Swimming Phase-Based Performance Evaluation Using a Single IMU in Main Swimming Techniques

Abstract

Comprehensive monitoring of performance is essential for swimmers and swimming coaches to optimize the training. Regardless of the swimming technique, the swimmer passes various swimming phases from wall to wall, including a dive into the water or wall push-off, then glide and strokes preparation and finally, swimming up to the turn. The coach focuses on improving the performance of the swimmer in each of these phases. The purpose of this study was to assess the potential of using a sacrum-worn inertial measurement unit (IMU) for performance evaluation in each swimming phase (wall push-off, glide, stroke preparation and swimming) of elite swimmers in four main swimming techniques (i.e. front crawl, breaststroke, butterfly and backstroke). Nineteen swimmers were asked to wear a sacrum IMU and swim four one-way 25 m trials in each technique, attached to a tethered speedometer and filmed by cameras in the whole lap as reference systems. Based on the literature, several goal metrics were extracted from the instantaneous velocity (e.g. average velocity per stroke cycle) and displacement (e.g. time to reach 15 m from the wall) data from a tethered speedometer for the swimming phases, each one representing the goodness of swimmer’s performance. Following a novel approach, that starts from swimming bout detection and continues until detecting the swimming phases, the IMU kinematic variables in each swimming phase were extracted. The highly associated variables with the corresponding goal metrics were detected by LASSO (least absolute shrinkage and selection operator) variable selection and used for estimating the goal metrics with a linear regression model. The selected kinematic variables were relevant to the motion characteristics of each phase (e.g. selection of propulsion-related variables in wall push-off phase), providing more interpretability to the model. The estimation reached a determination coefficient (R2) value more than 0.75 and a relative RMSE less than 10% for most goal metrics in all swimming techniques. The results show that a single sacrum IMU can provide a wide range of performance-related swimming kinematic variables, useful for performance evaluation in four main swimming techniques.

Monitoring weekly progress of front crawl swimmers using IMU-based performance evaluation goal metrics

“…Histograms of the changes in the goal metrics are displayed in Figure 3 . The root mean squared error ( RMSE ) for the estimation of each goal metric is extracted from our previous study ( Hamidi Rad et al, 2021a ) and shown specifically for each goal metric in vertical red lines in Figure 3 . The delta values lying inside the range of RMSE (± RMSE range) are too small to be valid as they might happen due the model errors and should be removed.…”

“…For technical reasons, only front crawl technique is examined here. However, based on our previous research ( Hamidi Rad et al, 2021a ), similar goal metrics can be extracted from other main swimming techniques (backstroke, butterfly, and breaststroke) to perform the same study. The lap time was recorded using stopwatch which is prone to human error and using more precise measurement methods such as cameras can increase the quality of this analysis.…”

“…The method uses motion biomechanics to identify the events corresponding to the beginning and end of each phase for lap segmentation. Subsequently, based on our phase-based performance evaluation method ( Hamidi Rad et al, 2021a ), a set of spatio-temporal parameters reflecting various aspects of swimmer’s performance were extracted from each phase. These parameters are categorized as propulsion, posture, efficiency and duration/rate to represent the most important aspects of performance.…”

“…The algorithms developed in this study take a macro-micro approach by swimming bouts detection, lap separation, and swimming style identification at the macro level, and then divide each lap into phases by detecting spatio-temporal events on IMU acceleration and angular velocity data at the micro level. Subsequently, a variety of kinematic parameters were extracted from each phase and used to estimate phase-based goal metrics ( Push maximum velocity, Glid end velocity, StPr average velocity, Swim average velocity and lap average velocity) for the swimmer’s performance evaluation ( Hamidi Rad et al, 2021a ), indicating how well the swimmer performed the corresponding phase. However, to fully utilize the IMU sensor for training, assessing the sensitivity of IMU-based goal metrics to performance progress is of utmost importance.…”

Prediction of Kick Count in Triathletes during Freestyle Swimming Session Using Inertial Sensor Technology

“…Since then, several studies reported in the literature have supported coaches in comprehensive monitoring of the performance of their athletes to develop and refine a training model. For example, in [15] numerous kinematic variables related to propulsion, posture, efficiency, and duration/rate of motion in four main swimming phases (wall push-off, glide, stroke preparation, and swimming) were extracted from an IMU attached to the sacrum. An IMU worn on the wrist or the upper-back was used to discriminate among the different swimming styles in [16,17], whereas in [18] the type of stroke and wall push-offs were detected by deriving pitch and roll angles from an accelerometer mounted on the swimmer's goggles.…”