Open access peer-reviewed chapter
Abstract
Football players are prone to sports injuries such as ankle sprain, groin pain, ACL injury, and so on. Muscle strain injury also frequently occurs in football games or practice. As previous studies show, previously injured players have altered muscle and neural functions as well as tissue properties associated with muscle strain injury. They have altered vibration sense, tissue stiffness, and increases in micro-muscle damage. However, training load or conditioning programs are provided the same as those for uninjured players in most cases. In this chapter, the conditioning strategies for players who have previous muscle injuries will be suggested according to the phenomenon after muscle strain injury.
Keywords
- muscle strain
- conditioning
- sensory
- monitoring
- training load
1. Introduction
Musculoskeletal injuries have occurred in over 30% of all players in the major professional football leagues. Furthermore, injured players lost almost 1/4 of total time due to injury in a season. Overall incidence is 8.1 injuries in 1000 hours of exposure; training-related incidence is 3.7 injuries in 1000 hours; and match-related incidence is 36 injuries in 1000 hours. Incidence rates are higher in lower extremity injuries compared with other regions [1].
Among the lower extremity injuries, hamstring strain is the most common injury, followed by groin injuries, ankle sprain, and ligamentous injuries of the knee [2].
Most studies of epidemiology are investigated in European league players with limited studies in other professional leagues such as Arian, American, or African leagues. Since body composition varies in races and affects injury ratio, more comprehensive regional studies are required [3].
Injuries have negative impacts both financially and physically on teams and players, and they, of course, decrease team success. The loss of the players due to injury is important for the team because of the limited funds or resources. An injury will lead to neuromuscular alteration and increase the risk of developing osteoarthritis later in an athlete’s career depending on the rehabilitation [4]. Therefore, injury prevention is a key factor to sustain the productivity of the team and a player’s performance [5].
In the last decade, a large number of studies related to injury prevention have been released in football. Literally, the study shows that the injury prevention program such as “The 11+” from FIFA has been recently implemented for professional, amateur, or youth football teams [6]. Nowadays, a lot of injury prevention techniques have been introduced; some examples are as follows.
Eccentric strength exercise prescription tends to decrease recurrent hamstrings’ muscle strain [5].
Core stability exercise intervention may decrease the rate of back and lower extremity injury [7].
Balance training, especially a soccer-specific balance training implementation, could reduce hamstring injury and tendinopathy [8].
Stretching is still suggested pre- and post-training. Even though stretching does not significantly reduce injury rate, it may slightly reduce the injury rate [9].
GPS monitoring and increase in training and gameplay intensity could predict soft tissue injuries [10].
However, here is a question: “Is Injury prevention actually possible for all players?” Even though some injury prevention programs seem to be effective, the recurrent ratio of muscle strain has not decreased [11].
At that point, injury prevention has been controversial, and football-related injuries may be inevitable. It is suggested that more reliable and valid studies in epidemiology or statistics will be necessary. Plus, it needs to be investigated for preventive programs or devices, for example, medical screenings, warming up, protective equipment, training programs, stretching methods, or team education [12].
Furthermore, in order to make an effective injury prevention program in some factors, the program should be modified or personalized.
One of the factors as to why all “effective” injury prevention programs do not work may be their behavioral aspects.
Any kind of program can be ineffective without the player’s active involvement. There have been a few studies related to injury prevention as a social and behavioral science [13]. The main theories are the Theory of Planned Behavior and the Self-Determination Theory [14]. The examination of injury prevention is utilized to integrate two theories, and it can positively predict intentions of injury prevention [15]. Behavioral factors should be considered in order to take injury prevention programs into effect.
The second factor is multicomponent injury prevention intervention.
Since only one injury prevention program may not be enough to reduce injuries, it has been suggested that injury prevention programs should combine two or more programs.
For example, stretching exercises and movement screening, which are part of the suggested programming, may not reduce injury incidence [9].
“Nothing is perfect.” Like Nordic hamstring exercise, which is one of the most reliable injury prevention exercises for hamstring muscle strain, but this exercise cannot perfectly prevent injury. Medical or conditioning staff in sports teams should not focus on only a single component of injury prevention intervention.
Another factor can be that previous injury history will influence effective programming. The recurrent injury rate in football has been high. Recurrent injury incidence is higher within two months after returning to training. These results are considered a premature return to play “too much too soon.” There is still a lack of evidence-based criteria for a safe return to play; therefore, the true reason for recurrent injury has been unknown [1]. Even so, a previous history of injury is considered a strong predictor of recurrent injury, such as hamstring strain injury, and it leads to some functional alterations such as neuromuscular function, sensory function, or tissue stiffness [16, 17].
To take into consideration these results, the players who have a previous history of muscle injury may receive different feedback from the players who do not have a history of injury. Moreover, players who have a previous history of muscle injury tend to have a raised urine titin fragment value compared to healthy players on the same training load condition. A fragile titin filament can lead to less resistance during muscle activity [18]. It may alter neuromuscular activities and lead to neuromuscular fatigue [19].
As excessive fatigue is associated with performance failure, decision-making ability, coordination and neuromuscular control with an overwhelming training load, and management of fatigue are important factors in preventing injury [20].
In the recent decade, the concept of workload management, in other words, “load/capacity” management, has appeared and been applied to the sports field.
Training workload is measured as a prescribed external training load, such as physical work, combined with internal training load such as physiological or perceptual response.
The concept of workload measurement has been controversial. It is suggested that there are further reliable parameters because of their inconsistent values; therefore, more individualized consideration will be required [21].
This chapter includes a proposal of reconsidering internal workload and conditioning strategy, especially for those players who have had a previous muscle injury, because previous injury may affect internal workload value.
2. Conditioning strategy
2.1 Workload
Workload measurement and training-load monitoring have been a part of standard conditioning methods in football teams. According to some studies [22, 23], they may reduce the injury ratio.
The concept of workload first appeared in the 1970s, and it proposes that the performance of an athlete in response to training can be estimated from the difference between fatigue and fitness, called the Fitness–Fatigue Model [24]. To strike an appropriate balance between the positive side, fitness, and negative side, fatigue, the ideal training stimulus does not exceed the capacity of training load.
The concept of ACWR is based on the Fitness–Fatigue Model, introduced in 2016.
ACWR attempts conditioning management, performance development, and injury prevention by the relationship between acute and chronic workload data. Inadequate training-load management and prescription is one of the risk factors for injury [20]. The key of ACWR application in the sports field should be comparing the acute training load to the chronic training load, which provides the feature of the conditioning. If the acute training load is low, it means less fatigue, and if the chronic training load is high, it means fit. As a result, athletes should be prepared to train or compete in this condition.
The optimal ACWR is in the range from 0.80 to 1.30, called the sweet spot, which is the lowest relative injury risk. When ACWR is too low (less than 0.80), it means under-training and a higher relative injury risk. When ACWR is too high (1.50 or more), called the danger zone, it is the highest relative injury risk [25] (Figure 1).
Figure 1.
If ACWR is above 1.5 “danger zone”, there is a significantly increased risk of injury within 1–4 weeks after rapidly increasing training load [
Both too low and too high ACWR may be necessary to manage the training load. The value of ACWR varies from sports to sports, such as Australian football, cricket, rugby, and soccer [26].
In fact, evidence shows that the lowest relative injury risk in the “sweet spot” can be between 1.00 and 1.25 in professional soccer; on the other hand, the sweet spot is 0.85 to 1.35 in rugby league [27].
Thus, when utilizing the ACWR concept, it should be a sports-specific or individualized monitoring protocol.
The ACWR is generally calculated by external training load and internal training load. External load is an objective measurement, that is, the external stimulus exposed to the athletes. External load commonly includes total distance, number of sprints, body load, or weight lifted. Recently, global positioning system (GPS) has been utilized for monitoring external loads.
Internal load is both an objective and subjective measurement, which is the individual’s physiological and/or psychological response to external loads. Internal load basically includes ratings of perceived exertion (RPE) applied as session ratings of perceived exertion (sRPE), heart rate, blood lactate concentrations, or creatine kinase measurement [28]. Most internal load measurements include RPE. RPE is a subjective measure of how hard an athlete feels during physical activity. The first appearance of RPE was created by Swedish psychologist Gunnar Borg as the RPE/Borg scale. The original Borg scale rates an athlete’s level of exertion on a scale from 6 to 20, with 6 being “very light” and 20 indicating “very difficult”. If the 6–20 rating was multiplied by 10. Borg also added a 10-point scale (Borg CR10 scale) that is utilized in medical or psychological fields in addition to the sports field. In an actual sports setting, athletes subjectively provide a 0–10, −100, or 6–20 rating on the intensity of the training session, and the intensity of the session is multiplied by the session. The unit of training load for RPE is calculated by the multiplication of training intensity and the length of training.
Training Load
Example: 60 min training with RPE of 7 (very hard)
Generally speaking, the range of training load is between 300 and 500 AU for lower intensity training and between 700 and 1000 AU for higher intensity training.
The acute workload is performed daily or weekly, but it is typically performed in one week. The chronic workload is calculated as 4 weeks’ accumulation divided by 4.
Example: week
To calculate ACWR, an acute workload of 420 AU is simply divided by a chronic workload of 565 AU, providing an ACWR of 0.74(420/565 = 0.74). Well-known research suggests that if an ACWR is between 0.8 and 1.3, it is called the “sweet spot” for less injury risk; however, if an ACWR is above 1.5, it is called the “danger zone” for highest injury risk. In addition, if an ACWR below 0.8, it also increases the injury risk due to under-training [25]. Therefore, by following the suggestion, the example athlete could increase injury risk by under-training.
Communication between athletes and coaches with personal oral and/or written feedback is important for identifying potential issues with motivation, stress, fatigue, and training. Behavioral aspects should be taken into consideration as crucial information in order to avoid motivational problems.
Models for ACWR calculation:
The Rolling Average Model
The Exponentially Weighted Moving Average Model
In the Rolling Averages Model, ACWR is calculated by dividing the acute workload by the chronic workload. Each workload in an acute and chronic period should be equal; therefore, all workloads in a given time period are seen as equivalent. If the chronic workload is greater than the acute workload, ACWR is lower. On the contrary, if the acute workload is greater than the chronic workload, ACWR is higher. The problem of this model is that it cannot accurately represent variations in the way loads accumulate.
The Exponentially Weighted Moving Average (EWMA) Model is calculated as below. EWMA
EWMA
EWMA describes the decaying nature of fitness because it emphasizes the most recent workload. It is considered as a variation in the manner in which loads are accumulated [29].
Even though both models show a high ACWR indicating increasing injury risk, the EWMA model is more sensitive to detect the injury risk due to the decaying nature of fitness [30].
Since ACWR measurements are popular in sprint sports such as football, running distance and number of sprints using GPS are mostly used for training-load measurement. However, other useful measures should be considered depending on the sports. For example, shoulder or elbow torques have been utilized in baseball.
https://www.ncbi.nIm.nih.gov/pmc/articles/PMC7534929/.
Moreover, if the team could have any adequate equipment for the examination of neuromuscular fatigue and recovery, for example, countermovement jump, exercise velocity, or musculoskeletal tests, those assessments would be useful for neuromuscular recovery and injury prevention.
2.2 Problem of the workload measurement
There are some issues with the ACWR monitoring and application for the actual sports field. The data for training load must be accurate and reliable in order to measure precise ACWR. However, Gabbett et al. pointed out mathematical errors of accumulation for ACWR data. If ACWR is 0.5–1.0, the midpoint of 0.75 is used as the ACWR score. As the endpoint is 0.5–2.0, any score below or above it is treated as 0.5 or 2.0 [31]. Furthermore, ACWR measurements are not enough for injury prevention because ACWR is a measurement for training load but not mechanical load for tissue damage. Plus, old injuries affect mechanical load and consequently increase tissue damage [32]. Therefore, the individual characteristics of each athlete, such as age, physical capacity, and injury history, should be taken into account when determining the training load in order to determine the training outcome. This is especially true for individual information about muscle injury history; it is linked to fatigue; therefore, it affects RPE as an internal load [33].
2.3 Consider previous injury
It is well-known that previous muscle injury history is the greatest risk factor for reinjury. Even though the relationship has not been clear, there are some alterations after muscle injury [34].
2.3.1 Neuromuscular inhibition
Due to scar tissue formation after muscle injury, it leads to the development of maladaptation including eccentric hamstring weakness, selective hamstring atrophy, and shifts in the knee flexor torque-joint angle relationship [35].
2.3.2 Muscle weakness
Previously injured players had muscle weakness on injured limbs. Besides, if the injured leg was nondominant, the dominant leg was much stronger before injury. That compensation will lead to improper sports performance [36].
2.3.3 Nerve conductivity
Athletes who had previous hamstring muscle strain had lower sciatic nerve conduction velocity. It may be due to damage in nerve tissue and the myelin sheath and/or axonal thinning. Because eccentric contraction during sprinting and sudden acceleration are incidences of muscle strain injury, alteration of sciatic nerve conductivity will affect the mechanism of muscle strain injury [37].
2.3.4 Decreased sensory function and increased tissue stiffness
The study shows that athletes who had previous muscle strain had decreased sensory input such as vibration sense in the injured area. Additionally, they had increased tissue stiffness [17]. This tissue stiffness is most likely from the stiffness of fascia, which contains viscoelastic ground substances [38]. Stiffer tissue affects the sliding system of the connective tissue, and it is important to sustain optimal tissue stiffness by tissue hydration [39]. Fascia contains abundant nerve receptors, free and encapsulated nerve endings such as Pacinian corpuscles, and Ruffini endings, which respond to mechanical forces [40]. Previous injury causes some damage in the fascia as well as muscle [41]. Stiffer tissue will change its mechanical properties and affect the sensory sensation through nerve receptors. Incorrect sensory feedback by nerve receptors alters muscle activation patterns and movement execution [42]. Therefore, these changes may lead to compensatory movement during sports activity.
2.3.5 Increased muscle damage
The study shows that larger urine titin fragment values are observed in previously injured football players after training compared to uninjured players [43].
Titin is a mechanical protein in muscle cells that has the function of stabilizing the filaments, preventing overstretching of the sarcomere, and recoiling the sarcomere as a spring effect [44]. Since the function is related to the mechanism of muscle strain injury that is mainly caused by eccentric contraction during lengthening of the muscle fiber, it is considered to have an important role in muscle strain injury. Chronic musculoskeletal injury will change structurally or physiologically as a response to the adaptation of neurophysiological processes. Fragile titin filament may lead to less resistance during lengthened activation of contracted muscle. As a result of this phenomenon, neuromuscular activity is altered in previously injured players.
In addition, the kinematic change from the micro damage will lead to fatigue response to muscle activities. This means that previously injured players experience more fatigue in response to training or games than uninjured players. Therefore, team staff should be careful to manage their training response more than uninjured players.
2.4 Reconsider internal load calculation for previously injured players
Currently, the internal load mostly counts for RPE, heart rate, and heart rate variability. Nowadays, football has become a more explosive sport, requiring more running distance and a greater number of sprints compared to the last several decades. Thus, heart rate might be a useful measure for internal load. However, from the point of injury prevention, most injuries are musculoskeletal injuries rather than cardiovascular problems. In this respect, heart rate may not be the right measurement for internal load in football [45]. In addition, the activity of muscles and tendons, power output, and force sustention during a football game or training can be a part of the aspects of the load in RPE score.
Therefore, RPE is a better representative measure of the internal load in order to prevent injury as well as evaluate the performance [46].
RPE is a very simple and feasible way to subjectively assess how hard an athlete feels to gauge the training and game intensity in the actual sports situation.
Previously injured players tend to have high micro-muscle fiber damage and may feel more fatigued compared to uninjured players. They have more muscle fiber damage, mostly within a day, and they may react more sensitively following the training [47]. Therefore, previously injured players may require daily training-load monitoring rather than weekly monitoring.
Body awareness is the key to know how previous injury affects RPE.
Body awareness is how conscious and connected to the body, the awareness of the position and movement, or feeling of body parts is.
As abovementioned problems with previous injuries, previously injured players tend to alter their proprioceptive function. Thus, they may feel an awkward sensation of their movements. Another expression of body awareness is “interoceptive awareness”. Interoceptive awareness is the conscious awareness of internal physiological states. Interoception is described as a physiological sensation such as touching, joint motion, or vasomotor reaction.
Interoception has become a popular concept in health-care professions, especially those who follow the subject of fascia, because the sensory receptors of interoception are mainly free nerve endings, and these are seen in fascial tissues.
Such free nerve endings are unmyelinated sensory nerve endings, and they transmit to the insular cortex and not to the somatosensory cortex, which is considered as the proprioceptive center [48].
Interoceptive sensations produce feelings that are not only sensory but also affective and motivating and are related to sustaining homeostasis. They are linked to behavioral motivations that are necessary for maintaining the physiological integrity of the human body. In competitive sports, athletes should be encouraged to focus on the task of identifying primary internal sensations of tiredness as RPE. In particular, previously injured players tend to damage micro-muscle fibers more compared to uninjured players, as mentioned before [43]. Such players tend to express higher sRPE depending on the condition of the injured area.
An internal sensation may be only part of the interoceptive sensations. Internal perception is sometimes almost entirely focused on proprioceptive refinement. Recent research has shown that interoceptive sensation is sensitive to pleasant touch [49].
When the nerve receptors in fascial tissues are stimulated, they provide sensory feedback such as tissue stretching, vibration, or light touch to the brain, and this information will be integrated for the movement and achieve neuromuscular control. It has been demonstrated that if athletes change the sensory input from the tissue, they will exhibit movement changes during sports activities. Because previously injured players could impair their sensory feedback, assessment of the sensory information could be a useful tool for interoceptive sensation of previously injured players.
Previously injured players tend to have stiffer tissue around their injured tissue [38]. The stiffness is mostly affected by the viscoelastic ground substances [50]. Stiffer viscoelastic properties may affect the sliding movement of connective tissues and change the biomechanics of the soft tissues by force transmission of FTs when the tissue is stretched [51]. Therefore, feeling the stretching on the stiff tissue can be an assessment tip for previously injured players.
Optimally, the sensory function should be evaluated by a somatosensory evoked potential test (SEP), and stiffness should be evaluated by an examination device such as elastography or soft tissue stiffness meter. However, it takes time and costs; therefore, it is not realistic to apply to actual sports-team management. In addition, the overwhelming data can be the pitfall of the ACWR calculation. The team staff finds it hard to manipulate mountains of data.
Taking the abovementioned factors that can alter proprioceptive and interoceptive feedback integration, I would like to suggest the following two internal load measurements for previously injured players (Figure 2).
Figure 2.
Flowchart of alterations due to previous muscle injury.
Subjective scale for feeling
Sensory
As athletes monitor their RPE, previously injured players monitor “how they feel” on their injured area. The simplest and feasible way is touching their injured area. The less the feeling, the more is the altering in their sensory input (Figure 3).
Figure 3.
Athletes put their hand or a cotton pad to the injured area and scale “how they feel?” from 1 to 10.
Stretching.
Same as monitoring sensory, monitoring “how they feel” with stretching as a function of injured muscle. The less the feeling, the more is the altering in their sensory input (Figure 4).
Figure 4.
Athletes stretch a muscle of their injured area and scale “how they feel?” from 1 to 10.
3. Discussion and conclusion
Management of the training load is considered one of the most recognized and useful tools for sports-team conditioning. Actually, it has been considered as an injury prevention method in current studies. However, current monitoring methods may require some improvement; for example, they should be more individualized.
Oftentimes, each athlete is suffering from other difficulties rather than sports activities and external stressors; for example, friendship relationships, financial difficulties, family-related psychological stress, fatigue, sleep quality, or lack of motivation [52].
Furthermore, current analysis has been more focused on external load and ratio, but internal load, especially for previously injured players, tends to be high and alters their subjective feeling in response to the training.
The monitoring should be simplified in order to easily apply to teams. However, to maximize compliance, the monitoring has to be adjusted individually for optimal conditioning.
The goal of conditioning is to optimize the performance of the athlete and minimize the risk of injury. Each athlete has a different reaction following the training. Physical Stress Theory describes this as “changes in the relative level of physical stress cause a predictable adaptive response in all biological tissues.” That means the body adapts and reacts to a given stimulus, and mechanical stress levels change in response to the amount of that stimulus. They alter sensory function and increase muscle fiber damage associated with fatigue, which causes their biological adaptation to mechanical stimulus to differ from that of uninjured players.
Previous muscle injury is the strongest risk factor for football injury and causes some changes.
When muscle strain injury occurs in fascial tissues that contain abundant proprioceptive receptors, there are two possible alterations. First, it can be damage of the loose component that affects the sliding system between different layers. Another one is the damage of the fibrous component that affects the capacity of loading transmission [53]. Furthermore, accompanied with damage to proprioceptive receptors, it can be alteration of the collagen fiber composition, a transformation of fibroblasts into myofibroblasts, or changes in ground substance [54]. This consequently changes some problems after suffering from muscle strain injury. Athletes with changed tissue stiffness and sensory input may change their body awareness, coordination of movement, and muscle activation patterns and achieve neuromuscular control [53, 54], resulting in incorrect movements during sports.
These alterations can lead to a possibly increased risk of a subsequent sport injury. Moreover, because previously injured players have more muscle fiber damage associated with the training, they may feel more fatigued, which is one of the internal risk factors of muscle injury.
The definition of conditioning is “the process of training to become physically fit by a regimen of exercise, diet, and rest.”
Training load is currently calculated by mostly combining external load such as GPS and internal training load such as RPE. However, because previously injured players are more likely to have a different internal load compared to uninjured players and since their feelings of being “physically fit” and recovery from “rest” can be different from those of uninjured players, their conditioning strategy will be more individualized and specific.
Therefore, I strongly suggest that previously injured players should “never ignore their body sign” and monitor their sensory function as a simple and feasible way in order to sustain their optimum condition.
References
- 1.
López-Valenciano A, Ruiz-Pérez I, Garcia-Gómez A, Vera-Garcia FJ, Croix MD, Myer GD, et al. Epidemiology of injuries in professional football: A systematic review and meta-analysis. British Journal of Sports Medicine. 2020; 54 (12):711-718 - 2.
Ekstrand J, Hagglund M, Walden M. Epidemiology of muscle injuries in professional football (soccer). American Journal of Sports Medicine. 2011; 39 :1226-1232 - 3.
Chomiak J, Junge A, Peterson L, Dvorak J. Severe injuries in football players. Influencing factors. The American Journal of Sports Medicine. 2000; 28 (5 Suppl):S58-S68 - 4.
Emery C, Meeuwisse W. The effectiveness of a neuromuscular prevention strategy to reduce injuries in youth soccer: A cluster-randomised controlled trial. British Journal of Sports Medicine. 2010; 44 (8):555-562 - 5.
Owen A, Wong D, Dellal A, Paul D, Orhant E, Collie S. Effect of an injury prevention program on muscle injuries in elite professional soccer. Journal of Strength and Conditioning Research. 2013; 27 (12):3275-3285 - 6.
Kristiansen J. Elite professional soccer players’ experience of injury prevention. Cogent Medicine. October 2017; 4 :1389257 - 7.
Willson J, Dougherty C, Ireland M, Irene, Davis I. Core stability and its relationship to lower extremity function and injury. The Journal of the American Academy of Orthopaedic Surgeons. 2005; 13 (5):316-325 - 8.
Kraemer R, Knobloch K. A soccer-specific balance training program for hamstring muscle and patellar and achilles tendon injuries: An intervention study in premier league female soccer. The American Journal of Sports Medicine. 2009; 37 (7):1384-1393 - 9.
Lauersen J, Bertelsen D, Andersen L. The effectiveness of exercise interventions to prevent sports injuries: A systematic review and meta-analysis of randomised controlled trials. British Journal of Sports Medicine. 2014; 48 (11):871-877 - 10.
Ehrmann F, Duncan C, Sindhusake D, Franzsen W, Greene D. GPS and injury prevention in professional soccer. Journal of Strength and Conditioning Research. 2016; 30 (2):360-367 - 11.
Brukner P. Hamstring injuries: Prevention and treatment-an update. British Journal of Sports Medicine. 2015; 49 (19):1241-1244 - 12.
Parkkari J, Kujala U, Kannus P. Is it possible to prevent sports injuries? Review of controlled clinical trials and recommendations for future work. Sports Medicine. 2001; 31 (14):985-995 - 13.
McGlashan A, Finch C. The extent to which behavioural and social sciences theories and models are used in sport injury prevention research. Sports Medicine. 2010; 40 (10):841-858 - 14.
Keats M, Emery C, Finch c. Are we having fun yet? Fostering adherence to injury preventive exercise recommendations in young athletes. Sports Medicine. 2012; 42 (3):175-184 - 15.
Chan D, Martin S, Hagger M. Transcontextual development of motivation in sport injury prevention among elite athletes. Journal of Sport & Exercise Psychology. 2012; 34 :661-682 - 16.
Pietrosimone B, McLeod M, Lepley A. A theoretical framework for understanding neuromuscular response to lower extremity joint injury. Sports Health. 2012; 4 (1):31-35 - 17.
Kawai T, Takamoto K, Bito I. Previous hamstring muscle strain injury alters passive tissue stiffness and vibration sense. Journal of Bodywork and Movement Therapies. 2021; 27 :573-578 - 18.
Nishikawa K, Dutta S, DuVall M, Nelson B, Gage M, Monroy J. Calcium-dependent titin-thin filament interactions in muscle: Observations and theory. Journal of Muscle Research and Cell Motility. 2020; 41 :125-139 - 19.
Areia C, Barreira P, Montanha T, Oliveira J, Ribeiro F. Neuromuscular changes in football players with previous hamstring injury. Clinical Biomechanics (Bristol, Avon). 2019; 69 :115-119 - 20.
Soligard T et al.: How much is too much? (Part 1) International Olympic Committee consensus statement on load in sport and risk of injury. British Journal of Sports Medicine. 2016-096581 - 21.
Zouhal H, Boullosa B, Ramirez-Campillo R, Ali A, Granacher U. Editorial: Acute: Chronic workload ratio: Is there scientific evidence? Frontiers in Physiology. 2021; 12 :669687 - 22.
Bowen L, Gross A, Gimpel M, Bruce-Low S, Li F. Spikes in acute:Chronic workload ratio (ACWR) associated with a 5-7 times greater injury rate in English premier league football players: A comprehensive 3-year study. British Journal of Sports Medicine. 2018; 54 (12):099422 - 23.
Hulin B, Gabbett T, Lawson D, Caputi P, Sampson J. The acute:Chronic workload ratio predicts injury: High chronic workload may decrease injury risk in elite rugby league players. British Journal of Sports Medicine. 2016; 50 (4):231-236 - 24.
Banister E, Calvert T, Savage M. A systems model of training for athletic performance. Australia Journal of Sports Medicine. 1975; 7 :57-61 - 25.
Gabbett T. The training-injury prevention paradox: Should athletes be training smarter and harder?.British. Journal of Sports Medicine. 2016; 50 (5):273-280 - 26.
Griffin A, Kenny I, Comyns T, Lyons M. The association between the acute: Chronic workload ratio and injury and its application in team sports: A systematic review. Sports Medicine. 2020; 50 (3):561-580 - 27.
Malone S, Owen A, Newton M, Mendes B, Collins K, Gabbett T. The acute: Chronic workload ratio in relation to injury risk in professional soccer. Journal of Science and Medicine in Sport. 2017; 20 (6):561-565 - 28.
Huxley D, O’Connor D, Healey P. An examination of the training profiles and injuries in elite youth track and field athletes. European Journal of Sport Science. 2013; 14 (2):185-192 - 29.
Williams S, West S, Cross MJ, et al. Better way to determine the acute:chronic workload ratio? British Journal of Sports Medicine. Published Online First: 20 Sep 2016-096589 - 30.
Murray N, Gabbett T, Townshend A, Blanch P. Calculating acute: Chronic workload ratios using exponentially weighted moving averages provides a more sensitive indicator of injury likelihood than rolling averages. British Journal of Sports Medicine. 2016; 51 (9):749-754 - 31.
Blanch P, Gabbett TJ. Has the athlete trained enough to return to play safely? The acute:Chronic workload ratio permits clinicians to quantify a player’s risk of subsequent injury. British Journal of Sports Medicine. 2016; 50 (8):471-475 - 32.
Baker B. An old problem: Aging and skeletal-muscle-strain injury. Journal of Sport Rehabilitation. 2017; 26 (2):180-188 - 33.
Dugan S, Frontera W. Muscle fatigue and muscle injury. Physical Medicine and Rehabilitation Clinics of North America. 2000; 11 (2):385-340 - 34.
Fulton J, Wright K, Kelly M, Zebrosky B, Zanis M, Drvol C, et al. Injury risk is altered by previous injury: A systematic review of the literature and presentation of causative neuromuscular factors. International Journal of Sports Physical Therapy. 2014; 9 (5):583-595 - 35.
Fyfe J, Opar D, Williams M, Shield A. The role of neuromuscular inhibition in hamstring strain injury recurrence. Journal of Electromyography and Kinesiology. 2013; 23 (3):523-530 - 36.
O’Sullivan K, O’Ceallaigh B, O’Connell K, Shafat A. The relationship between previous hamstring injury and the concentric isokinetic knee muscle strength of irish gaelic footballers. BMC Musculoskeletal Disorders. 2008; 9 :30 - 37.
Brockett CL, Morgan DL, Proske U. Predicting hamstring strain injury in elite athletes. Medicine and Science in Sports and Exercise. 2004; 36 :379-387 - 38.
Kawai T, Takahashi M, Takamoto K, et al. Hamstring strains in professional rugby players result in increased fascial stiffness without muscle quality changes as assessed using shear wave elastography. Journal of Bodywork and Movement Therapies. 2021; 27 :34-41 - 39.
Guimberteau J, Delage J, McGrouther D, Wong J. The microvacuolar system: How connective tissue sliding works. Journal of Hand Surgery (European Volume). 2010; 35 :614-622 - 40.
Stecco C, Gagey O, Belloni A, Pozzuoli A, Porzionato A, Macchi V, et al. Anatomy of the deep fascia of the upper limb. Second part: Study of innervation. Morphologie: Bulletin de l’Association des Anatomistes. 2007; 91 :38-43 - 41.
Wilke J, Hespanhol L, Behrens M. Is it all about the fascia? A systematic review and meta-analysis of the prevalence of Extramuscular connective tissue lesions in muscle strain injury. Orthopaedic Journal of Sports Medicine. 2019; 7 :2325967119888500 - 42.
Proske U, Gandevia SC. The kinaesthetic senses. Journal of Physiology. 2009; 587 :4139-4146 - 43.
Kawai T, Takamoto K, Bito I. Post-training urinary titin fragment concentration increases in athletes with previous muscle strain injury: A pilot study in soccer players. The Journal of Physical Fitness and Sports Medicine. 2021; 10 (5):263-268 - 44.
Prado L, Makarenko I, Andresen C, Krüger M, Opitz C, Linke W. Isoform diversity of giant protein in relation to passive and active contractile properties of rabbit skeletal muscles. The Journal of General Physiology. 2005; 126 :461-480 - 45.
Impellizeri F, Rampini E, Marcora S. Physiological assessment of aerobic training in soccer. Journal of Sports Sciences. 2005; 23 (6):583-592 - 46.
Zourdos MC, Klemp A, Dolan C, Quiles JM, Schau KA, Jo E, et al. Novel resistance training-specific rating of perceived exertion scale measuring repetitions in reserve. Journal of Strength and Conditioning Research. 2016; 30 (1):267-275 - 47.
Maruyama N, Asai T, Abe C, Inada A, Kawauchi T, Miyashita K, et al. Establishment of a highly sensitive sandwich ELISA for the N-terminal fragment of titin in urine. Scientific Reports. 2016; 6 :39375 - 48.
Berlucchi G, Aglioti SM. The body in the brain revisited. Experimental Brain Research. 2010; 200 :25-35 - 49.
McGlone F, Wessberg J, Olausson H. Discriminative and affective touch: Sensing and feeling. Neuron. 2014; 82.4 :737-755 - 50.
Gao Y, Waas A, Faulkner J, Kostrominova T, Wineman A. Micromechanical modeling of the epimysium of the skeletal muscles. Journal of Biomechanics. 2008; 41 :1-10 - 51.
Huijing P, Baan G. Myofascial force transmission causes interaction between adjacent muscles and connective tissue: Effects of blunt dissection and compartmental fasciotomy on length force characteristics of rat extensor digitorum longus muscle. Archives of Physiology and Biochemistry. 2001; 109 :97-109 - 52.
Foster C. Monitoring training in players with reference to overtraining syndrome. Medicine and Science in Sports and Exercise. 1998; 30 (7):1164-1168 - 53.
Piero G, Pavan P, Stecco A, Stern R, Stecco C. Painful connections: Densification versus fibrosis of fascia. Current Pain and Headache Reports. 2014; 18 :441 - 54.
Stecco C, Macchi V, Porzionato A, Morra A, Parenti A, Stecco A, et al. The ankle retinacula: Morphological evidence of the proprioceptive role of the fascial system. Cells, Tissues, Organs. 2010; 192 (3):200-210