Body in Motion CQ3

Biomechanics

Biomechanics

a science concerned with forces and the effect of these forces on and within the human body.

Enables:

• Development and alterations of techniques to achieve optimal performance.

• Reduce the risk of injury by improving movement patterns.

• Design and use equipment that contributes to improved performance.

Motion

Motion describes the movement and path of a particular body (i.e. athlete, equipment).

Linear Motion

Linear motion occurs when the human body, a human limb or an object propelled by a human move in the same direction (usually at the same speed). Examples: swimming and 100m sprint events, where competitors follow a straight line from start to finish.

Angular motion

occurs when a body and all parts of it travel along a circular path through the same angle and in the same direction. The axis of rotation is an imaginary line about which the body or part of a body rotate e.g. gymnast rotating on the high bars.

General motion

is a combination of angular and linear motion. It is the most common motion in athletic events because movement of body segments such as arms and legs are complex. Additionally, most athletic movement requires directional change e.g. 400m sprint.

Velocity

refers to the rate of positional change of an object and is calculated as displacement divided by the time taken (V = D/ T). Displacement is the movement of a body from one location to another in a straight line (direction). This differs from distance, which is the total recorded length of the path taken between two points (not necessarily in a straight line).

Speed

Speed is the time rate at which an object is moving along a path and is calculated using distance divided by the time taken (S = d / T). Distance varies from displacement, as changes in path or direction means that the distance between too points may not be linear

Acceleration

Acceleration relates to the rate of change in velocity in a given time period (A = V2 (final velocity) – V1 (initial velocity) / T). Acceleration can be positive, whereby the final velocity recorded is higher than the initial velocity recorded. In contrast, deceleration occurs when the final velocity recorded is lower than the initial velocity. If there is no change in velocity, then acceleration will be zero.

Momentum

Momentum is a product of mass and velocity (M = m x V). The application of the principle of momentum is significant in sporting contexts, particularly in impact or collision situations. An object or body with greater momentum is more difficult to stop.

Balance and Stabilty

The concepts of stability and balance are closely related to equilibrium. Stability is concerned with the resistance of a body to changes in its equilibrium. When an individual can assume a stable position and then control that position, they are said to be in a state of balance. There are two types of balance: static (no movement) and dynamic (movement).

Centre of gravity

The centre of gravity (COG) of an object is the point at which all the weight is evenly distributed and about which the object is balanced. Knowing the position of the COG is very important to improving sport performance, at it can allow for the modifications of technique to maximise movement efficiency. For the average human, the COG is at the centre of the pelvis (anatomical position). However, the composition of the body will mean that the COG will be different for all athletes.

Line of gravity

Line of gravity (LOG) is an imaginary vertical line passing through the COG and extending to the ground. It indicates the direction that gravity is acting on the body. When standing erect, the LOG dissects the COG so that balance is achieved. An object is most stable when the line of gravity falls through the centre of the base of support.

Base of support

The base of support (BOS) of a body is the region bounded by the body parts in contact with a surface that is applying a reactive force against the applied force of the body. It affects athlete stability and the ability to control equilibrium. A narrow BOS allows the COG to fall close to the edge of the BOS. This results in only needing a small amount if force to make the person lose their balance and movement to occur. A wide BOS is essential for stability because the COG is contained well within the boundaries.

Example: gymnast performing a pirouette has a very narrow BOS and must work hard to ensure that their COG remains within the base.

Fluid mechanics

Fluid mechanics is a branch of physics that is concerned with properties of gases and liquids.

Flotation

(buoyancy) is the upward force experienced by a body/object when immersed in water. Archimedes’ Principle states that a body that is partially or totally immersed in a fluid will experience buoyancy that is equal to the weight of the volume of fluid displaced by that body/object. Body density, or its mass per unit volume, also impacts on the ability to float. Density is an expression of how tightly a body’s matter is enclosed within itself (body composition).

High Buoyant Force

A body/object that has a higher buoyant force than the force of its weight (mass x gravity) will float in water.

Low Buoyant Force

A body/object will sink if the mass of the water it displaces is less than its own weight.

Centre of buoyancy

the point in the body where the amount of volume under the water is equally distributed on either side. The COB tends to be higher in your body than the COG, because of the effects of dense legs at one end and low-density lungs towards the other end.

Fluid resistance

Drag is the resistance force that opposes movement of a body/object as it moves through a fluid environment (i.e. air, water).

Profile Drag

Profile drag (also called form or pressure drag) refers to drag created by the shape and size of a body or object. As they move through fluids, bodies or objects cause the medium to separate, resulting in pressure differences at their front (high pressure) and rear (low pressure).

Surface Drag

Surface drag is caused by friction (movement of air particles between two objects/bodies) between the surface of an object and the fluid surrounding it. The amount of surface drag is caused by the density of fluid, the speed of movement, the smoothness of the object and the amount of surface area in contact with the fluid

Magnus Effect

The Magnus Effect occurs when a spinning object (ball) creates rotating air (whirlpool) around it, resulting in a force that moves ball from a conventional flight path. Velocity increases on one side of the object, where the fluid travels in the same direction as the whirlpool. As the velocity of a fluid increases, the pressure exerted by the fluid will decrease. The opposite side of the object experiences decreased velocity as the motion of the whirlpool is reversed. This difference in pressure creates a force on the ball that causes it to spin, which is relevant in a range of sports.

Examples of Magnus Effect

TENNIS Backhand slice (backspin) Keeps ball low to the ground, hard for the opponent to generate speed.

CRICKET Swing bowling (out / in) Ball moves in the air at pace, forcing batsmen error such as edging or missing the ball.

GOLF Drive (draw / fade) Shapes the ball left/right, avoids obstacles to hit fairway or green.

FOOTBALL Free kick (curved) Imparting spin/curve on the ball moves it out of the path of the goalkeeper, making it harder to save.

Force

is the action of pushing or pulling an object, to make it change its position.

1. Inertia: Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. That is, an object will remain in constant velocity until a force acts upon it to change the velocity.

2. Acceleration: The rate of change of momentum of an object is proportional to the force causing the change and occurs in the direction of the force.

3. Reaction: For every action, there is an equal and opposite reaction. This means that when a force is applied to an object, the object applies the same force upon that which is applying the original force, but in the opposite direction.

How the body applies force

Internal Force: Forces that develop within the body through the contraction of a muscle group (e.g. force generated by the quadriceps when kicking a football).

• External Force: Forces from outside the body and act on it in one way or another (e.g. gravity causing a high jump athlete to land on the mat).

• Action Force: Forces applied to external objects (e.g. running track or ball). They usually involve the application of internal forces within the body to produce a specific movement.

• Reaction Force: This relates directly to Newton’s 3rd Law (for every action, there is an equal and opposite reaction). The applied force to an external object/surface will produce a reaction force of equal value.

How do Joints/Muscles absorb force

Forces exerted on the body can be absorbed through the joints, which bend or flex in response to the impact, as well as through the muscles. When an athlete lands on a floor or similar surface (action force), that surface exerts a reaction force of equal value. If the athlete does not absorb the force through flexion of the knees/hips and allow a slow, controlled dissipation of the forces by the muscles, the risk of injury to the joint is increased.

Technique absorb force

Alteration of technique acknowledges the direction of an action force. When skills are executed that prolong the impact or collision time, the reaction force in the opposing direction is reduced. This is shown through the example of a cricketer catching a ball, where their hands ‘give’ with the direction of the ball so that is does not bounce out.

How does equitment absorb force

The use of specific proective equipment reduces the amount of force experienced by athletes. The density and surface area of the protective equipment dissipates the reaction force, as opposed to the muscles, bones or joints.

Force Quantity

The greater the amount of action force applied to the object, the greater the acceleration of the object. This ultimately will also determine other factors such as momentum, velocity and distance.

Object Mass

The greater the mass of the object, the greater the force required to move it. This is particularly relevant in sports such as weightlifting, where athletes need to continue increase the amount of force applied to the barbell in order to lift it.

Force Direction

The direction and manner in which the force is applied will determine the path of the object. This refers to directly to the Magnus Effect, as the application of the direction of force to a ball will result in spin.