Biomechanics of Resistance Exercise

Everett Harman, PhD, CSCS, NSCA-CPT

chapter

4Biomechanicsof ResistanceExercise

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.

A Lever

• Figure 4.2 (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 armof 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 4.2

Key Term

• mechanical advantage: The ratio of the moment arm through which an applied force acts to that through which a resistive force acts. A mechanical advantage greater than 1.0 allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque. A mechanical advantage of less than 1.0 is a disadvantage in the common sense of the term.

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 (the Forearm)

• Figure 4.3 (next slide)– The slide shows elbow extension against resistance (e.g., a triceps

extension exercise).

– 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, which, being less than 1.0, is a disadvantage.

– The depiction is of a first-class lever because muscle force and resistive force act on opposite sides of the fulcrum.

– During isometric exertion or constant-speed joint rotation, FM · MM = FR · MR .

– Because MM is much smaller than MR, FM must be much greater than FR; this illustrates the disadvantageous nature of this arrangement.

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.

A Second-Class Lever (the Foot)

• Figure 4.4 (next slide)– The slide shows 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 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.

Planes of the Human Body

• Figure 4.10 (next slide)– The three planes of the human body in the

anatomical position

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.

Key Terms

• pennate muscle: A muscle with fibers that align obliquely with the tendon, creating a featherlike arrangement.

• angle of pennation: The angle betweenthe muscle fibers and an imaginary line between the muscle’s origin and insertion;0° corresponds to no pennation.

Muscle Fiber Arrangements

• Figure 4.11 (next slide)– Muscle fiber arrangements and an example of each

Figure 4.11

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.

Force-Velocity Curve

• Figure 4.13 (next slide)– Force–velocity curve for eccentric and concentric

actions

Figure 4.13

Reprinted, by permission, from Jorgensen, 1976.

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 when

it 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.

“Fluid Ball”

• Figure 4.15 (next slide)– The “fluid ball” resulting from contraction of the deep

abdominal muscles and the diaphragm

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.