William E. Prentice, PhD, PT, ATC, FNATA
After reading this chapter,
the athletic training student should be able to:
- Define muscular strength, endurance, and power, and discuss their importance in a program of rehabilitation following injury.
- Discuss the anatomy and physiology of skeletal muscle.
- Discuss the physiology of strength development and factors that determine strength.
- Describe specific methods for improving muscular strength.
- Differentiate between muscle strength and muscle endurance.
- Discuss differences between males and females in terms of strength development.
Following all musculoskeletal injuries, there will be some degree of impairment in muscular strength and endurance. For the athletic trainer supervising a rehabilitation program, regaining and, in many instances, improving levels of strength and endurance are critical for discharging and returning the patient to a functional level following injury.
By definition, muscular strength is the ability of a muscle to generate force against some resistance. Maintenance of at least a normal level of strength in a given muscle or muscle group is important for normal healthy living. Muscle weakness or imbalance can result in abnormal movement or gait and can impair normal functional movement. Resistance training plays a critical role in injury rehabilitation.
Muscular strength is closely associated with muscular endurance. Muscular endurance is the ability to perform repetitive muscular contractions against some resistance for an extended period. As we will see later, as muscular strength increases, there tends to be a corresponding increase in endurance. For the average person in the population, developing muscular endurance is likely more important than developing muscular strength because muscular endurance is probably more critical in carrying out the everyday activities of living. This statement becomes increasingly true with age.
Clinical Decision-Making Exercise 9-1
A softball pitcher was out for a whole season for rehabilitation following shoulder surgery. Why is it important that she regain all 3 aspects of muscular fitness?
TYPES OF SKELETAL MUSCLE CONTRACTION
Skeletal muscle is capable of 3 different types of contraction: isometric, concentric, and eccentric.21 An isometric contraction occurs when the muscle contracts to produce tension, but there is no change in muscle length. Considerable force can be generated against some immovable resistance even though no movement occurs. In a concentric contraction, the muscle shortens in length while tension increases to overcome or move some resistance. In an eccentric contraction, the resistance is greater than the muscular force being produced, and the muscle lengthens while producing tension. Concentric and eccentric contractions are considered dynamic movements.47
An econcentric contraction combines both a controlled concentric and a concurrent eccentric contraction of the same muscle over 2 separate joints.16 An econcentric contraction is possible only in muscles that cross at least 2 joints. An example of an econcentric contraction is a prone, open kinetic chain hamstring curl. The hamstrings contract concentrically to flex the knee, while the hip tends to flex eccentrically, lengthening the hamstring. Rehabilitation exercises have traditionally concentrated on strengthening isolated single-joint motions, despite the fact that the same muscle is functioning at a second joint simultaneously. Consequently, it has been recommended that the strengthening program includes exercises that strengthen the muscle in the manner in which it contracts functionally. Traditional strength-training programs have been designed to develop strength in individual muscles in a single plane of motion. However, because all muscles function concentrically, eccentrically, and isometrically in 3 planes of motion, a strengthening program should be multiplanar, concentrating on all 3 types of contraction.12
FACTORS THAT DETERMINE LEVELS OF MUSCLE STRENGTH, ENDURANCE, AND POWER
Size of the Muscle
Muscular strength is proportional to the cross-sectional diameter of the muscle fibers. The greater the cross-sectional diameter or the bigger a particular muscle, the stronger it is, and thus the more force it is capable of generating. The size of a muscle tends to increase in cross-sectional diameter with resistance training. This increase in muscle size is referred to as hypertrophy.50 A decrease in the size of a muscle is referred to as atrophy.
Number of Muscle Fibers
Strength is a function of the number and diameter of muscle fibers composing a given muscle. The number of fibers is an inherited characteristic; thus, a person with a large number of muscle fibers to begin with has the potential to hypertrophy to a much greater degree than does someone with relatively few fibers.50
Strength is also directly related to the efficiency of the neuromuscular system and the function of the motor unit in producing muscular force.17 Initial increases in strength during the first 8 to 10 weeks of a resistance training program can be attributed primarily to increased neuromuscular efficiency.24 Resistance training will increase neuromuscular efficiency in 3 ways: there is an increase in the number of motor units being recruited, in the firing rate of each motor unit, and in the synchronization of motor unit firing.35
Strength in a given muscle is determined not only by the physical properties of the muscle, but also by biomechanical factors that dictate how much force can be generated through a system of levers to an external object.30,58
Position of Tendon Attachment
If we think of the elbow joint as one of these lever systems, we would have the biceps muscle producing flexion of this joint (Figure 9-1). The position of attachment of the biceps muscle on the forearm will largely determine how much force this muscle is capable of generating.58 If there are 2 individuals, A and B, and A has a biceps attachment that is closer to the fulcrum (the elbow joint) than does B, then A must produce a greater effort with the biceps muscle to hold the weight at a right angle because the length of the effort arm will be greater than that for B.
The length of a muscle determines the tension that can be generated. By varying the length of a muscle, different tensions can be produced.58 Figure 9-2 illustrates this length– tension relationship. At position B in the curve, the interaction of the cross-bridges between the actin and myosin myofilaments within the sarcomere is at maximum. Setting a muscle at this particular length will produce the greatest amount of tension. At position A, the muscle is shortened, and at position C, the muscle is lengthened. In either case, the interaction between the actin and myosin myofilaments through the cross-bridges is greatly reduced, thus the muscle is not capable of generating significant tension.46
The ability to generate muscular force is also related to age.63 Both men and women seem to be able to increase strength throughout puberty and adolescence, reaching a peak around age 20 to 25 years, at which time, this ability begins to level off and, in some cases, decline. After about age 25 years, a person generally loses an average of 1% of his or her maximal remaining strength each year. Thus, at age 65 years, a person would have only about 60% of the strength he or she had at age 25 years.36 This loss in muscle strength is definitely related to individual levels of physical activity. People who are more active, or perhaps continue to strength train, considerably decrease this tendency toward declining muscle strength.63 In addition to retarding this decrease in muscular strength, exercise can also have an effect in slowing the decrease in cardiorespiratory endurance and flexibility, as well as slowing increases in body fat. Thus, strength maintenance is important for all individuals regardless of age for achieving total wellness and good health as well as in rehabilitation after injury.19
Overtraining in a physically active patient can have a negative effect on the development of muscular strength. Overtraining is an imbalance between exercise and recovery, in which the training program exceeds the body’s physiologic and psychological limits. Overtraining can result in psychological breakdown (staleness) or physiologic breakdown that can involve musculoskeletal injury, fatigue, or sickness. Engaging in proper and efficient resistance training, eating a proper diet, and getting appropriate rest can all minimize the potential negative effects of overtraining.
Fast-Twitch vs Slow-Twitch Fibers
All fibers in a particular motor unit are either slow- or fast-twitch fibers. Each kind has distinctive metabolic and contractile capabilities.36
Slow-twitch fibers are also referred to as Type I or slow-oxidative fibers. They are more resistant to fatigue than fast-twitch fibers; however, the time required to generate force is much greater in slow-twitch fibers.50 Because they are relatively fatigue resistant, slow-twitch fibers are associated primarily with long-duration, aerobic-type activities.
Fast-twitch fibers are capable of producing quick, forceful contractions but have a tendency to fatigue more rapidly than slow-twitch fibers. Fast-twitch fibers are useful in short-term, high-intensity activities, which mainly involve the anaerobic system. Fast-twitch fibers are capable of producing powerful contractions, whereas slow-twitch fibers produce a long endurance force. There are 2 subdivisions of fast-twitch fibers. Although both types of fast-twitch fibers are capable of rapid contraction, Type IIa fibers, or fast-oxidative-glycolytic fibers, are moderately resistant to fatigue, whereas Type IIb fibers, or fast-glycolytic fibers, fatigue rapidly and are considered the “true” fast-twitch fibers. Recently, a third group of fast-twitch fibers, Type IIx, has been identified in animal models. Type IIx fibers are fatigue resistant and are thought to have a maximum power capacity less than that of Type IIb but greater than that of Type IIa fibers.50
Ratio in Muscle
Within a particular muscle are both types of fibers, and the ratio of the 2 types in an individual muscle varies with each person.50 Muscles whose primary function is to maintain posture against gravity require more endurance and have a higher percentage of slow-twitch fibers. Muscles that produce powerful, rapid, explosive strength movements tend to have a much higher percentage of fast-twitch fibers.
Because this ratio is genetically determined, it can play a large role in determining ability for a given sport activity. Sprinters and weightlifters, for example, have a large percentage of fast-twitch fibers in relation to slow-twitch fibers.38 Conversely, marathon runners generally have a higher percentage of slow-twitch fibers. The question of whether fiber types can change as a result of training has, to date, not been conclusively resolved.60 However, both types of fibers can improve their metabolic capabilities through specific strength and endurance training.7
THE PHYSIOLOGY OF STRENGTH DEVELOPMENT
There is no question that resistance training to improve muscular strength results in an increased size, or hypertrophy, of a muscle. What causes a muscle to hypertrophy? A number of theories have been proposed to explain this increase in muscle size.51
First, some evidence exists that there is an increase in the number of muscle fibers (hyperplasia) as a result of fibers splitting in response to training.50 However, this research has been conducted in animals and should not be generalized to humans. It is generally accepted that the number of fibers does not seem to increase with training.50
Second, it has been hypothesized that, because the muscle is working harder in resistance training, more blood is required to supply that muscle with oxygen and other nutrients. Thus, it is thought that the number of capillaries is increased. This hypothesis is only partially correct: no new capillaries are formed during resistance training; however, a number of dormant capillaries might become filled with blood to meet this increased demand for blood supply.50
A third theory to explain this increase in muscle size seems the most credible. Muscle fibers are composed of primarily small protein filaments, called myofilaments, which are contractile elements in muscle. Myofilaments are small contractile elements of protein within the sarcomere. There are 2 distinct types of myofilaments: thin actin and thicker myosin myofilaments. Fingerlike projections, or cross-bridges, connect the actin and myosin myofilaments. When a muscle is stimulated to contract, the cross-bridges pull the myofilaments closer together, thus shortening the muscle and producing movement at the joint that the muscle crosses50 (Figure 9-3).
Clinical Decision-Making Exercise 9-2
A new high school track coach wants to train his best distance runner to compete in hurdling events. Based on what you know about muscle physiology, why might this be a difficult task?
As stated in Chapter 2, it is well accepted that satellite cells play a critical role in the ability of the muscle cell to hypertrophy.45 These self-renewing cells can generate a population of myoblasts that are able to fuse with existing myofibers to help in facilitating growth.59
These myofilaments increase in size and number as a result of resistance training, causing the individual muscle fibers to increase in cross-sectional diameter.58 This increase is particularly present in men, although women will also see some increase in muscle size. More research is needed to further clarify and determine the specific reasons for muscle hypertrophy.
If resistance training is discontinued or interrupted, the muscle will atrophy, decreasing in both strength and mass. Adaptations in skeletal muscle that occur in response to resistance training can begin to reverse in as little as 48 hours. It does appear that consistent exercise of a muscle is essential to prevent reversal of the hypertrophy that occurs from strength training.
Other Physiologic Adaptations to Resistance Exercise
In addition to muscle hypertrophy, there are a number of other physiologic adaptations to resistance training.20,31 The strength of noncontractile structures, including tendons and ligaments, is increased. The mineral content of bone is increased, thus making the bone stronger and more resistant to fracture. Maximal oxygen uptake is improved when resistance training is of sufficient intensity to elicit heart rates at or above training levels. However, it must be emphasized that these increases are minimal and that, if increased maximal oxygen uptake is the goal, aerobic exercise rather than resistance training is recommended. There is also an increase in several enzymes important in aerobic and anaerobic metabolism.21,31 All of these adaptations contribute to strength and endurance.
Clinical Decision-Making Exercise 9-3
Two football players of the same age have been following the exact same training plan. One is consistently able to perform a hamstring curl using more weight than the other. What could possibly be making him stronger at this task?
TECHNIQUES OF RESISTANCE TRAINING
Resistance training has been demonstrated to be a superior exercise modality for increasing muscle strength, local muscular endurance, power, hypertrophy, and motor performance.30 There are a number of different techniques of resistance training for strength improvement, including functional strength training, isometric exercise, progressive resistance exercise, isokinetic training, circuit training, plyometric exercise, and bodyweight exercise. Regardless of the specific strength-training technique used, the athletic trainer should integrate functional strengthening activities that involve multiplanar, eccentric, concentric, and isometric contractions.12
The Overload Principle
Regardless of which of these techniques is used, one basic principle of reconditioning is extremely important. For a muscle to improve in strength, it must be forced to work at a higher level than it is accustomed to. In other words, the muscle must be overloaded. Without overload, the muscle will be able to maintain strength as long as training is continued against a resistance to which the muscle is accustomed, but no additional strength gains will be realized. This maintenance of existing levels of muscular strength may be more important in resistance programs that emphasize muscular endurance rather than strength gains. Many individuals can benefit more in terms of overall health by concentrating on improving muscular endurance. However, to most effectively build muscular strength, resistance training requires a consistent, increasing effort against progressively increasing resistance.20,46
Resistance exercise is based primarily on the principles of overload and progression. If these principles are applied, all of the following resistance training techniques will produce improvement of muscular strength over time.
In a rehabilitation setting, progressive overload is limited to some degree by the healing process. If the athletic trainer takes an aggressive approach to rehabilitation, the rate of progression is perhaps best determined by the injured patient’s response to a specific exercise. Exacerbation of pain or increased swelling should alert the athletic trainer that the rate of progression is too aggressive.
Functional Strength Training
For many years, the strength-training techniques in conditioning or rehabilitation programs have focused on isolated, single-plane exercises used to elicit muscle hypertrophy in a specific muscle. These exercises have a very low neuromuscular demand because they are performed primarily with the rest of the body artificially stabilized on stable pieces of equipment.12 The central nervous system controls the ability to integrate the proprioceptive function of a number of individual muscles that must act collectively to produce a specific movement pattern that occurs in 3 planes of motion. If the body is designed to move in 3 planes of motion, then isolated training does little to improve functional ability. When strength training using isolated, single-plane, artificially stabilized exercises, the entire body is not being prepared to deal with the imposed demands of normal daily activities (walking up or down stairs, getting groceries out of the trunk, etc).49 Functional strength training provides a unique approach that has revolutionized the way the sports medicine community thinks about strength training. To understand the approach to functional strength training, the athletic trainer must understand the concept of the kinetic chain and must realize that the entire kinetic chain is an integrated functional unit. The kinetic chain is composed of not only muscle, tendons, fasciae, and ligaments, but also the articular and neural systems.
All of these systems function simultaneously as an integrated unit to allow for structural and functional efficiency. If any system within the kinetic chain is not working efficiently, the other systems are forced to adapt and compensate; this can lead to tissue overload, decreased performance, and predictable patterns of injury. The functional integration of the systems allows for optimal neuromuscular efficiency during functional activities.12 During functional movements, some muscles contract concentrically (shorten) to produce movement, others contract eccentrically (lengthen) to allow movement to occur, and still other muscles contract isometrically to create a stable base on which the functional movement occurs. These functional movements occur in 3 planes. Functional strength training uses integrated exercises designed to improve functional movement patterns in terms of not only increased strength and improved neuromuscular control, but also high levels of stabilization strength and dynamic flexibility.6
Unlike traditional strength-training techniques, which use barbells, dumbbells, or exercise machines and single-plane exercises day after day, a primary principle of functional strength training is to make use of training variations to force constant neural adaptations instead of concentrating solely on morphologic changes. Exercise variables that can be changed include the plane of motion, body position, base of support, upper or lower extremity symmetry, the type of balance modality, and the type of external resistance.49 Table 9-1 lists these exercise training variables. Figure 9-4 provides examples of functional strengthening exercises.
An isometric exercise involves a muscle contraction in which the length of the muscle remains constant while tension develops toward a maximal force against an immovable resistance61 (Figure 9-5). An isometric contraction provides stabilization strength that helps maintain normal length–tension and force– couple relationships that are critical for normal joint arthrokinematics. Isometric exercises are capable of increasing muscular strength. However, strength gains are relatively specific, with as much as a 20% overflow to the joint angle at which training is performed. At other angles, the strength curve drops off dramatically because of a lack of motor activity at that angle. Thus, strength is increased at the specific angle of exertion, but there is little corresponding increase in strength at other positions in the range of motion (ROM).
The use of isometric exercises in injury rehabilitation or reconditioning is widely practiced. There are a number of conditions or ailments resulting from trauma or overuse that must be treated with strengthening exercises. Unfortunately, these problems can be exacerbated with full ROM resistance exercises. It might be more desirable to make use of positional or functional isometric exercises that involve the application of isometric force at multiple angles throughout the ROM. Functional isometrics should be used until the healing process has progressed to the point that full-range activities can be performed.
During rehabilitation, it is often recommended that a muscle be contracted isometrically for 10 seconds at a time at a frequency of 10 or more contractions per hour. Isometric exercises can also offer significant benefit in a strengthening program.5
There are certain instances in which an isometric contraction can greatly enhance a particular movement. For example, one of the exercises in power weightlifting is a squat. A squat is an exercise in which the weight is supported on the shoulders in a standing position. The knees are then flexed, and the weight is lowered to a three-quarter squat position, from which the lifter must stand completely straight once again.
It is not uncommon for there to be one particular angle in the ROM at which continuous, smooth movement is difficult because of insufficient strength. This joint angle is referred to as a sticking point. A power lifter will typically use an isometric contraction against some immovable resistance to increase strength at this sticking point. If strength can be improved at this joint angle, then a smooth, coordinated power lift can be performed through a full range of movement.
Clinical Decision-Making Exercise 9-4
A weightlifter has been progressing his maximum bench press weight. However, he still requires a spotter to get him through the full ROM. He gets “stuck” at about 90 degrees of elbow extension. What can he do to progress through this limitation?
Progressive Resistance Exercise
A second technique of resistance training is perhaps the most commonly used and most popular technique for improving muscular strength in a rehabilitation program. Progressive resistance exercise uses exercises that strengthen muscles through a contraction that overcomes some fixed resistance such as with dumbbells, barbells, various exercise machines, or resistive elastic tubing. Progressive resistance exercise uses isotonic, or isodynamic, contractions in which force is generated while the muscle is changing in length.
Concentric vs Eccentric Contractions
Isotonic contractions can be concentric or eccentric. In performing a bicep curl, to lift the weight from the starting position, the biceps muscle must contract and shorten in length. This shortening contraction is referred to as a concentric or positive contraction. If the biceps muscle does not remain contracted when the weight is being lowered, gravity would cause this weight to simply fall back to the starting position. Thus, to control the weight as it is being lowered, the biceps muscle must continue to contract while, at the same time, gradually lengthening. A contraction in which the muscle is lengthening while still applying force is called an eccentric or negative contraction.
It is possible to generate greater amounts of force against resistance with an eccentric contraction than with a concentric contraction because eccentric contractions require a much lower level of motor unit activity to achieve a certain force than do concentric contractions. Because fewer motor units are firing to produce a specific force, additional motor units can be recruited to generate increased force. In addition, oxygen use is much lower during eccentric exercise than in comparable concentric exercise. Thus, eccentric contractions are less resistant to fatigue than concentric contractions. The mechanical efficiency of eccentric exercise can be several times higher than that of concentric exercise.47
Traditionally, progressive resistance exercise has concentrated primarily on the concentric component without paying much attention to the importance of the eccentric component.18 The use of eccentric contractions, particularly in rehabilitation of various sport-related injuries, has received considerable emphasis in recent years. Eccentric contractions are critical for deceleration of limb motion, especially during high-velocity dynamic activities.58 For example, a baseball pitcher relies on an eccentric contraction of the external rotators of the glenohumeral joint to decelerate the humerus, which might be internally rotating at speeds as high as 8000 degrees per second. Certainly, strength deficits or an inability of a muscle to tolerate these eccentric forces can predispose an injury. Thus, in a rehabilitation program, the athletic trainer should incorporate resistance training using eccentrically dominated movement patterns that have been shown to be effective in increasing maximal strength and power.39 Eccentric contractions are possible with all free weights, with the majority of isotonic exercise machines, and with most isokinetic devices. Eccentric contractions are used with plyometric exercise discussed in Chapter 11 and can also be incorporated with functional proprioceptive neuromuscular facilitation (PNF) strengthening patterns discussed in Chapter 14.