Skeletal muscle and exercise

Strength, endurance, and speed during exercise depend largely on the muscle’s ability to produce energy and force. This section examines how muscle accomplishes this task.

Muscle fiber types

Not all muscle fibers are alike. A single skeletal muscle contains fibers having different speeds of shortening and strength; slow-twitch, or type I, fibers, and fast-twitch, or type II, fibers. Slow-twitch fibers take approximately 110ms to reach peak tension when stimulated. Fast-twitch fibers, on the other hand, can reach peak tension in about 50ms. While the terms “slow twitch” and “fast twitch” continue to be used, scientists now preffer to use terminology tupe I and type II, as is the case here.

Although only one form of type I fiber has been identified, type II fibers can be further classified. The two major forms of type II fibers are fast-twitch type a(type IIa) and fast-twitch type x(IIx). Type IIx fibers in humans are approximately the equivalent of type IIb fibers in animals. If you look under the electronic microscope, type I fibers will be stained as black; type IIa fibers appear to be white, and type IIx fibers appear to be gray. Although not visible under the microscope, there is also third subtype of fast-twitch fibers that has been identified, IIc fibers.

The differences among the type IIa, type IIx, and type IIc fibers are not fully understood, but type IIa fibers are believed to be the most frequently recruited. Only type I fibers are recruited more frequently than type IIa fibers. Type IIc fibers are the least often used. On the average, most muscles are composed of roughly 50% type I fibers and 25% type IIa fibers. The remaining 25% are mostly type IIx, with type IIc fibers making only 1-3% of the muscle. Cause knowledge about type IIc is limited, there will be no further discussion about that thema. The exact percentage of each fiber types varies greatly in various muscles and among individuals, so the numbers listed here are only average values. This extreme variation is most evident in the athletes, as we will see later when comparison table appears.

Characteristics of type I and type II fibers

Different muscle fiber types play different roles in physical activity. This is largely due to differences in their characteristics.

ATPase

The type I and type II fibers differ in their speed of contraction. This difference results primarily from different forms of myosin ATPase. Recall that myosin ATPase in the enzyme that splits ATP to release energy to drive contraction. Type I fibers have a slow form of myosin ATPase, whereas type II fibers have a fast form. In response to neural stimulation, ATP is split more rapidly in type II fibers than in type I fibers. As a result, cross-bridges cycle more rapidly in type II fibers.

One of the methods used to classify muscle fibers is a chemical staining procedure applied to a thin slice of tissue. This staining technique measures the ATPase activity in the fibers. Thus, the type I, type IIa, and type IIx fibers stain differently. This technique makes it appear that each muscle fiber has only one type of ATPase, but fibers can have a mixture of ATPase types. Some have a predominance of type I-ATPase, but others have mostly type II-ATPase. Their appearance in a stained slide preparation should be viewed as a continuum rather than as absolutely distinct types.

A newer method for identifying fiber types is to chemically separate the different types of myosin molecules(isoforms) by using a process called gel electrophoresis. The isoforms are separated and stained to show the bands of protein that characterize type I, type II and type IIx fibers. Although our discussion here cathegorizes fiber type simply as slow twitch(type I) and fast twitch(type IIa and type IIx), scientists have further subdivided these fiber types. The use of electrophoresis has led to the detection of myosin hybrids or bigers that possess two or more forms of myosin. With this method of analysis, the fibers are classified as I; Ic(I/IIa); IIc(IIa/I); IIa; IIax; IIxa; and IIx.

The table below shows the characteristics of different muscle fiber types. The table also includes alternative names that are used in other classification systems to refer to the various muscle fiber types.

Table 1. Classification of muscle fiber types

Fiber classification
System 1	Type I		Type IIa		Type IIx
System 2	Slow twitch(ST)		Fast twitch a(FTa)		Fast twitch x (FTx)
System 3	Slow oxidative(SO)		Fast oxidative/glycolitic(FGO)		Fast glycolytic(FG)
Characteristics of fiber types
Oxidative capacity		High		Moderately High	Low
Glycolitic capacity		Low		High	Highest
Contractile speed		Slow		Fast	Fast
Fatique resistance		High		Moderate	Low
Motor unit strength		Low		High	High

Sarcoplasmic reticulum

Type II fibers have a more highly developed SR than do type I fibers. Thus, type II fibers are more adept at delivering calcium into the muscle cell when stimulated. This ability is thought to contribute to the faster speed of contraction(V_o) of type II fibers. On average, human type II fibers have V_o a that is five to six times faster than of type I fibers. Although the amount of force(P_o) generated by type II and type I fibers having the same diameter is about the same, the calculated power(μN x fiber length^-1 x s^-1) of a type II fiber is three to five times greater than that of type I fiber because of a faster shortening velocity. This may explain in part why individuals who have a predominance of type II fibers in their leg muscles tend to be better sprinters than individuals who have a high percentage of type I fibers.

Motor units

Recall that a motor unit is composed of a single alpha-motor neuron and muscle fibers it innervates. The alpha motor neuron appears to determine whether the fibers are type I or type II. The alpha motor neuron in a type I motor unit has smaller cell body and typically innervates a cluster of less or equal 300 muscle fibers. This difference in the size of motor units means that when a single type I alpha-motor neuron stimulates its fibers, far fewer muscle fibers contract than when a single type II alpha-motor neuron stimulates its fibers. Consequently, type II muscle fibers reach peak tension faster and collectively generate more force than type I fibers.

Distribution of fiber types

As mentioned earlier, the percentages of type I and type II fibers are not the same in all the muscles of the body. Generally, a person’s arm and leg muscles have similar fiber compositions. Studies have shown that people with a predominance of type I fibers in their leg muscles will likely have a high percentage of type I fibers in their arm muscles as well. A similar relationship exists type II fibers. There are some exceptions, however. The soleus muscle(beneath the gastrocnemius in the calf), for example, is composed of a very high percentage of type I fibers in everyone.

Fiber type and exercise

Because of these differences in type I and type II fibers, one might expect that these fiber types would also have different functions when people are physically active. Indeed, this is the case.

Type I fibers

In general, type I muscle fibers have a high level of aerobic endurance. Aerobic means “in the presence of oxygen”, so oxidation is an aerobic process. Type I fibers are very efficient at producing ATP from the oxidation of carbohydrate and fat.

Recall that ATP is required to provide the energy needed for muscle fiber contraction and relaxation. As long as oxidation occurs, type I fibers continue producing ATP, allowing the fibers to remain active. The ability to maintain muscular activity for a prolonged period is known as muscular endurance, so type I fibers have high aerobic endurance. Because of this, they are recruited most often during low-intensity endurance events(e.g. marathon running) and during most daily activities for which the muscle force requirements are low(e.g. walking).

Type II fibers

Type II muscle fibers, on the other hand, have relatively poor aerobic endurance when compared to type I fibers. They are better suited to perform anaerobically(without oxygen). This means that in the absence of adequate oxygen, ATP is formed through anaerobic pathways, not oxidative pathways.

Type IIa motor units generate considerably more force than do type I motor units, but type Iia motor units also fatique more easily because of their limited endurance. Thus, type Iia fibers appear to be the primary fiber type used during shorter, higher-intensity endurance events, such as the mile run or the 400m swim.

Although the significance of the type IIx fibers is not fully understood, they apparently are not easily activated by the nervous system. Thus they are used rather infrequently in normal, low-intensity activity but are predominantly used in highly explosive events such as the 100m dash and the 50m sprint swim. Characteristics of the various fiber types are set in the table below.

Table 1.2. Structural and functional characteristics of muscle fiber types

	Fiber type
Characteristic	Type I	Type IIa	Type IIb
Fibers per motor neuron	≤ 300	≥300	≥300
Motor neuron size	smaller	larger	larger
Nerve conduction velocity	slower	faster	faster
Contraction speed(ms)	110	50	50
Type of myosin ATPase	slow	fast	fast
Sarcoplasmic reticulum development	low	high	high

Determination of fiber type

The characteristics of muscle fibers appear to be determined early in life, perhaps within the first few years. Studies with identical twins have shown that muscle fiber type, for the most part, is genetically determined, changing little from childhood to middle age. These studies reveal that identical twins have nearly identical fiber types, whereas fraternal twins differ in their fiber type profiles. The genes we inherit from our parents likely determine which alpha-motor neurons innervate our individual muscle fibers. After innervation is established, muscle fibers differentiate(become specialized) according to the type of alpha-motor neuron that stimulates them. Some recent evidence, however, suggests that endurance training, strength training, and muscular inactivity may cause a shift in the myosin isoforms. Consequently, training may induce a small change, perhaps less than 10%, in the percentage of type I and type II fibers. Further, both endurance and resistance training have been shown to reduce the percentage of type IIx fibers while increasing the fraction of type Iia fibers.

Studies of older men and women have shown that aging may alter the distribution of type I and type II fibers. As we grow older, muscles tend to lose type II motor units, which increases the percentage of type I fibers.

Muscle fiber recruitment

When an alpha-motor neuron carries an action potential to the muscle fibers in the motor unit, all fibers in the unit develop force. Activating more motor units produces more force. When little force is needed, only a few motor units are stimulated to act. Recall from our earlier discussion that type Iia and type Iix motor units contain more muscle fibers than type I motor units do. Skeletal muscle contraction involves a progressive recruitment of type I and then type II motor units, depending on the requirements of the activity being performed. As the intensity of the activity increases, the number of fibers recruited increases in the following order, in an additive manner: type I à type IIa à type IIx.

Orderly recruitment of muscle fibers and the size principle

Most researchers agree that motor units are generally activated on the basis of a fixed order of fiber recruitment. This is known as the principle of orderly recruitment, in which the motor units within a given muscle appear to be ranked. Let’s use the biceps brachii as an example: assume a total of 200 motor units, which are ranked on a scale from 1 to 200. For an extremely fine muscle contraction requiring very little force production, the motor unit ranked number 1 would be recruited. As the requirements for force production increase, numbers 2,3,4; and so on would be recruited, up to a maximal muscle contraction that would activate most, if not all, of the motor units. For the production of a given force, the same motor units are usually recruited each time and in same order.

A mechanism that may partially explain the principle of orderly recruitment is the size principle, which states that the order of recruitment of motor units is directly related to their motor neuron size. Motor units with smaller motor neurons will be recruited first. Because the type I motor units have smaller motor neurons, they are the first units recruited in graded movement(going from very low to very high rates of force production). The type II motor units then are recruited as the force needed to perform the movement increases. It is unclear at this time how the size principle relates to complex athletic movements.

During events that last several hours, exercise is performed at a submaximal pace, and the tension in the muscles is relatively low. As a result, the nervous system tends to recruit those muscle fibers best adapted to endurance activity: the type I and some type IIa fibers. As the exercise continues, these fibers become depleted of their primary fuel supply(glycogen), and the nervous system must recruit more type IIa fibers to maintain muscle tension. Finally, when the type I and type IIa fibers become exhausted, the type IIx fibers may be recruited to continue exercising.

This may explain why fatique seems to come in stages during events such as the marathon, a 42km run. It also may explain why it takes great conscious effort to maintain a given pace near the finish of the event. This conscious effort results in the activation of muscle fibers that are not easily recruited. Such information is of practical importance to our understanding of the specific requirements of training and performance.

Use of muscles

Types of muscle contraction

Muscle movement generally can be cathegorized into three types of contractions – concentric, static and eccentric. In many activities, such as running and jumping, all three types of contraction may occur in the execution of a smooth, coordinated movement. For the sake of clarity, though, we will examine each type separately.

A muscle’s principal action, shortening, is reffered to as a concentric contraction, the most familiar type of contraction. To understand muscle shortening, recall our earlier discussion of how the thin and thick filaments slide across each other. In a concentric contraction, the thin filaments are pulled toward the center of the sarcomere. Because joint movement is produced, concentric contractions are considered dynamic contractions.

Muscles can also act without moving. When this happens, the muscle generates force, but its length remains static(unchanged). This is called a static, or isometric, muscle contraction, because the joint angle does not change. A static contraction occurs, for example, when one tries to lift an object that is heavier than the force generated by the muscle, or when one supports the weight of an object by holding it steady with the elbow flexed. In both cases, the person feels the muscles tense, but there is no joint movement. In a static contraction, the myosin cross-bridges form and are recycled, producing force, but the external force is too great for the thin filaments to be moved. They remain in their normal position, so shortening can’t occur. If enough motor units can be recruited to produce sufficient force to overcome the resistance, a static contraction can become a dynamic one.

Muscles can exert force even while lengthening. This movement is an eccentric contraction. Because joint movement occurs, this is also a dynamic contraction. An example of an eccentric contraction is the action of the biceps brachii when one extends the elbow to lower a heavy weight. In this case, the thin filaments are pulled further away from the center of the sarcomere, essentially stretching it.

Generation of force

Whenever muscles contract, whether the contraction is concentric, static or eccentric, the force developed must be graded to meet the needs of the task or activity. Using golf as an example, the force needed to tap in a 1m(~40 in.) putt is far less than that needed to drive the ball 100m (109yd) from the tee to the middle of the fairway. The amount of muscle force developed is dependent on the number and type of motor units activated, the frequency of stimulation of each motor unit, the size of the muscle, the muscle fiber and sarcomere length, and speed of muscle contraction.

Motor units and muscle size

More force can be generated when more motor units are activated. Type II motor units generate more force than type I motor units because a type II motor unit contains more muscle fibers than a type I motor unit. In a similar manner, larger muscles, having more muscle fibers, can produce more force than smaller muscles.

Frequency of stimulation of the motor units: rate coding

A single motor unit can exert varying levels of force dependent on the frequency at which it is stimulated. The smallest contractile response of a muscle fiber or a motor unit to a single electrical stimulus is termed a twitch. A series of three stimuli in rapid sequence, prior to complete relaxation from the first stimulus, can elicit an even greater increase in force or tension. This is termed summation. Continued stimulation at higher frequencies can lead to the state of tetanus, resulting in the peak force or tension of the muscle fiber or motor unit. Rate coding is the term used to describe the process by which the tension of a given motor unit can vary from that of a twitch to that of tetanus by increasing the frequency of stimulation of that motor unit.

Muscle fiber and sarcomere length

There is an optimal length of each muscle fiber relative to its ability to generate force. Recall that a given muscle fiber is composed of sarcomeres connected end to end and that these sarcomeres are composed of both thick and thin filaments. The optimal sarcomere length is defined as that length where there is optimal overlap of the thick and thin filaments, thus maximizing cross-bridge interaction. When a sarcomere is fully stretched(A) or shortened(E), little or no force can be developed since there is little cross-bridge interaction.

Speed of contraction

The ability to develop force also depends on the speed of muscle contraction. During concentric(shortening) contractions, maximal force development decreases progressively at higher speeds. When people try to lift a very heavy object, they tend to do it slowly, maximizing the force they can apply to it. If they grab it and quickly try to lift it, they will likely fail, if not injure themselves. However, with eccentric(lengthening) contractions, the opposite is true. Fast eccentric contractions allow maximal application of force. Eccentric contractions are shown at the left and concentric at the right.

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney