Biomechanics of Resistance Exercise

Biomechanics of Resistance Exercise chapter4 Biomechanicsof ResistanceExercise Everett Harman, PhD, CSCS, NSCA-CPT

Chapter Objectives • Identify the major bones and muscles of the human body. • Differentiate among the types of levers of the musculoskeletal system. • Calculate linear and rotational work and power. • Describe the factors contributing to human strength and power. • Evaluate resistive force and power patterns of exercise devices. (continued)

Chapter Objectives (continued) • Recommend ways to minimize injury risk during resistance training. • Analyze sport movements and design movement-oriented exercise prescriptions.

Section Outline • Musculoskeletal System • Skeleton • Skeletal Musculature • Levers of the Musculoskeletal System • Variations in Tendon Insertion • Anatomical Planes of the Human Body

Key Terms • anatomy: The study of components that make up the musculoskeletal “machine.” • biomechanics: The mechanisms through which these components interact to create movement.

Musculoskeletal System • Skeleton • Muscles function by pulling against bones that rotate about joints and transmit force through the skin to the environment. • The skeleton can be divided into the axial skeleton and the appendicular skeleton. • Skeletal Musculature • A system of muscles enables the skeleton to move. • Origin = proximal (toward the center of the body) attachment • Insertion = distal (away from the center of the body) attach-ment

Human Skeletal Musculature • Figure 4.1 (next slide) • (a) Front view of adult male human skeletal musculature • (b) Rear view of adult male human skeletal musculature

Figure 4.1

Key Terms • agonist: The muscle most directly involved in bringing about a movement; also called the prime mover. • antagonist: A muscle that can slow down or stop the movement.

Musculoskeletal System • Levers of the Musculoskeletal System • Many muscles in the body do not act through levers. • Body movements directly involved in sport and exercise primarily act through the bony levers of the skeleton. • A lever is a rigid or semirigid body that, when subjected to a force whose line of action does not pass through its pivot point, exerts force on any object impeding its tendency to rotate.

Key Term • first-class lever: A lever for which the muscle force and resistive force act on opposite sides of the fulcrum.

Figure 4.3

Key Term • second-class lever: A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm longer than that through which the resistive force acts. Due to its mechanical advantage, the required muscle force is smaller than the resistive force.

Figure 4.4

Key Term • third-class lever: A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm shorter than that through which the resistive force acts. The mechanical advantage is thus less than 1.0, so the muscle force has to be greater than the resistive force to produce torque equal to that produced by the resistive force.

A Third-Class Lever (the Forearm) • Figure 4.5 (next slide) • The slide shows elbow flexion against resistance (e.g., a biceps curl exercise). • FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = momentarm of the resistive force. • Because MM is much smaller than MR, FM must be much greater than FR.

Figure 4.5

The Patella and Mechanical Advantage • Figure 4.6 (next slide) • (a) The patella increases the mechanical advantage of the quadriceps muscle group by maintaining the quadriceps tendon’s distance from the knee’s axis of rotation. • (b) Absence of the patella allows the tendon to fall closer to the knee’s center of rotation, shortening the moment arm through which the muscle force acts and thereby reducing the muscle’s mechanical advantage.

Figure 4.6 Reprinted, by permission, from Gowitzke and Milner, 1988.

Moment Arm and Mechanical Advantage • Figure 4.7 (next slide) • During elbow flexion with the biceps muscle, the perpendicular distance from the joint axis of rotation to the tendon’s line of action varies throughout the range of joint motion. • When the moment arm (M) is shorter, there is less mechanical advantage.

Figure 4.7

Moment Arm • Figure 4.8 (next slide) • As a weight is lifted, the moment arm (M) through which the weight acts, and thus the resistive torque, changes with the horizontal distance from the weight to the elbow.

Figure 4.8

Key Point • Most of the skeletal muscles operate at a considerable mechanical disadvantage. Thus, during sports and other physical activities, forces in the muscles and ten-dons are much higher than those exerted by the hands or feet on external objects or the ground.

Musculoskeletal System • Variations in Tendon Insertion • tendon insertion: The points at which tendons are attached to bone. • Tendon insertion farther from the joint center results in the ability to lift heavier weights. • This arrangement results in a loss of maximum speed. • This arrangement reduces the muscle’s force capability during faster movements.

Tendon Insertion and Joint Angle • Figure 4.9 (next slide) • The slide shows changes in joint angle with equal increments of muscle shortening when the tendon is inserted (a) closer to and (b) farther from the joint center. • Configuration (b) has a larger moment arm and thus greater torque for a given muscle force, but less rotation per unit of muscle contraction and thus slower movement speed.

Figure 4.9 Reprinted, by permission, from Gowitzke and Milner, 1988.

Musculoskeletal System • Anatomical Planes of the Human Body • The body is erect, the arms are down at the sides, and the palms face forward. • The sagittal plane slices the body into left-right sections. • The frontal plane slices the body into front-back sections. • The transverse plane slices the body into upper-lower sections.

Figure 4.10

Section Outline • Human Strength and Power • Basic Definitions • Biomechanical Factors in Human Strength • Neural Control • Muscle Cross-Sectional Area • Arrangement of Muscle Fibers • Muscle Length • Joint Angle • Muscle Contraction Velocity • Joint Angular Velocity • Strength-to-Mass Ratio • Body Size

Human Strength and Power • Basic Definitions • strength: The capacity to exert force at any given speed. • power: The mathematical product of force and velocity at whatever speed.

Human Strength and Power • Biomechanical Factors in Human Strength • Neural Control • Muscle force is greater when: (a) more motor units are involved in a contraction, (b) the motor units are greater in size, or (c) the rate of firing is faster. • Muscle Cross-Sectional Area • The force a muscle can exert is related to its cross-sectional area rather than to its volume. • Arrangement of Muscle Fibers • Variation exists in the arrangement and alignment of sarcomeres in relation to the long axis of the muscle.

Human Strength and Power • Biomechanical Factors in Human Strength • Muscle Length • At resting length: actin and myosin filaments lie next to each other; maximal number of potential cross-bridge sites are available; the muscle can generate the greatest force. • When stretched: a smaller proportion of the actin and myosin filaments lie next to each other; fewer potential cross-bridge sites are available; the muscle cannot generate as much force. • When contracted: the actin filaments overlap; the number of cross-bridge sites is reduced; there is decreased force generation capability.

Muscle Length and Actin and Myosin Interaction • Figure 4.12 (next slide) • The slide shows the interaction between actin and myosin filaments when the muscle is at its resting length and when it is contracted or stretched. • Muscle force capability is greatest when the muscle is at its resting length because of increased opportunity for actin-myosin cross-bridges.

Figure 4.12

Human Strength and Power • Biomechanical Factors in Human Strength • Joint Angle • Amount of torque depends on force versus muscle length, leverage, type of exercise, the body joint in question, the muscles used at that joint, and the speed of contraction. • Muscle Contraction Velocity • Nonlinear, but in general, the force capability of muscle declines as the velocity of contraction increases. • Joint Angular Velocity • There are three types of muscle action.

Key Term • concentric muscle action: A muscle action in which the muscle shortens because the con-tractile force is greater than the resistive force. The forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it.

Key Term • eccentric muscle action: A muscle action in which the muscle lengthens because the contractile force is less than the resistive force. The forces generated within the muscle and acting to shorten it are less than the external forces acting at its tendons to stretch it.

Key Term • isometric muscle action: A muscle action in which the muscle length does not change because the contractile force is equal to the resistive force. The forces generated within the muscle and acting to shorten it are equal to the external forces acting at its tendons to stretch it.

Human Strength and Power • Biomechanical Factors in Human Strength • Strength-to-Mass Ratio • In sprinting and jumping, the ratio directly reflects an athlete’s ability to accelerate his or her body. • In sports involving weight classification, the ratio helps determine when strength is highest relative to that of other athletes in the weight class.

Human Strength and Power • Biomechanical Factors in Human Strength • Body Size • As body size increases, body mass increases more rapidly than does muscle strength. • Given constant body proportions, the smaller athlete has a higher strength-to-mass ratio than does the larger athlete.

Section Outline • Sources of Resistance to Muscle Contraction • Gravity • Applications to Resistance Training • Weight-Stack Machines • Inertia • Friction • Fluid Resistance • Elasticity • Negative Work and Power

Sources of Resistanceto Muscle Contraction • Gravity • Applications to Resistance Training • When the weight is horizontally closer to the joint, it exerts less resistive torque. • When the weight is horizontally farther from a joint, it exerts more resistive torque. • Weight-Stack Machines • Gravity is the source of resistance, but machines provide increased control over the direction and pattern of resistance.

Cam-Based Weight-Stack Machines • Figure 4.14 (next slide) • In cam-based weight-stack machines, the moment arm (M) of the weight stack (horizontal distance from the chain to the cam pivot point) varies during the exercise movement. • When the cam is rotated in the direction shown from position 1 to position 2, the moment arm of the weights, and thus the resistive torque, increases.

Figure 4.14

Sources of Resistance to Muscle Contraction • Inertia • When a weight is held in a static position or whenit is moved at a constant velocity, it exerts constant resistance only in the downward direction. • However, upward or lateral acceleration of the weight requires additional force. • Friction • Friction is the resistive force encountered when one attempts to move an object while it is pressed against another object.

Sources of Resistanceto Muscle Contraction • Fluid Resistance • Fluid resistance is the resistive force encountered by an object moving through a fluid (liquid or gas), or by a fluid moving past or around an object or through an orifice. • Elasticity • The more an elastic component is stretched, the greater the resistance. • Negative Work and Power • Negative work refers to work performed on, rather than by, a muscle. • The rate at which the repetitions are performed determines the power output.

Section Outline • Joint Biomechanics: Concerns in Resistance Training • Back • Back Injury • Intra-Abdominal Pressure and Lifting Belts • Shoulders • Knees

Joint Biomechanics:Concerns in Resistance Training • Back • Back Injury • The lower back is particularly vulnerable. • Resistance training exercises should generally be performed with the lower back in a moderately arched position. • Intra-Abdominal Pressure and Lifting Belts • The “fluid ball” aids in supporting the vertebral column during resistance training. • Weightlifting belts are probably effective in improving safety. Follow conservative recommendations.

Figure 4.15

Biomechanics of Resistance Exercise