chapter Biomechanics of Resistance Exercise 4 Biomechanics of Resistance Exercise 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. Figure 4.5 Figure 4.7 Figure 4.8 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. 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 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 Key Term • Valsalva maneuver: The glottis is closed, thus keeping air from escaping the lungs, and the muscles of the abdomen and rib cage contract, creating rigid compartments of liquid in the lower torso and air in the upper torso. Joint Biomechanics: Concerns in Resistance Training • Shoulders – The shoulder is prone to injury during weight training because of its structure and the forces to which it is subjected. – Warm up with relatively light weights. – Follow a program that exercises the shoulders in a balanced way. – Exercise at a controlled speed. • Knees – The knee is prone to injury because of its location between two long levers. – Minimize the use of wraps. Joint Biomechanics: Concerns in Resistance Training • How Can Athletes Reduce the Risk of Resistance Training Injuries? – Perform one or more warm-up sets with relatively light weights, particularly for exercises that involve extensive use of the shoulder or knee. – Perform basic exercises through a full ROM. – Use relatively light weights when introducing new exercises or resuming training after a layoff of two or more weeks. – Do not ignore pain in or around the joints. (continued) Joint Biomechanics: Concerns in Resistance Training • How Can Athletes Reduce the Risk of Resistance Training Injuries? (continued) – Never attempt lifting maximal loads without proper preparation, which includes technique instruction in the exercise movement and practice with lighter weights. – Performing several variations of an exercise results in more complete muscle development and joint stability. – Take care when incorporating plyometric drills into a training program. Section Outline • Movement Analysis and Exercise Prescription Major Body Movements • Figure 4.16 (next two slides) – Planes of movement are relative to the body in the anatomical position unless otherwise stated. – Common exercises that provide resistance to the movements and related sport activities are listed. Figure 4.16 Reprinted, by permission, from Harman, Johnson, and Frykman, 1992. Figure 4.16 (continued) Reprinted, by permission, from Harman, Johnson, and Frykman, 1992. Key Point • Specificity is a major consideration when one is designing an exercise program to improve performance in a particular sport activity. The sport movement must be analyzed qualitatively or quantitatively to determine the specific joint movements that contribute to the whole-body movement. Exercises that use similar joint movements are then emphasized in the resistance training program.
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