Measuring jumping performance in a controlled setting on sand and rigid surface with two different devices

Hendker, A., Szwajca, S., Eils, E.

Abstract

INTRODUCTION: In the last decade, beach sports such as -volleyball, -handball, and -soccer have become increasingly popular which demand specific attributes for the athletes. Sport-specific diagnostic, e.g. jumping performance on sand surfaces need valid measurements to evaluate training interventions. Jump performance diagnostics mainly takes place under controlled conditions e.g. in a lab on rigid surfaces questioning ecologic validity in sand sports. It is necessary to facilitate the “in field” diagnostic to make results feasible for athletes and the trainer. Therefore, the aim of the present investigation was to measure jumping performance in a controlled setting on sand and rigid surfaces with two different devices. One reliably method was set to as the gold standard (3D motion analysis with markers) and compared with a system that is easy to use in the field. METHODS: A sandbox (1.25x1.25mx30cm) was constructed to measure counter-movement (CMJ) and drop- jumps (DJ) on a sand surface in the lab. Seventeen healthy novices (28.4±4.7 years, 181±8.4cm, and 76±11.2kg) perform CMJ and DJ on sand and rigid surface in a balanced order. Jumping height was measured using a 3D motion capture system (Q) (Qualisys, Göteborg, Sweden) and compared with an accelerometer-based system (H) (Humotion SmarTracks Diagnostics, Muenster, Germany). Validity of the in-field system was calculated using differences of the measured jumping performance compared to Q. Furthermore, 95% confidence intervals (CI) and Bland-Altman-Plots were used to assess validity. RESULTS: Mean vertical jump results were 37.8±6.7cm (95%CI: 34.6-40.9) (Q) and 35.8±6.9cm (32.5-39.1) (H) for CMJ on sand, 39.4±6.9cm (36.1-42.7) (Q) and 38±8.2cm (34.1-41.9) (H) for CMJ on rigid surface, 28.4±8.2cm (24.5-32.3) (Q) and 27.5±7.6cm (23.9-31.1) (H) for DJ on sand, 31.3±7.1cm (28.0-34.7) (Q) and 30.4±7.6cm (26.8-34.1) (H) for DJ on rigid surface. Mean differences between Q and H were 1.9±2.4cm for CMJ on sand, 1.4±2.8cm for CMJ on floor, 0.9±2.5cm for DJ on sand, 0.9±1.8cm for DJ on floor, respectively. CONCLUSION: Although mean values and differences appear to be comparable with a systematic error between systems, high standard deviations and large confidence intervals suggest that individual results deviate greatly from the mean (Humotion does not constantly underestimate the gold standard). Bland-Altman-plot evaluation strongly suggests that results spread in an unsystematic manner out of the range of confidence intervals on either side. Single deviations up to 9.3cm were found between systems, and 15% of all jumps had to be repeated due to wrong automatic or non-detection of jumps via H especially on sand surfaces. Therefore, the validity of the accelerometer based system for the assessment of vertical jump performance is questionable on rigid as well as on sand surfaces. This is of particular importance when focusing on elite athletes ́ performance.