ISSA Chapter 5: Concepts of Biomechanics
ISSA Chapter 5: Concepts of Biomechanics 1

Chapter Goals:

  • Discuss the vocabulary terms for movement and anatomical position.
  • Know Newton’s laws of motion.
  • Discuss the classes of levers and be able to give examples for each.
  • Be able to explain the key concepts of muscles as the prime movers of the body.

The nervous system, bones, muscles, and connective tissue work together to produce movement.

Personal trainers need a firm grasp on healthy human movement so that they may correctly build a safe and effective training program.

Two important fields of study will be biomechanics and kinesiology.

Biomechanics is the study of the mechanical laws that govern movement in living organisms.

Kinesiology is the study of the mechanics of human movement.

Anatomical Reference Terms

Fitness training and biomechanics make use of anatomical position to reference where something is located.

Proper anatomical position is seen with the neutral body facing forward with the arms at the side of the body and the palms and toes pointing straight ahead.

From there, there are many terms to know so you may reference a location.

  • Anterior/ventral – Front of the body or toward the front relative to another reference point
  • Posterior/dorsal – Back of the body or toward the back relative to another reference point
  • Proximal – Position closer to the center of the body relative to a reference point
  • Inferior – Below a reference point
  • Superior – Above a reference point
  • Distal – Position farther from the reference point
  • Medial – Position relatively closer to the midline of the body
  • Lateral – Position relatively farther from the midline of the body
  • Prone – Lying facedown
  • Supine – Lying on one’s backside
  • Superficial – Closer to the surface relative to another reference point
  • Deep – Further beneath the surface relative to another reference point
  • Volar – Relating to the palm of the hand or sole of the foot
  • Cephalic – Toward the head
  • Caudal – Toward the bottom
  • Contralateral – On the opposite side
  • Ipsilateral – On the same side
  • Bilateral – Refers to both sides
  • Unilateral – Refers to only one side

Anatomical Movement Terms

These terms are also universal in health and fitness and describe how the muscles of the body act on the skeleton and generate movement.

  • Abduction – Movement away from the midline
  • Adduction – Movement toward the midline
  • Flexion – Movement decreasing the angle between two body parts
  • Extension – Movement increasing the angle between two body parts
  • Retraction – Adduction of the scapula
  • Protraction – Abduction of the scapula
  • Lateral Flexion – Flexion in the frontal plane
  • Elevation – Movement in a superior direction
  • Depression – Movement in an inferior direction
  • Plantar Flexion – Extension of the foot downward (inferiorly)
  • Dorsiflexion – Flexion of the foot upward (superiorly)
  • Eversion – Movement of the sole of the foot away from the median plane
  • Inversion – Movement of the sole of the foot toward the median plane
  • Circumduction – Circular movement of a limb extending from the joint where the movement is controlled
  • Internal Rotation – Rotational movement toward the midline
  • External Rotation – Rotational movement away from the midline
  • Pronation – Turning the palm or arch of the foot down
  • Supination – Turning the palm or arch of the foot up
  • Hyperextension – Position that extends beyond anatomical neutral
  • Ispilateral – Same-side movement
  • Contralateral – Opposite-side movement
  • Lateral – Situated away from the midline
  • Medial – Situated toward or closer to the midline

Planes of Motion

The anatomical planes of motion are used for describing directions of movement.

The Frontal plane is an imaginary line that divides the body into anterior and posterior halves.

The Sagittal plane divides the body into left and right halves.

The Transverse plane is an imaginary line that divides the body into inferior and superior halves.

Everyday activites involve movement in all three of these planes. Thus, it is important that we train and incorporate these three planes into our programs.

Range of motion is a measure of movement around a specific joint or body part.

Some common movements in the frontal plane are trunk lateral flexion and extension, scapula depression, scapula elevation, scapular upward rotation, and scapular downward rotation.

Some common movements within the sagittal plane would be knee flexion / extension, trunk flexion / extension, and shoulder flexion / extension.

Some movements in the transverse plane include torso rotation, head and neck rotation, scapular retraction, and scapular protraction.

Balance, Equilibrium, and Stability

Balance involves an even distribution of weight that enables someone or something to keep its center of gravity within a base of support.

Equilibrium is a state where the opposing forces or influences are balanced.

Stability is the ability to control and maintain control of joint movement or body position.

The Center of gravity is the hypothetical position in the body where the combined mass appears to be concentrated and the point around which gravity seems to act.

The base of support is the area beneath an object or person that includes every point of contact that the object or person makes with the supporting face.

Gravity is the attraction between objects and the earth.

Muscular force involves the contraction of a muscle while exerting a force and performing work. This can be concentric, eccentric, or isometric.

Dynamic balance is the ability to remain upright and balanced when the body and arms and legs are in motion.

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Static balance is the ability to remain upright and balanced when the body is at rest.

Mass is the amount of matter in an object, and weight is the gravitational force of attraction on an object.

The line of gravity is a vertical line straight through the center of gravity.

Training for balance has been shown to be beneficial in improving dynamic joint stabilization.

Joint Mobility and Stability

Human movement requires the joints to be both mobile and stable.

Healthy joints have the ability to move through the proper range of motion with control.

Muscular efficiency is producing the right amount of force with the right muscles at the right time.

Joint mobility is the degree of movement around a joint before movement is restricted by surrounding tissues.

Joint stability is the ability of the muscles around a joint to control movement or hold the joint in a fixed position.

The mobility and stability allowed by joints are important for keeping proper posture and function.

Movement dysfunctions are derived from underactive and overactive musculature, which affects the movement at a joint.

Some limiting factors for joint mobility will be things like age, genetics, body type, and the shape of joints.

The Laws of Motion

The laws of motion are laws of physics that describe movement.

These laws of physics relate to an object’s motion to the forces acting on it and gravity.

Newton’s First Law: Inertia

This law states that a body in motion tends to stay in motion while a body at rest tends to stay at rest unless acted on by an outside force.

Inertia is the resistance to action or change and describes the acceleration and deceleration of the human body.

Acceleration is the rate of change of velocity. Deceleration is a special type of acceleration where a person or object is slowing down.

Newton’s Second Law: Acceleration

This law has two parts:

  1. A change in acceleration of mass happens in the same direction of the force causing it.
  2. The change in acceleration is directly proportional to the mass of the body.

Acceleration is equal to change in velocity divided by change in time.

Velocity is the speed of an object and the direction in which it is moving.

Force is equal to mass times acceleration.

This law is absolutely essential in the world of exercise.

It also brings together the idea of the force-velocity curve. This represents the inverse relationship between force and velocity in muscle contraction.

Power is equal to force times velocity.

Momentum is a measure of how much motion is in a moving body and it is a product of mass and velocity.

Newton’s Third Law: Action and Reaction

The third law states that for every action, there is an equal and opposite reaction.

This means that for the human body to provide force for movement, there is a reactionary force to a surface or thing.

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Changing training environments from a smooth and flat ground to a beach, would show this law well, as overcoming the sang under your feet is different than the ground.

Speed is the ability to move the body in one direction as fast as possible.

The ground reaction force is the force the ground exerts on a body it is in contact with.

Friction is the resistance of relative motion that one surface or object encounters when moving over another one.

Compression, tensile, and shear forces can happen between two objects.

Compression force happens when two surfaces press toward one another and this causes them to be compacted.

Tensile force is the opposite, and the two contact surfaces pull apart.

And shear force is created when two surfaces move and glide across one another.

The muscles in the human body are only able to pull for the creation of movement. Thus the voluntary motion of the human body is always done via muscular contraction.

Motion can be linear or angular. Angular motion rotates around an axis, while linear motion moves in a line, whether it is curved or straight.

Displacement is the distance that an object moves from its starting point.

Distance is the total length that the object travels.


Friction is a physical force that affects the ability for the body to create force, accelerate, and decelerate.

Friction is the force that is made by two surfaces rubbing across one another.

This force can be static, sliding, or rolling.

Static friction sees that the object does not move. Sliding friction sees that one or both objects move. Rolling friction is the force resisting a surface rolling across another like a ball bearing or wheel on a road.

Principles of Biomechanics

Kinetics is the study of force acting on a mechanism.

Biomechanics includes stability, maximum effort, linear motion, and angular motion.

Angle of Muscle Pull

During strength exercises, the strength at particular spots in the range of motion will vary based on the angle that the muscle is pulling from.

It is more difficult to start a curl with the elbow fully extended than it is with a slight flex.

Mechanical advantage is when the ratio of force creates a meaningful movement compared to the force applied to generate the movement.


Moving the body and exercising are ways to show measurable work.

Work is equal to force times distance.


Power is the amount of work done in a unit of time.

The key to this is the amount of time needed to accomplish the task.

Power is a combo of both speed and strength. It is the ability for muscles to generate max tension as quickly as possible.

Mechanical work is the amount of energy that is transferred by force, so the product of force and distance.


Levers are a rigid or semirigid bar rotating around a fixed point when force is applied to one end.

These levers cannot be altered, but the body is able to use them to be more efficient.

A lever has a rigid bar and an axis that the lever moves around.

Levers rotate around their axis as a result of force applied to move weight or applied against a resistance.

The fulcrum is the point where a lever rest or is supported on which it pivots.

First-Class Lever

For this type of lever, the fulcrum will be located between the effort and the load.

This type of lever creates balanced movements when the fulcrum is centrally located between the effort and load. This is like a seesaw.

A good example of this lever would be with flexion and extension of the neck, the fulcrum is the base of the skull.

Second-Class Lever

For this lever, the load is located between the fulcrum and the effort.

Force movements require very little effort to move the resistance.

In the human body, the ankle and plantar flexion is the best example of this. Large forces can be moved with little effort.

Third-Class Lever

For this lever, the effort is between the fulcrum and the load. Speed and range of motion are used well in these levers, and it is the most common lever in the body.

A good example of this lever would be elbow flexion driven by the biceps brachii with the elbow as a fulcrum.


Torque is applied with rotation. Since most joints happen due to some form of rotation, this is rather common in force output.

Torque is found by multiplying the force by the length of the force arm, which is the distance between the fulcrum and the force.

Torque can be increased by lengthening the force arm and increasing the leverage to move the load.

Muscles as Movers

Muscles create movement by generating force and transferring force to the attached bones by way of connective tissue.

The origin of a muscle is where it attaches to a bone, closest to the midline of the body. The insertion point is the opposing end of the muscle.

Some muscles have multiple heads and thus have multiple points of connection.

The muscles of the human body are arranged in groups or pairs based on their actions and the joints that exert force.

Agonist muscles are the prime movers, so they are the primary muscle involved in joint movement.

Synergist muscles are secondary muscles supporting the action of the prime mover.

The antagonist is one that opposes the prime mover for a given movement. It relaxes and is inhibited when the agonist muscle is activated.

Stabilizer muscles are the muscles that work to stabilize joints and support joint movement, but they do not contribute much to joint motion.

Muscle synergies are the activation of a group of muscles to generate movement around a certain joint.

ISSA Chapter 5: Concepts of Biomechanics 2
ISSA Chapter 5: Concepts of Biomechanics 3
ISSA Chapter 5: Concepts of Biomechanics 4

Tyler Read

Tyler Read, BSc, CPT. Tyler holds a B.S. in Kinesiology from Sonoma State University and is a certified personal trainer (CPT) with NASM (National Academy of sports medicine), and has over 15 years of experience working as a personal trainer. He is a published author of running start, and a frequent contributing author on Healthline and Eat this, not that.

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