(The information in this article was taken or adapted from the High Performance Coaching Program Study Guide.)
The physiological demands of tennis and how a player’s body responds to meet these demands are not as clearly understood as they are for some other sports and activities. Tennis is characterized by intermittent bouts of activity of variable intensities and durations. Moreover, the length of a match can be less than an hour to more than four hours. Tennis can also be played on a variety of surfaces with a wide range of accompanying environmental conditions. All of these factors play a significant role in defining the energy demands on the body and how a player must physiologically respond. Understanding these perspectives are important to a coach, so that s/he can help any player to appropriately and optimally train, compete and recover from play.
Many of the physiological processes related to play involve converting stored chemical energy to mechanical energy to in order to run, hit the ball, and recover. In addition, after each one of these bouts of muscular activity, the chemical energy in the muscles must be rapidly restored. These processes go on throughout a match and a player’s ability to continue playing depend in large part on one’s physiological capacity to efficiently and rapidly convert, utilize, and replenish energy as it is needed. This can become a great challenge as a match get goes on and the intensity of play increases. Moreover, as body temperature goes up, energy replacement and effective utilization becomes even more difficult.
One of the key components to maintaining optimal energy conversion and restoration throughout a match (i.e., endurance) is a player’s ability to take in and utilize oxygen. With a high aerobic capacity, the cardiovascular system is able to deliver oxygen effectively. Muscles will also adapt to be able to utilize oxygen more efficiently and rapidly. Thereby, a player can continue to play longer without fatiguing as rapidly or having to reduce the intensity of play as much. This, combined with the body’s ability to provide energy to the muscles at a very high rate to maintain power, speed, and strength (via adaptations of other physiological systems), will give a player a tremendous advantage over one who is less physiologically well trained and conditioned.
MEETING THE ENERGY DEMAND
The body has three systems that continually function together to provide the energy needed for muscle activity and replenish energy stores. These are the immediate, non-oxidative glycolysis, and oxidative metabolic systems. The former are considered as anaerobic (not dependent on oxygen) energy systems and the third refers to the body’s aerobic (in the presence of oxygen) energy system. All three are important during tennis play.
ATP – The Energy Building Block
The immediate source of energy for all muscle contractions is a chemical called adenosine triphosphate (ATP). ATP is a high-energy molecule that stores a considerable amount of energy within the chemical bonds that hold the molecule together. The energy needed for muscle contraction is produced when an ATP molecule is broken down into adenosine diphosphate (ADP) and phosphate. Typically, for ADP to be “recharged” so that it can produce more energy, the ADP and phosphate must be chemically reattached to reform ATP. The three energy system described below all function to replenish the ATP stores in a muscle.
Immediate Energy System
As ATP is used, another chemical called creatine phosphate (CP) is available to be broken down (into creatine and phosphate) to almost immediately replenish a muscles’ ATP supply. The ongoing processes of ATP breakdown and restoration of ATP by the separation of CP are anaerobic – in other words no oxygen is required for, or directly involved in, either of these processes. The CP system is used to fuel short, intense activities that last for several seconds or so. The CP reserve is about 5-6 times that of ATP, so a player has plenty of CP to effectively restore energy (ATP) during most points and movements on the court. One molecule of ATP is reformed for every molecule of CP that is broken down.
The CP system is very effective and sufficient process for meeting the short-term energy demands of tennis. However, to keep going, the assistance of two other energy-providing systems is needed. Moreover, these other support energy systems are relied on as well to restore muscle energy during and between points when the demand for energy in a given muscle or muscle group is less.
Carbohydrate, in the form of glucose or glycogen (stored form in the liver or muscles) can be biochemically broken down to provide energy in the form of ATP via a process called glycolysis. This energy system is also an anaerobic system and does not require the presence of oxygen to create ATP. The fast glycolytic system provides most of the energy for activities lasting up to 2 minutes. The CP system, as mentioned, provides the initial burst of energy in any activity. However, if the demand for muscle energy requires ATP to be provided at a fast rate for a longer period of time, then the fast-glycolytic system will be used. Using the fast-glycolysis system, a molecule of glucose is used to produce 2 molecules of ATP.
This system is also sometimes referred to as the lactic acid system, since lactate is a byproduct of this form of metabolism. This energy system might be used when a player runs hard from side to side in order to hit a series of ground strokes. If the duration and intensity of points continues to increase (or stay high), then the proportionate reliance on this lactic acid producing system increases. Even though the oxidative (aerobic) metabolism system will also function to provide energy to working muscles, the rate of ATP production via aerobic metabolism is not fast enough to keep up with this temporary high demand. Fortunately, such a high demand is not usually sustained for very long periods of time. If it did, then a high level of lactic acid would accumulate in the muscles and fatigue would set in quickly. Because, tennis is so intermittent and a repeated high demand in a given group of muscles occurs so irregularly and infrequently, the lactic acid level in a tennis player during play rarely gets very high.
As effective as the immediate and non-oxidative glycolysis systems are for providing rapid energy during tennis, the oxidative (aerobic) metabolism system can supply substantially more energy and plays a critical role throughout play. As an example, each molecule of glucose that is broken down by aerobic metabolism produces 38 molecules of ATP. Carbohydrate, fat, and even protein can be used to fuel aerobic metabolism. Use of these nutrients to provide energy via the utilization of oxygen will lessen the burden on glycolysis alone and thus a player will be able to provide and restore energy for a much longer period before feeling the effects of carbohydrate depletion or lactic acid accumulation. Again, even when the intensity of play is high and anaerobic glycolysis is called upon to meet the greater muscle energy demand, oxidative metabolism continues at the same time. When the intensity of a point is lessened (and certainly between those hard points), aerobic metabolism is already working to rapidly help the muscle cells to recover. A sufficient aerobic capacity will help a player to recover faster – during and between points. Overall, the aerobic (oxidative) metabolism system is the primary source of ATP production and energy restoration during a tennis match – even though the other anaerobic (non-oxidative) systems are continually functioning as well.
Despite the intermittent nature of tennis, continuous singles play (i.e., without a lot of extended breaks and rest periods) can provide, in itself, a cardiorespiratory conditioning effect, if one plays often and regularly enough. This not only yields an increase in a player’s aerobic power (capacity), but there can be myriad other beneficial physiological adaptations as well that could enhance performance and reduce certain health risks (e.g., lower body fat, enhanced heart efficiency, lower blood pressure, and enhanced blood lipid profile). However, to meet the on-court physiological demands and develop the level of aerobic fitness required of today’s high-level players, a certain amount of supplemental aerobic conditioning is often necessary.
There is a wide array of on- and off-court exercises that will enhance a player’s aerobic capacity (endurance). Endurance training can include moderate- to high-volume, interval or continuous, on- or off-court tennis-specific drills, as well as the more traditional forms of distance running and cycling. The key to enhancing the aerobic system is keeping the heart rate elevated for a fairly long time (either continuous or with higher intensity intervals and short rest periods between). Generally, the player should keep the intensity below the lactic acid threshold (or the intensity when the aerobic system can not meet the player’s energy needs and more energy is produced via fast-glycolysis). In other words, you do not want to go so hard that your body has to rely on anaerobic glycolysis to a great degree, where you end up producing a lot of lactic acid. Breathing rate (if it is too high) can be an indicator or crossing the anaerobic threshold. However, monitoring heart rate is usually a more effective way, especially without having the option of using more sophisticated measuring tools. You and your players need to be able to monitor training intensities to ensure that the aerobic energy system is the one being primarily and sufficiently stimulated and stressed. Whether you use a high-tech heart rate monitor or simply feel the radial (wrist) or carotid (neck) pulse, simply monitoring heart rate can give you and your players invaluable information about exercise intensity.
The recommended range for heart rate during an activity to promote aerobic fitness is between approximately 60 and 85% of the individual’s maximum heart rate. Despite the variability in maximal heart rate within specific age ranges, the following formula can be used to estimate one’s heart rate range for enhancing aerobic fitness:
(220 – age of player) x .60 = lowest heart rate (60%)
(220 – age of player) x .85 = highest heart rate (85%)
For example, a 20-year-old player would want to train at an intensity where his or her heart rate would be between 120 and 170 beats per minute. Keep in mind, when you are training in the heat, there is usually a compensatory increase in heart rate, due to the body having to maintain cardiac output while regulating temperature at the same time. In addition, such training should occur several times a week for a duration of at least 20-30 minutes each time, depending on a player’s current fitness and aerobic enhancement goals.
Some additional tips for improving aerobic fitness can be found from the American College of Sports Medicine’s Position Statement (1998) that gives general recommendations for improving health and fitness levels, as well as other related sources. See www.acsm.org (American College of Sports Medicine).
Anaerobic exercise is characterized by brief, high-intensity periods of activity, where energy metabolism during the activity periods is predominantly provided by the systems that do not involve oxygen. Anaerobic training would include a series of these activities, each one followed by a specified period of rest to allow for replenishment of the anaerobic energy stores within the working muscles. Such exercises can include a variety of high-intensity, short-duration activities on the court or a variety of running drills/sprints at a track or open field. The work-to-rest ratio does not have to mimic that of tennis (e.g., 10 seconds on, 25-30 seconds off). In fact, just like with aerobic conditioning, to get the body to adapt in ways beyond what tennis in itself would do (i.e., enhance anaerobic capacity and tolerance further), a player needs to extend beyond the demands that would be encountered on the court (i.e., overload). For example, high intensity running intervals could range from 2 seconds to 30 seconds. Just keep in mind that the longer the activity (work) interval is, the longer the recovery (rest) interval should be. Also keep in mind that the higher the intensity, the more repeated exercises you perform, and the shorter the rest intervals are – the greater demand there is on the body. As important and beneficial as anaerobic interval training can be for helping to optimally develop a tennis player, such training is very demanding. Without sufficient preparation, progression (within the session and over longer periods of time), warm-up, and recovery, too much of this type of training can readily lead to injury.
Activities that can be used to enhance a player’s anaerobic capacity could include:
• Wind sprints
• Line drills
• Spider drill
• Alley hops
• Side shuffles
• Ladder drills
• Split-step progressions
• Hexagon drill
• Circuit training
FLEXIBILITY TRAINING (STRETCHING)
One of the most misunderstood training modes in most players’ programs is stretching. Having tennis players stretch not only can prevent injuries but also can enhance performance. Flexibility is simply defined as the ability of the body’s muscles and soft tissue to elongate or lengthen to allow movement around a joint. The two primary forms of stretching are static and dynamic, both of which can be used effectively and safely by tennis players. Other types of stretching include proprioceptive neuromuscular facilitation and ballistic stretching.
Static stretching is one of the safest and most effective forms of stretching. It has been used by athletes for many years and has been proven to increase muscle length and motion of the joints. Characteristics of static stretches include the following:
• The body is positioned to isolate a muscle or group of muscles.
• Slow, controlled movements are used.
• A period of holding or static posturing is used to stretch the muscle at its end range or near the limit of available motion.
Static stretching typically is done in a particular sequence that has been found to enhance the muscles’ ability to elongate:
• Perform a general body warm-up prior to performing static stretching. This can be doing light calisthenics, riding a cycle, or jogging slowly around the court until a very light sweat appears.
• Perform static stretches using positions that safely place a muscle or muscle group in an elongated position.
• Hold the stretch for approximately 15 to 30 seconds. Research has not demonstrated any benefit to holds longer than 30 seconds, nor have periods of stretching less than 10 seconds produced effective tissue lengthening.
• Repeat the stretch.
While recent research has brought into question the effectiveness of static stretching immediately before a sport or explosive movement (such as running or jumping at maximum intensity), sports medicine professionals still endorse the use of static stretching due to its ability to improve muscles’ flexibility. Whenever possible, players should stretch both before and especially after exercise training or tennis play/competition. Stretching the muscles after exercise or tennis play helps to minimize the post-exercise soreness and stiffness players often encounter after a hard workout or lengthy match.
Stretching should become a regular part of the players’ training program. An evaluation by a sports medicine professional can assist players in identifying areas that need particular attention in their stretching programs, with regular testing and evaluation being an important part of any player’s long-range development.
Dynamic stretching is a form of stretching that is used to prepare the body’s musculature for specific activities. Instead of specific postures and static positions, dynamic stretching actually uses sport-specific movement patterns, which are typically performed after a general warm-up, such as riding a stationary bike. The movements used in a dynamic stretching routine can be actual tennis strokes, jogging in place, side-to-side shuffles, butt kicks, or movements mimicking actual tennis movement patterns.
The idea behind dynamic stretching exercises is to warm up the body and raise a player’s core temperature. At the same time, these exercises move muscles and joints through movement patterns that actually will be encountered during the sport activity. This prepares the muscles and joints for the stresses of the game.
Flexibility programs typically incorporate both static and dynamic flexibility exercises, as in this example:
• Dynamic stretches
• Sport activity
• Static stretches after sport activity
Proprioceptive Neuromuscular Facilitation
An advanced concept in stretching that is now used by many coaches and trainers is proprioceptive neuromuscular facilitation (PNF). This type of stretching uses a partner and involves contracting and relaxing a muscle or muscle group to elicit a greater stretch. During this type of stretching the athlete is actually contracting the muscle isometrically near the end of the available movement pattern, then subsequently relaxing it as the partner moves the limb farther into the stretch pattern. Here are the steps you would take as a partner in this type of stretching:
• Take the limb to right near the end of the available range of motion.
• Ask the player to isometrically contract the muscle for up to 6 seconds as you are stretching and to resist any movement during the contraction.
• Have the player relax the muscle, then slowly move the limb farther in the direction you are trying to stretch for 15 to 30 seconds.
• Repeat the sequence.
The theory behind this type of stretching is that a period of deeper muscle relaxation occurs immediately following a contraction of that muscle. Neurologically speaking, the muscle will best be able to relax after it has been stimulated or asked to contract. This method of stretching takes advantage of this relaxation period to obtain a better stretch of the muscle or muscle group. It can be particularly effective for players who are having difficulty with a particular muscle group or who find it very difficult to relax their muscles while stretching.
One type of stretch that is not recommended is the ballistic stretch. Ballistic stretches involve fast, sudden movements at the end of a joint’s range of movement that are intended to improve flexibility. While some sources report improvement using ballistic stretches, doing this type of stretch increases the chances of injury. Safer forms of static and dynamic stretches that use smoother, less jerky movements without bouncing are preferable.
You will be better able to assist players with their strength training programs if you understand several key concepts: the types of muscle contractions, the body’s adaptations to strength training, how to determine the appropriate resistance level for strength training, and overload and progression.
Types of Contractions
Muscular contractions occur in three main forms: isometric, concentric, and eccentric.
Isometric muscular contractions occur when the muscle contracts but no joint movement occurs. Such contractions are used by the body to stabilize joints; but, as no joint motion is produced, they are not used to move body segments. Tennis players can perform isometric training as part of their training programs, but it is not considered very useful, as tennis requires complex movements and is a very dynamic activity.
Concentric muscular contractions occur when the muscle fibers actually shorten. This produces a shortening movement between where the muscle originates (starts) and inserts (ends, attaching to a bone). An example of a concentric muscular contraction would be the action of the bicep muscle during an arm curl exercise as the weight is brought up toward the shoulder or face. Concentric muscular contractions are typically used during the acceleration of body segments. For example, when the arm is accelerating forward during a serve, muscles in the front of the body, such as the pectorals, are contracting concentrically.
Eccentric muscular contractions occur when muscle fibers lengthen. This allows the body to decelerate or break a segment. For example, during the arm curl exercise mentioned earlier, the bicep muscle works eccentrically as the weight is lowered from the shoulder. If we were unable to perform eccentric contractions, the body would be unable to slowly control the lowering of a weight or object and uncontrolled shaky movements would occur. In tennis, eccentric contractions are used by the muscles in the back of the shoulder such as the rhomboids and rotator cuff during the follow-through phase of the serve (following ball impact) to slow the arm and maintain joint integrity and control. In general, eccentric contractions are thought to be stabilizing contractions that serve to absorb force and decelerate body segments.
Eccentric contractions are harder on the body than concentric contractions in terms of the stress imparted to tissue. While a person perceives that he or she is working less while walking down a steep hill or into the Grand Canyon, the muscle fibers and the structures around them are actually being stressed to a greater degree than when the individual is walking up a hill or out of the Grand Canyon. It is important to keep this in mind, as excessive eccentric muscle work can lead to extreme muscle soreness. This soreness typically presents itself 36 to 72 hours after exercise and has been termed delayed onset muscle soreness (DOMS). For this reason, eccentric training is usually used in a more limited fashion and combined with concentric muscle training to minimize the effects from delayed onset muscle soreness.
Strength training is an important component of a complete conditioning program for a tennis player, and it can greatly enhance performance when applied correctly. Here are a few general recommendations for training:
• Use both concentric and eccentric contractions.
• Have players perform three sets of 12 to 15 repetitions to train strength and endurance in the working muscles.
• Set rest periods between sets of exercise that is specific to the sport of tennis, generally between 20 and 30 seconds.
• Do not strength train immediately before skill-oriented practice or tennis play.
• Hold strength-training sessions after tennis or on days when tennis play is lighter or tennis is not played at all.
• Do not recommend maximal lifts, due to the risk of injury.
Adaptations to Strength Training
Two primary phases or adaptations occur in the body in response to strength training. These responses are termed neurological and morphological.
The neurological response occurs as the body’s nerves become more organized and better at communicating with the working muscles during a particular exercise or movement pattern. This response can occur quite rapidly and is usually the reason that players may say that they feel stronger in as little as two weeks after starting a strength-training program. This adaptation is very important, as the body is dependent on proper recruitment or communication between the nerves and the working muscles during all activities.
The morphological or structural adaptation involves actual change or adaptation within the muscles being exercised or trained. This adaptation is thought to involve both the enlargement of the existing muscle fibers that comprise a muscle and the splitting and development of new or more muscle fibers within a muscle. Scientists are still unsure of the exact amount and extent of each of these two adaptations; however, structural changes do occur within the muscle. This change takes at least six to eight weeks, however, and so people must adhere to a strength-training program for an extended period to see this result.
Setting the Resistance Level for Strength Training
One of the most difficult concepts for both players and coaches to understand is how to determine the amount of weight appropriate for strength training. One widespread recommendation is to apply the repetition maximum (RM) concept. This concept allows a player or coach to set the proper resistance level based on the goal of the exercise. In tennis, as stated previously, 12 to 15 reps per set are often used to promote muscular endurance and strength.
Therefore, using the RM concept, a player or coach can select a weight at which the player can perform 12 repetitions (12RM) or 15 repetitions (15RM) continuously, without breaking form, and at the conclusion of which the working muscles are fatigued.
To illustrate this concept, let us examine an example of a player doing a bicep curl with a free weight. If the player chooses an 85-pound weight, he or she may be able to do only one rep or may not even be able to lift the weight off the rack! This weight would not allow the player to perform 12 or 15 reps. Likewise, if the player chose a 2-pound weight, he or she could perform 12 to 15 reps and feel no fatigue afterward. Therefore, carefully selecting a resistance level somewhere between these two extremes would result in the player’s finding the optimum resistance level for the RM level being performed. In general, for a sport like tennis, it is safer to fatigue a muscle with repetitions than with excessive weight. Most players use too much weight to impress coaches and other players, and this practice can lead to early fatigue or even injury.
Overload and Progression
The previous discussion leads us to two final important concepts in physiology: overload and progression. These concepts apply to any type of training program. The concept of overload is that an exercise or activity must provide some type of overload to the system in order to improve the system’s function in terms of strength, endurance, speed, agility, or other qualities. For example, appropriate means for overloading in strength training might be free weights, tubing, weight machines, or one’s own body weight.
Here is an example of applying overload to a program. A player initially is given an alley hop-type exercise to improve lateral movement and lower extremity strength. However, after the player does the alley hop exercise in different formats for several workouts, she or he will need more resistance in order to overload the system. The coach can place a stretch cord or tubing around the player’s waist to resist the movement in each direction, forcing the player’s musculature to work against a greater resistance or overload.
The concept of progression is also critically important when designing any type of training program for tennis players. This concept states that any program must be continually progressed to allow the player to develop and improve beyond the initial improvements. Program progression includes increasing the duration, frequency, or intensity of the program being performed. Without progression, a resistance-training program becomes boring and does not continue to stimulate the muscles to develop additional strength or endurance. Both the amount of resistance and the number of repetitions can be increased to provide progression to a program.
KEY POINTS FOR SPORTS PHYSIOLOGY
• The three systems that continually function together to sustain muscle activity and restore energy are the immediate (ATP-CP), non-oxidative glycolysis, and oxidative metabolism systems.
• Traditional aerobic activities are ones in which large muscle groups are used in a rhythmic, repeated fashion for a prolonged period (e.g., distance running and cycling).
• Anaerobic activities are characterized by brief all-out performances followed by periods of rest and recovery.
• Monitor aerobic exercise intensity by checking the athlete’s heart rate during exercise. It should fall within the recommended heart rate range of 60 to 85% of maximal heart rate.
• For anaerobic exercise, monitor the work-to-rest ratio and be sure to not over work your players too much.
• Use both static and dynamic stretching to improve range of motion and prevent injury. You may also want to try proprioceptive neuromuscular facilitation. Avoid ballistic stretching techniques, which can cause injury.
• The three types of muscular contractions are isometric, concentric, and eccentric. Emphasize both concentric and eccentric muscular contractions when designing a strength-training program for tennis players, as both of these types of contractions are needed in high-level tennis play. Use a low-resistance, high-repetition format such as three sets of 12 to 15 repetitions to promote local muscular endurance.
• The body adapts to strength training in two ways: neurological adaptations, which can occur rather quickly upon initiation of a strength training program, and morphological adaptations, which take at least six to eight weeks.
• Set the resistance level for strength training at a weight at which the player can perform 12 to 15 repetitions and feel fatigue in the muscles after completing the exercise.
• Every strength-training program must provide overload and progression.
American College of Sports Medicine. 1998. The recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness, and flexibility in healthy adults. Medicine and Science in Sports and Exercise, 30(6): 975-991.