DEFINITION
Load, overload, and recovery are key issues for team physicians treating and caring for athletes. Load is an inevitable result of athletic conditioning, training, and competition. Load in activity may be defined as a stimulus experienced and responded to by an individual before, during, or after participation. Load creates a demand or stress (both physiological and psychological) and has internal and external components. Load that is safely managed may result in improved athletic capacity and performance, and injury and illness risk reduction.
Overload, as defined in this document, is load that is excessive or not well managed. It will result in anatomical, physiological, and/or psychosocial conditions that will manifest as altered performance and injury and illness. Identification and modification of load and minimizing overload have been advocated as central parts of optimal performance and injury and illness and prevention strategies.
Recovery is the period and process during which the body responds to load. Adequate recovery may result in positive adaptations for athletic capacity, performance, and injury and illness risk, whereas inadequate recovery may result in maladaptations for athletic capacity, performance, and injury and illness.
GOAL
The goal of this document is to help the team physician improve the care of the athlete by understanding load, overload, and recovery issues in athletes. To accomplish this goal, the team physician should have knowledge of and be involved with the following:
• Current concepts in load, overload, and recovery
• Medical issues related to load, overload, and recovery
• Musculoskeletal issues related to load, overload, and recovery
• Psychosocial and mental health issues
• The role of measurement and monitoring in load, overload, and recovery
• Intervention strategies to reduce overload injury and illness risk
• The role of modalities in recovery
SUMMARY
This document provides an overview of the concepts of load, overload, and recovery, as well as the musculoskeletal and psychosocial issues related to overload injuries and illnesses that are important for the team physician to understand. In addition, understanding the role of wearable devices, technology, and other tools used to measure load, overload, and recovery is important. Interventions to manage load may improve performance and reduce the risk of overload injuries and illnesses. This document is not intended as a standard of care and should not be interpreted as such. It is only a guide and, as such, is of a general nature, consistent with the reasonable, objective practice of the health care professional. Individual treatment will turn on the specific facts and circumstances presented to the physician. Adequate insurance should be in place to help protect the physician, the athlete, and the sponsoring organization.
This statement was developed by a collaboration of six major professional associations concerned about clinical sports medicine issues; they have committed to forming an ongoing project-based alliance to bring together sports medicine organizations to best serve active people and athletes. The organizations are the American Academy of Family Physicians, the American Academy of Orthopaedic Surgeons, the American College of Sports Medicine, the American Medical Society for Sports Medicine, the American Orthopaedic Society for Sports Medicine, and the American Osteopathic Academy of Sports Medicine.
EXECUTIVE COMMITTEE
Stanley A. Herring, M.D., Chair, Seattle, Washington
W. Ben Kibler, M.D., Lexington, Kentucky
Margot Putukian, M.D., Princeton, New Jersey
EXPERT PANEL
David J. Berkoff, M.D., Chapel Hill, North Carolina
Jeff Bytomski, D.O., Durham, North Carolina
Eric Carson, M.D., Charlottesville, Virginia
Cindy J. Chang, M.D., San Francisco, California
David Coppel, Ph.D., Seattle, Washington
R. Rob Franks, D.O. Marlton, New Jersey
Peter Indelicato, M.D., Gainesville, Florida
Neeru Jayanthi, M.D., Atlanta, Georgia
Mark Kovacs, Ph.D., Atlanta, Georgia
Jason Matuszak, M.D., Amherst, New York
Claude T. Moorman, III, M.D., Charlotte, North Carolina
CURRENT CONCEPTS IN LOAD, OVERLOAD, AND RECOVERY
Load
A variety of factors affect the body’s response to training load. Excessive training may place the athlete at risk for injury, illness, and decreased performance. In some cases, undertraining for the expected demands of activity and/or competition may also leave an athlete susceptible to injury and decreased performance.
Load has been measured by factors such as number of pitches thrown, distance run, length of season, hours of training, and/or amount of weight lifted. To determine the point of overload, these measurements are evaluated in association with the types of injury and illness or effect on performance to which they contribute (e.g., pitch count and length of season with upper extremity injuries, hamstring weakness with hamstring muscle injury, lower extremity landing mechanics with knee injury, duration and intensity of exercise with respiratory illness, and increased anxiety with decreased performance).
Overload
Overload occurs when the balance between external load and internal load is altered so that the body’s adaptive capacity is inadequate, resulting in manifestations of altered performance and injury and/or illness.
There may be an absolute overload, in which even if the body is well conditioned, the load is too high (e.g., lifting too much weight). There may also be relative overload, in which the load is normal but the body’s capacity is diminished (e.g., running sprints with a hamstring injury).
Current Concepts
Current concepts more completely define factors related to load, overload, and recovery. Athletes respond individually to internal and external load, the result of which determines the magnitude of the load stimulus experienced.
• External load is the work completed by the athlete during training and competition. External loads create physical, physiological, and psychosocial demands, which are affected by frequency, intensity, and duration of the activity, among other factors. Demands may be sport specific (Table 1).
;)
TABLE 1: External loads create physical, physiological, and psychosocial demands.
• Internal loads are individual physical, physiological, and psychosocial characteristics that respond to external load. Identical external loads may elicit considerably different internal loads in two athletes with different internal characteristics (e.g., age, sex, body composition, fitness, range of motion [ROM], history of prior injury, and psychosocial health). Internal loads vary over time, requiring ongoing monitoring (Table 2).
TABLE 2: Internal loads are individual characteristics that respond to external load.
• Recovery is a process with the goal of optimizing physical, physiological, and psychosocial adaptation to internal and external loads (Table 3).
TABLE 3: Recovery optimizes adaptation to internal and external loads.
MEDICAL ISSUES RELATED TO LOAD, OVERLOAD, AND RECOVERY
Load and overload may result in medical problems, including compromised immune system, illnesses, altered sleep patterns, and overtraining (Figs. 1A and 1B).
FIGURE 1: A, Relationship between load and injury risk. B, Proposed model of adaptation to load.
Medical issues may result from too much load, too intense training, or increased training intensity for athletes not properly conditioned to the load stimulus. Some research suggests an increased susceptibility to upper respiratory infection, thought to be related to an immune level response. Athletes at different levels respond to similar loads in varied ways.
Increased load may negatively affect sleep volume and quality. Numerous studies suggest that sleep deprivation or poor sleep quality may affect athletic performance, recovery from exercise, cognition, reaction time, and mood. Decreased sleep duration and training load increases may increase risk for injury. Poor sleep quality is common in athletes and may be more prevalent leading up to a competition (1).
Sleep deprivation results in autonomic (sympathetic/parasympathetic) disturbances and could decrease the ability to tolerate load resulting in decreased performance and increased risk of injury and illness. Although data remain mixed, the literature suggests that “competitive calendar congestion” (concentrating increasing competition exposures into a shorter time frame, or having shorter periods of rest) increases injury risk. Long-distance air travel, especially across time zones, may increase susceptibility to illness or negatively influence athletic performance; however, there has been no demonstrated link to injury risk (2,3).
Overtraining is often characterized by a performance plateau not improved with the usual rest and recovery cycle; it is highly individual with physical symptoms such as elevated heart rate (HR), weight loss, muscle pain/soreness, elevated blood pressure, gastrointestinal distress, delayed recovery from exertion, loss/decrease in appetite, severe fatigue, disturbed sleep, overuse injuries, and immune system deficits. There are physical symptoms of overtraining (including cardiovascular, respiratory, hormonal, and immunologic) as well as associated psychosocial symptoms (see section below).
Cardiovascular
• Resting HR typically decreases with fitness but can be elevated or reduced in overtraining.
• Athletes’ achievable HRmax will decrease.
• HR variability (HRV), or beat-to-beat variability, has been used as a measure of overload. HRV is decreased in the overtrained athlete in some studies. Others are inconclusive. The interpretation of HRV’s association with overtraining is evolving.
Respiratory
• Respiratory exchange ratio is lower in overtraining athletes, as is V˙O2max.
Hormonal
• Single hormonal measure cannot be used to diagnose overtraining, but many are affected by acute and chronic training.
• Increased resting norepinephrine may be found in overtraining.
• Cortisol levels may be altered. Current studies are inconclusive; some report elevated levels, whereas others report reduced depending on acuity of exercise and chronicity.
∘ Acute strain usually causes an elevation in cortisol, whereas chronic strain usually leads to decreased levels
• In overtraining, there is often a decreased ratio of testosterone (free T) to cortisol. Endogenous testosterone levels will often go down.
Immunity
• IgA levels may increase.
• Enhanced expression of T-cell surface markers (CD45 RO+) has been reported.
Enzymes and blood markers:
• Creatine kinase (CK), urea, uric acid, and ammonia may become elevated.
∘ CK is a marker of mechanical muscular strain. CK is affected by intensity and duration of exercise, as well as some genetic factors. Elevations can be small or large and is not necessarily directly associated with overtraining.
○ Urea is a marker of protein catabolism, and stimulated gluconeogenesis from increased training load and levels can be elevated in overload situations.
• Lactate levels at submaximal effort are lower in overtraining.
It is essential that the team physician
• understand medical issues, such as sleep disturbance, immune dysfunction, and overtraining, resulting from load, overload, and recovery.
It is desirable that the team physician
• be involved in the management of medical illnesses affected by load, overload, and recovery;
• work with the athletic care network to manage medical manifestations of overload;
• work with the athlete care network to enable acute adjustments to the training and competition loads for athletes if internal overload factors are unfavorable.
MUSCULOSKELETAL ISSUES RELATED TO LOAD, OVERLOAD, AND RECOVERY
Pathophysiology
Overload injuries occur when the athlete is inadequately prepared for the stress the muscle–tendon–bone units to manage training and competition.
Mechanical stresses on either soft tissue or bone that exceed the ability to protect/restore the damage resulting from excessive load will result in microscopic or macroscopic damage. Positive adaptations result in strengthening the soft tissue or the bone. Maladaptations may result in muscle injury/tendinopathy or bone stress injury (BSI).
BSI
• BSI results from limited time for recovery causing imbalance of osteoclast and osteoblast activity.
When bone homeostasis is not maintained due to abrupt or persistent training or competition stress, the ability of the osteoblasts to lay down new bone is exceeded by the resorptive activity of the osteoclasts in remodeling the cortex. This results in cortical weakening that manifests first as stress reaction with the spectrum continuing to stress fracture and, ultimately, acute fracture if not addressed.
Physeal Injury
The physis is particularly susceptible to load and overload injuries in the skeletally immature athlete. Traction loads and repetitive tensile loads to the apophysis may also result in injury.
Muscle Injury
Myocytes will also respond adversely to chronic overload. This overload leads to reduced muscular force development and reduced contraction velocity.
Tendinopathy
• Tendinopathy can occur when tenocytes are unable to manage and recover from and excessive load.
• When abrupt or persistent load is applied to tendon in excess of the healing capacity of the fibroblasts and tenocytes, tendon breakdown manifests as tendinopathy.
• Tendon units crossing two joints appear to be particularly susceptible due to constant functioning in eccentric mode.
Injuries
Musculoskeletal injuries are affected by internal and external load factors and may be modified by internal characteristics. Several studies have shown that overload or sudden change in load is associated with increased injury risk. Examples include the following:
• BSI of the lower extremity
○ External load: volume and intensity of running, running surface, and shoewear
○ Internal load: energy balance, muscle strength, and balance
○ Internal characteristics: age, sex, anatomical alignment, and history of stress injury
• Hamstring tendinopathy
○ External load: volume and intensity of training, change in training volume, sprinting
○ Internal load: hamstring flexibility and strength
○ Internal characteristics: previous lower extremity injury
• Patellofemoral injury
○ External load: running and jumping sport
○ Internal load: core strength, quadriceps flexibility and strength, hamstring flexibility
○ Internal characteristics: hip and knee alignment, sex
• Youth shoulder injuries
○ External load: age-appropriate pitch/serve count, length of season, repetitive overhead motion while in pain or fatigued, inadequate recovery, and frequency of competition
○ Internal load: poor mechanics, core weakness, scapular motion, and shoulder ROM
○ Internal characteristics: open physes, hip and knee alignment
• Youth elbow injuries
○ External load: age-appropriate pitch/serve count, length of season, pitching with pain or fatigue, and frequency of competition
○ Internal load: poor mechanics, core weakness, and quadriceps strength
○ Internal characteristics: open physes, hip and knee alignment
It is essential that the team physician
• understand the influence of internal and external load and internal characteristics on musculoskeletal injuries.
It is desirable the team physician
• use the concepts of load and overload in diagnosing and managing musculoskeletal injuries;
• understand the sport- and age-specific implications of overload injuries;
• educate the athletic care network (4) concerning the predisposing factors that result in overload;
• work with the athlete care network to enable acute adjustments to the training and competition loads for athletes if internal overload factors are unfavorable.
PSYCHOSOCIAL AND MENTAL HEALTH ISSUES
Psychosocial factors and issues are present in athletic injuries and illnesses and their recovery (2,5). The load or stress that athletes experience during training can have psychosocial and physical components. In both cases, load can result in positive adaptations and/or maladaptations. Although the physical demands or load of training are seen as a stress, the experience of training can produce additional stressors, such as acute fatigue, pain, susceptibility to illness, and emotional symptoms. Nontraining loads can impose stress on athletes and may be either internal (e.g., preexisting or emerging clinical level emotional symptoms) or external (e.g., psychosocial and/or interaction difficulties, academic struggles, and general adjustment issues). There can be significant interpersonal and intrapersonal-variability in response to loads that may reflect factors such as age, sex, fitness level, fatigue level, metabolic or hormonal status, and genetic factors. The athlete’s subjective responses, including RPE and mood, have been shown to have superior sensitivity and consistency over some objective measures in determining acute and chronic changes in athlete well-being related to load (6).
Psychosocial Issues Related to Overload
Psychosocial overload occurs when load overwhelms coping resources. Poor management of loads and/or recovery can interact with sport-related and non-sport-related major and minor life stress events and create significant maladjustment. Sport-related stress can include inadequate rest cycle, musculoskeletal pain, and concern over capability or progress. These stressors can interact with personality variables, which may include anxiety, somaticizing tendency, mood problems, motivation, and maladaptive coping.
Finding the optimal balance point between load stress and recovery is important. Imbalance often results in overtraining. Psychosocial symptoms of overtraining can include loss of confidence, fatigue, apathy, irritability, emotional or motivational changes, sadness, anger/hostility, confusion, concentration difficulties, and boredom.
Psychosocial Issues Related to Recovery
Recovery strategies to address internal and external load to reestablish physiological and psychological balance include the following: developing resilience strategies to help minimize the effect of negative life events, educating athletes in stress management techniques to minimize effects of stress, reducing training load and/or competition load and intensity, implementing periodic stress assessments, and addressing sleep dysfunction (7). All of these strategies could inform adjustment of an athlete’s training and/or competition loads
It is essential that the team physician
• understand the psychological components of load, overload, and recovery.
It is desirable that the team physician
• be involved in the management of psychological issues related to load, overload, and recovery;
• be involved in referral to the mental health network as indicated;
• understand maladaptations and injury and illness due to load and overload may be influenced by sport and nonsport stressors;
• work with the athlete care network to monitor load by implementing subjective load measures of internal overload factors;
• work with the athlete care network to enable acute adjustments to the training and competition loads for athletes if internal overload factors are unfavorable.
THE ROLE OF MEASUREMENT AND MONITORING IN LOAD, OVERLOAD, AND RECOVERY
There are varying levels of accuracy in tools and technology developed to measure load, overload, and recovery. Even with accurate measurement of internal and external load, further research is necessary to determine their utility in improving athletic performance and reducing risk of injury and illness in sport.
There are a variety of measurement techniques, including new technologies and wearable devices. Categories include the following:
• Laboratory: blood work (e.g., cortisol, CK, hemoglobin, ferritin, iron, testosterone, lactate, vitamin D, and white blood cell count) and urinalysis (i.e., hydration status)
• Questionnaires: psychological (e.g., POMS, REST-Q Sport, Pittsburgh Sleep Quality Index, and scales that measure stress), perceived exertion (e.g., RPE), wellness scales (e.g., muscle soreness scale), and nutritional (e.g., food journal)
• Nonwearable technologies: force plates, V˙O2max, imaging (e.g., MRI and ultrasound), isokinetic testing, equipment/devices to measure velocity and power (e.g., Wingate test), video analysis systems, and internal/external measurements of core temperature (e.g., ingestive thermometer)
• Wearable technologies: global positioning system, HR monitors, HRV devices, velocity (accelerometers), wearable motion-capture devices (e.g., wrist-worn and clothing-based technology)
Although there is broad interest in these technologies and devices, more research is needed for many of these to understand their application. Main limitations include the following:
• Limited large-scale, independent, longitudinal data
• The need to place devices at specific anatomical locations
• Movement artifact
• Frequency of data sampling
• Monitoring of a few selected variables (as opposed to a suite of variables)
• Lack of measurement of environmental factors (e.g., temperature, humidity, altitude, and UV radiation)
• Inconsistencies and accuracy in algorithms that collect, analyze, and distribute data
• Variability of data interpretation by interested parties, including athletes, coaches, researchers, and medical personnel
• Inability to transmit data indoors, underwater, and in built-up areas, and interference from other physiological responses (e.g., vasoconstriction and hypovolemia).
It is essential that the team physician understand the following:
• There are components of load, overload, and recovery that are measurable.
It is desirable that the team physician understand
• the need for further research to determine the utility of tools and technology in improving athletic performance and reducing risk of injury and illness in sport;
• the categories of measurement techniques;
• the limitations in the data obtained from the technology, particularly in its accuracy;
• the limitations in the interpretation of the data;
• the need to work with the athletic care network to effectively monitor athletes using technology that provides valid and reliable information;
• the need to work with the athlete care network to enable acute adjustments to the training and competition loads for athletes if internal overload factors are unfavorable.
INTERVENTION STRATEGIES
Intervention strategies may decrease overload factors and improve recovery. There are tactics that can be applied to manage internal and external load and overload.
An effective tactic to manage external load is a customized periodization program with monitoring (1).
An effective tactic to manage internal load is an ongoing training program that addresses physical, physiological, and psychosocial demands.
Training
The role of this program is designed to modify the athlete’s response (the internal load) to the imposed external load. The athletic care network may use a variety of strategies to evaluate load to improve performance and decrease injury and illness.
Athletes with excessive exposure to external load may be more susceptible to both injury and illness because of their internal characteristics, level of conditioning, and coping resources. Athletes with inadequate exposure to external load may also be at a higher risk. Newer studies suggest that the balance between acute and chronic training loads, or the acute–chronic workload ratio (8), may be an important determinant of injuries.
Load and overload can be managed by training strategies, including modifying volume and intensity, adding variety to avoid monotony, providing emotional support, and allowing for rest and recovery. Examples include the following:
• Optimizing sleep, nutrition (3,9), and mental health
• Limiting weekly hours of training or mileage
• Enforcing a pitch/serve count and recovery periods in overhead athletes
• Limiting number of games and tournaments and allowing for recovery between competition
• Implementing neuromuscular training programs (e.g., FIFA 11)
• Following rules and recommendations of national, state and/or local governing bodies regarding play and recovery
It is essential that the team physician understand
• Intervention strategies may decrease overload factors and improve recovery.
It is desirable that the team physician
• be involved in the planning and implementation of intervention strategies related to load, overload, and recovery;
• understand the concepts of load management as part of training strategies for optimal athlete response, such as acute–chronic workload ratio, competitive calendar congestion, training variety, and periodization;
• educate the athletic care network on intervention strategies;
• work with the athlete care network to enable acute adjustments to the training and competition loads for athletes if internal overload factors are unfavorable.
THE ROLE OF MODALITIES IN RECOVERY
Recovery is best accomplished with a comprehensive, multifaceted approach that addresses musculoskeletal, medical, and psychological issues.
Many types of modalities are passive treatments and are used as adjuncts in aiding recovery. More research is needed to understand their efficacy. Categories include the following:
• Physical modalities
○ Electricity
○ Thermal agents
○ Electromagnetic agents
• Mechanical agents (e.g., hydrotherapy)
• Stretching
• Traction
• Massage/soft tissue manipulation/myofascial release
• Compression
• Taping
It is essential that the team physician understand the following:
• Modalities are frequently used as adjuncts to treatment in recovery.
• Limited data exist on the efficacy of modalities used in recovery.
It is desirable that the team physician understand
• The benefits of a comprehensive, multifaceted approach to address musculoskeletal, medical, and psychological issues in recovery.
Figures provided courtesy of Mark Kovacs, Ph.D.
REFERENCES
1. Herring SA, Bergfeld JA, Boyajian-O’Neill L, et al. The team physician and strength and conditioning of athletes for sports: a consensus statement.
Med Sci Sports Exerc. 2015;47(2):440–5.
2. Herring SA, Kibler WB, Putukian M, et al. Selected issues in injury and illness prevention and the team physician: a consensus statement.
Med Sci Sports Exerc. 2016;48(1):159–71.
3. Herring SA, Kibler WB, Putukian M, et al. Selected issues for nutrition and the athlete: a team physician consensus statement: 2013 update.
Med Sci Sports Exerc. 2013;4(12):2378–86.
4. Herring SA, Kibler WB, Putukian M, et al. Team physician consensus statement: 2013 update.
Med Sci Sports Exerc. 2013;45(8):1618–22.
5. Herring SA, Kibler WB, Putukian M, Coppel D, Cavanna A, Chang C. Psychological issues related to illness and injury in athletes and the team physician: a consensus statement: 2016 update.
Med Sci Sports Exerc. 2017;49(5):1043–54.
6. Saw AE, Main LC, Gastin PB. Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review.
Br J Sports Med. 2016;50(5):281–91.
7. Schwellnus M, Soligard T, Alonso JM, et al. How much is too much? (Part 2) International Olympic Committee consensus statement on load in sport and risk of illness.
Br J Sports Med. 2016;50(17):1043–52.
8. 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.
Br J Sports Med. 2016;50(8):471–5.
9. Thomas DT, Erdman KA, Burke LM. Nutrition and athletic performance.
Med Sci Sports Exerc. 2016;48(3):543–68.
Additional Team Physician Consensus Series Readings
Herring SA, Kibler WB, Putukian M, et al. Female athlete issues for the team physician: a consensus statement—2017 update. Med Sci Sports Exerc. 2018;50(5):1113–22.
Herring SA, Kibler WB, Putukian M, et al. The team physician and the return-to-play decision: a consensus statement-2012 update. Med Sci Sports Exerc. 2012;44(12):2446–8.
Herring SA, Kibler WB, Putukian M, et al. Sideline preparedness for the team physician: a consensus statement-2012 update. Med Sci Sports Exerc. 2012;44(12):2442–5.
Herring SA, Cantu RC, Guskiewicz KM, Putukian M, Kibler WB. Concussion (mild traumatic brain injury) and the team physician: a consensus statement—2011 update. Med Sci Sports Exerc. 2011;43(12):2412–22.
Herring SA, Kibler WB, Putukian M, et al. Selected issues for the master athlete and the team physician: a consensus statement. Med Sci Sports Exerc. 2010;42(4):820–33.
Herring SA, Bergfeld JA, Bernhardt DT, et al. Selected issues for the adolescent athlete and the team physician: a consensus statement. Med Sci Sports Exerc. 2008;40(11):1997–2012.
Herring SA, Bergfeld JA, Boyajian-O’Neill L, et al. Mass participation event management for the team physician: a consensus statement. Med Sci Sports Exerc. 2004;36(11):2004–8.
Recommended Readings
Soligard T, Schwellnus M, Alonso JM, et al. How much is too much? (Part 1) International Olympic Committee consensus statement on load in sport and risk of injury. Br J Sports Med. 2016;50(17):1030–41.
Hislop MD, Stokes KA, Williams S, et al. The efficacy of a movement control exercise programme to reduce injuries in youth rugby: a cluster randomised controlled trial. BMJ Open Sport & Exerc Med. 2016;2:e000043.
Impellizzeri FM, Bizzini M, Dvorak J, Pellegrini B, Schena F, Junge A. Physiological and performance responses to the FIFA 11+ (part 2): a randomised controlled trial on the training effects. J Sports Sci. 2013;31(13):1491–502.
Gleeson M. The scientific basis of practical strategies to maintain immunocompetence in elite athletes. Exerc Immunol Rev. 2000;6:75–101.
Knowles SB, Marshall SW, Miller T, et al. Cost of injuries from a prospective cohort study of North Carolina high school athletes. Inj Prev. 2007;13(6):416–21.
Olsen SJ, Fleisig GS, Dun S, Loftice J, Andrews JR. Risk factors for shoulder and elbow injuries in adolescent baseball pitchers. Am J Sports Med. 2006;34(6):905–12.
Nissen CW, Westwell M, Ounpuu S, Patel M, Solomito M, Tate J. A biomechanical comparison of the fastball and curveball in adolescent baseball pitchers. Am J Sports Med. 2009;37(8):1492–8.
Drew MK, Finch CF. The relationship between training load and injury, illness and soreness: a systematic and literature review. Sports Med. 2016;46(6):861–83.
Gabbett TJ. The training-injury prevention paradox: should athletes be training smarter and harder? Br J Sports Med. 2016;50(5):273–80.
Drew MK, Cook J, Finch CF. Sports-related workload and injury risk: simply knowing the risks will not prevent injuries: narrative review. Br J Sports Med. 2016;50(21):1306–8.
Bourdon PC, Cardinale M, Murray A, et al. Monitoring athlete training load: consensus statement. Int J Sports Physiol Perform. 2017;12(2)161–70.
Nédélec M, Halson S, Abaidia AE, Ahmaidi S, Dupont G. Stress, sleep and recovery in elite soccer: a critical review of the literature. Sports Med. 2015;45(10):1387–400.
Fullagar HH, Skorski S, Duffield R, Hammes D, Coutts AJ, Meyer T. Sleep and athletic performance: the effects of sleep loss on exercise performance, and physiological and cognitive responses to exercise. Sports Med. 2015;45(2):161–86.
Taylor L, Chrismas BC, Dascombe B, Chamari K, Fowler PM. Sleep medication and athletic performance—the evidence for practitioners and future research directions. Front Physiol. 2016;7:83.
Gattie E, Cleland JA, Snodgrass S. The effectiveness of trigger point dry needling for musculoskeletal conditions by physical therapists: a systemic review and meta-analysis. J Orthop Sports Phys Ther. 2017;47(3):133–49.
Halson SL. Recovery techniques for athletes. Sports Science Exchange. 2013;26(120):1–6.
Malanga GA, Yan N, Stark J. Mechanisms and efficacy of heat and cold therapies for musculoskeletal injury. Postgrad Med. 2015;127(1):57–65.
DiFiori JP, Benjamin HJ, Brenner JS, et al. Overuse injuries and burnout in youth sports: a position statement from the American Medical Society for Sports Medicine. Br J Sports Med. 2014;48(4):287–8.
Fleisig GS, Andrews JR. Prevention of elbow injuries in youth baseball pitchers. Sports Health. 2012;4(5):419–24.
Jayanthi NA, Feller E, Smith A. Junior competitive tennis: ideal training and tournament recommendations. J Med Sci Tennis. 2013;18(2):30–6.
Jayanthi NA, Tzakis E. Return to play tennis on-court stroke modifications following injury in junior competitive tennis players. J Med Sci Tennis. 2016;21(1):28–35.
Buchheit M, Allen A, Poon TK, Modonutti M, Gregson W, Di Salvo V. Integrating different tracking systems in football: multiple camera semi-automatic system, local position measurement and GPS technologies. J Sports Sci. 2014;32(20):1844–57.
Anderson L, Orme P, Di Michele R, et al. Quantification of training load during one-, two- and three-game week schedules in professional soccer players from the English Premier League: implications for carbohydrate periodisation. J Sports Sci. 2016;34(13):1250–9.
Achten J, Jeukendrup AE. Heart rate monitoring: applications and limitations. Sports Med. 2003;33(7):517–38.
Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Front Physiol. 2014;5:73.
Halson SL. Monitoring training load to understand fatigue in athletes. Sports Med. 2014;44(2):139–47.
Daanen HA, Lamberts RP, Kallen VL, Jin A, Van Meeteren NL. A systematic review on heart-rate recovery to monitor changes in training status in athletes. Int J Sports Physiol Perform. 2012;7(3):251–60.
Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Med. 2013;43(9):773–81.
Marino M, Li Y, Rueschman MN, et al. Measuring sleep: accuracy, sensitivity, and specificity of wrist ActiGraphy compared to polysomnography. Sleep. 2013;36(11):1747–55.
Ferguson T, Rowlands AV, Olds T, Maher C. The validity of consumer-level, activity monitors in healthy adults worn in free-living conditions: a cross-sectional study. Int J Behav Nutr Phys Act. 2015;12:42.
de Zambotti M, Baker FC, Colrain IM. Validation of sleep-tracking technology compared with polysomnography in adolescents. Sleep. 2015;38(9):1461–8.
Gabbett TJ. Quantifying the physical demands of collision sports: does microsensor technology measure what it claims to measure? J Strength Cond Res. 2013;27(8):2319–22.
Bartlett JD, O’Connor F, Pitchford N, Torres-Ronda L, Robertson SJ. Relationships between internal and external training load in team-sport athletes: evidence for an individualized approach. Int J Sports Physiol Perform. 2017;12(2):230–4.
Leeder L, Glaister M, Pizzoferro K, Dawson J, Pedlar C. Sleep duration and quality in elite athletes measured using wristwatch ActiGraphy. J Sports Sci. 2012;30(6):541–5.
Hausswirth C, Louis J, Aubry A, Bonnet G, Duffield R, LE Meur Y. Evidence of disturbed sleep and increased illness in overreached endurance athletes. Med Sci Sports Exerc. 2014;46(5):1036–45.
Chan ED, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med. 2013;107(6):789–99.
Windsor JS, Rodway GW. Con: pulse oximetry is useful in predicting acute mountain sickness. High Alt Med Biol. 2014;15(4):442–3.
Fullagar HH, Skorski S, Duffield R, Hammes D, Coutts AJ, Meyer T. Sleep and athletic performance: the effects of sleep loss on exercise performance, and physiological and cognitive responses to exercise. Sports Med. 2015;45(2):161–86.
