Abstract
The purpose of this study was to examine the combined effects of drop-height and surface
condition on drop jump (DJ) performance and knee joint kinematics. DJ performance,
sagittal and frontal plane knee joint kinematics were measured in jump experienced
young male and female adults during DJs on stable, unstable and highly unstable surfaces
using different drop-heights (20, 40, 60 cm). Findings revealed impaired DJ performance
(Δ5–16%; p<0.05; 1.43≤d≤2.82), reduced knee valgus motion (Δ33–52%; p<0.001; 2.70≤d≤3.59),
and larger maximum knee flexion angles (Δ13–19%; p<0.01; 1.74≤d≤1.75) when using higher
(60 cm) compared to lower drop-heights (≤40 cm). Further, lower knee flexion angles
and velocity were found (Δ8-16%; p<0.01; 1.49≤d≤2.38) with increasing surface instability.
When performing DJs from high (60 cm) compared to moderate drop-heights (40 cm) on
highly unstable surfaces, higher knee flexion velocity and maximum knee valgus angles
were found (Δ15–19%; p<0.01; 1.50≤d≤1.53). No significant main and/or interaction
effects were observed for the factor sex. In conclusion, knee motion strategies were
modified by the factors ‘drop-height’ and/or ‘surface instability’. The combination
of high drop-heights (>40 cm) together with highly unstable surfaces should be used
cautiously during plyometrics because this may increase the risk of injury due to
higher knee valgus stress.
Key words
drop jump - knee flexion angle - knee valgus angle - knee valgus motion
