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skeletal muscle. By having insertions near the joints, changes in muscle length are amplified at the end of the bone being removed. This enables the muscle to cause large movements of the skeleton while always functioning near its own Lo. On the other hand, the muscle must develop forces larger than those that must be overcome at the end of the bone that the muscle must move. (Modified from Guyton AC. Textbook of medical physiology, 7th ed. Philadelphia: W.B. Saunders, 1986.)

skeletal muscle. By having insertions near the joints, changes in muscle length are amplified at the end of the bone being removed. This enables the muscle to cause large movements of the skeleton while always functioning near its own Lo. On the other hand, the muscle must develop forces larger than those that must be overcome at the end of the bone that the muscle must move. (Modified from Guyton AC. Textbook of medical physiology, 7th ed. Philadelphia: W.B. Saunders, 1986.)

also means that these muscles must generate forces greater than those that would be needed to directly affect the load at the end of the bone being moved; however, such an attachment has the advantage that small changes in muscle length are translated into large movements of the load.

Skeletal muscles differ from one another in their force-velocity relationships. Some (e.g., extensor digi-torum longus) contract more quickly than others (e.g., soleus). This difference is due to variations in the number and types of muscle cells that make up the various muscles in the body. Although there is a spectrum of velocities among various muscle cells, they have been divided into two main groups—fast twitch and slow twitch. Fast-twitch muscle cells generally are found in those muscles associated with rapid movement; slow-twitch cells are found in muscles associated more with endurance and posture. Many muscles are composed of a mixture of fast- and slow-twitch cells. Fast- and slow-twitch muscle cells differ in the

TABLE 1 Skeletal Muscle Cell Types

Fast twitch Slow twitch

Vmax High LOW

Myosin ATPase High Low

Glycolytic metabolism High Low

Oxidative metabolism Low High

Mitochondrial content Low High

Myoglobin content Low High contractile protein isoforms that are present and in the ATPase activities of the myosin isoforms; other differences are given in Table 1.

Although force-velocity relationships in any given muscle do not change acutely, they can change over long periods of time (days to years). This is seen during development from embryo to adult and in muscle that undergoes hypertrophy. In these conditions, the contractile protein isoforms that are expressed by the cells change. This in turn affects velocity of shortening.

Cardiac Muscle

Cardiac muscle contractions are graded in both frequency and force such that the cardiac output can vary from approximately 5 L/min at rest to 20 L/min during exercise. Factors that affect frequency influence the pacemaker and conducting cells of the heart (discussed in Chapter 11). The potential mechanisms that alter force have been presented above; however, as with skeletal muscle, not all mechanisms are operative. Force of cardiac contraction is not influenced by variations in the number of stimulated myocardial cells in the heart because the cells are electrically coupled and are excited as a unit (or syncytium), nor is force altered by the occurrence of summation and tetanus because these cannot occur. On the other hand, force is influenced by the length of the muscle before contraction and by changes in contractility.

Myocardial cells are not attached to rigid structures such as the skeleton, but form chambers (the atria and ventricles) that can be distended. Thus, myocardial cell length can change significantly. The length of cardiac muscle cells before contraction is determined by the volume of blood contained in the cardiac chambers at the end of diastole. In the normal case, the more blood, the greater the length; the greater the length, the more forceful the contraction. For this to be true, the volume of blood must never be enough to stretch the cells to lengths greater than Lo. Such extensive stretch is prevented by the appreciable passive force at lengths at and below Lo. The importance of this length-force relationship in regulating cardiac output is discussed in Chapter 14.

The force and velocity of myocardial cell contractions also are affected by the level of contractility that exists during the contraction. The level of contractility normally is determined by heart rate, by the levels of circulating hormones such as epinephrine, and by the level of activity of the sympathetic nervous system. Contractility is decreased in diseases such as congestive heart failure. Regulation of myocardial contractility also is discussed in more detail in Chapters 13 and 14.

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