Anatomy & Physiology Essentials 2e, Textbook Page 178 (194 of 624)

178 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. the nature of the work or exercise being done, how often the muscle is used, the muscle fiber composition of the muscle, and the temperature and humidity of the environment. Muscle fatigue is characterized by reduced ability to produce force and slowed contraction speed. Also, movements can vary significantly, the definition of muscular endurance is specific to each physical activity. In general, muscle fatigue can be thought of as the opposite of muscular endurance. The faster a muscle fatigues, the less endurance it has. A variety of factors affect the rate at which a muscle fatigues, including Calculating Muscle Force Fm (muscle force) W (weight) dW dF Joint center of rotation Do you know how much force your muscle must generate to hold a 5-pound weight in the position shown in the illustration? To hold the weight in this position, the torque at the elbow joint generated by the muscle (muscle torque) must balance the torque produced by the weight (weight torque) at the elbow. Muscle torque is the product of muscle force and the perpendicular distance of that force from the center of rotation at the joint. The formula is: Tm = Fm × dF where Tm = muscle torque, Fm = muscle force, and dF = the perpendicular distance. Weight torque is the product of the weight and the perpendicular distance of that weight from the center of rotation at the joint. This formula is: TW = W × dW where TW = weight torque, W = weight, and dW = the perpendicular distance. Suppose that the weight in the illustration is 5 pounds, and that it is being held at a distance (dW) of 12 inches from the center of the joint. The distance of the muscle attachment from the joint center (dF) is 1 inch. How much force must the muscle produce to support the weight? Tm = TW Fm × dF = W × dW Fm × 1″ = 5 lb × 12″ Fm = 60 lb Are you surprised? To support just 5 pounds in the hand, the muscle must generate 60 pounds of force. Because muscles attach so closely to joints, the human musculoskeletal system is designed more for movement speed than for strength. Now you try: Suppose that the weight (W) in this picture is 10 pounds, the dW is 15 inches, and the dF is 1 inch. What is the Fm? Figure 6.10 © Body Scientific International

Previous Page Next Page

Extracted Text (may have errors)

178 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. the nature of the work or exercise being done, how often the muscle is used, the muscle fiber composition of the muscle, and the temperature and humidity of the environment. Muscle fatigue is characterized by reduced ability to produce force and slowed contraction speed. Also, movements can vary significantly, the definition of muscular endurance is specific to each physical activity. In general, muscle fatigue can be thought of as the opposite of muscular endurance. The faster a muscle fatigues, the less endurance it has. A variety of factors affect the rate at which a muscle fatigues, including Calculating Muscle Force Fm (muscle force) W (weight) dW dF Joint center of rotation Do you know how much force your muscle must generate to hold a 5-pound weight in the position shown in the illustration? To hold the weight in this position, the torque at the elbow joint generated by the muscle (muscle torque) must balance the torque produced by the weight (weight torque) at the elbow. Muscle torque is the product of muscle force and the perpendicular distance of that force from the center of rotation at the joint. The formula is: Tm = Fm × dF where Tm = muscle torque, Fm = muscle force, and dF = the perpendicular distance. Weight torque is the product of the weight and the perpendicular distance of that weight from the center of rotation at the joint. This formula is: TW = W × dW where TW = weight torque, W = weight, and dW = the perpendicular distance. Suppose that the weight in the illustration is 5 pounds, and that it is being held at a distance (dW) of 12 inches from the center of the joint. The distance of the muscle attachment from the joint center (dF) is 1 inch. How much force must the muscle produce to support the weight? Tm = TW Fm × dF = W × dW Fm × 1″ = 5 lb × 12″ Fm = 60 lb Are you surprised? To support just 5 pounds in the hand, the muscle must generate 60 pounds of force. Because muscles attach so closely to joints, the human musculoskeletal system is designed more for movement speed than for strength. Now you try: Suppose that the weight (W) in this picture is 10 pounds, the dW is 15 inches, and the dF is 1 inch. What is the Fm? Figure 6.10 © Body Scientific International

Help

Chapter 6 The Muscular System 179 Copyright Goodheart-Willcox Co., Inc. the period during which the motor units must rest before responding to another nerve stimulus becomes longer. A muscle fiber reaches absolute fatigue once it is unable to develop tension when stimulated by its motor axon. SELF CHECK 1. What is measured to determine muscular strength? 2. What is measured to determine muscular power? 3. What is muscular endurance? 4. What influences muscular endurance? sirtravelalot/Shutterstock.com Figure 6.11 Success in the hurdles, combining running speed and jumping, requires muscular power. Supplements You Do Not Want to Take Some athletes and bodybuilders have succumbed to the temptation to take drugs to increase their muscle size, strength, and power production. The results have included devastating physiological and psychological side effects, as well as ruined professional careers and public disgrace. Famous athletes such as Lance Armstrong, Mark McGwire, Alex Rodriguez, José Canseco, Ben Johnson, and Marion Jones have been exposed for using banned drugs. The World Anti-Doping Code prohibits supplementing with all forms of anabolic agents at all times for athletes. Anabolic means “building up,” and refers to increasing bone and muscle mass. Different types of anabolic drugs have been developed for medical treatment of certain conditions. They are not intended for use by athletes and bodybuilders to build muscle. For example, anabolic androgenic steroids are synthetic versions of the male hormone testosterone. These drugs are designed for medical treatment of delayed puberty, some forms of impotence, and diseases that cause wasting of muscle tissue. When used inappropriately, however, these drugs can cause severe, long-lasting, and sometimes irreversible negative effects on health, including: • liver damage, including blood-filled cysts and tumors • kidney failure • elevated blood pressure and other cardiovascular effects contributing to early heart attack and stroke • stunted growth due to premature closure of epiphyses in long bones • abnormally increased aggressiveness • depression and suicide In children, these steroids can disrupt puberty. In adolescents, stunted growth and accelerated puberty changes can result from steroid use. Males may experience breast development, shrinking of the testicles, impotence, reduced sperm production, and an increased risk of prostate cancer. Females may experience irreversible deepening of the voice, cessation of breast development, male pattern hair growth, enlarged clitoris, and abnormal menstrual cycles. Another banned anabolic agent is human growth hormone (HGH). Synthetic human growth hormone is approved by the FDA for specific uses in children and adults. In children, HGH injections are approved for treating medical conditions that impair normal growth. In adults, HGH is used to treat conditions that prevent normal absorption of nutrients in the intestines or cause muscle wasting. Negative side effects of HGH include severe headaches, vision loss, acromegaly, high blood pressure, heart failure, diabetes, tumors, and crippling arthritis. Diuretics are also banned for use as anabolic agents. However, they are medically used for treatment of hyper- tension, kidney disease, and congestive heart failure. These drugs are sometimes taken by athletes trying to reduce body weight, notably in sports that classify competition by weight. The side effects of diuretics are dehydration, muscle cramps, dizziness or fainting, low blood pressure, and a loss of coordination and balance. Use of all of these drugs for purposes other than medical treatment is not only inadvisable, but also dangerous. Many of the identified side effects can also result in death. The intelligent, safe way to increase muscle size and strength is through a program of progressive resistance training. Clinical Application

Previous Page Next Page

Chapter 6 The Muscular System 177 Copyright Goodheart-Willcox Co., Inc. The more torque a muscle generates at a joint, the greater the tendency for movement of the bones at the joint. Machines called dynamometers measure joint torque. Joint torque, which is a measure of strength, is based solely on the resistance moved or matched. The speed with which a resistance is moved is not relevant to the strength measurement. Muscular Power The definition of mechanical power is force multiplied by velocity (force × velocity). Muscular power, then, is muscle force multiplied by muscle-shortening velocity during contraction. Notice, however, that neither muscle force nor shortening velocity can be measured from outside the body. Research dynamometers generate estimates of muscular power based on the resistance moved and the speed of the movement. Like muscular strength, muscular power is typically generated by several different muscles working collec- tively. Sprinting, along with the jumping and throwing events in track and field, are good examples of activities that require muscular power (Figure 6.11). Because force production and movement speed contribute equally to muscular power, the sprinter with the greatest leg strength may not necessarily be the fastest. Muscular Endurance Muscular endurance is the ability of a muscle to pro- duce tension over a period of time. The tension may be constant (for example, when a gymnast holds a motionless handstand), or it may vary cyclically (for example, during running, cycling, or rowing). Generally, the longer the physical activity is main- tained, the greater the required muscular endurance. Because the force and speed requirements of different Strength, Power, and Endurance In everyday conversation, people sometimes use the words strength and power interchangeably. However, muscular strength and muscular power are quite different concepts. This section examines that difference, as well as muscle fatigue and the related concept of muscular endurance, which is a little more complicated. Muscular Strength It may be tempting to think that muscular strength is the amount of force a given muscle can produce. It is impossible, however, to measure muscle force directly without penetrating the body. So to avoid invasive procedures, researchers use external measures, such as the amount of resistance a person can move, to establish an indirect measure of muscle strength. Remember that most joints in the human body are crossed by more than just one muscle. Additionally, many exercises involve more than one joint. This means that an index-of-strength measure such as maximum bench press actually assesses the collective work of several muscles that cross the shoulder and elbow. The main muscles that work during execution of a bench press include the pectoralis major, pecto- ralis minor, anterior deltoid, and triceps brachii. A more precise assessment of the strength of a muscle group at a given joint is the amount of torque, or rotary force, that the muscles can generate. Torque is the product of the size of a force and the perpendicular distance of that force from an axis of rotation. For the joint shown in Figure 6.10, the torque produced by a muscle is the product of muscle force and the perpendicular distance from the muscle attachment to the center of rotation at the joint. Unipennate Bipennate Multipennate B Pennate fiber arrangements Fusiform Bundled Triangular A Parallel fiber arrangements © Body Scientific International Figure 6.9 Fibers within a muscle may be arranged so that they are largely parallel (A) or pennate (B).

Previous Page Next Page

Extracted Text (may have errors)

Help

loading