Wednesday, June 19, 2024

The Fundamentals of Biomechanics & Mechanical Principles


Mechanics is a branch of Physical science dealing with the effect of energy and forces on the body. Biomechanics is the study of mechanics in the human body- the effects of internal and external forces on the body in movement and at rest. Biomechanics studies the biological function from an application of mechanical principles.

In this article, we will learn about the fundamentals of biomechanics & mechanical principles.


The Force, in Biomechanics & Mechanical Principles, is that which alters the state of a body at rest or its uniform motion in a straight line. Force produces or modifies the motion.

The application of force is defined by:

  1. The direction of the force- represented by the direction of the arrow
  2. The magnitude of the force- may be represented by the length of the arrow

Force = Mass (kg) x Acceleration (m/s2)

The SI Unit of force is N (newton). 1 newton is defined as the force required to move 1-kilogram weight for a distance of 1 meter.

Types of Force in Biomechanics

There are mainly 4 types of forces in Biomechanics & Mechanical Principles:

  1. Linear Forces- When two forces are acting along the same line and produce motion in a straight line, they are termed as liner forces.
    • If two forces are acting in the same direction and at a common point, the resultant force is a single one equivalent to the sum of the magnitudes of those two forces. This can lead to either pushing an object or pulling.
    • Two equal forces acting on a body in opposite directions will lead to a state of equilibrium
    • If two unequal forces are acting on the same body in opposite directions, the body will move in the direction of the greater force.
  1. Parallel Forces- when two equal forces are acting in the same direction and the same plane against a counterforce in the middle, it is called parallel forces.
    • Example– two children with equal weight sitting on either end of a see-saw. The fulcrum of the see-saw is giving the counterforce, and the whole see-saw is maintained in equilibrium.
  1. Rotational Force- in the parallel forces if any one of the two forces is greater, then it leads to rotation and the force is called rotational force.
    • Also, if two forces of different magnitudes, act on different points of a body in opposite directions it results in a twisting effect- rotation movement. And the forces are called force couple.
  1. Concurrent Force- When two forces are acting on an object in different directions and are perpendicular to each other, the resultant force is between the two forces. It is diagrammatically represented by the Parallelogram Method.


Gravity is a force by virtue of which all bodies are attracted to the earth. The force of Gravity continuously acts on the human body and if it is unopposed, the body will fall to the ground. Gravitational force is counterbalanced when a force equal or greater acts in the opposite direction. Such as support of a plinth, the buoyancy of the water, or isometric muscle contraction- forces equal and opposite to gravity. If force greater than and opposite to gravity is employed- movement occurs.

Example– From a standing position, one can raise heels from the ground by contracting the calf musclesHere the force of contraction of calf muscles exceeds that of gravity and thus the movement (raising the heel) occurs.

Movements of joints in the human body occur by a combined work of gravity and muscular action. Each controls the effect of the other.

Centre of Gravity (COG)

COG is an imaginary point at which the mass of any object is concentrated. The earth’s attraction acts through this point no matter what the position of the body is. Symmetrical objects have their COG in the center. Whereas in asymmetrical objects, COG is more towards the heavier side

The COG of the human body lies in the vicinity of the body of the second sacral vertebra. However, its position may vary according to the anatomical structure of the individual. COG is higher in men and children than an average woman because most of their weight is located in the upper half of the body.

Line of Gravity (LOG)

An imaginary vertical line passing through the center of gravity of a body is called the line of gravity.  When a human body is in the fundamental standing position the LOG passes through the body of the second sacral vertebra, through the vertex, and a point between the feet, level with transverse-tarsal joints.

Base of Support

The base for a body is defined as the area which is supported. Example- For a cube, the base is the face on which it rests.

In the human body base of support changes with the change in positions. In standing position, the base of support is the area covered by the heels posteriorly, and by a line joining the tips of toes anteriorly. Similarly, in a lying position, the whole posterior aspect of the body forms the base.

The base of support is directly proportional to the stability, i.e., a body will be more stable if it has a greater base of support. And the greater the base of support, the lower will be the center of gravity of that body.

COG LOG & BOS in standing, sitting, and lying


Equilibrium is defined as a state of rest or balance of a body. It is achieved when all the forces acting on a body are perfectly balanced.

There are 3 types of equilibrium in the Biomechanics & Mechanical Principles, which are:

  1. Stable Equilibrium– If the forces acting on a body are such that they tend to restore its original position even after the body is displaced, then that body is said to be in stable equilibrium.
    The equilibrium condition is more stable when the COG of the body is as low as possible, and the line of gravity falls near the center of a large base of support.
    Example– book placed on a table. When force is applied to the book to displace it, it regains its original state as soon as the force is removed.
  1. Unstable Equilibrium- When a body at rest is given an initial displacement and the forces acting on that body increase this initial displacement, then that body is said to be in unstable equilibrium. Minimal force can alter the equilibrium.
    A high center of gravity and a small base lead to unstable equilibrium as even minimal force can cause the line of gravity to fall outside the base of support.
    Example– pen balanced to stand. Even a smaller force can make it fall down.
  1. Neutral Equilibrium- Even if the forces acting on a body changes the position or causes displacement; the body is said to be in neutral equilibrium if the height and position of its center of gravity remain the same in relation to the base of support.
    Example– ball rolling on the floor


Biomechanical Principles – UO Biomechanics

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