Ballistic skills assessment in semi-professional football players through inertial sensors: the effects of COVID-19 forced rest period

This study reports the instrumental analysis of ballistic skills in semi-professional football players in the month immediately after the COVID-19 lockdown. The quantitative assessment was based on wearable magneto-inertial measurement units (MIMUs). Eight male semi-professional football players participated in the study, which was characterized by a football-specific fatiguing protocol in an uncontrolled environment and a test on vertical jump (i.e., Bosco test). Six kinematic parameters were selected as suitable indicators for ballistic skill assessment (i.e., maximum height, mean exerted power, relative strength index, leg stiffness, contact time and flight time). The temporal analysis of their variations allowed to monitor the athletes' recovery. A significant effect of fatigue on measured variables was confirmed during the four weeks for three parameters in the Bosco test. Friedman's test confirmed significant differences across the four weeks for the aforementioned parameters (p < 0.05). From the Cliff's delta effect size, it can be confirmed that after four weeks the fatigue protocol effects were no longer detectable by our instrumental parameters, suggesting a full recovery of athletes' ballistic ability after the COVID-19 forced rest period. Indeed, the mean estimated effect size were 0.73 in the first week, 0.98 in the second one, 0.58 in third and 0.17 in the last one. Our solution is proposed as a reliable tool able to quantify the ballistic skill and its changes due to efforts in an ecological way, directly on the playground, thanks to the use of wearable sensors.