Biomechanics is a study of how natural laws and forces affect the body and objects in sports movement and performance. You will understand the mechanical cause-effect relationships that determine human movement and apply the biomechanical principles such as stability, summation of force and projectile motion in physical activities to improve movement.

At the end of this section, you will gain the following:

 Key Understanding

You will understand that:

• force, motion and stability affect movement and performance in exercise and sports; and

• proper application of biomechanical principles will lead to improved and refined human movement.

Knowledge

You will know:

• the effect of the natural laws and forces on the human body in movement and performance;

• the biomechanical principles such as stability, summation of forces and projectile motion; and

the movement phases of skill performance for analysis. 

Skills

You will be able to:

• apply biomechanical principles such as force, stability, summation of forces and projectile motion to analyse movement for refinement and improvement; and

• observe, analyse and evaluate efficiency of movement.

Essential Questions

• How can efficient movement be achieved? 

Distance and DISPLACEMENT

Distance and displacement measure how far a body or an object has travelled.

Distance is how far a person or an object has travelled between the start and end positions. It is the length of the path taken as the person or object moves from one position to another. It is measured in metres (m).

Displacement is the change in overall position of a person or an object. It is the length of the straight line between the start and end positions after movement. It is measured in metres (m). 

Speed and Velocity

Speed and velocity measure how fast a body or an object moves. 

Speed is the rate of change of distance without considering direction. It is measured in metres per second (m/s or ms-1).

Speed (m/s) = Distance (m) / Time (s)

Velocity is how fast a body travels in a certain direction, which is the rate of change of displacement. Velocity is a more precise representation of motion as it measures how fast a body is moving and in what direction. It is measured in metres per second (m/s or ms-1).\

Velocity (m/s) = Displacement (m) / Time (s)

ACCELERATION and DECCELERATION

Acceleration and deceleration measure the rate of change of velocity. Acceleration occurs when rate of change of velocity increases and deceleration occurs when rate of change of velocity decreases. Both are measured in metres per second (m/s2 or ms-2).

Acceleration (m/s2) = Change in velocity (m/s) / Time (s)

FORCES

A force is a push or pull that changes, or tends to change, the state of motion of a body or an object. Force has magnitude and direction. It is measured in newtons (N). 

There are two type of force: internal force and external force.

• Internal force originates from the contraction of skeletal muscles. 

  • An example of internal force is when a person contracts the pectoralis majors and triceps to generate the force needed to perform a push-up.

• External force originates from outside the body. 

  • Examples of external force include gravity, air resistance and friction.

Force is essential to create movement. Below is a detailed description of how force can be seen in a penalty shot.

Mass and Weight

Mass is a physical quantity expressing the amount of matter or substance in a body or an object. In the body, mass is made up of bone, muscle, fat, tissue and fluid. It is measured in kilograms (kg). Mass is also a measure of an object’s inertia. Inertia refers to the reluctance of a body to change its state of motion or rest.

Weight is the amount of force exerted on the mass of a body or an object due to gravity. It is measured in newtons (N). Gravity is the acceleration of a body and is taken as a constant 9.8 ms-2.

Weight (N) = Mass (kg) x Gravity (9.8 ms−2)

Examples IN SPORTS CONTEXT

Centre of Gravity (Centre of Mass)

The centre of gravity is an imaginary point which may lie inside or outside of a body, about which all of the body’s mass is equally distributed. However, this point may change at times, based on the relative position of the body parts.

Stability

Stability is the ability of a body to resist disruption to its state of equilibrium.

1. Body Mass

If all other factors relating to stability remain unchanged, a heavier athlete is  more stable than a lighter athlete. In sports that require athletes to be fast, too much body mass can be a liability.

2. Size of the Base of Support

The base of support refers to the area enclosed by the points of contact with the athlete’s body. The bigger the base of support, the greater the stability.

3. Height of the Centre of Gravity

Athletes with a higher centre of gravity tend to be less stable than those with a lower centre of gravity.

4. Position of the Line of Gravity Relative to the Base of Support

The line of gravity is an imaginary vertical line passing through the centre of gravity. An athlete’s balance is maintained as long as the line of gravity falls inside the perimeter of the base of support. The closer to the centre of the base the line of gravity falls, the more stable the athlete becomes. The closer to the edge of the base the line of gravity falls, the more unstable the athletes becomes.

Bones, ligaments, and muscles are structures that form levers in the body to create human movement.

A lever has three parts: the fulcrum, effort (force) and the load (resistance). There are three classes of levers. While all three types are found in the body, third-class levers are the most common.

Identify the Fulcrum, Load, Effort

How to Identify Levers in The Human Body

Step by step

Identify and locate 

1. the joint where the movement takes place - this is the fulcrum

2. the moving body part - this is the load

3. the insertion of the muscle that cause the movement - this is the effort

Levers in The Human Body

Summation of forces refers to the sum of all forces generated by each body part to produce an optimal or a maximal force. Summation of forces can occur simultaneously or sequentially.

Simultaneous summation of forces

• Optimal or maximal force is generated by activating all the relevant body parts (and muscles) at the same time to perform an action.

• In some sporting examples, there are times when body parts are moved at the same time to generate force to perform an action. For example, in a 100m race, a sprinter explosively moves all relevant body parts such as the calves, thighs, hips, knees, ankles, elbows and shoulders at the same time to generate maximal force when pushing off from the starting block at the start of the race.

Projectile motion refers to the motion of a projected object (e.g., a shot put, a javelin, a human body during pole-vaulting) acted on by forces of gravity and air resistance. In sports, once the projected object is released or thrown in the air, control of the object is no longer possible. Therefore, it is important to consider the following factors that affect the flight path of the projectile prior to its release:

Learning Goals

• Determine how each parameter (initial height, initial angle, initial speed, mass, diameter, and altitude) affects the trajectory of an object, with and without air resistance

• Predict how varying the initial conditions will affect a projectile’s path, and provide an explanation for the prediction

• Estimate where an object will land, given its initial conditions.

• Discuss projectile motion using common vocabulary (such as: launch angle, initial speed, initial height, range, time).

https://phet.colorado.edu/en/simulations/projectile-motion

Height at release 

• Relative projection height refers to the difference in the height from which the body is projected and that at which it lands or stops.

Angle of release

• refers to the angle at which the projectile is thrown

• the direction of projection with respect to the horizontal determines the shape of flight

Velocity at release

• The speed of projection determines the length of trajectory (range)

For round projectile objects such as balls, it is also possible to influence the flight path by spinning the object. 

When a spinning ball moves through the air, it creates a difference in pressure between the top and bottom of the ball. 

The interaction between the air layer on the side of the ball that is rotating in the same direction as the surrounding air, but against the direction that the ball is moving in, causes an area of low pressure to form. 

The other side of the ball, where the air layer is moving against the direction of the surrounding air, slows the air down and thus creates an area of high pressure.

The difference in pressure creates what is called the Magnus force, a force directed from the high-pressure zone to the low-pressure zone. 

The Magnus force affects the flight path of the spinning ball as it travels through the air, causing the moving ball to deviate gradually along the direction of the spin (i.e., towards the low-pressure zone).

This deviation is known as the Magnus effect.

Exams Question

At the end of this section, you will gain the following:

 Knowledge

• Know the phases (during preparation, action and follow through) of performances for analysis.

 Skills

• Apply biomechanical principles to analyse movement for refinement and improvement.

 Learning Outcomes

• Recognise the phases (during preparation, action and follow through) of performance and use a biomechanical analysis to analyse physical performance.

• Apply concepts in biomechanics to modify physical performance responses for improvement.

Movement Phases of Performance

An analysis of a movement or sport skill can result in a better understanding of how to improve the technique for better performance and prevent potential injury.

One method of analysing the biomechanics of sports movements is to study movement phases during performance. A sports movement such as hitting, throwing, kicking or jumping generally contains three main movement phases:

a. Preparation

• Contains all the movements that get the athlete ready to perform a skill (e.g., the run-up to a long jump, the backswing of the leg while kicking).

b. Action

• Performance of the actual movement that often includes a point of contact with an object (e.g., a tennis forehand) or the release of an object (e.g., throwing a javelin).

c. Follow through

• Movements after the action phase (e.g., the forearm moving downwards after a smash) that slow down the body’s momentum to prevent injury and get the body ready for another movement.

Analysis of Skill Performance for Improvement

How will you analyse your movement?