A Wearable System for Jump Detection in Inline Figure Skating

Abstract

This article presents the design and experimental evaluation of a non-invasive wearable sensor system that can be used to acquire crucial information about athletes' performance during inline figure skating training. By combining distance and time-of-flight sensors and gyroscopes, the system is able to detect when jumps are performed and provides a live view of the data (e.g., the number and height of jumps) through a graphical user interface. The main novelty of our approach lies in the way in which the optical sensors are orientated. Typically, the sensors are orientated horizontally and positioned in pairs on the ground, where they measure the time interval between the moment the athlete leaves the ground and the moment they land. In our system, an optical sensor is placed under each foot and is vertically orientated so as to constantly measure the distance from the ground. In addition, a gyroscope sensor is placed on the athlete's back, which provides information on the direction and angular momentum of the movement. By combining this data, the system provides the accurate detection of various jumps and technical elements without any constraints on the training ground. In this paper, the system is also compared to similar platforms in the literature, although there are no other specific systems that are available for inline figure skating. The results of the experimental evaluation, which was performed by high profile athletes, confirm its effectiveness in correctly detecting jumps, especially considering its compromise between precision and the overall cost of the equipment.

Keywords: human movement analysis; inline figure skating; sports biomechanics; wearable sensors.

Figure 1

An overview of the system’s software and hardware components.

Figure 2

The positioning of the VL53L0X sensor on the boot.

Figure 3

The positioning of the control unit on the athlete.

Figure 4

The web-based GUI. The graph shows jump heights sampled in real time by the sensors.

Figure 5

A box plot of the deviations between the sensor data and the belt reference measurement for each athlete. Empty dots represent the mean and black segments are the median. Despite the different physical characteristics of the athletes, the deviations were quite stable, with an overall mean of around 1.61 cm and a relatively low variance.

Figure 6

Double Salchow. Max height detected: 28.8 cm.

Figure 7

Flying camel spin. Max height detected: 23.6 cm. The element was discarded by the algorithm as it detected a fast spin after landing.

Figure 8

A sample calculation of vertical height $\overline{OB}$ given the tilt angle $\widehat{AOB}$ and the distance from the ground $\overline{OA}$.

Figure 9

The statistics box plot of the 40 tilt angles measured from the video frames. The empty dot represents the mean and the black segment represents the median.

Figure 10

Inferential statistics on the population of tilt angles: the estimated density function in the range 0–90 degrees (left) and the cumulative distribution function (right).

Figure 11

A comparison between a double Lutz (upper chart) and a flying sit spin with foot change (lower chart). The gyroscope hard limit was reached while jumping (the cyan line in the B area above) rather than after landing (the cyan line in the D area below).

Figure 12

The green and blue lines relate to the left and right foot, respectively. The red dots highlight when both feet surpassed the threshold, with three red dots indicating that a valid jump was detected. This chart shows three successful detections of three technical elements (double Lutz).

Figure 13

A zoom-in of the third (rightmost) double Lutz of the series of three in Figure 12. After the flying phase, marked by the three red dots and regularly detected as a valid jump, the left foot went parallel to the ground, hence the high green peak reaching the hard limit of the laser sensor, whereas the right foot (blue line) remained on the ground. This scenario corresponded to the exit phase of the element and was not detected as a jump.