The body as a machine
Biomechanics applies the ideas of physics, mainly Newton’s laws, torque and the properties of materials, to living bodies. A skeleton gives the body its frame, but to a physicist it is a set of rigid levers connected by joints, with muscles supplying the pulling force. The same reasoning used for cranes and bridges helps explain why a person can lift a heavy box, why posture matters and why joints bear so much stress.
Levers in the body
A lever is a rigid bar that pivots around a fixed point, the fulcrum. Two forces act on it: an effort (here, a muscle pull) and a load. The classic classes depend on the order of those three points.
- First-class lever: The fulcrum lies between effort and load. Nodding the head is a common example, with the neck muscles at the back pulling against the weight of the front of the head, and the joint at the top of the spine as the pivot.
- Second-class lever: The load lies between the fulcrum and the effort. Rising onto the toes is often given as an example, with the ball of the foot as the pivot and the calf muscle pulling at the heel.
- Third-class lever: The effort lies between the fulcrum and the load. Flexing the elbow is the standard example, and most of the limb muscles work this way.
Real joints are more complicated than textbook diagrams, since muscle angles change as a joint moves and several muscles typically work together. Still, the classes are useful for seeing the trade-offs.
Torque and mechanical advantage
What matters at a joint is torque, the turning effect of a force, equal to the force multiplied by its perpendicular distance from the pivot. For a limb to be held steady, the torques on either side must balance.
Many muscles attach quite close to the joint they move. That gives up force for speed and range: a small shortening of the muscle moves the hand a long way, but the muscle must pull much harder than the weight it holds.
A worked example (hypothetical numbers)
Suppose a person holds a 2 kg object in the hand with the forearm horizontal. Assume the object is 30 cm (0.30 m) from the elbow, and the biceps attaches 4 cm (0.04 m) from the elbow and pulls straight up. Ignore the weight of the forearm for simplicity, and take gravity as 9.8 m/s².
- Weight of the object: 2 × 9.8 = 19.6 N
- Torque from the object about the elbow: 19.6 × 0.30 = 5.88 N·m
- Muscle force needed to balance: 5.88 ÷ 0.04 = 147 N
So the muscle pulls with about 147 N, roughly 7.5 times the object’s weight (30 ÷ 4 = 7.5), equivalent to the weight of about a 15 kg mass. These numbers are illustrative only. Real muscle forces also depend on the weight of the forearm, the angle of pull and the activity of other muscles.
Muscle as a force generator
Muscle fibers produce force by shortening, and the force available depends on factors including the muscle’s cross-sectional area, its length at the time and the speed of contraction. A muscle generates the most force near a middle length, and less when very stretched or very shortened. Faster shortening produces less force, which is why heavy lifts are slow.
Muscles pull through tendons, strong cords of connective tissue that can also store and return elastic energy, as in the Achilles tendon during running. Muscles can only pull, never push, so joints are typically controlled by opposing pairs, such as biceps and triceps.
Bone as a structural material
Bone is a composite of mineral and protein fibers, which makes it both stiff and somewhat tough. Basic material properties explain its behavior: bone is generally stronger under compression than under tension or twisting, and it is stronger along its length than across.
Its structure suits its load. The shafts of long bones are hollow tubes, a shape that resists bending well for the amount of material used. The ends contain a spongy lattice, whose struts tend to line up along the main lines of stress. When people move around under Earth’s gravitational force, every step sends forces up through the legs, and joints and bones must absorb them. Cartilage at joints adds cushioning and low friction.
Bone adapts to its loads
Living bone remodels continually. An observation often called Wolff’s law holds that bone tends to adapt to the loads placed on it, becoming stronger where stress is regular and losing material where it is not. This is why weight-bearing activity is generally associated with healthier bones, and why long periods of inactivity or weightlessness may lead to losses. The extreme case is seen in astronauts, discussed in terms of bone and muscle loss in space. Vitamin D and calcium also matter for bone mineral, and regular exercise is widely recommended for bone and muscle health.
Posture, balance and stress
Standing upright requires the body’s center of mass to stay over its base of support, which is why people lean back when carrying a load in front. Spine curvature, the pelvis and the arches of the feet spread loads across structures. Poor alignment can concentrate stress in particular tissues, though the link between posture and pain is more complicated than often claimed, and the signals that register discomfort are discussed under pain signals.
This article is a physics-based overview, not medical or fitness advice.