Describing motion vs. explaining it
So far in this rung you have named the parts — bones, joints, the muscle team, the planes that movement happens in. Now we ask a different kind of question: not what moves, but why it moves that way, and what it costs. Biomechanics is simply the physics of the body, and the first useful split it offers is between two ways of looking at the same step. You can describe the motion — how far the knee bends, how fast the heel rises, the path the toe traces through the air — without saying one word about what caused it. Or you can talk about the pushes and pulls that produced it. The glossary calls this distinction kinematics versus kinetics.
Kinematics is the geometry of movement: positions, angles, speeds, directions, with no mention of force. When a clinician measures how many degrees an elbow flexes — building on the range of motion you met earlier — that is pure kinematics. Kinetics is the cause side: the forces and turning effects that make the motion happen or hold it still. A camera can capture kinematics. To get at kinetics you need to reason about loads, often with a force plate under the foot or, more usually, with a trained eye and a mental model. Both matter, and they answer different clinical questions: kinematics tells you the movement looks abnormal; kinetics begins to tell you why.
Force, and the turning kind: torque
A force is just a push or a pull — gravity pulling your forearm down, a biceps pulling it up, the floor pushing back against your feet. Force has a size and a direction. But the body almost never moves in a straight line; it rotates around joints. So the quantity that really runs the body is not bare force but the *turning effect* of a force, which the glossary calls force, torque, and moment. Torque and moment are two names for the same thing: how hard a force twists something around a pivot. You meet it every day. Pushing a door near its hinge barely moves it; pushing the same force at the far edge swings it easily. Same force, very different turning effect.
The intuition to keep is this: turning effect = how hard you push multiplied by how far out from the pivot you push. That distance from the pivot is the moment arm, and it is where most of the surprises live. Hold a light book in your hand with your elbow bent at a right angle, and it feels manageable. Now straighten your arm and hold the same book out at full reach — it suddenly feels heavy, even though the book has not gained an ounce. Nothing about the force of gravity changed; what changed is the moment arm. Held far from your shoulder, the book has a long lever to twist your arm down, and your muscles must produce far more torque to oppose it.
This single idea quietly governs an enormous amount of clinical reasoning. It is why we teach someone with back pain to lift a box held close to the trunk rather than at arm's length: close in, the box has a short moment arm and demands little spinal torque; far out, it demands a great deal. It is why a heavy bag carried in one hand makes you lean, and why a small weight on the far end of a long limb can be exhausting to lift in therapy while the same weight near a joint is easy. You do not need the equation. You need the picture: force times distance-from-the-pivot.
Bones are levers: the three classes
Put torque together with the skeleton and you get the lever — a rigid bar that turns about a pivot. In the body the rigid bar is a bone, the pivot is a joint, the effort is the pull of a muscle, and the load is the weight being moved (the limb itself plus anything it holds). The relationship between where these three things sit decides how much force the muscle must produce, and whether the arrangement favours strength or favours speed and range. This is the substance of levers and mechanical advantage, and engineers sort levers into three classes by what sits in the middle.
CLASS middle element body example trade-off 1st fulcrum (joint) head nodding on the spine (atlas) balanced; like a seesaw 2nd load standing on tiptoe (ball of foot) favours force; rare in body 3rd effort (muscle) biceps flexing the elbow favours speed/range; MOST joints Mechanical advantage = effort moment arm / load moment arm > 1 -> less muscle force needed (strength) < 1 -> more muscle force needed, but more speed & range at the hand/foot
Look at the elbow again. The biceps attaches just a few centimetres past the joint, while the hand is a long way out. That is a third-class lever, and it means the muscle sits at a mechanical *disadvantage*: to support a small weight in the hand, the biceps must pull with several times that weight. The body "chose" this on purpose. By giving up brute force, it gains range and speed — a tiny shortening of the muscle whips the hand through a wide, quick arc. Your body is built for reaching, throwing, and walking, not for being a crowbar. This is the deep reason muscles are so strong relative to the loads they visibly move: most of their effort is spent paying the lever tax.
Levers also explain why we put braces where we do. An ankle-foot orthosis does not have a motor; it controls a joint purely through geometry, pressing at three points along a limb to resist an unwanted torque — the glossary's three-point pressure system. And the load arms vary joint by joint, which is why the same muscle weakness shows up so differently at the shoulder, hip, and ankle. Tracing how each major joint trades force for motion is exactly the work of the next guide on the biomechanics of major joints.
Centre of mass over base of support
Step back from single joints to the whole body, and balance comes down to two ideas working together — the heart of the centre of mass and base of support. Your centre of mass is the single point where, on average, all your body's weight can be thought to act; in a standing adult it sits roughly in the pelvis, just in front of the sacrum. It is not fixed: raise your arms, lean, or carry a child on your hip and it shifts, sometimes right out beyond your skin. Your base of support is the whole area enclosed by whatever is touching the ground — for a person standing, the two feet plus the patch of floor between them.
The rule for staying upright is beautifully simple: keep the vertical line dropping from your centre of mass inside your base of support. While that line stays within the footprint, gravity merely presses you down. The instant it crosses the edge — lean too far forward, and the line passes beyond your toes — gravity gains a moment arm and starts to turn you over the edge of your feet. That is the precise mechanical meaning of "losing your balance." It is also why every trick for steadier standing is really a trick to widen the base or keep the line centred: feet apart, a cane or walker adding new contact points and enlarging the base, or simply not reaching for something on a high shelf while standing on tiptoe.
Why we sway, and why we fall
Here is the unsettling truth about standing still: you never actually do. Your centre of mass sits high above a small base, balanced on flexible ankles, like a tall pole held upright on a fingertip. Left alone it would topple. So the body runs a continuous, invisible correction — sensing tiny tilts through the eyes, the inner ear, and pressure and stretch sensors in the feet and joints, then firing muscles, mostly at the ankle, to nudge the centre of mass back over the base. This is postural sway, and it never stops. Standing is not a static pose; it is a slow, ceaseless act of catching yourself, the topic the glossary treats as postural control and alignment.
Falls happen when this correction loop fails to keep the centre of mass over the base in time. And it can fail at any link. The senses can dim — fading eyesight, an inner-ear disorder, numb feet from a neuropathy that no longer report where the ground is. The processing can slow with age or after a stroke. Or the muscles can be too weak or too slow to make the catch, so the ankle strategy fails and the person must step — and if the step is too late or too short, they go down. This is why falls are rarely about one thing, and why a careful fall assessment looks across all of it rather than blaming clumsiness.
Picture an older woman, recovering at home, who has had two falls in the kitchen. The team does not just hand her a walker. They ask what gave way: the eyes, the inner ear, the sensation in her feet, the ankle strength, the medications that slow reaction, the loose rug that shrank her safe path. Clinicians put numbers on this with tools like the Berg Balance Scale and the Timed Up and Go, which probe exactly these limits of keeping the line over the base — a central concern in geriatric rehabilitation. Be honest about the limits, though: no score predicts a fall with certainty, and reducing falls means changing the body, the task, and the environment together, not chasing a single number.
Carrying the picture forward
Notice what you now have. Movement is forces producing torques about joints that act as levers, while the whole body keeps its centre of mass shepherded over its base. These are not separate facts; they are one machine seen from different distances. The torque you compute at a single joint and the sway you feel when standing on one leg are the same physics at two scales. Walking is where they fuse: every step is a controlled, repeated near-fall, the centre of mass thrown forward beyond the base and caught by the next foot. That is the story the gait cycle tells, and it is the subject the rest of this rung builds toward.
One last honest word about scope. Biomechanics is a powerful lens, but it is only a lens. The body is not a clean steel linkage: tendons stretch and store energy, muscle force changes with length and speed, friction and soft tissue absorb shocks, and above all a living nervous system is choosing the strategy moment by moment. The lever diagrams in textbooks are deliberate simplifications that illuminate the dominant effect, not the whole truth. Hold them lightly, use them to reason, and stay curious about where the simple picture stops fitting the real person in front of you — because that gap is exactly where good rehabilitation thinking begins.