Article
Assessment of Biomechanical Response to Fatigue through
Wearable Sensors in Semi-Professional Football Referees
Luigi Truppa
1,2,
*, Michelangelo Guaitolini
1,2
, Pietro Garofalo
3
, Carlo Castagna
4,5
and Andrea Mannini
1,2,6
Citation: Truppa, L.; Guaitolini, M.;
Garofalo, P.; Castagna, C.; Mannini, A.
Assessment of Biomechanical Response
to Fatigue through Wearable Sensors
in Semi-Professional Football Referees.
Sensors 2021, 21, 66. https://dx.doi.
org/10.3390/s21010066
Received: 16 November 2020
Accepted: 21 December 2020
Published: 24 December 2020
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1
The BioRobotics Institute, Scuola Superiore Sant’Anna, 56127 Pisa, Italy;
michelangelo.guaitolini@santannapisa.it (M.G.); a.mannini@santannapisa.it (A.M.)
2
Department of Excellence in Robotics & AI, Scuola Superiore Sant’Anna, 56127 Pisa, Italy
3
TuringSense EU Lab s.r.l., 47121 Forlì, Italy; pietro.garofalo@turingsense.com
4
School of Sport and Exercise Sciences, Università di Tor Vergata, 00118 Rome, Italy; castagnac@libero.it
5
Italian Football Federation (FIGC) Technical Department, Football Training and Biomechanics Laboratory,
50135 Firenze, Italy
6
IRCCS Fondazione don Carlo Gnocchi, 50143 Firenze, Italy
* Correspondence: luigi.truppa@santannapisa.it
Abstract:
Quantifying muscle fatigue is a key aspect of everyday sport practice. A reliable and
objective solution that can fulfil this task would be deeply important for two main reasons: (i) it would
grant an objective indicator to adjust the daily training load for each player and (ii) it would provide
an innovative tool to reduce the risk of fatigue-related injuries. Available solutions for objectively
quantifying the fatigue level of fatigue can be invasive for the athlete; they could alter the performance
or they are not compatible with daily practice on the playground. Building on previous findings
that identified fatigue-related parameters in the kinematic of the counter-movement jump (CMJ),
this study evaluates the physical response to a fatigue protocol (i.e., Yo-Yo Intermittent Recovery
Test Level 1) in 16 football referees, by monitoring CMJ performance with wearable magneto-inertial
measurement units (MIMU). Nineteen kinematic parameters were selected as suitable indicators
for fatigue detection. The analysis of their variations allowed us to distinguish two opposites but
coherent responses to the fatigue protocol. Indeed, eight out of sixteen athletes showed reduced
performance (e.g., an effective fatigue condition), while the other eight athletes experienced an
improvement of the execution likely due to the so-called Post-Activation Potentiation. In both cases,
the above parameters were significantly influenced by the fatigue protocol (p < 0.05), confirming
their validity for fatigue monitoring. Interesting correlations between several kinematic parameters
and muscular mass were highlighted in the fatigued group. Finally, a “fatigue approximation index”
was proposed and validated as fatigue quantifier.
Keywords:
fatigue detection; counter-movement jump; wearable inertial sensors; football; biomechanics
1. Introduction
Muscle response after a preload stimulus is the result of its adaptability. Indeed, mus-
cles are characterized by an intrinsic plasticity that allows them to conveniently calibrate
their force, endurance and contractile speed according to the effort they are subjected to.
This phenomenon is particularly evident in sport, where the specific adaptation of the
muscle tissue after consecutive training sessions allows the athlete’s performance to be
maximized [
1
] (i.e., improved jumps [
2
]). In addition to a long-term plasticity, the muscles
are able to perform a temporary adaptation during and after endurance exercises and
activities involving high speed and power (i.e., a football match) by recruiting an increased
number of higher order motor units [
3
]. This process is called Post-Activation Potentiation
(PAP), and it generally causes an increase in the low-frequency force exerted by the athlete
during an endurance exercise, but it can also increase the force at higher frequencies in
concentric versus isometric contractions (e.g., jumping, swimming and cycling) [
4
]. On the
Sensors 2021, 21, 66. https://dx.doi.org/10.3390/s21010066 https://www.mdpi.com/journal/sensors
