JMIR Publications

ABSTRACT

Background:

Vertical jump height is widely used in healthcare and sports fields to assess muscle strength and power from lower limbs muscular groups. Different approaches have been proposed for vertical jump height-measurement. Some commonly used approaches need no sensor at all, however, these methods tend to overestimate the height reached by the subjects. There are also novel systems using different kind of sensors like force sensitive resistors, capacitive sensors, inertial measurement units, among others, to achieve more accurate measurements.

Objective:

The objective of this study is twofold. The first objective is to validate the functioning of a developed low-cost system able to measure vertical jump height. The second objective is to assess the effects on obtained measurements when the sampling frequency of the system is modified.

Methods:

The system developed in this study consists of a matrix of force sensitive resistor (FSR) sensors embedded on a mat, and the electronics that allows a full scan of the mat. This mat detects pressure exerted on it. The system calculates the jump-height by using the flight-time (FT) formula, and the result is sent through Bluetooth to any mobile device or a PC. Two different experiments were performed. In the first experiment a total of 38 volunteers participated, with the objective of validating the performance of the system against a high-speed camera used as reference (120fps). In a second experiment a total of 15 volunteers participated. In this experiment raw data is obtained in order to assess the effects of different sampling frequencies on the performance of the system with the same reference device. Different sampling frequencies were obtained by performing offline-downsampling of raw data. In both experiments CMJ and CMJAS techniques were performed.

Results:

In the first experiment an overall mean relative error (MRE) of 1.98% and a mean absolute error (MAE) of 0.38cm were obtained. Bland-Altman and correlation analysis were also performed, obtaining a coefficient of determination equal to R^2=0.996. On the second experiment sampling frequencies of 200Hz, 100Hz, and 66.6Hz show similar performance with MRE below 3%. Slower sampling frequencies show an exponential increase in MRE. On both experiments, when dividing jump trials in different heights-reached, a decrease in MRE with higher-heights trials suggests that the precision of the proposed system increases as height-reached increases.

Conclusions:

From first experiment it is concluded that the results between the proposed system and the reference are systematically the same. From second experiment it is appreciated the relevance of a high-enough sampling frequency, especially for jump trials whose height is below 10cm. For trials with heights above 30cm, MRE decreases in general for all sampling frequencies, suggesting that at higher heights-reached the impact of high sampling frequencies is lesser.