Biomechanics: the physics your bones and muscles were already obeying

The body produces three kinds of motion, and most skills combine them:

• Linear motion — the whole body travels as a unit, every part covering the same distance in the same time. A skater gliding; a sprinter's centre of gravity moving down the track.
• Angular motion — rotation about an axis. A gymnast on a bar, a diver somersaulting, and — importantly — every single joint action from Lesson 1. Flexion is angular motion.
• General motion — the combination, and therefore what nearly all real movement is. A runner's body travels linearly down the track because the limbs are rotating angularly about the hips, knees and ankles.

That last point is the connection between this lesson and the previous two. The body produces linear motion by means of angular motion at joints. Muscles cannot push the ground directly; they rotate bones about joints, and the sum of those rotations moves the body in a line.

Newton's three laws govern all of it, and each has a direct movement application:

• First law, inertia. A body stays at rest or in uniform motion unless acted on by an external force. A rugby player at speed is hard to stop, and a heavier player harder still — inertia increases with mass. It is also why the start of a sprint costs more energy than maintaining it.
• Second law, acceleration. Force equals mass multiplied by acceleration, so acceleration is proportional to the force applied and inversely proportional to mass. To accelerate faster you apply more force or carry less mass, and there is no third option.
• Third law, action and reaction. For every action there is an equal and opposite reaction. A sprinter drives down and backwards into the blocks, and the ground pushes them up and forwards with equal force. The ground provides the propulsion. This is why sprinting on sand is slow: soft ground deforms, so some of the force is absorbed rather than returned.

Three terms do all the work here, and they must be kept separate:

• Centre of gravity — the point where the body's mass is evenly distributed. In a standing adult it sits roughly at the level of the navel, slightly in front of the sacrum. It moves when you move: raise your arms and it rises, bend forward and it can travel outside the body entirely.
• Line of gravity — the vertical line dropping from the centre of gravity to the ground.
• Base of support — the area enclosed by your points of contact with the ground.

The governing rule is one sentence: you are balanced while the line of gravity falls within the base of support, and you fall over when it does not. Everything else is a way of making that harder or easier to achieve.

Force is a push or a pull that changes an object's motion, and in human movement there are three separate questions about it: how you generate it, where you apply it, and how you absorb it. The third is the one that keeps people uninjured and the one most often left out of an answer.

Summation of forces. Maximum force comes from using body parts in sequence, starting with the largest and slowest and finishing with the smallest and fastest, with each contribution timed to begin as the previous one peaks. A cricket throw runs legs, hips, trunk, shoulder, elbow, wrist, fingers. Two conditions have to hold: the parts must be used in the right order, and the timing must be continuous — a pause anywhere and the force already generated is lost. This is why "throwing with just your arm" produces so little: it recruits the last and smallest link only.

Application of force. Three things matter — the magnitude, the direction, and the point of application. Force applied through the centre of gravity produces linear motion; force applied off-centre produces rotation. That is not a detail, it is a choice: a footballer punting for distance strikes through the ball's centre, and a player wanting swerve deliberately does not.

Absorption of force. To reduce the risk of injury, spread the force over a longer time and a larger area. Both reduce the force experienced at any instant and at any point.

• Longer time. Bending the knees on landing, giving with the hands when catching, rolling out of a fall. The eccentric contractions from Lesson 2 are how the body buys that time.
• Larger area. Landing on the whole foot rather than the heel, catching with two hands rather than one, and the entire design principle of protective padding.

Levers. A bone turning about a joint is a lever, and the body uses all three classes. The class is set by the arrangement of the fulcrum (the joint), the effort (the muscle's pull) and the resistance (the load).

The trade-off in the third row is why the body is built the way it is. Muscles insert very close to joints, which is mechanically inefficient for lifting — the biceps must generate far more force than the weight it holds. What it buys is speed and range at the end of the limb: a few centimetres of muscle shortening becomes a hand travelling a metre. For a species that throws, that is the better trade.

Air and water are both fluids, and anything moving through a fluid is affected by it. The effects are small in air at walking pace and enormous in water at any pace, which is why swimming technique is overwhelmingly a fluid-mechanics problem.

Drag is the resistive force opposing motion, and it comes in three kinds:

• Form drag — caused by the shape and frontal area presented to the fluid. The biggest single factor for a swimmer or cyclist. Reduced by streamlining: a horizontal body position in the water, a tucked position on a bike.
• Surface drag — friction between the fluid and the surface of the body. Reduced by smooth suits, shaving, and tight-fitting clothing.
• Wave drag — created at the boundary between water and air by a swimmer's own waves. This is why underwater phases after a start or turn are faster than surface swimming, and why they are limited by the rules.

Lift is a force acting perpendicular to the direction of motion, generated when a fluid flows faster over one side of an object than the other, producing a pressure difference. A discus or javelin held at the right angle of attack generates lift and flies further than a projectile alone would.

The effect of spin. A spinning ball drags a thin layer of air around with it, so the air moves faster on one side than the other, creating a pressure difference and deflecting its path — the Magnus effect.

• Topspin makes a ball dip sharply — the reason a tennis player can hit hard and still land the ball in.
• Backspin makes it stay in the air longer and land more softly.
• Sidespin curves it sideways, which is the banana kick in football and the away-swinger in cricket.

Buoyancy matters in the water: an upward force equal to the weight of fluid displaced. Body composition affects it, and a swimmer whose legs sink presents a larger frontal area and therefore more form drag — which is why body position, not effort, is usually the first thing a coach fixes.

This is the half of the dot point that carries the marks, because it is where biomechanics is connected back to specific muscles, bones and joints. Every recommendation below is a principle applied to an anatomical structure, and an answer must show both.

Lifting — and here is the answer to Card 01. The critical quantity is torque: the turning force on the lumbar spine, which increases with the horizontal distance between the load and the spine. Hold a load at arm's length and the torque on the lower back is several times greater than holding the same load against your chest.

• Keep the load close to the body. This shortens the lever arm and reduces torque on the lumbar vertebrae and intervertebral discs — the cartilaginous joints from Lesson 1, which are not built to resist large shear forces.
• Use the hips and knees to share the work. The quadriceps and gluteus maximus contribute strongly during a lift, while the trunk and spine also move and bear load. No single posture guarantees that a lift is injury-free.
• Manage the load through a controlled movement. Spinal position changes how load is shared across the back; it does not, by itself, determine whether someone will develop low-back pain.
• Widen the base of support and turn with the feet when practical. A wider base can improve stability. Avoiding a loaded twist is one way to manage task demands, but it is not a single explanation for most lifting injuries.

Squatting. Same principles in a different arrangement: keep the bar's line of gravity over the middle of the foot so it stays within the base of support, let the knees track in line with the toes so the hinge joint is loaded in the plane it moves in rather than being forced into abduction, and hinge at the hips so the gluteus maximus and hamstrings share the load with the quadriceps.

Walking. Efficient gait absorbs force over time and area: the heel contacts first, the foot rolls through to the toes to spread the impact, and the ankle, knee and hip flex slightly to lengthen the absorption. Footwear extends the absorption time further. The centre of gravity should oscillate as little as possible — every centimetre it rises and falls is energy spent going nowhere useful.

Efficiency and specific needs. The syllabus names people with disability, and biomechanics is central rather than incidental here.

• Wheelchair propulsion. Seat height and axle position determine the angle at which the arm meets the push rim. Set correctly, the shoulder works through a mid-range where the deltoid and triceps generate force efficiently; set badly, the athlete pushes at the end of range, generates less force per stroke, needs more strokes, and accumulates the repetitive shoulder injury that is the commonest complaint among wheelchair users.
• Prosthetic alignment. A prosthesis has to place the line of gravity within the base of support through the whole gait cycle, and running blades are shaped to return elastic energy at push-off, which is the third law being used deliberately.
• Adaptive technique generally. Where a joint action is unavailable, movement is rebuilt from the principles: a different base of support, a different lever, a different sequence for summing force. The principles do not change; only which structures are available to apply them does.

Watch someone lift something heavy from the floor — a training partner, a family member, or yourself filmed on a phone from the side. Film it if you can, because the diagnosis depends on seeing the side view.

Choose one everyday or sporting movement — getting out of a chair, a basketball chest pass, a swimming start, carrying shopping. Then choose one constraint:

• the person cannot bear weight through one leg;
• the person uses a wheelchair;
• the person has significantly reduced grip strength;
• the person has limited shoulder range of motion.

Now rebuild the movement. Work through the four principle areas in order — motion, balance and stability, force, and fluid mechanics where it applies — and for each, state what changes and which muscles, bones or joints now do the work instead.

Finish with the honest paragraph: what does your redesign cost? Almost every adaptation trades something away — speed for stability, range for safety, efficiency for accessibility. Naming the trade-off is the analytical move, and pretending there isn't one is the commonest weakness in answers on this sub-point.