Biomechanics of movement (SACE Stage 2 Physical Education)

What this dot point is asking

You must apply biomechanical principles - force, levers, Newton's laws, projectile motion and stability - to analyse movement and suggest improvements.

Force

Force is a push or pull that can start, stop, speed up, slow down or change the direction of motion. Key applications include:

• Summation of forces: maximum force comes from using as many body parts as possible, in the correct sequence (large/slow muscles first, then smaller/faster), with correct timing. A cricket throw uses legs, then trunk rotation, then shoulder, elbow and wrist in sequence.
• Force application and the impulse-momentum relationship: applying force over a longer time produces a greater change in momentum. Following through on a throw or bending the knees on landing both increase the time over which force acts.

Levers

The body moves through lever systems made of bones (the lever), joints (the fulcrum), and muscles (which apply the effort) against a resistance (load).

• First-class lever: fulcrum between effort and load (the head nodding at the neck).
• Second-class lever: load between fulcrum and effort (rising onto the toes).
• Third-class lever: effort between fulcrum and load - by far the most common in the body (the forearm flexing at the elbow). Third-class levers sacrifice force to gain speed and range of motion at the end of the limb.

Lengthening a lever (a straight arm in a tennis serve, a longer racquet) increases the speed at the end of the lever, generating more velocity but requiring more force and control.

Newton's laws of motion

• First law (inertia): a body stays at rest or in uniform motion unless acted on by a net force. A stationary shot put will not move until force is applied.
• Second law (acceleration): acceleration is proportional to the force applied and inversely proportional to mass ($F = ma$). A larger force, or a lighter object, produces greater acceleration.
• Third law (action-reaction): every action has an equal and opposite reaction. A sprinter pushes back and down into the blocks; the ground pushes them forward and up (ground reaction force).

Projectile motion

Once an object (or the body) is airborne, its flight path is determined by three release factors:

• Speed (velocity) of release - the most important factor for distance.
• Angle of release - around 45 degrees gives maximum range when release and landing heights are equal; the optimal angle is lower when the object is released from above the landing point (such as a shot put or a basketball shot).
• Height of release - a greater release height increases flight time and range.

Air resistance and gravity then act on the projectile; gravity is the only force that affects the vertical component once airborne.

Stability and balance

Stability is the ability to resist a change in the body's state of motion. It improves with:

• a lower centre of mass (bending the knees),
• a wider base of support,
• keeping the line of gravity within the base of support, and
• a greater mass.

A rugby player braces for a tackle by widening their stance and lowering their hips; a sprinter on the blocks raises their centre of mass and shifts the line of gravity toward the edge of the base to be deliberately unstable and ready to move.