jumps, regardless of squat depth.
The relationship between peak force and jump height was inverse and low, corroborating previous reports of peak force as unreliable in predicting jump performance (2,3). The relationship of peak power and jump height was inconsistent between the 2 types of jumps and not as clear. A salient point, though, was that peak power, which for the static jump was best at the self-selected depth, did not coincide with the highest jump heights.
All the findings of this study are within the context of having unlimited time per jump to produce the highest peak vertical velocity; the participants were not given any time restriction for each attempt at a maximal jump height. Under circumstances involving time constraints though, the athlete may be forced to self-select a shallower depth for the best possible jump height. Nonetheless, athletes and coaches should note the important contribution of squat depth to vertical jump height and reexamine their self-selected depth, especially if fairly shallow; it might not constitute the best approach to maximal vertical displacement. Practitioners who administer vertical jump tests on a force plate to evaluate training adaptations may want to allocate higher priority to vertical impulse measurement, considering it as a test variable under various loaded conditions.
1. Kirby TJ, McBride JM, Haines TL, Dayne AM. Relative net vertical impulse determines jumping performance. J Appl Biomech 27: 207–214, 2011.
2. McBride JM, McCaulley GO, Cormie P. Influence of preactivity and eccentric muscle activity on concentric performance during vertical jumping. J Strength Cond Res 22: 750–757, 2008.
3. Nuzzo JL, McBride JM, Cormie P, McCaulley GO. Relationship between countermovement jump and multijoint isometric and dynamic tests of strength. J Strength Cond Res 22: 699–707, 2008.