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).