Several research has been performed about fatigue in football (4,9,15,20,25) and particular attention should be paid to its association with injury risk. Does fatigue only influences soft tissue injury or has repercussions in other injury types? In this FootballMedicine article we reply to some of the questions that we believe are frequently brought up to discussion when fatigue is the main topic. We discuss some of the issues around fatigue in football, such as its association with different injury types (and not exclusively soft tissue ones). In this context we also discuss how the adaptation to training and playing loads imposed to players plays a fundamental role in fatigue occurrence in game days, ultimately leading to higher injury risk. Together with this also some of the current ways of monitoring fatigue and how it should affect training and rehabilitation periodization strategies are also mentioned.
What is fatigue?
Fatigue has been defined in numerous ways. In football it can be defined as a state that enables the performance of a given task due to the inability of our body to recruit its muscles and move the different segments in an efficient way. It is a sensation with a detrimental impact at a physiological, functional and perceptual level, which can last several days after exercise (20). One of levels at which fatigue occur after a significant physical stimulus is the peripheral one in which the skeletal muscles will be under the influence of local inflammatory phenomenon and a short term muscle damage (7,14). The other form includes a central level fatigue related to the decreased ability of the Central Nervous System to activate the neural pathway for the muscle contraction to occur (17).
Does fatigue influences injury risk?
Yes. The fact that most of injuries occur in the latter stages of games halves (6) has been associated with decrements in functional markers such as strength (12,21,24) or joint position sense (22), measured at half time and after 90 minutes of real match play or simulation protocols. Additionally to isolated variables quantification such as strength or joint positions sense, authors like Barrett et al. (2016) using a soccer specific simulation protocol have shown a loss of movement efficiency that occurs throughout a football match. In their study there was an increase of the ratio between the accelerometer-derived variable PlayerLoad™ and the total distance covered in the latter stages of each half. This suggests a loss of efficiency in the participant´s locomotive skills throughout the protocol, which theoretically can be associated to the increased injury incidence observed in these periods.
Does fatigue only influences soft tissue injury risk?
No. Although it seems logical that over-exerted muscles will become progressively insufficient and therefore at increased risk of injury, this decreases in the muscles strength and neural firing rate may also be detrimental to the players´ motor control abilities, with consequences to overall motor control. For example, the decrease in joint position sense throughout the game (22) will most likely be detrimental to the reflexive (and often protective) response to joint movements. This will be responsible for an increased likelihood of a traumatic joint injury, such as the anterior cruciate ligament (ACL) rupture.
Video 1. Ibrahimovic´s knee injury sustained close to the end of the game against RSC Anderlecht for UEFA Europa League.
Poor luck or poor preparation?
Commonly football coaches refer to “luck” as the main reason for having injuries, especially when their squads present high incidence of traumatic injuries to joints and not soft tissues. However, is this exclusively associated with luck? Insufficiencies in training methods have been associated often with soft tissue injury, but is there an association between fatigue (this one resulting from deficits in training) and traumatic injuries such as the ACL one?
At Football Medicine® we definitively believe so. As we referred previously in this text most of injuries are sustained during the latter stages of game halves, and this includes traumatic joint injuries were ACL is included. There are several reasons why players could be excessively fatigued during a game: 1) insufficient training stimulus (undertraining) leading to the player inability to cope with the game´s physical demands; 2) excessive training stimulus and insufficient recovery from training leading up to the game (overtraining), resulting in the player to start the game not fully recovered from the previous trainings; 3) sudden increases in fixture congestion with abrupt peaks in physical load leading to insufficient recovery between matches; and 4) a particular load increase in a game like (e.g.) the event of extra-time .
What determines the players´ level of fatigue?
Fatigue is a fundamental part of the training process. Reaching the state of fatigue means that the metabolic or/and mechanical stress placed on the player´s body is significant, working as a trigger to generate fundamental adaptations to reach a right level of physical performance. However, in order to maintain a proper level of fitness, working under some level of fatigue during training is necessary. Simultaneously, also during the game as the player reaches a certain level of tolerance to the game physical effort, there will be a tendency of pushing himself to further beyond, ending up working with some level of fatigue also during the game.
Although this process is well known, if the player´s fitness status of the player is the correct one, especially during the game day, the likelihood of the player to be exposed to extreme levels of fatigue will decrease, and therefore not only it will not impair his performance levels as also the will not increase his injury risk.
For this matter the training process is fundamental. First through a progressive exposition from the pre-season to the competitive period to a level of load that creates a positive adaptation in the player musculoskeletal system to the game demands (and obviously to the manager tactical demands). Unsurprisingly, extensive research using several approaches has establish a relation between training loads and injury occurrence in football and other team sports, regardless of its nature (2,3,5,10,11,13).
Second, using a periodization training model throughout the week, that depending on the number of days between games should guarantee a proper recovery from the previous game and preparation to the following one. The latter issue was previously discussed in this Football Medicine® article in which models of periodization are suggested.
TRAINING PERIODIZATION AND INJURY PREVENTION – MODEL A
TRAINING PERIODIZATION AND INJURY PREVENTION – MODEL B
How long should we expect players to recover from fatigue and return to regular levels?
Understanding how long the players´ body takes to return to baseline levels both in physiological and functional parameters in football has being of important matter of interest and research (4,7, 14,25). For example, just recently, a study from Thomas et al. (2017) using a soccer simulation protocol has shown that functional variables like maximum voluntary contractions and quadriceps potentiated twitch force were not at baseline levels until 72 hours post –protocol. Voluntary muscle activation was also not at regular levels at 48 hours post-exercise. In a metabolic rather than functional perspective, a study performed with professional French league players by Djaoui et al. (2016) showed that physiological markers like creatine kinase and heart rate at rest were only significantly higher during 24 hours post-match, whereas post effort recovery heart rate remained significantly different at 48 hours. Although is generally accepted that 48 hours is the standard time to recover from fatigue, different studies regarding functional and physiological markers not only show asynchronies between these to return to baseline levels , as also call the attention to factors such as the players age which may play a significant role on the recovery time.
This type of information is useful to establish training methods, and deciding when and how to apply physical stimulus to the players, understanding that pushing them too soon after a match may be detrimental and not beneficial to their fitness status, and negatively affect performance on the following match.
How can the players´ fatigue be monitored?
Several forms of monitoring can be performed in professional football to monitor the player´s fatigue status, with some of it being briefly addressed in this opinion article.
Neuromuscular fatigue is generally accessed through the use of countermovement and squat jumps where variables such as duration time, time to peak force/power or flight time:contraction time ratio have been shown valid for assessing neuromuscular fatigue status (27).
Biochemical markers to assess muscle damage include measurements of blood creatine kinase or myoglobin (1). Increases in creatine kinase in football were associated with the muscle damage provoked by sprinting and high intensity running that occurs in football (26). However some questioning around the factors leading to the increased levels of creatine kinase following a soccer match still remains (25). Other assessment methods include salivary cortisol and testosterone levels to determine immunity breakdowns following matches and stress hormone responses to soccer play (18, 26).
Also, in a review by Thorpe et al. (2017) several forms of monitoring are detailed, including the ones referent to the autonomic nervous system function such as submaximal heart rate, heart rate variability and heart rate recovery. This methods, although constitute promising approaches to monitor recovery and performance in athletes are still being a focus of research in order to understand its meaningfulness, especially in team sports.
Readiness questionnaires are also another common form of monitoring the players physical and psychological status for training and competing. Although valuable, this form of monitoring depends on the honesty of the athletes. In our experience at FootballMedicine, suspicions from the players that this may affect selection or personal discredit to this monitoring strategy may affect this honesty, creating a bias in this form of evaluation. However, apart from this fact, this form of assessment has shown better sensitivity than objective measures to variations in training load, both acute and chronic (23).
Several examples given by England Premier League and Championship clubs include monitoring strategies, together with other training methodology and injury prevention aspects.
Figure 1. Methods used for fatigue indicators monitoring, creatine kinase levels (left), vertical jump assessment (right).
What about injured players and re-conditioning/re-loading strategies? Should the medical staff consider all this also during rehabilitation in football?
Yes. Several forms of physical stimulus are part of a rehabilitation plan, such as strength training in a gymnasium and outfield re-conditioning involving (or that should inolve) football drills. Therefore, guidelines during rehabilitation regarding exercise are quite similar to a regular training regime, yet with some adjustments or increased need of recover between stimuli considering there is an on-going healing process together with a detraining period. The detraining period associated with the injury, which varies in duration depending on the injury type, will probably be responsible for extra recovery needs from physical stimulus once the player is introduced to significant load during rehabilitation. Overloading the athlete after this detraining together with a continuous loading of a recently injured area without recovery periods in between will only not impair his improvement as also could have detrimental effects in his rehabilitation.
For example, starting and outfield re-conditioning program will be affected by the players injured area and remaining body mechanical properties regarding load absorption and locomotion strategies. The latter are affected by variables like (e.g.) tissue stiffness (29), which in turn will be significantly affected by immobilization or detraining periods (16). These physical/biomechanical properties will directly affect the athlete´s body energy demands to propel his body forward and to overcome external forces acting on his body, and therefore determine his cardiovascular/metabolic response to support the metabolic requirements of the involved musculoskeletal structures (28). This is a good reason to establish a progression of loads, if possible based on the player´s individualized profile using portable monitoring devices such as GPS. Together with this it is also valuable to define periodization strategies considering a balance between physical stimulus and recovery, allowing a higher effectiveness to the load responsiveness and adaptation by the player.
“If you fail to prepare, you’re prepared to fail.”
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