Influence of Dynamic Strength Index on Countermovement Jump Force-, Power-, Velocity-, and Displacement-Time Curves.

John J Mcmahon,Paul A Jones,Thomas Dos'Santos,Paul Comfort

doi:10.3390/sports5040072

Abstract

The dynamic strength index (DSI), often calculated as the ratio of countermovement jump (CMJ) propulsion peak force to isometric mid-thigh pull (IMTP) peak force, is said to inform whether ballistic or maximal strength training is warranted for a given athlete. CMJ propulsion peak force is highly influenced by jump strategy, however, which is not highlighted by the DSI alone. This study aimed to quantitatively compare CMJ force-, power-, velocity-, and displacement-time curves between athletes who achieved high versus low DSI scores. Fifty-three male collegiate athletes performed three CMJs and IMTPs on a force platform. Athletes were ranked based on DSI score and the CMJ kinetic and kinematic-time curves of the bottom and top twenty athletes were compared. The low DSI group (0.55 ± 0.10 vs. 0.92 ± 0.11) produced greater IMTP peak force (46.7 ± 15.0 vs. 31.1 ± 6.6 N·kg−1) but a larger braking net impulse in the CMJ, leading to greater braking velocity and larger countermovement displacement. This strategy resulted in a similar CMJ propulsion peak force (25.9 ± 2.2 vs. 25.4 ± 3.1 N·kg−1) to the high DSI group. These results, taken together with those of previous studies, support the notion of ballistic versus maximal strength training likely being better suited to low versus high DSI scorers, respectively.

Highlights

To provide insight into an athlete’s training status, and inform future training focus, the ratio of propulsion peak force produced during ballistic vertical jumping (either the squat jump (SJ)or countermovement jump (CMJ)) to isometric peak force produced during the isometric mid-thigh pull (IMTP) has been recommended in the literature [1,2,3,4,5]
As isometric peak force capacity is higher than propulsion peak force capacity, a dynamic strength index (DSI) of ≤0.60 is suggested to indicate that ballistic training is warranted as only 60% of the athlete’s maximal isometric force capacity is being utilized during a ballistic jump, whereas a ratio of ≥0.80 indicates that maximal strength training is warranted as the athlete is utilizing
The low DSI group were stronger but performed the CMJ with a braking phase characterized by a larger net impulse, leading to greater braking velocity and countermovement displacement

Summary

Introduction

To provide insight into an athlete’s training status, and inform future training focus, the ratio of propulsion peak force produced during ballistic vertical jumping (either the squat jump (SJ)or countermovement jump (CMJ)) to isometric peak force produced during the isometric mid-thigh pull (IMTP) has been recommended in the literature [1,2,3,4,5]. To provide insight into an athlete’s training status, and inform future training focus, the ratio of propulsion peak force produced during ballistic vertical jumping (either the squat jump (SJ). Relative isometric force capacity (i.e., relative strength) must be considered alongside DSI values to better inform future training priorities for a given athlete, as one could produce a DSI of ≤0.60 and be very weak, maximal strength rather than ballistic (or concurrent) training may be more suitable [7]. Adopting a compliant leg strategy in the CMJ by increasing countermovement displacement or starting the SJ from a deeper squat position acts to decrease propulsion peak force, but can increase jump height through the application of a net impulse that is characterized by a longer time of force application [9,10]. A preferred CMJ strategy was recently shown to yield better DSI values compared to those attained for the SJ [5], illustrating that athletes tend to demonstrate a consistent CMJ technique

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