GENERAL MUSCLE BIOLOGY

The body of an individual muscle is surrounded by noncontractile connective tissue called the epimysium. Within the muscle are bundles of fibers called fasciculi, which are surrounded by another noncontractile connective tissue called the perimysium. The endomysium is a noncontractile connective tissue that surrounds each individual muscle fiber. The individual muscle fibers are composed of myofibrils that lie parallel to each other and the muscle fiber itself (Fig. 4-1, A). The structural components of the myofibrils are called myofilaments, and they comprise two predominant proteins, actin and myosin. The functional, or contractile, unit of a muscle fiber cell is called the sarcomere (Fig. 4-1, B). Myosin (a thick protein) and actin (a thin protein) are actively involved with the mechanics of muscular contraction, which involves a complex and highly structured series of chemical and mechanical events.

The extraordinarily complex biochemical excitation–contraction coupling and mechanical actions of muscular contraction are described in physiology textbooks. In simple terms, the neurologic stimulus to contract a muscle causes the release of acetylcholine, which initiates the release of calcium. The calcium ions bond with troponin and tropomyosin, two proteins within the actin filaments. This allows actin–adenosine triphosphate (ATP) to react with myosin–adenosine triphosphatase (ATPase), producing energy so the thick myosin and thin actin filaments can “slide” past each other, generating tension and producing contraction of the muscle.

Muscle Fiber Types

Generally two distinct types of muscle fibers have been identified in humans. These fibers are classified by their contractile and metabolic characteristics (Fig. 4-2).

Slow twitch (ST) (type I, red oxidative) muscle fibers possess relatively large and numerous mitochondria, triglycerides, and oxidative enzymes (succinic dehydrogenase [SDH]), which allow for aerobic work. They also have relatively low myosin-ATPase and glycolytic activity, as well as slower calcium handling ability and shortening speed. This type of fiber is specialized for muscular endurance activities. These fatigue-resistant fibers contract slowly but are highly efficient for prolonged aerobic events.

Fast twitch (FT) (type II, white glycolytic) muscle fibers, by contrast, are anaerobic. These fibers are not as vascular as type I fibers, but they fire, or contract, at a higher speed than type I fibers and with more force. These fibers have a very high level of myosin-ATPase, which provides energy for speed of contraction and tension; they also have low myoglobin content and very few mitochondria. However, they are larger in diameter than red fibers. These fibers are used mainly in activities that require speed, strength, and power.

Type II fibers can be further broken down into three distinct subclassifications: type IIA; type IIAB; and type IIB.49,73 These fiber types differ mainly in terms of contraction velocity and endurance and are classified as intermediate fiber types with both aerobic and anaerobic capacities.

 The type IIA (fast-oxidative-glycolytic [FOG]) fiber has a fast contraction speed and a moderate capacity for energy transfer from both aerobic and anaerobic sources.

 The type IIB (fast-glycolytic [FG]) fiber possesses the greatest anaerobic capacity and the fastest shortening speed.

 The type IIAB fiber is rare and undifferentiated and may contribute to reinnervation and motor unit transformation.

The motor unit is the basic unit of movement. It consists of the anterior motor neuron and all the muscle fibers it innervates. A motor unit contains only one specific muscle fiber type. Motor unit recruitment is the adding of motor units to increase force. The Henneman size principle proposes an orderly recruitment of motor units within a motor neuron pool during a defined movement task.^36a When a low force is needed, only the slow twitch motor units (type I) are activated; and with increasing force requirements, larger and faster motor units (type IIA, type IIB) are recruited. Therefore the orderly recruitment of muscle fibers during contraction proceeds according to increased force requirements, as shown in Fig. 4-3.

TYPES OF MUSCLE CONTRACTIONS

The three true types of muscle contractions are concentric, eccentric, and isometric. Two other terms have been used to describe muscle contractions: isotonic and isokinetic. These are not types of contractions but rather terms used to describe events.

Concentric

In a concentric contraction, tension is produced and shortening of the muscle takes place (Fig. 4-4, A). The action produced by a concentric contraction brings together or approximates the origin and insertion of the contracting muscle. In a concentric exercise, tension is developed and shortening of the muscle occurs to overcome an external force, such as a weight.

DEFINITIONS OF STRENGTH AND POWER

Strength is a broad term. Generally strength is the ability of a muscle to generate force, or more specifically the maximum force generated by a single muscle or related muscle group.⁵⁰ Other definitions of strength include “The ability to exert force under a given set of conditions defined by body position, the body movement by which force is applied, movement type, and movement speed”³⁶ and “The maximal force a muscle or muscle group can generate at a specified velocity.”⁴⁵ The American Physical Therapy Association defines muscle strength as “the muscle force exerted by a muscle or group of muscles to overcome resistance under specific set of circumstances”; muscle performance as “the capacity of a muscle or group of muscles to generate forces”; muscle power as “the work produced per unit time or the product of strength and speed”; and muscle endurance as “the ability to sustain forces repeatedly or to generate forces over a period of time.”⁵ Functional strength has been described as the ability of the neuromuscular system to produce, reduce, or control forces, contemplated or imposed, during functional activities, in a smooth, coordinated manner.⁴¹

To help clarify strength clinically, it is perhaps most useful to consider strength in terms that describe performance.

Overall, these terms help describe resultant muscular performance as they relate to the development of strength.

MEASURING STRENGTH

Strength can be measured by six methods:

Manual muscle testing (MMT) is a isometric method of muscle testing that is designed to measure muscle strength requiring no equipment other than the examiner’s hands. This technique was introduced in the early 1900s and its use is widely accepted in the health care professions.⁶¹ It is used to generally grade a muscle’s isometric contraction capacity at a specific joint angle against a manually applied force or gravity. Performing a MMT requires extensive time, effort, and attention to detail while performing the correct technique to ensure that the results obtained are as accurate as possible.⁶¹ The tester must have a comprehensive and detailed understanding of kinesiology to accurately and consistently reproduce manual grading of muscle strength (performance). The grading scale for this test is clinically easy to use and is outlined in Table 4-1. The disadvantage of isometric strength testing is that because muscle length is held constant, isometric strength testing provides muscle strength data at only one point in the range.⁵⁹ MMT is valid from grades 0 to 5; however, when MMT scores exceed grade 4, the MMT loses it ability to discriminate between gradations of strength. In cases where measurement and documentation of strength level is critical above a MMT grade 4, an alternative form of measuring strength should be used.⁵⁹

Cable tensiometry is used to isometrically measure a muscle’s strength (Fig. 4-5). Essentially this tool is a mechanical form of manual muscle testing. The tensiometer provides the advantage of versatility for recording force measurements at virtually all angles of a joint’s ROM and may be more sensitive for grading muscle strength above grade 4. This method is used primarily to measure strength in normal subjects in research projects. Many tests were developed in the 1950s to describe static force or isometric strength by use of the cable tension method.^15,16

Dynamometry is used extensively in physical therapy. Hand-held dynamometers (Fig. 4-6) are used to quantify grip strength, and the standing-back dynamometer is used to evaluate back extension strength. In this latter example, many factors contribute to the subject’s ability to generate tension or force during the back pull, including the patient’s motivation, degree of pain (if any), arm length, leg length, height, weight, and the obvious contribution from other muscle groups. These variables make dynamometry an unreliable, nonspecific testing tool.

An isotonic one-repetition maximum lift is used to test strength using commercially available exercise equipment or barbells and dumbbells. In this method the patient performs a single, full ROM lift, such as a bench press (Fig. 4-7), shoulder press, or arm curl, for a particular muscle group. Applying this method is difficult because the tester and patient must first establish a reasonable starting weight through trial and error, fatigue becomes a factor if many trials are needed, and precise performance or execution of the proper lift is determined subjectively by the tester. This method is best used for normal subjects, in a sports medicine environment, or with uninvolved body parts not necessary for stabilization of a disabled joint.

Perhaps the most widely used and clinically relevant method of objective, reproducible strength testing is through isokinetics. The data collected with isokinetic testing document strength (force production), torque, power, and work.⁶⁰ As stated, isokinetics employs a fixed speed, or velocity, of movement that allows for maximum loading throughout the full ROM. If a patient experiences pain during any part of the test, or does not apply a maximum force throughout the entire ROM, the velocity remains constant with a variable resistance that is totally accommodating to the individual.¹⁹ To test for strength, slow speeds (30° to 60° per second) are generally used.⁴⁸ Because isokinetic equipment can be interfaced with computers, a hard-copy graph of the data can be used for evaluation and exercise prescription. In addition to being a valid and reliable tool for strength testing, isokinetics also can evaluate neuromuscular endurance, speed of muscle contraction, and muscular power.⁴⁰

The determination of an individual’s readiness to return to normal levels of activity is a common issue in rehabilitation. To resolve this issue, clinicians have incorporated functional testing following rehabilitation. Functional testing involves the evaluation of broad skills necessary to perform complex movements versus traditional methods that focus on isolated joint testing. This is particularly important when dealing with athletes who may be returned to activity too soon after rehabilitation because of inaccuracies in the assessment of their functional ability resulting from more traditional assessment methods.¹⁸

Examples include the following:

COMPARISON OF MUSCLE CONTRACTION TYPES

Generally, muscle contractions are characterized by the amount of tension the contraction produces and the amount of energy liberated (ATP use) by the contraction. The most common clinically applicable way to strengthen muscle is with concentric and eccentric contractions using isotonic (isoinertial)⁵⁵ progressive resistive exercise (PRE). Ankle or cuff weights, hand-held weights (dumbbells), and weight machines are examples of isotonic equipment used in physical therapy practice.^∗ Elftman²³ has demonstrated that the production of maximal force of contraction by various methods occurs in a predictable fashion, as seen in Fig. 4-8.

The force of contraction is expressed as the amount of tension developed per unit of contractile tissue. In terms of energy liberated (ATP use), eccentric muscle contractions use the least ATP, and concentric contractions use the most (Fig. 4-9).³

Based on this information, it appears that eccentric muscle contractions are more energy efficient and produce greater tension per contractile unit than both concentric and isometric contractions. However, Davies¹⁹ points out that much of the tension produced by eccentric muscle contraction results from stress imposed on the noncontractile serial elastic components (perimysium, epimysium, and endomysium) of the muscle. Therefore eccentric muscle contractions stimulate both contractile and noncontractile elements, whereas concentric contractions and isometrics focus on the contractile elements.⁶⁰

The PTA must consider the context in which each muscle contraction type is used clinically. Fundamentally implementing multiple muscle contraction types during all phases of rehabilitation is well supported.8,35,69 In comparing muscle contraction types, it is best to view the decision concerning which type to use, when to use it, and in what pathologic conditions it should be used as a progression or continuum rather than a choice of one type over another. Davies¹⁹ has described a classic model of exercise progression (Box 4-1) that can be used as a general guide. Certain criteria must be established for the progression from one type of contraction to another.

First, exercise variables and parameters must be understood so that necessary adjustments can be made in a patient’s exercise prescription (Box 4-2).The criteria established for progressing from one exercise mode to another is based on many factors and is patient specific. In general, pain usually dictates the time frame for progression, although swelling also does to a lesser degree. The sequence proceeds from the least intense to more challenging exercises with increased joint forces and metabolic demands.

Some of the advantages and disadvantages of concentric and eccentric isotonic exercise and isokinetic exercise equipment are outlined for general comparison in Table 4-2.

MUSCLE RESPONSE TO EXERCISE

Strength training must be individually tailored to meet the goals of recovery. As stated by DeLee and colleagues,²¹ “Function increases with use; functions we do not use, we lose. The intensity, duration, and frequency of activity are all related to the functional capacity that is developed.”

Muscle tissue morphology is mutable; that is, it has the ability to change. Muscle mutability has two distinct categories, hypertrophy and atrophy.

The stimuli for adaptive changes in skeletal muscle are described as frequency, intensity, and duration.¹² Human skeletal muscle responds and adapts to these stimuli and is characterized by the nature, rate, magnitude, and duration of the stimulus.⁸ In a clinical situation, the stimulus provided to human muscle is the conditioning or training program. These programs are based on certain principles that lead to the necessary adaptive changes, which in turn affect function. The principles of overload, specificity, and reversibility²⁵ as well as progression and transfer of training⁶ provide the foundation for the strength training programs used in physical therapy and are as follows.

The overload principle⁶ is the guiding principle of exercise prescription. If muscle performance is to improve, a load that exceeds the metabolic capacity of the muscle must be applied. A muscle must be challenged to perform at a level greater than that to which it is accustomed.

The specific adaptations to imposed demands²⁵ (SAID) principle