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Why does a high jumper arch their back over the bar? Why do sprinters lean forward at the start? Why does a shot put travel farthest when released at a certain angle? The answers come from biomechanics: the study of the mechanics of human movement, applying the laws of physics to the body.
Biomechanics helps athletes improve technique, coaches design training, and everyone prevent injuries. This chapter explains body movements, Newton's laws, levers, balance, friction and projectiles, each with sports examples.
Learning goals
Explain the meaning and importance of biomechanics in sports.
Describe the types of body movements.
Apply Newton's three laws of motion to sports.
Classify levers and give examples from the body and sports.
Explain equilibrium and centre of gravity.
Explain friction and its role in sports.
Explain projectile motion and the factors affecting it.
Core ideas
1. Meaning and importance of biomechanics
Biomechanics is the science concerned with the internal and external forces acting on the human body and the effects produced by these forces.
Importance:
Improves technique by analysing movements (for example, the angle of take-off in a long jump).
Improves performance by making movements efficient.
Prevents injuries by identifying harmful movements and improving technique.
Helps design better sports equipment, shoes and clothing.
Helps coaches teach skills and correct errors; aids rehabilitation.
2. Types of body movements
Movement | Meaning | Example |
|---|---|---|
Flexion | Decreasing the angle between two body parts (bending) | Bending the elbow in a bicep curl |
Extension | Increasing the angle (straightening) | Straightening the knee when kicking |
Abduction | Moving a body part away from the midline | Raising the arm sideways in a jumping jack |
Adduction | Moving a body part towards the midline | Lowering the arm back to the side |
Rotation | Turning a body part around its long axis | Turning the head to say "no"; twisting in a discus throw |
Circumduction | Circular movement combining flexion, extension, abduction and adduction | Arm circles; bowling action in cricket |
Pronation | Turning the palm downward (or backward) | Dribbling a basketball |
Supination | Turning the palm upward (or forward) | Holding a bowl of soup |
Dorsiflexion | Bending the foot upward towards the shin | Walking on the heels |
Plantar flexion | Pointing the toes downward | Standing on tiptoe; taking off in a jump |
3. Newton's laws of motion in sports
Law | Statement | Sports applications |
|---|---|---|
First law (law of inertia) | A body remains at rest or in uniform motion in a straight line unless acted upon by an external force | A football stays on the spot until kicked; a sprinter needs great force to start from rest; a moving cyclist continues until brakes or friction stop them |
Second law (law of acceleration) | The acceleration of a body is directly proportional to the force applied and inversely proportional to its mass: F = m × a | A harder kick makes the ball go faster; a lighter shot put accelerates more with the same force |
Third law (law of reaction) | For every action there is an equal and opposite reaction | A swimmer pushes water backward and moves forward; a sprinter pushes the starting blocks backward and is propelled forward; a high jumper pushes down on the ground to rise |
Example (second law). A force of 60 N acts on a 0.4 kg ball: acceleration = F ÷ m = 60 ÷ 0.4 = 150 m/s².
4. Levers
A lever is a rigid bar that moves about a fixed point called the fulcrum (axis). In the body, bones are the levers, joints are the fulcrums, muscles provide the effort (force), and the weight of the body part or object is the load (resistance).
Class | Arrangement | Body example | Sports or everyday example |
|---|---|---|---|
First class | Fulcrum in the middle, between effort and load (E–F–L) | Nodding the head: the neck joint is the fulcrum, neck muscles the effort, and the weight of the head the load | See-saw, scissors, triceps extension at the elbow |
Second class | Load in the middle, between fulcrum and effort (F–L–E) | Standing on tiptoe: the ball of the foot is the fulcrum, body weight the load, and calf muscles the effort | Wheelbarrow, push-up |
Third class | Effort in the middle, between fulcrum and load (F–E–L) | Bicep curl: the elbow is the fulcrum, the biceps the effort, and the weight in the hand the load | Hitting with a bat or hockey stick, rowing, kicking |
Mechanical advantage = effort arm ÷ load arm.
Second-class levers always have mechanical advantage greater than 1 (they favour force).
Third-class levers have mechanical advantage less than 1 but favour speed and range of motion; most levers in the body are third class.
5. Equilibrium and centre of gravity
Equilibrium is a state of balance in which all forces acting on a body cancel out.
Static equilibrium: the body is at rest and balanced, as in a headstand or the "set" position of a shooter.
Dynamic equilibrium: the body is balanced while moving, as in running, cycling or skating.
Centre of gravity (COG): the point at which the entire weight of the body is considered to act. In a standing person, it lies roughly around the navel region; it shifts when the body changes position and can even lie outside the body (as in the Fosbury flop high jump, where the COG passes below the bar).
Principles of stability:
Principle | Application |
|---|---|
Broader base of support increases stability | Wrestlers and judokas widen their stance |
Lower centre of gravity increases stability | Bending the knees when receiving a tackle or in a sumo stance |
Line of gravity within the base gives balance | A gymnast on the beam keeps the COG over the beam |
Greater body mass increases stability | Heavier players are harder to push |
6. Friction
Friction is the force that opposes motion between two surfaces in contact.
Type | Example |
|---|---|
Static friction | Prevents a stationary object from starting to move |
Sliding (kinetic) friction | Acts when one surface slides over another |
Rolling friction | Acts when an object rolls, such as a ball on grass |
Fluid friction (drag) | Air or water resistance on a moving body |
Friction in sports:
Increasing friction (as an advantage): spikes in athletics shoes, studs in football boots, chalk on gymnasts' hands, rubber soles in basketball, grips on bats and rackets.
Reducing friction (as an advantage): waxed skis, smooth swimsuits, lubricated bicycle chains, ice in skating.
Disadvantages: friction causes wear of equipment and blisters, and slows movement.
7. Projectiles
A projectile is an object thrown or launched into the air that moves under the influence of gravity (and air resistance). Examples: shot put, javelin, discus, a football kick, and the athlete's body in a long jump.
Factors affecting the trajectory of a projectile:
Factor | Effect |
|---|---|
Angle of release | Theoretically, 45° gives the maximum horizontal distance when release and landing heights are equal; in practice, shot put is released at about 40–42° because it is released above ground level |
Speed (velocity) of release | Greater speed gives greater distance; it is the most important factor |
Height of release | A higher release point increases distance (tall shot putters have an advantage) and lowers the best angle |
Gravity | Pulls the projectile downward, giving it a curved (parabolic) path |
Air resistance | Slows the projectile; spin and shape (aerodynamics) matter, as with a javelin or discus |
Example. For the same speed of 10 m/s and equal release and landing height, the range is R = v² sin(2θ) ÷ g. At 45°: R = 100 × 1 ÷ 9.8 ≈ 10.2 m. At 30°: R = 100 × 0.866 ÷ 9.8 ≈ 8.8 m. This shows why the angle matters.
A reliable approach to questions
Define biomechanics and list its importance.
Name each movement with an example: flexion decreases the angle; abduction moves away from the midline.
State each of Newton's laws with two sports examples.
Identify lever class from the position of fulcrum, effort and load.
Apply stability principles (wide base, low COG).
List and explain the factors affecting a projectile.
Connections that matter
Physics: laws of motion explains Newton's laws, friction and projectiles.
Physiology and injuries in sport explains how muscles produce force.
Training in sports applies technique improvements.
Common mistakes
Mixing up abduction (away from the midline) and adduction (towards the midline).
Calling a bicep curl a second-class lever. It is third class (effort between fulcrum and load).
Saying a higher centre of gravity increases stability. A lower COG increases stability.
Saying 45° is always the best release angle. It is ideal only when release and landing heights are equal.
Revision checklist
I can explain the meaning and importance of biomechanics.
I can describe ten types of body movement with examples.
I can apply Newton's laws to sports.
I can classify levers with body and sports examples.
I can explain equilibrium, centre of gravity and stability.
I can explain friction and projectile factors.