William J. Kraemer, Ph.D., CSCS, FNSCA, FACSM, Shawn D. Flanagan, BA,
Gwendolyn A. Thomas, MA, CSCS Human Performance Laboratory
Department of Kinesiology University of Connecticut,
Storrs, Connecticut
Abstract
As tennis places demand on a multitude of physiological systems, several must be considered to better identify, implement, and assess strategies to minimize fatigue and optimize recovery after competitive and preparatory events. Damage and repair processes in skeletal muscle are varied and complex in nature and are central to fatigue and recovery considerations in response to stressful bouts of physical exertion like that seen in tennis. Fatiguing demand primarily originates from motor unit activation patterns, variable environmental factors, and psychological stress that may impact readiness to play even when the competitor is physically ready.
The demands of tennis play vary according to the characteristics of match play and the characteristics of the players themselves. Factors include playing surface and implement types, competitive game play variance, temperature and humidity, and player characteristics such as age, sex, and skill level. A better understanding of the physiological basis of fatigue, how it occurs in tennis, and how it is affected by the characteristics of match play and the competitors themselves will aid in the identification and implementation of practices that strengthen the physiological systems involved in recovery. Finally, strategies to minimize fatigue and optimize recovery including proper conditioning, hydration, and nutritional strategies are discussed in conjunction with recognition of the importance of psychological recovery.
Introduction
Fatigue & Recovery
Recovery is general term referring to the short and long-term adaptive responses occurring as a result of “overloading” activity (stress) that causes fatigue. Fatigue is a term that must be defined in a highly “operational manner” as it is directly related to the external demands and physical characteristics of the person experiencing stress. Understandably, the multifaceted nature of fatigue necessitates a
multifaceted conception of recovery. Before discussing the various known aspects of recovery, it is helpful to learn more about the phenomenon that creates the need for it: fatigue.
Fatigue is the consequence of “overloading” stress placed on physiological systems and is reflective of the conditions or demands of a given activity, which in turn dictates the recovery that is needed. When power output cannot be maintained or physiological homeostasis cannot be achieved in a given set of physiological systems, fatigue occurs 1. A host of physiological mechanisms have been used to try to explain this phenomenon but many links and sites of action from central to peripheral mechanisms are believed to contribute to fatigue 2-9. Most importantly, different demands are likely to cause and result in different forms of fatigue. As such, it is important to understand the generally accepted mechanisms of fatigue and the variables influencing the nature and extent of fatigue in each physiological
system. All tennis matches possess differing demands just as all conditioning sessions vary in their fatigue and recovery profiles.
Research in the area of fatigue is important, especially when dealing with competitive athletics and their methods of preparation. In such activities, management of fatigue and recovery may play a fundamental role in achieving optimal competitive outcomes. Each distinct source of stress causes a specific type of fatigue. Consequently, if optimal recovery is to occur, fatigue must be addressed relative to the exertion it was caused by. The fatigue resulting from tennis play is largely specific to tennis. At the same time, tennis-related fatigue is likely to affect general physiological functions (e.g., cardiovascular fitness, thermoregulation, neuromuscular function, endocrine function) and tennis-specific functions (e.g., shoulder rotator cuff strength, side to side balance of muscular strength and hypertrophy, serve velocity and accuracy) 10, 11.
The Demands of Activity Determine the Fatigue Experienced And Recovery Needed
By nature, vigorous physical activity demands levels of exertion exceeding those observed in sedentary lifestyles. In order for players to realize the ultimate goal of success in tennis competition (i.e., reaching peak performance capability): regular, intense, and extensive preparation, competition, and personal sacrifice must occur 12. Many studies have sought to establish the physical attributes that comprise and
determine the ability to perform in the sport of tennis 11-18. By comparison, few studies have explored the sources of fatigue and the role of recovery in improving performance 19.
Fatigue can be described in terms of the physiological system it impacts (e.g. muscular, neurological, nutritional, hydrational, psychological, thermoregulatory). Such systemic fatigue is believed to negatively affect performance in terms of speed, power, agility, coordination, and specialized skills (such as serving) 16.
Correspondingly, tennis players have been shown to suffer stroke accuracy decrement as high as 81% with increasing play durations 11. Fernandez and
colleagues 14 have suggested playing style, gender, training status, playing surface, ball type, and environment as the primary contributors to fatigue in tennis.
Some fatigue factors come from conditions that cannot be determined with certainty before play, such as the psychological readiness of the player in response to the match conditions and opponent. Factors that affect the demands of play commonly include varying environmental conditions such as temperature and
humidity as well as strategic play-style differences between competitors. Finally, the physiological status of the player largely affects the stress experienced from a given match. Optimized recovery occurs when stress of play is minimized and rate of recovery is maximized. Therefore, emphasis is placed on practices that develop physiological resistance to fatigue and improve the physiological ability to repair
damage to tissues. This is where conditioning, practice, and other preparatory strategies become exceedingly important 11, 17.
A normal fatigue to recovery response pattern takes on a type of undulating staircase pattern. Acute fatigue results in a decrease in function that is followed by a “recovery” phase back to baseline. Chronic fatigue may be a sign of functional overreaching (i.e., planned overload with withdrawal for supercompensation) in a training program or nonfunctional overreaching where mistakes are made in preparation and conditioning or the stress of competition/practice is beyond the athlete’s ability to recover. Functional overreaching may be desirable if planned correctly, leading to supercompensation in affected physiological systems and a subsequent improvement in competitive capabilities (see Figure 1). Conversely, repeated exposure to non-functional overreaching (staying below the baseline without the ability to make a positive recovery response) can lead to an overtraining syndrome, which can last for months20-23. Thus, fatigue is a physiological event as seemingly varied as the physiological systems it can affect 24, 25. With effective training programs (e.g., periodized resistance training) and careful management of fatigue and recovery, long-term positive adaptations can occur, even during
TIME
Supercompensation
Adaptation Fatigue
Functional Overreaching
Non-Functional Overreaching leading to an Overtraining Syndrome
Figure 1. Fatigue pushes the body’s physiological status below baseline but with
optimal recovery physiological status will typically return to baseline within 24-48 h. With training the baseline (solid line) will gradually increase to higher levels resulting in an upward trend in fitness level. Functional overreaching (dashed line) will
intentionally push physiological status down for an extended period of time. With reductions in the volume or intensity or added rest, fitness level rebounds to a higher level. Non-functional overtraining (dotted line) will cause a downward trend in
physiological status without recovery which can potentially lead to overtraining, which may take months recovery from. Overtraining typically occurs as a result of mistakes in the training program design or an excessively overloading competitive schedule.
Physiological Basis of Fatigue and Recovery Damage and Repair
Damage
Recovery is partially related to the repair of the neuromuscular system in which damage occurs in response to force production. The extent of the disruption of motor units (i.e., motor neuron and associated muscle fibers) is related to the demands of the match (e.g., 4 hours in the heat) or the specific conditioning activity (e.g., sprint intervals vs. heavy lifting). Acute skeletal muscle tissue damage is related to “mechanical forces”, and subsequent (additional) damage is related to “chemical forces” (i.e. excessive chemical compounds and free radical scavenging etc.) which continues to break down membrane structures for several days after the mechanical stress has occurred 27-30. The magnitude of each will be related to the specific forces the body is exposed the amount of metabolic stress (e.g.,
inflammatory response, noxious chemicals, high temperature, etc.), and the competitors physiological status before damaging exertion. The mechanisms involved with damage and repair represent a fertile area of research, as factors of initiation and resolution remain complex and largely unknown 31.
The amount of disruption or damage spans a continuum from normal,
planned disruption as a training strategy to complete dysfunction as a consequence of serious injury (e.g., Grade II/III muscle strain). Tennis practice, competition, and strength and conditioning protocols can elicit injury across this continuum. Macro- type trauma due to injury that stops play is typically what conditioning programs
attempt to prevent. The micro-trauma (microscopic tears) muscle damage/disruption caused by tennis competition, practice, or its conditioning activities are what the body is challenged to repair and recovery from. Dramatically elevated heart rates, blood lactate concentrations, and localized pH decreases are indicative of intense physical stress and may offer guidance in predicting the extent of tissue damage.
The magnitude of injury is related to the loading of muscle, most notably the eccentric component (i.e., the lengthening of muscle under load) and any
subsequent chemical insults. The heavier the eccentric load the greater the potential for soreness and injury 27. More dramatic muscle damage arises from stress with a high eccentric component (e.g., running downhill, performing a new novel exercise), and is especially pronounced when the stressed tissue has not been exposed to training stress previously. Ranges of racquet movements can result in tissue damage when outside of the trained movement patterns. Damage is proportional to the intensity of the tensile force and also depends on the
biomechanical orientation of the tissue subjected to overload (e.g., stress stain curve of the involved tissue).
Damage to skeletal muscle tissue is characterized by a loss of structural arrangements in muscle, which results in loss of force production capabilities. Mechanical stress causes damage to the basic contractile unit of the muscle called the “sarcomere”, which consist of the contractile proteins “actin and myosin” along with many non-contractile proteins (e.g., titin, nebulin, Z proteins) that hold the actin and myosin in proper alignment for optimal muscle contraction (i.e., shortening of the
muscle fiber). When the fiber subjected to extreme loads, especially as it elongates or contracts eccentrically, damage to the protein structure of the sarcomere can occur. Microscopically, damage to muscle tissue is easily observed in the loss of parallel positioning of Z-lines that bracket each sarcomere. This process is called Z- line streaming and results in the inability of sarcomeres to generate contractile forces. If enough sarcomeres are damaged the muscle fiber can become ineffective in producing force. When a complete muscle tear occurs, the entire muscle will lose all contractile capability.
Complete loss of muscle function as a result of exercise damage is rare, yet many athletes can experience soreness and weakness due to the demands of sport and conditioning programs. Mechanical damage initiates a cascade of inflammatory responses and noxious chemical release, which is likely to cause soreness, pain and swelling 31. The key factors then become the amount of damage produced by the activity and the speed with which repair can take place. The greater the damage, the longer the time for repair (in worst cases, athletes may have to compete before they are fully recovered). Conversely, trained tissue will recover more quickly. Repeated bouts of overload without adequate repair may lead to chronic overuse injuries or serious musculotendonous tears. Damaged tissue may suffer reductions in its’ stress-strain curve, meaning that lower amounts of force can cause the tissue to break.
Mechanical damage starts an inflammatory process that initiates the repair and healing process. This is part of recovery and the extent of its impact on training
and performance depends upon the magnitude of tissue damage and the development of physiological systems that facilitate the recovery process. The inflammatory process is complex and involves multiple systems (e.g., complement system, coagulation system, fibrinolysis system), the total process of which has been well documented (see 31. Historically, exercise physiologists spoke of “good inflammation” and “bad inflammation”, meaning that inflammation took place on a continuum of severity (completely relevant to our discussion on overload). Excessive inflammation leads to excessive swelling and free radical damage. Collateral
damage that cannot be stopped in a timely manner is treated with interventions that seek to reduce that amount of inflammation and speed its resolution (e.g. rest, ice, compression, elevation).
The well-known signs of inflammation are pain, redness, and swelling (edema) resulting from increased fluid flow to the area. Such fluids include blood (which carries attracted white blood cells such as leukocytes and macrophages), important proteins (e.g. fibrin), and cytokines needed in the clean up and repair of the tissue 31. Additionally, inflammation is worsened by “chemical damage”
mediated by free radicals that cause tissue membrane breakdown several days after the original mechanical overload 30. For example, a group of reactions resulting from mechanical damage can help to form the superoxide radical that combines with iron to form reactive hydroxyl radicals that attack the polyunsaturated fatty acids in cell membranes. Such lipid peroxidation reactions are the basis of the cell membrane disruptions that occur with exercise 3019 suggested the use of free radical
concentrations as a way to quantify the magnitude of damage from physical exertion and as a possible indicator of readiness for physical exertion.
The pain arising from the damage and repair processes associated with conditioning and competition is commonly referred to as delayed onset of muscle soreness (DOMS), a classic response to mechanical overloads or continued chemical damage arising from free radical scavenging of muscle tissue 27, 28. The duration of DOMS is directly related to the amount of overload, amount of tissue damage, and the fitness level of the musculature affected. Delayed onset muscle soreness can last as long as 10 days with intense overloading of the muscle in untrained individuals, although trained individuals should experience DOMS for 24- 72 hours after overloading eccentric exercise 27, 28. Tennis play and conditioning activities that utilize progressive, periodized heavy resistance training should cause the tissue adaptations that reduce or eliminate DOMS resulting from competition or conditioning.
Repair
Muscle repair includes processes that range from the activation of genes to the differentiation of satellite cells and integration of myoblasts into injured fibers 31, 32. Ultimately, the body will address and rebuild damaged tissue in addition to adding additional “damage preventing” protein. Repair is initiated by damage but some tissues (i.e., connective and skeletal muscle) will be damaged to the extent that repair is not possible and cell necrosis or death results. The repair process is
initially signaled by many of the inflammatory responses occurring after exercise stress. Connective structures such as tendon tissue respond to progressive heavy resistance training with increased thickness and strength. However, the deformation and injury of such tissue is not a typical consideration of most exercise-induced repair processes. Skeletal muscle tissue is most susceptible to exercise-induced damage, which is why it is the primary focus of the damage and repair process 33, 34.
For skeletal muscle, the mechanical forces of muscular activity initiate the activation and differentiation of satellite cells found between the sarcolemma and basal lamina. Satellite cells break away from the muscle fiber and are attracted (i.e. chemotaxis) to the injured areas of fibers. Satellite cells undergo differentiation once they reach damaged muscle tissue, becoming fiber-sealing myoblast cells 32. This repair process is important for both normal growth and heavy resistance exercise- associated muscle hypertrophy. Satellite cells can contribute daughter myonuclei, which are needed for repair and growth of muscle fibers (see Figure 2 below). If there is a large enough break in the damaged muscle fiber, the ends may not be bridged by myoblasts, and the innervated portion of the fiber will survive while the other end of the fiber will degrade. “Neural spouting” from another motor unit can provide the neural connection 35, 36 to “reengage” contractile ability.
In skeletal muscle, repair depends upon signaling from the injured tissue, interactions with cytokines (e.g., IL-1, IL-6) and hormones (e.g., IGF-I), positioning of the injured fibers, and potential rescue of fiber lengths with neural sprouting 37, 38. The repair process operates continually in highly resistance-trained tennis players
subjected to lower levels of exercise-induced muscle damage. Managing the design of the training program and optimizing other preparatory factors (such as nutrition and hydration) ensures relatively expedient recovery of tissue structures 39-42.
Re‐inervated muscle regains contractile capability, adopting qualities to those of the sprouted motor neuron. Stress from high load activity MUSCLE TISSUE Non‐intact damaged tissue cannot re‐ approximate endpoints Chemotaxis signals neural sprouting Neural sprouting re‐innervates severed muscle tissue without a motor neuron that otherwise would die, Intact‐damaged tissue creates signaling factors for immune cells that digest damaged proteins and cause inflammation, necrosis, and stimulate new protein formation Once wound site debris is removed, Quiescent Satellite cells are activated and proliferate, circulating to receptors on wound sites Satellite cells fuse to damaged tissue, aligning to form myotubules New contractile protein formation occurs with myotube formation, pushing nuclei from central to peripheral locations REPAIRED MUSCLE TISSUE
Figure 2. Muscle Damage and Repair Model – Muscle subjected to large and
repetitive loads (especially eccentric) may become damaged. Different levels of damage necessitate different repair processes. Repair of damaged muscle tissue is facilitated by tissues affected by trauma and ligands present in lymph and circulatory systems that reached damaged tissues.
FATIGUE ARISES FROM THREE (3)MAJOR DOMAINS OF COMPETITIVE DEMAND 1. Motor unit recruitment to activate skeletal muscle 2. Environmental
3. Psychological demands
The number and type of motor units recruited will dictate the extent of the involvement of support systems (e.g., cardiovascular system, endocrine system, metabolic systems etc.) needed for optimal performance and recovery. For example, hitting a single serve is accomplished using highly engrained motor unit recruitment patterns, but musculoskeletal support systems involvement will be minimal if that is the only exercise performed. However, motor unit recruitment patterns while hitting a single serve at the end of a 3-hour competitive match may be very different due to alterations in available metabolic support and the amount of skeletal muscle damage that may have already occurred.
It should also be obvious that environmental stress can alter the demands of physical exertion, with heat stress being the most common challenge to both
performance and recovery processes in tennis 43-45. Psychological stress is typically related to situational stress responses (e.g., increased epinephrine concentrations prior to a match), perceptions of recovery, and the possible interference of optimal motor skills occurring in consequence 25, 46-48. Thus, it is important to appreciate that fatigue from physical exertion is a multivariate paradigm that must be examined within the “context” of the specific set of conditions or demands the athlete is placed under.
1. Motor unit recruitment to activate skeletal muscle
The recruitment of motor units (i.e., one alpha motor neuron and the skeletal muscle it innervates) containing skeletal muscle fibers of various types (i.e., Type I slow twitch or Type II fast twitch fiber types), number of fibers in the motor unit, activation thresholds, and metabolic capabilities dictate the ability to meet external demands for force and power. Inherent to motor unit activation is the “size principle”