Despite the implementation of prevention strategies, the rate of muscle injuries in professional football has not decreased.1,2 The prevalence in youth players is particularly alarming and represents a significant health challenge.3 Although a relationship between the head coach's leadership style and injury rates has been suggested,4 there is a lack of studies relating muscle injuries with training methodologies. This brief opinion paper aims to alert that strength training may promote sports injuries in some players, and suggest the use of intermuscular connectivity measures to test the risk of muscle injuries.

The increase of hamstring injuries during the last decades in elite clubs has been linked to the high number of competitions per season and the rigorous demands of modern football.1,5 It is a fact that successful teams participate in national and international cups, and their players frequently compete with their national teams. The interaction among competitive stress, extensive travel, and other related factors can lead to fatigue, coordination deterioration, and an elevated risk of injury.6-8 However, preventive strategies, including nutrition, physiotherapy, and other interventions, specifically designed to alleviate these effects, have been notably implemented in recent years.9-11 Furthermore, while the interactions of physiological, biomechanical, and psychosocial sports injury risk factors have been discussed, albeit partially addressed,8,12 the impact of physical conditioning and training methodologies on muscle injuries keeps relatively understudied.

Although training soccer methodologies have notably evolved in recent years toward viewing players/teams as complex adaptive systems, strength and conditioning methodologies still treat players as sophisticated machines.13,14 It is common to separate preventive and performance training using summative and commutative suppositions such as: “strength training combined with skill training results in performing skills with more strength”, “strength training before field training has a preventive effect”, “game actions can be trained in the gym through localized and unspecific exercises.

In recent years, professional football clubs have widely adopted strength training programs as a crucial component of the daily routines for footballers with the primary aim of preventing injuries. They are often based on localized and repetitive exercises which are specific to the performance tests but non-specific with respect to football actions. The potential influence of such methodologies relying on series and repetitions of the same exercises on the development of muscle synergies remains underexplored.15,16 Traditional, contrast models, ballistic, plyometric, and eccentric programs, known for their substantial strength gains measured through performance tests, are among the most common applied methods.17 They are typically conducted in dedicated gym facilities using weights, elastic bands, Olympic bars, ropes, medicine balls, etc. and before the field training sessions. It is worth to distinguish these routines focused on strength training from the widely used warming-up FIFA 11+ injury prevention program.9

The strength training programs mentioned above often lack robust scientific evidence, as it is the case of eccentric programs,18-20 or lack evidence-based practice.21 Moreover, existing evidences are frequently based on probabilistic results obtained through experimental designs of group data means comparisons which are non-adequate for personalized recommendations.22,23

Driven by prevailing social beliefs and values associated with body image, strength training practices have gained popularity, in particular, among youngsters. Muscular bodies, aligning with contemporary aesthetics and representing the archetype of a fit person, are exhibited by football stars and linked implicitly to success. From a complex system perspective, players, teams and competitions are nested structures related through circular causality.24,14 If the conditional profile of professional football players changes, the game style and the competition changes as well. In turn, the game style and the competition constraints further the players profile. In this way, a generalized strength-oriented practice may change also the football competition, keeping aside the players with difficulties of adaptation to strength training.

Relying on the theoretical principles of the Network Physiology of Exercise,25,26 the potential adverse effects of overloading footballers with strength training programs will be explained, and monitoring methods based on connectivity measures will be proposed to evaluate and track players progress.

Football actions emerge from the intricate interaction between multilevel physiological networks (e.g., molecules, cells, organs, limbs) and the environment.27 Most muscle injuries occur performing actions that have been repeated many times without harmful consequences. To elucidate the nonlinear transition from safe to injurious actions, Pol et al. (2019) have proposed the connectivity of microinjuries hypothesis.28 According to it, microinjuries, produced by any muscle action of significant intensity, can accumulate and interconnect over time. In general, if the rate of workloads overcome the time of tissue reparation,29 clusters of microinjuries may create a context for the sudden emergence of a macroinjury. This means that one more repetition of a safe action can produce, through a percolation mechanism, a nonlinear effect (e.g., muscle strain or rupture).28 As there is no universal threshold-safe number of repetitions or workload values, determining how much workload is too much is a persistently unsolved problem.30,31 Several interacting factors (e.g. sleep, nutrition, life style) may delay the tissue repair requirements and increase the susceptibility to injury.32 In this scenario, the widespread GPS-based technologies to quantify and track players workload may have a poor preventive power.

Qualitative changes in training routines, undetected by the tech-based monitoring systems, can also increase the susceptibility to injuries: the replacement of coaches, changes in training methodologies, shifts in tactical strategies or playing positions (e.g. when competing with clubs and national teams), resuming training after a resting period (e.g., preseason). Such changes are often overlooked by injury prevention strategies. However, the consideration of such coordinative stress factors becomes crucial in developing more comprehensive and effective injury prevention approaches.

The accumulation of microinjuries induce coordinative changes in motor actions, similar to those produced by fatigue.28 The explanation can be found in the loss of connectivity among muscle fibers and the instability of neuromuscular interactions.33,34 The impact of fatigue on the muscular connectivity has been tested through the study of intermuscular coordination during exercise.35,36 The authors have evaluated how distinct muscle fiber types dynamically interact with each other and integrate as a network while performing strength exercises (e.g. push-ups, squats). Specifically, they have measured intermuscular cross-frequency interactions among distinct muscles by quantifying the spectral components of different frequency bands embedded in surface electromyogram (sEMG) time series. Specific links strength among embedded networks have identified characteristic network profiles.

Fig. 1 shows the progressive loss of connectivity of muscle networks during three consecutive maximal squat tests.36 The number and strength of connections among the muscle fibers is poorer in the third exercise compared to the first. However, due to synergetic muscle networks reorganization participants are able to continue performing the test requirements. This means that performance tests are not informative enough about changes in muscle connectivity and may poorly predict injury risks. When muscle fibers of the exercising muscle network compensate the lack of function of their neighbors, they become more and more overloaded and prone to increase and connect microinjuries.28 When the number and/or strength of muscle fiber links decreases, the network becomes unfunctional,25 and the susceptibility of suffering muscle injuries may increase. The empirical results of Fig. 1 strongly point to connectivity measures, obtained from spectral analysis of EMG time series of muscle groups during exercise, as a promising internal workload measure for preventing muscle injuries.

Fig. 1.

Networks of interactions among muscle fibers of erector espinae (up) and vastus lateralis (down) during 3 consecutive bouts of squat exercises performed until exhaustion. The muscle network reorganizes with fatigue accumulation. The network maps represent the coupling strength between frequency bands (color nodes) corresponding to fast, intermediate and slow fibers within each muscle group. Links strength is shown by the width of the lines (adapted from García-Retortillo et al., 2022).35

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The loss of functional connectivity with fatigue has been also related to sedentarism and injuries.25 In fact, severe muscle injuries have been associated with a high risk of reinjury.37 Fatigue and injuries share similar dynamic laws and can be interpreted as adaptive protective mechanisms enforcing time for recovery.28 In a similar manner that a poor muscle connectivity may induce a nonlinear effect like task failure,33 it may induce a macroinjury.28

As noticed in early times, faster and high gains of strength were rapidly lost with detraining. In contrast, moderate gains obtained over longer periods of time were better retained.38 In general, individual fitness profiles, which stabilize over generations, tend to decline after a temporary cessation of exercise at a rate inversely proportional to the accumulated training time.39,40 This means that poorly stabilized strength muscle synergies may increase the susceptibility to injuries and vice versa. In this way, the adaptive response to strength training is highly individual, and depends on previous exposure to strength training and genetic factors. In fact, a high heterogeneity has been found in the adaptive response to strength training in older adults identifying adverse responders.41

Adaptation to exercise is a consequence of functionally integrated multilevel physiological networks interacting with external workloads. These multilevel and multi timescale co-adaptive physiological networks interact with each other through circular causality during exercise.40 Each level, acting on different characteristic timescale, parametrize the muscle synergies. The faster evolving processes (e.g., enzymatic or protein signaling network activity, neural excitability etc.) may have already stabilized at increased values while the more slowly evolving variables (e.g., tendon and bone remodeling, muscle hypertrophy, capillarization, etc.) may not have stabilized yet. The resulting incoherence between these processes may eventually produce a poor stability of muscle synergies and may increase the susceptibility to injuries.28

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  Update the reductionist assumptions of physical conditioning by considering players as complex adaptive systems also at physiological level.

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  Avoid the fragmentation of prevention and performance training.

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  Evaluate the individual characteristics of players before imposing strength training programs.

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  Eschew strength training programs before field training sessions to protect susceptible players.

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  Promote moderate and long-term adaptations to strength training.

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  Avoid the homogenization of players pushing a common conditional profile based on strength.

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  Pay attention to changes in coordinative requirements not reflected in the common tracking control of workloads.

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  Introduce connectivity measurements (e.g. intermuscular connectivity) to test the injury risks.

This work was supported by the National Institute of Physical Education of Catalonia (INEFC), Generalitat de Catalunya.