Biomechanics

The study of mechanical principles that govern human movement. Branches of biomechanics include kinetics and kinematics.

Kinetics

The study of forces that cause motion.

Kinematics

The description of motion with reference to time, distance, and displacement.

Motion

Refers to the change in position of a body in relation to time. It is typically described as linear or angular, which relates to animate and inanimate bodies.

Linear motion

Motion that occurs either in a straight line or a curved path. Motion can either be rectilinear or curvilinear.

Rectilinear

Movement in a straight line. E.g. ski jumper travelling down the slope.

Curvilinear

Movement over a curved path. E.g. ski jumper off the ramp travelling through the air.

Angular motion

Motion that takes place when a body moving along a circular path. It occurs around some type of axis, which can either be internal or external

General motion

Combination of linear and angular motion. E.g. running in a straight line for a 100-meter sprint, with angular rotation of the arms and legs.

Angular and linear kinetic concepts of movement

Angular and linear kinetic concepts of human movement are mass, inertia, force, momentum, impulse, and Newton's three laws of motion.

Mass

The quantity of matter found in a particular body. Typically measured in kilograms. Mass and inertia are closely linked.

Weight

Weight is the measurement of the pull of gravity on an object.

Inertia

The resistance of a body to a change in its state of motion. Inertia can be categorised as static interia or dynamic inertia. For example, sumo wrestlers aim to overcome the inertia of opponents using a large mass.

Static inertia

For example, the reluctance of a heavy piece of sporting equipment to be moved, such as a barbell with weights, or the reluctance.

Dynamic inertia

For example, reluctance of a rugby player running quickly with the ball to be stopped.

Force

The product of mass x acceleration. It is an effect on one body that results from the interaction of a second body. Forces can be internal or external. 1N = 1m x 1a

Internal forces

Internal forces are those generated by the action of muscles and tendons within the human body.

External forces

External forces are gravity, air resistance, and friction.

Momentum

The product of mass and velocity. Describes the quantity of motion a particular body of mass has. p = mv The greater its momentum the further it will travel and the harder it is to slow or stop the object.

Velocity

The rate of the speed an object moves its position.

Conservation of momentum

The total momentum of two objects before impact or contact will equal the total momentum after impact. The object with the greatest momentum will be least affected by the collision.

Summation of momentum (force summation)

The correct timing and sequencing of body segments and muscles through a range of motion.

Simultaneous summation of momentum (simultaneous force summation)

The use of multiple body parts at the same time to produce force. Occurs when an explosive action of all body parts occurs at the same time.

Sequential summation of momentum (sequential force summation)

The activation of body parts that are used in sequence to produce force. By coordinating all the body parts that are involved in the movement, athletes are able to maximise velocity at the point of release.

Sequential summation of momentum principles

- Activating the stronger and larger muscles first. - Using as many body parts as possible. - Transferring momentum from one body part to another when at maximum velocity. - The presence of a stable base. - Ensuring appropriate follow-through.

Angular momentum

A product of moment of inertia and angular velocity.

Moment of inertia

The moment of inertia of a rotating object is a measure of its resistance to change, particularly its resistance to beginning angular motion or rotation.

Forces and angular momentum

A force that results in angular motion or rotation is called an eccentric force. This occurs when the force is applied away from the centre of gravity of the object.

Impulse

The product of force and time. Two factors affect impulse: - The magnitude (size) of the force. - The length of time the force is applied.

Newton's three laws of motion

Isaac Newton first compiled his three laws of motion in 1687. These laws describe the relationship between a body and the forces acting upon it.

Newton's first law of motion: Law of inertia

"An object will stay at rest or continue to travel in the same direction at a constant velocity unless acted on by an unbalanced force.”

Newton's second law of motion: Law of acceleration and momentum

"Any change in the motion of a body is directly proportional to the amount of force applied and takes place in the direction in which the force is applied."

Newton's third law of motion: Law of action and reaction

"For every action, there is an equal and opposite reaction."

Newton's first law of angular motion

The angular momentum of a body remains constant unless acted upon by an external torque (force).

Newton's second law of angular motion

A torque applied to an object will produce a change in angular motion in the direction of the applied torque that is directly proportional to the size of the torque and inversely proportional to the moment of inertia of the object.

Newton's third law of angular motion

For every torque is an equal and opposite torque.

Forms of kinematic motion

The three forms of kinematic motions are linear motion, angular motion and projectile motion.

Linear distance and displacement

Both distance and displacement measure how far an athlete has travelled.

Linear distance

How much ground an object covers throughout its motion.

Linear displacement

The difference between the initial position and final position of an object.

Linear speed and velocity

Both speed and velocity describe the rate at which something moves from one location to another.

Linear speed

The rate of motion.

It is the ratio of the distance covered to the time taken.

S = l/t

Linear velocity

The rate of the speed an object moves its position.

The ratio of displacement over time taken.

V = d/t

Linear acceleration

The rate of velocity change experienced by an object over time

a = vf - vi/t

Angular distance

The total of all angular changes that result from an object or body part angle between the starting and finishing position.

Angular displacement

The difference in degrees between the object or body part's initial and final positions.

It includes the magnitude and direction of the motion.

Angular speed

The angular distance covered divided by the time taken.

Angular speed = angular distance/ time

Angular velocity

Measured by dividing the angular displacement by the time taken, with mention of a clockwise or anticlockwise direction.

Angular velocity (ω) = angular displacement (θ)/ time (t)

ω = θ/t

Angular acceleration

The rate of change of angular velocity over time.

Angular acceleration (a) = final velocity (wf) - initial velocity (wi)/ time (t)

a =wf - wi/t

Projectile motion

An object or body that is launched into the air and affected only by the forces of gravity and air resistance.

Gravity

The cause of acceleration towards the earth's surface.

Has an impact on the vertical projectile motion.

Air resistance

Frictional force acting against a moving object.

As an object moves, the air resistance slows the object down, impacting on the horizontal projectile motion.

Factors that influence a projectile's motion through air resistance

The factors are velocity, mass, shape, surface area, and nature of the surface area.

Projectile motion velocity

The higher the velocity, the greater the air resistance.

Projectile motion mass

The lower the mass, the greater the air resistance.

Projectile motion shape

Objects considered streamlined will experience less air resistance than those that are not.

Streamlined shapes allow air to flow over then with less drag.

Projectile motion surface area

Refers to the area of an object which is exposed to the air.

The greater the surface area the greater the air resistance.

Projectile motion nature of surface area

Smooth surfaces decrease drag and are therefore less affected by air resistance, while rough surfaces are slowed more readily.

Three factors affecting the path of a projectile

The three factors are angle of release, speed of release, and height of release.

Angle of release

The angle in respect to the horizontal plane, that an object is projected into the air.

If height is the goal, then the angle of release should be closer to 90 degrees.

If distance is the goal, an angle of release closer to 45 degrees is preferred.

Speed of release

The speed at which an object is thrown, kicked or propelled into the air.

The greater the force is applied, the greater the speed.

Height of release

The height at which an object/ body is released from.

Equilibrium

Refers to a state in which there is a balance of forces or influences in opposition to each other.

There are two types of equilibrium: static equilibrium, and dynamic equilibrium.

Static equilibrium

The state in which a body has zero velocity and zero acceleration.

E.g. standing on one leg

Dynamic equilibrium

The state in which a body is in motion with a constant velocity.

E.g. a floor routine in gymnastics, or a cartwheel.

Stability

Refers to the degree to which a body resists changing its equilibrium.

Balance

The ability to control the state of equilibrium.

There are two types of balance: static balance, and dynamic balance.

Static balance

When an object is at rest and is not moving

E.g. a gymnastic handstand or starting block in sprints.

Dynamic balance

When an object is in motion and is moving with either linear or angular motion.

E.g. speed skating, wheelchair basketball, team sports.

Factors affecting balance and stability

The main factors are the centre of gravity, the line of gravity, size of the base of support, the height of the centre of gravity, the mass of the body, and friction.

Centre of gravity

Gravity always acts through the centre of an object's mass.

The centre of gravity changes positions depending on the actions and movements of the body.

Line of gravity

The line of gravity is an imaginary vertical line passing downwards through the centre of gravity.

When the line of gravity acts through the centre of the base of support, stability is increased.

Size of the base of support

The larger the base of support, the more stability.

When the line of gravity is centrally located within the base of support, balance should be secure.

Height of the centre of gravity

The higher the centre of gravity, the less stable the object.

Mass of the body

The greater the weight pressing vertically downwards, the more stable the body and the greater the force required to move it.

Friction

Increasing friction increases stability.

E.g. surf wax, running spikes, and gymnastics chalk.

Levers

A rigid structure that rotates around an axis and to which forces are applied to.

Leverage

The action or advantage of using a lever.

Components of a lever

A lever consists of the axis, force, and resistance.

The axis

The turning point of the lever.

The force

The point where force is applied.

The resistance

The weight of whatever a person is trying to move (either the force of objects or the weight of body parts)

First Class Lever (F.A.R)

Axis is between the point of force application and the resistance.

E.g. see-saw.

E.g. head and neck; to prevent the weight of your head bringing it forward, your neck muscles that sit posteriorly apply a force against the head (resistance).

Second Class Levers (A.R.F)

The resistance is located between the axis and the point of force application.

E.g. wheelbarrow.

E.g. stepping; axis is the point of impact, resistance is the weight on the body, force is the hamstring muscle.

Third Class Levers (A.F.R.)

The point of application of force is between the axis and the resistance.

Force is applied by muscles to change the angle of the joint (axis)

E.g. bicep curls; force is the bicep muscle, the axis is the elbow, the resistance is the weight.

E.g. extension of lower leg when kicking a football.

Mechanical advantage of levers

Mechanical advantage is determined by the type of lever, and length of the force arm and resistance arm.

Force arm/ resistance arm

Mechanical advantage greater than 1

If the force arm is greater than the resistance arm, the mechanical advantage ratio is greater than 1.

This means less effort is required to move the resistance.

All second-class levers are greater than 1.

Mechanical advantage less than 1

All third class levers have a mechanical advantage less than 1.

More force is required, but a larger range of motion is achieved, resulting in increased angular speed.