Chapter 4:Basic Biomechanics

National Academy of Sports Medicine

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Basic Biomechanics

Objectives

After completing this chapter you will be able to:

• Define basic anatomical locations, planes of motion, and joint motion.

• Describe the muscle action spectrum.

• Classify muscles as movers.

• Describe and understand levers.

• Describe and understand common muscle synergies.

Key Terms

Biomechanics

Superior

Inferior

Proximal

Distal

Anterior

Posterior

Medial

Lateral

Contralateral

Ipsilateral

Sagittal

Flexion

Extension

Frontal plane

Abduction

Adduction

Transverse plane

Internal rotation

External rotation

Scapular retraction

Scapular protraction

Scapular repression

Scapular elevation

Muscle action spectrum

Eccentric

Isometric

Concentric

Agonist

Synergist

Stabilizer

Antagonist

First-class levers

Second-class levers

Third-class levers

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Basic Biomechanics

Introduction to BiomechanicsBiomechanics uses principles of physics to quantitatively study

how forces interact within a living body. Specifically, this chapter

focuses on the motions that the kinetic chain produces and the

forces that act upon it. (1, 2) This includes basic anatomical

terminology, planes of motion, joint motions, muscle action,

functional anatomy, levers, and common muscle synergies.

TerminologyAll industries have language that is specific to their needs. Because Health and Fitness Professionals deal with

human motion and the human body, they must understand the basic anatomical terminology to allow effective

communication. This section will review anatomical locations, planes of motion, and joint motions.

Anatomical location refers to terms that describe locations on the body. These include medial, lateral,

contralateral, ipsilateral, anterior, posterior, proximal, distal, inferior, and superior.

Figure 4.1 Anatomical locations

Biomechanics:

The scientific study of internal or external forces placed upon the body.

Proximal

Distal

Superior

Lateral

Medial

Inferior

Contralateral

Ipsilateral

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Basic Biomechanics

Superior refers to a position above a reference point. The

femur (thigh bone) is superior to the tibia (shin bone).

Inferior refers to a position below a reference point. The foot

is inferior to the knee.

Proximal refers to a position nearest the center of the body

or point of reference. The proximal portion of the femur (thigh

bone) is located at the hip.

Distal refers to a position farthest from the center of the body

or point of reference. The distal portion of the femur (thigh

bone) is located at the knee.

Anterior refers to a position on the front or toward the

front of the body. The quadriceps are located on the anterior

aspect of the thigh.

Posterior refers to a position on the back or toward the

back of the body. The hamstrings are located on the posterior

aspect of the thigh.

Superior:

Positioned above a point of reference.

Inferior:

Positioned below a point of reference.

Proximal:

Positioned nearest the center of the body, or point of reference.

Distal:

Positioned farthest from the center of the body, or point of reference.

Anterior:

On the front of the body.

Posterior:

On the back of the body.

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Medial refers to a position relatively closer to the midline of

the body. The medial side of the knee is the side closest to

the other knee.

Lateral refers to a position relatively farther away from the

midline of the body or toward the outside of the body. The

lateral side of the knee is the outside of the knee.

Contralateral refers to a position on the opposite side of the

body. The right foot is contralateral to the left hand.

Ipsilateral refers to a position on the same side of the body.

The right foot is ipsilateral to the right hand.

Medial:

Positioned near the middle of the body.

Lateral:

Toward the outside of the body.

Contralateral:

Positioned on the opposite side of the body.

Ipsilateral:

Positioned on the same side of the body.

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Planes of Motion and Joint MotionsThe universally used method of describing human movements in three dimensions is based on a system of

planes and axis. Three imaginary planes are positioned through the body at right angles so they intersect

at the body’s center of mass. They include the sagittal, frontal, and transverse planes. Movement is said

to occur more predominantly in a specific plane if it is actually along the plane or parallel to it. Although

movements can be one-plane dominant, no motion occurs strictly in one plane of motion. Movement in a

plane occurs on an axis running perpendicular to that plane, much like the axle that a car wheel revolves

around. This is known as joint motion, or arthrokinematics. Joint motions are termed for their action in each

of the three planes of motion.

Figure 4.2 Planes of motion

Sagittal Plane

Transverse Plane

Frontal Plane

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Sagittal plane

The sagittal plane bisects the body into right and left halves and

consists of predominantly front-to-back movements. Movements

in the sagittal plane are termed flexion and extension. Flexion

is a bending movement where the relative angle between two

adjacent segments decreases. (2, 3) Extension is a straightening

movement where the relative angle between two adjacent seg-

ments increases. (2, 3) Flexion and extension occur in many

joints in the body, including vertebral, shoulder, elbow, wrist,

hip, knee, foot, and hand. At the ankle, flexion is referred to as

dorsiflexion, and extension is plantarflexion. (1–3) Examples of

predominantly sagittal-plane movements include biceps curls,

triceps pushdowns, squats, front lunges, calf raises, walking, run-

ning, vertical jumps, climbing stairs, and shooting a basketball.

Figure 4.3 Figure 4.4 Figure 4.5 Figure 4.6 Vertebral flexion Vertebral extension Shoulder flexion Shoulder extension

Figure 4.7 Figure 4.8 Figure 4.9 Figure 4.10 Elbow flexion Elbow extension Hip flexion Hip extension

Sagittal:

An imaginary bisector that divides the body into left and right halves.

Flexion:

The bending of a joint, causing the angle to the joint to decrease.

Extension:

The straightening of a joint, causing the angle to the joint to increase.

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Figure 4.11 Figure 4.12 Knee flexion Knee extension

Figure 4.13 Figure 4.14 Ankle plantarflexion Ankle dorsiflexion

Frontal plane

The frontal plane bisects the body to create front and back halves and predominately consist of side-to-

side motions. Movements in the frontal plane include abduction and adduction in the limbs (relative to the

trunk), lateral flexion in the spine, and eversion and inversion

at the foot and ankle complex. (1–4) Abduction is a movement

away from the midline of the body or, similar to extension, it

is an increase in the angle between two adjoining segments,

but in the frontal plane. (1–4) Adduction is a movement of the

segment toward the midline of the body or, like flexion, it is

a decrease in the angle between two adjoining segments, but

in the frontal plane. (1–4) Lateral flexion is the bending of the

spine from side to side. Eversion and inversion follow the same

principle but relate more specifically to the movement of the

foot and ankle in the frontal plane. Examples of predominantly

frontal plane exercises include side lateral dumbbell raises, side

lunges, and side shuffling.

Frontal plane:

An imaginary bisector that divides the body into front and back halves.

Abduction:

Movement of a body part away from the middle of the body.

Adduction:

Movement of a body part toward the middle of the body.

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Figure 4.15 Figure 4.16 Figure 4.17 Figure 4.18 Shoulder abduction Shoulder adduction Hip abduction Hip adduction

Figure 4.19 Figure 4.20 Figure 4.21 Foot eversion Foot inversion Lateral spine flexion

Transverse plane

The transverse plane bisects the body to create upper and

lower halves and consists primarily of rotational movements.

Movements in the transverse plane include internal rotation

and external rotation of the limbs, right and left rotation of

the head and trunk, and radioulnar (forearm) pronation and

supination. (1, 2, 4) Examples of transverse plane exercises

include cable rotations, turning lunges, throwing a ball, golfing,

and swinging a bat.

Transverse plane:

An imaginary bisector that divides the body into top and bottom halves.

Internal rotation:

Rotation of a joint toward the middle of the body.

External rotation:

Rotation of a joint away from the middle of the body.

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Figure 4.22 Figure 4.24 Shoulder external Figure 4.23 Hip external Figure 4.25 rotation Internal rotation rotation Hip internal rotation

Figure 4.26 Figure 4.27

Figure 4.28

Radioulnar pronation Radioulnar supination

Trunk rotation

Scapular motion

Motions of the shoulder blade (or scapulae) are important for

the fitness professional to be familiar with to ensure proper

movement of the shoulder complex. Scapular movements are

primarily retraction, protraction, elevation, and depression.

Scapular retraction occurs when the shoulder blades come

closer together. Scapular protraction occurs when the shoulder

blades move farther apart. Scapular depression occurs when

Scapular retraction:

Shoulder blades are pulled together.

Scapular protraction:

Shoulder blades are pulled apart.

Scapular depression:

Shoulder blades are pulled together and downward.

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the shoulder blades move downward while scapular elevation

occurs when the shoulder blades move upward toward the

ears. The photographs below illustrate these four scapular

movements.

Figure 4.29 Figure 4.30 Figure 4.31 Figure 4.32 Scapula retraction Scapula protraction Scapula depression Scapula elevation

Scapular elevation:

Shoulder blades are pulled apart and upward.

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Muscle ActionsMuscles produce a variety of actions to effectively manipulate gravity, ground reaction forces, momentum,

and external resistance. The three different contractions that muscles produce are:

• Eccentric

• Isometric

• Concentric

This range of muscle action is known as the muscle-action

spectrum and is necessary to produce efficient movement.

Eccentric

When a muscle contracts eccentrically, it is exerting less force

than is being placed upon it. This results in a lengthening of the

muscle. In reality, the lengthening of the muscle usually refers

to its return to a resting length and not actually increasing in its

length as if it were being stretched. (3) An eccentric motion is commonly referred to as the “negative” in

gym and health club settings. An eccentric motion is synonymous with deceleration and can be observed

in many movements, such as landing from a jump. More commonly, it is seen in a gym when lowering a

weight during a resistance exercise.

Isometric

When a muscle contracts isometrically, it is exerting force equal

to that placed upon it. This results in no appreciable change

in the muscle length. (2, 3) In functional activities, such as

daily movements and/or sports, isometric actions are used

to dynamically stabilize the body. For example, the rotator cuff muscles help stabilize the shoulder joint

during a pushup. An isometric contraction can easily be observed when an individual pauses during a

resistance-training exercise in between the lifting and lowering phases.

Isometric:

A muscle maintaining a certain length.

Muscle-action spectrum:

Combination of eccentric, isometric, and concentric muscle actions.

Eccentric:

The lengthening of a muscle.

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Basic Biomechanics

Concentric

When a muscle contracts concentrically, it is exerting more

force than is being placed upon it. This results in a shortening

of the muscle. A concentric muscle action is synonymous with

acceleration and can be observed in many movements, such as

jumping upward and the lifting phase during resistance-training

exercises.

Let’s use the example of a bicep curl exercise to illustrate muscle actions. The initial movement requires

the bicep to shorten to generate force to overcome the weight of the dumbbell in the individual’s hand,

allowing the dumbbell to move up toward the front of the shoulder (see Figure 4.33). This is the concentric

phase of the exercise. Once the dumbbell is raised to the front of the shoulder, the individual holds this

position. Because the length of the muscle does not change while holding this position, this is considered

the isometric portion of the exercise. As one lowers the dumbbell back down to the starting position, the

muscle must now lengthen (under the control of the nervous system) to decelerate the force of lowering

the dumbbell. This is the eccentric portion of the exercise (see Figure 4.34).

Figure 4.33 Figure 4.34 Bicep curl concentric Bicep curl eccentric

Concentric force production is emphasized in many traditional routines. It is important, however, for

Health and Fitness Professionals to train muscles to be strong not only concentrically, but also eccentri-

cally and isometrically to maximize strength potential, maintain proper joint range of motion, and prevent

injury. In essence, the entire muscle action-spectrum must be emphasized.

Concentric:

The shortening of a muscle.

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Functional AnatomyThe traditional perception of muscles is that they work concentrically and predominantly in one plane

of motion. However, in order to more effectively understand motion and design efficient training pro-

grams, it is imperative to view muscles functioning in all planes of motion and through the entire muscle-

contraction spectrum (eccentrically, isometrically, and concentrically). The following section describes the

origin (proximal attachment toward the center of the body), the insertion (distal attachment away from

the center of the body), and muscle-action spectrum of the major muscles of the body. (5, 6)

Anterior tibialis

Origin: Outside the tibia

Insertion: Top of foot below the big toe

Muscle actions

Concentrically accelerates dorsiflexion

Eccentrically decelerates plantarflexion

Isometrically stabilizes the arch of the foot

Anterior tibialis

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Gastrocnemius

Origin: Back and lower portion of femur

Insertion: Back of the heel (calcaneous)

Muscle actions

Concentrically accelerates plantarflexion

Decelerates ankle dorsiflexion

Isometrically stabilizes the foot and ankle complex

Gastrocnemius

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Quadriceps

Origin: Front of pelvis, front of femur

Insertion: Front and top of tibia

Muscle actions

Concentrically accelerates knee extension, hip flexion

Eccentrically decelerates knee flexion, hip extension

Isometrically stabilizes the knee, hip and low back

Quadriceps

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Hamstrings

Origin: Ischium

Insertion: Tibia and fibula

Muscle actions

Concentrically accelerates knee flexion, hip extension, and lower leg external rotation

Eccentrically decelerates knee extension, hip flexion, and lower leg internal rotation

Isometrically stabilizes the hips, low back, and knee

Hamstrings

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Adductors

Origin: Pubis (lower portion of pelvis) and ischium (lower back part of hip bone)

Insertion: Inside and back of femur

Muscle actions

Concentrically accelerates hip adduction, flexion, and internal rotation

Eccentrically decelerates hip abduction, extension, and external rotation

Isometrically stabilizes the hip, low back, and knee

Adductors

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Gluteus maximus

Origin: Ilium (dorsal, upper, and largest of the three principal pelvic bones) sacrum (lower part of the

vertebral column)

Insertion: Back and top of femur and IT band (large piece of fascia running down the lateral aspect of the

thigh)

Muscle actions

Concentrically accelerates hip extension and external rotation

Eccentrically decelerates hip flexion and internal rotation

Isometrically stabilizes the hips, low back, and knee

Gluteus maximus

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Gluteus medius

Origin: Side of the ilium (dorsal, upper, and largest of the three principal pelvic bones)

Insertion: Side and top of femur

Muscle actions

Concentrically accelerates hip extension and external rotation

Eccentrically decelerates hip flexion and internal rotation

Isometrically stabilizes the hips, low back, and knee (during side-to-side movements)

Gluteus medius

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Erector spinae

Origin: Pelvis, thoracic, and cervical spine

Insertion: Spine and back of skull

Muscle actions

Concentrically accelerates spine extension

Eccentrically decelerates spine flexion

Isometrically stabilizes the hips and low back

Erector spinae

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Multifidus

Origin: Sacrum, lumbar, thoracic, and cervical spine

Insertion: Lumbar, thoracic, and cervical spine

Muscle actions

Concentrically accelerates spine extension and rotation

Eccentrically decelerates spine flexion and rotation

Isometrically stabilizes the spine

Multifidus

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Rectus abdominus

Origin: Pubis

Insertion: Ribs and sternum

Muscle actions

Concentrically accelerates spine flexion

Eccentrically decelerates spine extension

Isometrically stabilizes the hips, low back, and trunk

Rectus abdominus

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Internal oblique

Origin: Top and back of ilium, thoracolumbar fascia (fascia covering deep muscles of low back)

Insertion: Pubis, ribs, linea alba (fibrous band in the center of the anterior abdominal wall)

Muscle actions

Concentrically accelerates spine flexion, rotation

Eccentrically decelerates spine extension, rotation

Isometrically stabilizes the spine and pelvis

Internal oblique

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Transverse abdominus

Origin: Top and back of ilium

Insertion: Pubis, linea alba (fibrous band in the center of the anterior abdominal wall)

Muscle actions

Isometrically stabilizes the pelvis and lumbar spine (low back)

Transverse abdominus

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External oblique

Origin: External surface of ribs 5 through 12

Insertion: Anterior iliac crest of pelvis, linea alba (fibrous band in the center of the anterior abdominal wall)

Muscle actions

Concentrically accelerates spinal flexion, lateral flexion, and contralateral rotation

Eccentrically decelerates spinal extension, lateral flexion, and rotation

Isometrically stabilizes the hips and low back

External oblique

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Latissimus dorsi

Origin: Pelvis, ribs, lumbar, and thoracic spine

Insertion: Front of humerus, scapulae

Muscle actions

Concentrically accelerates shoulder extension, adduction, and internal rotation

Eccentrically decelerates shoulder flexion, abduction, and external rotation

Isometrically stabilizes the shoulder and low back

Latissimus dorsi

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Pectorals

Origin: Clavicle and sternum

Insertion: Front of humerus

Muscle actions

Concentrically accelerates shoulder flexion, horizontal adduction, and internal rotation

Eccentrically decelerates shoulder extension, horizontal abduction, and external rotation

Isometrically stabilizes the shoulder

Pectorals

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Deltoids (anterior, middle, posterior)

Origin: Clavicle and scapulae

Insertion: Side of humerus

Muscle actions

Anterior: Concentrically accelerates shoulder flexion and internal rotation

Anterior: Eccentrically decelerates shoulder extension and external rotation

Anterior: Isometrically stabilizes the shoulder girdle

Middle: Concentrically accelerates shoulder abduction

Middle: Eccentrically decelerates shoulder adduction

Middle: Isometrically stabilizes the shoulder girdle

Posterior: Concentrically accelerates shoulder extension and external rotation

Posterior: Eccentrically decelerates shoulder flexion and internal rotation

Posterior: Isometrically stabilizes the shoulder girdle

Anterior deltoid Medial deltoid Posterior deltoid

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Trapezius (upper, middle, lower)

Origin: Bottom of skull and cervical and thoracic spine

Insertion: Clavicle and scapulae

Muscle actions

Upper: Concentrically accelerates scapular elevation

Upper: Eccentrically decelerates scapular depression

Upper: Isometrically stabilizes the cervical spine and scapula

Middle: Concentrically accelerates scapular retraction

Middle: Eccentrically decelerates scapular elevation

Middle: Isometrically stabilizes the scapulae

Lower: Concentrically accelerates scapular depression

Lower: Eccentrically decelerates scapular elevation

Lower: Isometrically stabilizes the scapulae

Upper trapezius Middle trapezius Lower trapezius

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Biceps brachii

Origin: Scapulae

Insertion: Radius (lateral forearm bone)

Muscle actions

Concentrically accelerates elbow and shoulder flexion

Eccentrically decelerates elbow and shoulder extension

Isometrically stabilizes the elbow and shoulder girdle

Biceps brachii

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Triceps brachii

Origin: Scapulae and back of humerus

Insertion: Ulna (medial forearm and elbow bone)

Muscle actions

Concentrically accelerates elbow and shoulder extension

Eccentrically decelerates elbow and shoulder flexion

Isometrically stabilizes the elbow and shoulder girdle

Triceps brachii

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Muscles as MoversNow that we have a better understanding of the individual capabilities of the major muscles of the body,

let’s take a closer look at how muscles work together to perform movement. Muscles provide the human

body with a variety of functions that allow for the manipulation of forces placed on the body, such as pro-

ducing and decelerating movement. These muscle functions categorize a muscle as an agonist, antagonist,

synergist or stabilizer.

Agonist muscles are muscles that act as prime movers; in other

words, they are the muscles most responsible for a particular

movement. For example, the triceps muscle is an agonist for

elbow extension (as seen in a triceps extension exercise).

Figure 4.35 Triceps extension

Antagonist muscles perform the opposite action of the

prime mover. For example, the triceps are the antagonist of

elbow flexion (as seen in rowing). See Table 4-1 for more

examples.

Figure 4.36 Row

Agonist:

Muscles that act as prime movers.

Antagonist:

Muscles that perform the opposite action of the prime mover.

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Synergist muscles assist prime movers during movement.

For example, the hamstrings are synergistic with the gluteals

during a squat.

Figure 4.37 Squat

Stabilizer muscles support or stabilize the body (or joint),

while the prime movers and the synergists perform the move-

ment patterns. For example, the rotator cuff stabilizes the

shoulder joint during a pushup.

Figure 4.38 Pushup

Synergist:

Muscles that assist prime movers during movement.

Stabilizer:

Muscles that support or stabilize the body.

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Table 4-1

Muscles as Movers

Muscle Type Muscle Function Exercise Muscle(s) Used

Agonist Prime mover Chest press Pectoralis major

Overhead press Deltoid

Row Latissimus dorsi

SquatGluteus maximus, quadriceps

Synergist Assist prime mover Chest press Anterior deltoid, triceps

Overhead press Triceps

Row Posterior deltoid, biceps

Squat Hamstrings

Stabilizer Stabilize while prime mover and synergist work

Chest press Rotator cuff

Overhead press Rotator cuff

Row Rotator cuff

Squat Transversus abdominus

Antagonist Oppose prime mover

Chest press Posterior deltoid

Overhead press Latissimus dorsi

Row Pectoralis major

Squat Psoas (hip flexor)

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LeversIn addition to identifying the classification of muscles during movement, understanding human movement

also requires a rudimentary knowledge of levers. The musculoskeletal system is comprised of bones,

muscles, tendons, and ligaments, all of which create a series of levers and pulleys that generate force

against external objects. Skeletal muscles are attached to bones by tendons and produce movement by

bending the skeleton at moveable joints. Joint motion is caused by muscles pulling on bones, because

muscles cannot actively push. Particular attachments of muscles to bones will determine how much force

the muscle is capable of generating. For example, the quadriceps muscles can produce more force than

muscles of the hand.

Most motion uses the principle of levers. A lever consists of a rigid “bar” that pivots around a stationary fulcrum (pivot point). In the human body, the fulcrum is the joint axis, bones are the levers, muscles cre-

ate the motion (effort), and resistance can be the weight of a body part or of an object (such as a barbell

and dumbbell). (7)

Levers are divided into first, second, and third class, depending upon the relations among the fulcrum,

the effort, and the resistance.

First-class levers have the fulcrum in the middle, like a seesaw.

Nodding the head is an example of a first-class lever, with the

top of the spinal column as the fulcrum (joint axis).

Figure 4.39 First-class lever

First-class lever:

Has the fulcrum in between the effort and resistance.

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Second-class levers have a resistance in the middle (with the ful-

crum and effort on either side), like a load in a wheelbarrow. The

body acts as second-class lever when one engages in a full-body

pushup. The foot is the fulcrum, the body weight is the resistance,

and the effort is applied by the hands against the ground.

Figure 4.40 Second-class lever

Third-class levers have the effort placed between the resis-

tance and the fulcrum. The effort always travels a shorter dis-

tance and must be greater than the resistance. Most limbs of the

human body are operated as third-class levers. (7) An example

of a third-class lever is the human forearm: the fulcrum is the

elbow, the effort is applied by the biceps, and the load is in the

hand, such as dumbbell, when performing a biceps curl.

Figure 4.41 Third-class lever

Second-class lever:

Has the resistance in between the fulcrum and effort.

Third-class lever:

Has the effort in between the resistance and fulcrum.

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Muscle SynergiesNow that we have an understanding of muscle actions, levers, and classification of muscles as movers, it is

equally important to understand how muscles work together in a synergistic fashion to produce optimum

movement. Muscles produce a force that is transmitted to bones through connective tissues (tendons).

However, muscles rarely work in isolation, but rather, in groups (controlled by the nervous system). This

simplifies movement by allowing muscles and joints to operate as a functional unit. In the end, through

practice of proper movement patterns (proper exercise technique), these synergies become more fluent

and automated. Table 4-2 illustrates common muscle synergies for some popular exercises.

Figure 4.42

Figure 4.43 Figure 4.44

Figure 4.45 Squat

Overhead press Cable row

Chest press

Table 4-2

Common Muscle Synergies

Exercise Muscle Synergies

SquatQuadriceps, hamstrings, gluteus maximus

Overhead press Deltoid, rotator cuff, trapezius

Cable rowLatissimus dorsi, rhomboids, posterior deltoid

Chest pressPectoralis major, anterior deltoid, triceps brachii

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SummaryBiomechanics uses principles of physics to quantitatively study how forces interact within a living body.

In order to understand the body and communicate about it effectively, a Health and Fitness Professional

must know the terminology for the various anatomical locations. It is also important to know and express

how the body moves in all planes of motion and associated joint motions, the muscle-action spectrum,

levers, and muscle synergies.

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FA Davis Company; 1992.

3. Luttgens K, Hamilton N. Kinesiology: Scientific Basis of Human Motion. 9th ed. Dubuque: Brown &

Benchmark Publishers; 1997.

4. Kendall FP, McCreary EK, Provance PG. Muscles Testing and Function. 4th ed. Baltimore: Lippincott

Williams & Wilkins; 1993.

5. Brooks VB. The Neural Basis of Motor Control. New York: Oxford University Press; 1986.

6. Gambetta V. Everything in Balance. Training and Conditioning. 1996;1(2):15–21.

7. Harman E. The Biomechanics of Resistance Exercise. In: Baechle TR, ed. Essentials of Strength

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