Basic physics use in exercise therapy

Physical therapy focuses on movement and the recovery of that movement.physics is applicable to torque and forces as the body bends, stretches and moves. physics can be applied to a number of subjects in kinesiology, physiology, and biomechanics.

Think of the body’s skeletal system as a system of pulleys and levers. the exercise that are used in therapy can be defined by newton’s laws of motion, examples-

  • the first law: performing aerobic exercise on rubber mats instead of wood floors.
  • the second law: resistance band exercise.
  • the third law: strength training

Because of the science behind physics, the physical therapy field is able to thrive and advance in method of learning about the body and how to treat it.

The mechanical principles utilized in exercise therapy are defined here to ensure


Force is that which produces the motion or modifies the motion of an object and moves it for a particular distance. it is a vector quality i.e. magnitude and direction.

Force=mass(kg) × acceleration(m/s²)

Unit of force: newton and pound. [ one-newton is defined as the force applied in the 1kg weight object and moves it for 1 meters distance.]

Force may act on object in 2 way:-

  1. Static- when object in rest.
  2. Dynamic- when object is in motion

Types of force acting on object:-

A) Linear force- when 2 forces act along same line and produce the straight line motion is referred as linear force.

  1. Pushing linear force→when two force are acting on an object in same direction produces linear motion.
  2. pulling linear force→ when the two forces pull an object in same direction it produces the linear motion.
  3. equal magnitude (equillibrium)→when the two force are acting opposite to each other with the equal magnitude.
  4. unequal magnitude→when the two forces are acting opposite to each other with unequal magnitude, the movement occurs towards the greater magnitude.

B) Parallel force- when two equal magnitude are acting in the same direction and same plane with the counter force in the middle.

example; sea -saw , the children are applying the two forces in the end, and the fulcrum gives counter force to maintain the see-saw in neutral position.

example in human body- when the deltoid muscle contracts during shoulder abduction, the action of the anterior and posterior fibers of the muscles being compound to work with the middle fibers and so vastly increasing their power.

C) Rotational force- in parallel force when first force is greater then second force, rotation occurs. two unequal forces acting at different points and in opposite directions produce rotation of the body.

example in human body- when the trapezius muscle and serratus anterior muscle contract to rotate the scapula.

With respect to body forces are two types

1)External force- force arise from sources outside the body.example-

  • Gravity
  • external device added to body, like wet-cuff, spring,by physio pulley etc.

2)Internal force- force that act on structure of body that aries from the body’s own structure. example muscle, ligament, bone. this force developed within the body.


Gravity is the force by which a planet or other body draws objects toward its center. force by which all the bodies are attracted to earth is gravity.

What else does gravity do?

Why do you land on the ground when you jump up instead of floating off into space? Why do things fall down when you throw them or drop them? The answer is gravity: an invisible force that pulls objects toward each other. Earth’s gravity is what keeps you on the ground and what makes things fall.

An animation of gravity at work. Albert Einstein described gravity as a curve in space that wraps around an object—such as a star or a planet. If another object is nearby, it is pulled into the curve.

Anything that has mass also has gravity. Objects with more mass have more gravity. Gravity also gets weaker with distance. So, the closer objects are to each other, the stronger their gravitational pull is.


COG is an imaginary point at which the mass of the object is concentrated.

  • COG depend on the size, shape, structure, height and portion of particular object.
  • in symmetrical object COG located at center of object.
  • in asymmetrical object COG located at heavier end.

LINE OF GRAVITY [LOG]                                                                                                                   

  •  Imaginary vertical line passing through the COG called LOG.
  • when the human body is in the standing position the line of gravity passes from the vertex through the second sacral vertebra to just in front of the ankle joint and between the feet.


  • BOS means the area supported beneath the object.
  • if base of support is more the stability will be more.
  • In the lying position the posterior aspect of the whole body forms the base.
  • in stride standing it is an area as wide as the feet.
  • the fundamental position of standing the BOS is lesser than the lying, so COG in the standing position it is in the higher level whereas in the lying posture.


A state when the forces acting upon a body are perfectly balanced and the body remain at rest. it is the state of an object where all the force acting on it are equal in magnitude and opposite in direction.


  1. stable equilibrium:- where two forces acting on an object are equal. example- book placed on the table. the BOS is more and COG lies in the lower part.
  2. unstable equilibrium:- when two forces acting on an object are force is greater than other.example-pen is balanced to stand on the floor,when force applied on pen it alters its original state. BOS is less and COG is in higher position.
  3. neutral equilibrium:-when the movement or position change in the object by the force exerted on it, there will not be any change in COG of object. example- ball rolling on floor.


Lever is a rigid bar/ part which is capable of movement about a fixed point called as fulcrum (F). in lever work is done when a force of effort (E) applied at one point on the lever acts upon another force or weight (w) acting at a second point on the lever.

  • To perform an activity or a movement the bones act as the lever and the joint act as the fulcrum (F).
  • The movement will be created by two types of forces i.e. weight (w) and effort (E).
  • in which the weight(W) is the opposing force and the effort(E) is action achieving force.
  • W is the weight of the body parts and effort may be the pull of the muscles.

Types of lever:- There are three classes of lever-

A) First order lever-

  • In this type of lever fulcrum is between the effort and weight.
  • fulcrum may be shifted centrally or towards weight or effort.
  • example- see-saw, scissors, nail-cutter.
  • important feature of first order lever is stability.
  • example in human body ATLANTO-OCCIPITAL JOINT. during the nodding movement of the head; neck extensors act as the→effort(E); the facial part is the→weight(W); atlanto-occipital joint as the→fulcrum(F).

B) Second order lever-

  • In these weight is in between the fulcrum and effort.
  • example-Wheelbarrow.
  • These are the lever of strength but not the speed and ROM.
  • example in human body- standing on toes; in which the ball of the toes→fulcurm(F); the tendo-achilles tendon is →effort(E); the body weight is passing through the middle part of the foot→weight(W).

C) Third order lever-

  • In this effort is in between fulcrum and weight.
  • example- Bettle nut cutter, chapati press.
  • these lever having mostly mechanical disadvantage.
  • but the third order levers are seen plenty in the human body.
  • example in human body- elbow flexion; in which the elbow→fulcrum (F); the biceps insertion→effort(E);  the forearm and hand→weight.


A pulley is a grooved wheel which is rotated about a fixed axis by a rope which passes round it.

A pulley is a basic device or machine made of a wheel with a rim that a cord or rope fits around. The wheel and axle of a pulley make it easier to lift heavy objects with the rope.

These are different types of pulley systems:

  • Fixed: A fixed pulley has an axle mounted in bearings attached to a supporting structure. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. Mechanical advantage is gained by combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter.
  • Movable: A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two. this type of pulley is used in the suspension therapy units which makes easy to lift the body parts.
  • Compound: A combination of fixed and a movable pulleys forms a block and tackle. A block and tackle can have several pulleys mounted on the fixed and moving axles, further increasing the mechanical advantage.

Pulley in human body example-


Anatomical plane is a hypothetical plane used to transect the human body, in order to describe the location of structures or the direction of movement.

Three principal planes are used:

  • THE SAGITTAL PLANE OR MEDIAN PLANE( longitudinal, anteroposterior) is a plane parallel to the sagittal suture. it divided the body into left and right.
  • THE CORONAL PLANE OR FRONTAL PLANE is perpendicular to the ground, in humans it separates the anterior from the posterior, the front from the back, the ventral from the dorsal.
  • THE TRANSVERSE PLANE OR AXIAL(HORIZONTAL) PLANE is parallel to the ground, in human it separates the superior from the inferior, the cranial from the caudal.


An axis is a straight line around which an object rotates. Movement at the joint take place in a plane about an axis. There are three axis of rotation.

  • Sagital axis – passes horizontally from posterior to anterior and is formed by the intersection of the sagital and transverse planes.
  • Frontal axis – passes horizontally from left to right and is formed by the intersection of the frontal and transverse planes.
  • Vertical axis – passes vertically from inferior to superior and is formed by the intersection of the sagital and frontal planes.
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