is the study of forces and their effects whilst Biomechanics is the study of the
structure and functions of biological system of living organism using the
principles of physics, engineering, anatomy and physiology. Therefore
biomechanics concerns the interrelations of the skeleton, muscles and joints.
Applying mechanics to the
human systems can be achieved by considering the two aspects of movement:
Statics and dynamics. Statics is the study of a system when all external forces
on the body are equal and there is no change in motion (at rest) or moving with
a constant velocity. In contrast, Dynamics is the study of a system when the
system is in a state of acceleration or deceleration.
In addition, both
kinematics and kinetics aspects of action are also considered in describing
motion. Kinematics is a branch of mechanics that concerns with providing an
appropriate description of movement with respect to time and space as well as
dealing with the geometry of the motion or action of objects; including
displacement, velocity, and acceleration without taking into account the forces
that produce the motion. Kinetics however focuses on the forces that are
associated with movements, that is between the force system acting on a body
and the changes it produces in a body motion.
All movements and changes in movements arise from the action of
forces, both internal and external. A change in the force acting on an object
is necessary to move an object from a stationary position or move with
velocity. The amount of change in the velocity of an object also depends on the
magnitude and direction of the applied force. Newton’s laws of motion give a
clear relationship between the changing force and the resultant change in
movement, and this is applicable to all forms of movement, including human
Axes of Movement
An axis is a line around which motion occurs which are
related to planes of reference, and the cardinal axes are oriented at right
angles to one another. It is expressed as a three-dimensional coordinate system
with x, y, and z to mark the axes (Figure 1). This coordination system is
significant in defining or locating the extent of the types of movement
possible at each joint— translational, rotational and curvilinear motion. All
movements that occur about an axis are considered rotational,
whereas linear movements along an axis and through a plane are called translational.
Curvilinear motion occurs when a translational movement accompanies
1: Three-Dimensional Coordination system for translational and rotational
movement. (Courtesy of Joints Anatomy and Biomechanics)
Basic Principles of Biomechanics
of Gravity, Stability, and Balance
One of the most commonly monitored locations for any body when
conducting a kinematic analysis of a human movement is the center of gravity,
which also called the center of mass. However, the center of gravity in a human
body is not fixed as the body is made up of so many segments that can be move
or remain stationary at any moment in time.
There are two methods that are extensively used in locating the center
of gravity for the human body.
The reaction board;
when the body remains motionless, which then makes use of the principle of
moments which states that the sum of moments acting on a body in equilibrium is
zero. The segmental method;
for when a body is in a state of motion assuming that the location of the
body’s center of gravity is a function of the center of gravity of each of the
segments (e.g. upper arm, lower leg) of the body. The heavier segments of the
body will exert a greater influence on the position of the whole-body center of
gravitational force acts on the levers of the body to create torque at
various body segments and joints. The Center of Gravity in the anatomical
position is just a frontal to the second sacral vertebrae. However,
it’s important to note that center of gravity is theoretical which is
constantly changing with motion. With every movement and change of position
the Center of Gravity changes the way joints react and muscles perform.
Stability can be defined as resistance to a disruption
in the equilibrium of the body or a quality relating to the degree to which a
body resists being upset or moved. Some of the major factors that affect a
person’s stability are:
The area of the base of supportThe relation of the line of gravity to the edge of the baseThe height of the center of gravity andThe mass of the person.
Balance, on the other hand, is a physical ability to
control stability. There are two types of balance:
Static balance, when a person remains motionless, andDynamic balance, when a person is in motion.
Therefore, lowering the centre
of gravity increases balance and stability and the greater the mass of
an object, the greater the stability.
Linear Motion – Linear Kinematics and Projectile Motion
motion which is also called translational, occurs when the body moves in such a
way that all parts of it travel in the same distance and time as well as the
same directional path. There are several different variables that need to be
considered in examining the linear motion.
by drawing a straight line from the initial point of an object to its end
location.Distance covered by a
by dividing the distance covered by the time it takes to cover the distance.Velocity; the amount
of displacement across time.Acceleration; the change
in velocity over time.
For most sports and human movements involving projectiles,
there is a range of angles that results in best performance. The Projection
Principle refers to the angle(s) that an object is projected to achieve a
particular goal (e.g., a figure skater executing a triple axel jump).
Linear kinetics focus on the concept of force, a push or a pull
acting on a body. Force also allows inertia to be overcome, thus affords an
individual the opportunity to change the state of motion of a system (e.g. the
human body). Forces have both magnitude and direction and it is common to
distinguish between an internal or external force. An internal force exists
within the system being examined. In the human system, the contraction of
muscle causing a force to be created at the bone would be considered as
internal. Whereas, an external force is a function of something outside the
system being studied such as the air resistance, gravity or any contact with
other objects (e.g. the ground).
By examining Newton’s Three Laws of motion, it will able us to
get a better understanding on how it relates to the biomechanics of the human
Newton’s First Law; Inertia
states that “a body continues to be in a state of rest or in uniform motion
until an external force of sufficient magnitude is acted upon to disturb this
state”. For example, a dumbbell will remain stationary until the weightlifter
applies an appropriate force.
Newton’s Second Law; Acceleration
states that “the acceleration of the body is proportional to the force exerted
on it and inversely proportional to its mass.”
Force = Mass x Acceleration
This law provides a description on the relationship between the
force, acceleration and mass. Both acceleration and force must have the same vector direction. For example, as the mass of the
soccer ball is increased, the force needed to kick the ball is also increased
to ensure a particular acceleration is achieved.
In a case of football or
soccer, kicking uses a
lot of muscles and joints as well as balance, accuracy, skill and power.
Kicking involves a lower-body activity while maintaining balance and stability.
Kicking a soccer ball requires the coordination of the hips, torso, feet, legs,
head and arms to ensure the proper form and therefore provide balance. The hip
joint, which connects the femur to the pelvis, serves as the intersection for a
kinetic chain that transmits power to the soccer ball.
In addition, momentum plays
an important part in this matter as momentum is the product of a body’s mass
and its velocity. As stated by Newton’s Second Law, a force can be applied to a
mass that results in a change of velocity of that mass, therefore when an external
force is exerted on the body to change the velocity; the force is also creating
a change in the body’s momentum.
Third Law, “For every action there is
an equal and opposite reaction”. This law refers to the way in which forces act
against each other. Example of how Newton’s Third Law is applied in
biomechanics; – the reaction force provided by the surface on which a person is
walking or standing. The foot will produces a force against the ground, and in
accordance with Newton’s third law, the ground generates a ground reaction force in the
opposite direction but of equal magnitude.
of Angular Motion
Linear motion occurs when the
direction of an external force that is applied to the body is in line with the
body’s center of gravity. However it is also essential to consider the
possibilities when a force is applied along a line at which not at the body’s
center of gravity. Therefore this type of motion is called the rotary motion.
In angular motion, the term
angular distance and displacement must first be distinguished as Angular
distance is equal to the angle between the initial and final positions when
measured by the following the path taken by the body whereas, the angular
displacement is the change of location of the rotating body. Therefore to
evaluate the angular motion, appropriate units need to be used: degrees,
revolutions and radians.
basketball shooting as an example. The basketball shooting arm has three rigid
planar links with rotational joints imitating an upper arm, forearm and hand
with shoulder, elbow and wrist joints. The kinematics of the shooting arm is
solved to calculate the joint angles, and the velocity and angular velocity of
links at release. There are many angular displacement and velocity combinations
of shoulder, elbow and wrist joints to produce the optimal release speed, angle
and backspin of the ball at release. Shoulder rotation contributes to the
vertical component of release velocity of the ball and elbow and wrist actions
mostly produce the horizontal component of release velocity and backspin of the
ball when the forearm and hand are nearly vertical at release.
Newton’s first law is also called the law of inertia as mentioned
is the resistance of a body to a change in its current state of linear motion,
which also means that it is related to the amount of energy required to alter
the velocity of a body. The inertia of a body is directly proportional to its mass. In
addition to the object’s mass, its resistance to angular motion will be
dictated by how far the mass is distributed away from the axis of rotation, and
this is called object’s moment of inertia.
Taking the case of a baseball, it
will take more effort to initially swing the bat when the moment of inertia is
greater. The moment of inertia can be increased by either increasing the mass
of the bat or by distributing the bulk of the mass away from the axis of
rotation. However, a body
is unlikely to lose its mass during a movement but it can change its distance
or distribution from the axis of rotation. If the mass moves closer to the axis
of rotation then the moment of inertia decreases, resulting in increase of
In addition, the body also
possesses certain amount of angular momentum which is central to most swinging,
kicking or any throwing activities. Angular Momentum is a movement of a mass
when it is rotating that is calculated by the product of the body’s moment of
inertia and its angular velocity. The angular momentum is initiated by an
external force or torque. Once the external torque is exerted, the body will
continue to rotate until a new torque is applied to the system or experienced a
resistive force that is referred to the conservation of angular momentum.
of Fluid Mechanics
Fluid Mechanics is the study
of the application of the law of force and motion to liquid that develop when
an object moves through a fluid environment. I.e. in liquids (water) or
gases (air). In addition, the study of fluid that is in motion is called Fluid
Kinematics. Such that Fluid Statics is the study of fluids at rest at an
absence of shear force whilst Fluid Dynamics is the study of forces in fluids motion.
A fluid is generally
a substance which distorts continuously under the application of a shear
stress; force per unit area, which is acting on an infinitesimal surface
element. Stresses have both magnitude (force per unit area) and direction, and
the direction is relative to the surface on which the stress acts.
However, the primary
forces that exist in the fluid environments that will be address in this report
are drag and buoyancy.
is a fluid resistance which opposes the forward motion of the body and reduces
the body’s velocity. One simple demonstration of the effect of drag in air is
when taking out one hand from the
window of moving car perpendicularly to the air flow, there is a considerable
pressure against the hand. However, there is a decrease in the air pressure when
the hand is placed in a horizontal position. Therefore, the drag force at which
the hand is at vertical position exerted a force that is directly against the
frontal surface area of the hand impeding the forward motion. The other example of the effect of drag
in liquid is swimming. Swimmers push against water to
move forward, and water pushes back to slow them down.
Swimming makes use of upper and
lower joints activity, e.g. glenohumeral joint, tibiofemoral
joint, subtalar joint and talocrural joint. Swimmers generally flex their tibio-femoral joint excessively
while kicking in water which in this case is considered kicking from the knee.
This creates large amounts of drag and causes swimmer’s legs to sink low in the
water when in fact; swimmers should kick from the hip at the iliofemoral
joint with a relatively extended leg while move them simultaneously
up and down.
Figure 2: Forces exist in swimming. (Courtesy
of Askey Physics)
There are three types of drag
that influences the human performance:
Surface Drag: Resistance experienced by a body that is a result
from fluid rubbing against the surface of a body. E.g. Swimmer gliding through
water.Profile Drag: Exerting a severe limitation on performance in a
number of different sporting activities. E.g. High-speed activities such as
cycling, skiing and running.Wave Drag: Particularly important to swimmer as they move through
air and water at the same time. When the swimmer id in the air, during a dive,
or submerged under water just after a dive, the swimmer is subject to both
surface and profile drag. However, when the swimmer begins to swim along the
surface of the water, a new drag is created by the production of waves.
is the primary force against gravity that acts as an upward force, caused by
fluid pressure therefore keeps objects afloat. For an object to float in
water it must be less dense (mass per unit of volume) than the water. When an
object is placed in water it causes the water to be displaced (move upwards).
gravitational force on the water (weight) is greater than the force on the
object, then the water will create a buoyant force that will push the object
upwards against gravity (shown on Figure 2 above). Once the two forces become
equal the object will float in this position known as the point of equilibrium.
This is as in accordance to Archimedes’s Principle; the magnitude of the
buoyancy force is equal to the weight of water displaced by the floating body.
Flotation and centre
of buoyancy relate to performance because the higher an object floats in
the water, the less resistance the water will create to its movement. The
centre of buoyancy is the point in the body through which buoyancy acts. This
is particularly crucial in determining the way in which the body will lie when
it eventually coming to rest in a floating position. This applies to various water
sports, including: surfing, synchronised swimming, swimming and others.