# Association of multi-phase rates of force development during an isometric leg press with vertical jump performances

## TL;DR

This study shows how the speed of force development during a leg press relates to jump performance in athletes, highlighting the importance of early-phase force production.

## Contribution

The study introduces a novel method to assess explosive leg strength using multi-phase force development during an isometric leg press.

## Key findings

- Early-phase IRFDs (onset to 150 ms) correlate with jump height and reactive strength index.
- Peak force-independent IRFDs during onset to 100 ms are strongly linked to rebound jump performance.
- The earliest IRFD (onset to 50 ms) is uniquely associated with shorter contact times during jumps.

## Abstract

This study aimed to elucidate characteristics of explosive force-production capabilities represented by multi-phase rate of force developments (IRFDs) during isometric single-leg press (ISLP) through investigating relationships with countermovement (CMJ) and rebound continuous jump (RJ) performances.

Two-hundred-and-thirty male athletes performed ISLP, CMJ with an arm swing (CMJAS), and RJ with an arm swing (RJAS). IRFDs were measured during ISLP using a custom-built dynamometer, while CMJAS and RJAS were measured on force platforms. The IRFDs were obtained as rates of increase in force across 50 ms in the interval from the onset to 250 ms. Jump height (JH) was obtained from CMJAS, while RJAS provided JH, contact time (CT), and reactive strength index (RSI) values.

All IRFDs were correlated with CMJAS-JH (ρ = 0.20–0.45, p ≤ 0.003), RJAS-JH (ρ = 0.22–0.46, p ≤ 0.001), RJAS-RSI (ρ = 0.29–0.48, p < 0.001) and RJAS-CT (ρ = −0.29 to −0.25, p ≤ 0.025). When an influence of peak force was considered using partial rank correlation analysis, IRFDs during onset to 150 ms were correlated with CMJAS-JH (ρxy/z = 0.19–0.36, p ≤ 0.004), IRFDs during onset to 100 ms were correlated with RJAS-JH and RJAS-RSI (ρxy/z = 0.33–0.36, p < 0.001), and IRFD during onset to 50 ms was only correlated with RJAS-CT (ρxy/z = −0.23, p < 0.001).

The early phase (onset to 150 ms) IRFDs measured using ISLP enabled the assessment of multiple aspects of leg-extension strength characteristics that differ from maximal strength; these insights might be useful in the assessment of the athletes’ leg-extension strength capabilities.