Biomechanics of Resistance Exercise chapter 2 Biomechanics of Resistance Exercise Jeffrey M. McBride, PhD

Chapter Objectives Identify the major components of skeletal musculature Differentiate the various types of levers of the musculoskeletal system Identify primary anatomical movements during sport activities and exercises Calculate linear and rotational work and power (continued)

Chapter Objectives (continued) Describe the factors contributing to human strength and power Evaluate resistive force and power patterns of exercise devices Identify factors of importance for joint biomechanics with exercise

Key Term biomechanics: The mechanisms through which components interact to create movement.

Skeletal Musculature 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) attachment.

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. synergist: A muscle that assists indirectly in a movement

Skeletal Musculature 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.

A Lever Figure 2.1 (next slide) The lever can transmit force tangential to the arc of rotation from one contact point along the object’s length to another. FA = force applied to the lever; MAF = moment arm of the applied force; FR = force resisting the lever’s rotation. MRF = moment arm of the resistive force. The lever applies a force on the object equal in magnitude to but opposite in direction from FR.

Figure 2.1

Key Term mechanical advantage: The ratio of the moment arm through which an applied force acts to that through which a resistive force acts. Greater than 1.0 means a person can apply less (muscle) force than the resistive force to produce an equal amount of torque. Less than 1.0 means a person must apply greater (muscle) force than the amount of resistive force present, creating a disadvantage for the muscle.

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

A First-Class Lever Figure 2.2 (next slide) O = fulcrum FM = muscle force FR = resistive force MM = moment arm of the muscle force MR = moment arm of the resistive force Mechanical advantage = MM /MR = 5 cm/40 cm = 0.125 Less than 1.0, so is a disadvantage

Figure 2.2

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 the muscle’s mechanical advantage, the required muscle force is smaller than the resistive force.

A Second-Class Lever Figure 2.3 (next slide) Plantarflexion against resistance (e.g., a standing heel raise exercise) FM = muscle force FR = resistive force MM = moment arm of the muscle force MR = moment arm of the resistive force When the body is raised, the ball of the foot, the point about which the foot rotates, is the fulcrum (O) Because MM is greater than MR, FM is less than FR

Figure 2.3

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 Figure 2.4 (next slide) Elbow flexion against resistance (e.g., a biceps curl exercise). FM = muscle force FR = resistive force MM = moment arm of the muscle force MR = moment arm of the resistive force Because MM is much smaller than MR, FM must be much greater than FR

Figure 2.4

The Patella and Mechanical Advantage Figure 2.5 (next slides) (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 2.5(a)

Figure 2.5(b)

Moment Arm and Mechanical Advantage Figure 2.6 (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 2.6

Moment Arm Figure 2.7 (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 2.7

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 tendons are much higher than those exerted by the hands or feet on external objects or the ground.

Skeletal Musculature 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 2.8 (next slides) Changes in joint angle with equal increments of muscle shortening (a) The tendon is inserted closer to the joint center. (b) The tendon is inserted 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 2.8(a)

Figure 2.8(b)

Anatomical Planes and Major Body Movements 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. (continued)

Anatomical Planes and Major Body Movements (continued) Figure 2.9 (next slide) The three planes of movement of the body in the anatomical position

Figure 2.9

Anatomical Planes and Major Body Movements Figure 2.10 (next slides) Major body movements 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 2.10 (continued)

Figure 2.10 (continued)

Human Strength and Power Basic definitions Strength: The capacity to exert force at any given speed. Acceleration: The change in velocity per unit of time. This is associated with resistive force by Newton’s second law (Force = Mass × Acceleration). (continued)

Human Strength and Power (continued) Positive work and power Power: Outside of the scientific realm, power is loosely defined as “explosive strength.” Also defined as the time rate of doing work. Power = Work / Time Work: The product of force exerted on an object and the distance the object moves in the direction the force is exerted. Work = Force × Displacement (continued)

Human Strength and Power (continued) Negative work Work performed on, rather than by, a muscle Occurs during eccentric muscle actions (continued)

Human Strength and Power (continued) Angular work and power Angular displacement The angle through which an object rotates Rotational work = Torque × Angular displacement

Key Points Although the word strength is often associated with slow speeds and the word power with high velocities of movement, both variables reflect the ability to exert force at a given velocity. The sport of weightlifting (Olympic lifting) has a much higher power component than the sport of powerlifting, due to the higher movement velocities with heavy weights of the weightlifting movements.

Human Strength and Power Biomechanical factors in human strength Neural control Recruitment affects maximal force output by determining which and how many motor units are involved in a muscle contraction. Rate coding affects maximal force output by determining the rate at which the motor units are fired. Muscle cross-sectional area (continued)

Human Strength and Power (continued) Biomechanical factors in human strength Arrangement of muscle fibers Pennate muscle: A muscle with fibers that align obliquely with the tendon, creating a featherlike arrangement. Angle of pennation: The angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion; 0° corresponds to no pennation. (continued)

Human Strength and Power (continued) Figure 2.11 (next slide) Muscle fiber arrangements and an example of each

Figure 2.11

Human Strength and Power Biomechanical factors in human strength Muscle length (at resting length) Actin and myosin filaments lie next to each other. A maximal number of potential crossbridge sites are available. The muscle can generate the greatest force. (continued)

Human Strength and Power (continued) Biomechanical factors in human strength Muscle length (when stretched) A smaller proportion of the actin and myosin filaments lie next to each other. Fewer potential crossbridge sites are available. The muscle cannot generate as much force. (continued)

Human Strength and Power (continued) Biomechanical factors in human strength Muscle length (when contracted) The actin filaments overlap. The number of crossbridge sites is reduced. There is decreased force generation capability.

Muscle Length and Actin and Myosin Interaction Figure 2.12 (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 crossbridges.

Figure 2.12

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 contractile 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.

Force–Velocity Curve Figure 2.13 (next slide) Force–velocity curve for eccentric and concentric actions

Figure 2.13

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. (continued)

Human Strength and Power (continued) 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.

Key Point In sport activities such as sprinting and jumping, the ratio of the strength of the muscles involved in the movement to the mass of the body parts being accelerated is critical. Thus, the strength-to-mass ratio directly reflects an athlete’s ability to accelerate his or her body.

Sources of Resistance to 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.

Key Point Exercise technique can affect the resistive torque pattern during an exercise and can shift stress among muscle groups.

Sources of Resistance to Muscle Contraction Gravity Weight-stack machines Gravity is the source of resistance, but machines provide increased control over the direction and pattern of resistance. Figure 2.14 (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 2.14

Sources of Resistance to Muscle Contraction Inertia Though the force of gravity acts only downward, inertial force can act in any 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. (continued)

Sources of Resistance to Muscle Contraction (continued) 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 opening. Elasticity The more an elastic component is stretched, the greater the resistance.

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.

Key Point The risk of injury from resistance training is low compared to that with other sport and physical conditioning activities.

Joint Biomechanics: Concerns in Resistance Training Back 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.

“Fluid Ball” Figure 2.15 (next slide) The “fluid ball” resulting from contraction of the deep abdominal muscles and the diaphragm

Figure 2.15

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. (continued)

Joint Biomechanics: Concerns in Resistance Training (continued) Knees The knee is prone to injury because of its location between two long levers. Minimize the use of wraps. Elbows and wrists The primary concern involves overhead lifts. However, the most common source of injury to these areas is from overhead sports such as throwing events or the tennis serve.