Biomechanics: The Physics of the Human Body

Your Body Obeys Newton

Every time you walk, lift a cup, or catch a ball, your body solves complex physics problems in real time. Your brain coordinates hundreds of muscles, each applying forces through lever systems, storing energy in elastic tendons, and managing the fluid dynamics of blood and air — all without you solving a single equation.

But the equations are there. The human body obeys the same laws of mechanics that govern bridges, rockets, and planetary orbits. Biomechanics — the physics of living systems — reveals just how elegantly evolution has engineered the human machine.

Bones and Levers

Archimedes said “give me a lever long enough, and I shall move the world.” Your skeleton is a system of over 200 levers.

A lever consists of a rigid bar (bone), a fulcrum (joint), an effort force (muscle), and a load (the weight being moved). The mechanical advantage depends on where these elements are positioned relative to each other.

Most human joints are third-class levers — the muscle attaches between the fulcrum and the load. The biceps, for example, attaches to the radius about 5 cm from the elbow joint, while the hand (carrying the load) is about 35 cm away. The mechanical advantage is 5/35 ≈ 0.14 — meaning the biceps must exert about 7 times the weight it lifts.

This seems inefficient, but the trade-off is speed and range. A 1 cm contraction of the biceps moves the hand 7 cm — a sevenfold amplification of motion. Evolution has optimised the musculoskeletal system for speed and dexterity, not raw force.

First-class levers also appear — the skull balanced on the atlas vertebra (the neck muscles pull down at the back to tilt the head forward, like a seesaw). Second-class levers are rarer — the calf muscles raising the body onto the toes use the ball of the foot as fulcrum, with the body weight between fulcrum and muscle.

Torque and Joint Mechanics

Rotation around joints is governed by torque — the rotational equivalent of force. Torque equals force times the perpendicular distance from the axis of rotation: τ = F × d.

When you hold a 5 kg dumbbell with your arm outstretched, gravity exerts a torque about the shoulder joint. If your arm is 0.6 metres long, the gravitational torque is approximately 5 × 9.8 × 0.6 ≈ 29 N·m. Your deltoid muscle, which attaches only about 0.15 m from the shoulder, must generate 29/0.15 ≈ 193 N of force — about 20 kg equivalent — to hold a mere 5 kg weight in place.

This is why holding objects at arm’s length is so tiring. The physics of torque means muscles near joints must produce enormous forces to balance moderate external loads. It also explains why weightlifters keep heavy loads close to the body — reducing the moment arm reduces the torque and the required muscle force.

Tendons as Energy Stores

Tendons connect muscles to bones and transmit force. But they also function as energy storage devices — elastic springs that save and return mechanical energy.

When you run, each footstrike involves decelerating your body. The impact energy is not entirely wasted — about 35% is stored as elastic potential energy in stretched tendons, primarily the Achilles tendon and the plantar fascia of the foot. During push-off, this stored energy is released, propelling you forward.

The physics is identical to a compressed spring: E = ½kx², where k is the stiffness and x is the stretch. The Achilles tendon has a stiffness of roughly 200 N/mm and can stretch by about 6% of its length during running, storing approximately 35 joules per stride. This elastic recoil reduces the metabolic cost of running by about 35–50% compared to what muscles alone would require.

Kangaroos take this further — their enormous tendons store so much elastic energy that hopping at high speed actually requires less energy per metre than walking slowly. The physics of springs, not muscle power, explains their efficiency.

Fluid Dynamics of Blood

The circulatory system is a pressurised fluid network governed by the same physics as any piping system.

The heart is a pump, generating pressure (~120/80 mmHg in the aorta) that drives blood through approximately 100,000 km of vessels. Blood flow follows Poiseuille’s law: flow rate is proportional to the pressure difference and the fourth power of the vessel radius, and inversely proportional to viscosity and vessel length.

The fourth-power dependence on radius is critical. A 10% narrowing of an artery (from plaque buildup, for example) reduces flow by about 34%. A 50% narrowing reduces flow by 94%. This is why atherosclerosis — even partial blockage — can have severe consequences.

Blood is not a simple Newtonian fluid. Its viscosity depends on shear rate (it is shear-thinning), red blood cell concentration (haematocrit), and vessel size. In capillaries — with diameters of 5–10 micrometres, barely wider than red blood cells — flow physics is dominated by the deformability of individual cells squeezing through the vessel. This is biophysics at the cellular level.

The total cross-sectional area of the capillary network is about 600 times that of the aorta. By the continuity equation (A₁v₁ = A₂v₂), blood velocity drops from about 40 cm/s in the aorta to less than 0.1 cm/s in capillaries — slow enough for oxygen and nutrients to diffuse into surrounding tissues.

Impact Physics and Injury

Understanding injury requires understanding the physics of impact. When you fall, the kinetic energy of your body must be dissipated. The crucial variable is deceleration time — the duration over which the impact occurs.

Force = mass × acceleration. A 70 kg person falling from 1 metre hits the ground at about 4.4 m/s. If they stop in 0.01 seconds (landing on concrete), the average force is about 31,000 N — roughly 45 times body weight. If they stop in 0.2 seconds (rolling, bending the knees), the average force drops to 1,540 N — about 2.2 times body weight.

This is why martial artists learn to roll when falling, why cars have crumple zones, and why gymnasts bend their knees on landing. The physics is identical in each case: increasing the deceleration time reduces the peak force. Helmets, airbags, and running shoes all work on this principle — they extend the deceleration distance and time, reducing the force transmitted to the body.

Breathing: Gas Physics in the Lungs

Respiration is governed by gas laws and pressure physics. The diaphragm and intercostal muscles expand the chest cavity, reducing pressure in the lungs below atmospheric pressure by about 3 mmHg. Air flows in (from high to low pressure). Relaxation compresses the lungs, raising pressure above atmospheric, and air flows out.

The lungs contain approximately 300 million alveoli — tiny air sacs with a total surface area of about 70 m² (roughly the size of a tennis court). Gas exchange occurs by diffusion across membranes only 0.2–0.5 micrometres thick — thin enough that oxygen molecules cross in about 0.25 seconds.

Surface tension at the air-liquid interface in the alveoli would collapse these tiny structures (by the Laplace equation, smaller bubbles have higher internal pressure). The lungs solve this with surfactant — a phospholipid that reduces surface tension by a factor of 10–15, preventing alveolar collapse and reducing the work of breathing by about 65%.

The Body as Physics Laboratory

Every movement you make is a solved physics problem. Your bones are levers optimised for speed. Your tendons are springs that recycle energy. Your heart is a pressure pump driving non-Newtonian fluid through a network governed by Poiseuille’s law. Your lungs are diffusion chambers with surfactant-stabilised interfaces.

The human body did not study physics — but 3.8 billion years of evolution, subjected to the same physical laws that govern all matter, produced solutions that any engineer would recognise. Biomechanics is not a metaphor. It is physics, applied to the most complex machine in the known universe — the one you inhabit.