What Is Biomechanics?

For Young Learners

The Principle

Biomechanics is what you get when you use physics to study a living body. Forces, levers, springs and friction are all involved, and so are bones, muscles and ligaments.

Every time you move, something pushes and something else resists. Biomechanics is the study of that pushing and resisting.

The word has two halves. Bio means life, mechanics means forces and motion.

At the Knee

The knee is a good place to start, because it is a joint you can feel working. Stand up, sit down, and you have just used it as a hinge, a shock absorber and a lever.

Your thigh bone sits on your shin bone. Muscles pull on both. Ligaments stop the bones sliding too far. Cartilage stops them wearing out. Each part is doing a mechanical job as well as a biological one.

When you walk, the force going through your knee is more than your body weight, because your muscles are pulling at the same time as gravity is pushing. 3.4 covers the numbers.

For Patients

The Principle

Biomechanics is the study of how forces move through the body. It is the part of knee medicine that explains why things happen rather than just what happens.

Two kinds of question belong to it. How does a normal knee move, and what forces does it carry? Both have measurable answers.

It matters to patients because almost every treatment for the knee is mechanical. A brace changes a force, exercise changes a muscle pull, and an operation changes the shape something moves in.

At the Knee

The knee is not a simple hinge. It bends and straightens, and it also rotates a little and slides a little as it does so. The bones roll and glide on each other rather than turning around a fixed pin.

Four ligaments and a ring of muscles control that movement. The menisci spread the load across the joint surface so that the cartilage is not loaded at a point.

When the pattern changes, through injury, weakness or wear, the forces move to parts of the joint that were not carrying them before.

Clinical Relevance

Understanding the mechanics explains why pain is often felt in a particular activity rather than all the time. Different movements load different parts of the knee.

It also explains why strengthening helps. A muscle that works earlier and harder takes load off the structures that hurt.

And it explains why alignment matters. A knee that is slightly bow legged puts more load through its inner side with every step, which 3.9 covers.

For Medical Students

The Principle

Biomechanics applies the mechanics of rigid bodies and deformable materials to living tissue. At the knee it divides into kinematics, the description of motion, and kinetics, the forces that cause it. 3.3 and 3.4 cover each.

Motion is described in six degrees of freedom: three rotations and three translations. The knee uses all six, which is why calling it a hinge is a simplification. 3.2.1 covers them.

Tissue behaviour is the second half of the subject. Ligament, tendon, cartilage and bone each respond to load differently, and each is viscoelastic, meaning its behaviour depends on how fast the load is applied.

At the Knee

The tibiofemoral joint combines rolling and gliding. Pure rolling would roll the femur off the back of the tibia within a few degrees; the cruciate ligaments convert part of that roll into a glide. 3.3.2 covers it.

The patellofemoral joint is a pulley. The patella increases the lever arm of the quadriceps, which is why it is there and why removing it weakens extension.

The menisci convert a point contact between two curved surfaces into an area contact, which reduces contact pressure. 3.6 covers meniscal mechanics.

Stability comes from four sources: bone shape, ligaments, muscles and the menisci. Their relative contributions change through the range of movement.

What Changes It

Flexion angle changes everything. Ligament tension, contact area, lever arms and muscle efficiency all vary through the range, which is why a mechanical statement without a flexion angle is incomplete.

Load rate matters. Tissue is stiffer when loaded quickly, which is why the same force applied slowly and suddenly produces different injuries.

Alignment changes the distribution. A varus knee loads the medial compartment more in stance, and the difference is present at every step rather than occasionally.

Muscle activity changes the joint reaction force. Co-contraction stabilises the joint and increases the force through it at the same time.

How It Is Measured

Kinematics is measured with motion capture, fluoroscopy or navigation, each trading accuracy against how natural the movement is.

Kinetics is measured with force plates and inverse dynamics, which calculate joint forces from external measurements rather than measuring them directly.

Cadaveric testing measures what a structure restrains by cutting it and remeasuring, which is where most ligament sectioning figures come from.

Every method has a boundary. A cadaver has no muscle activity, a laboratory gait is not a street gait, and an instrumented implant measures one patient's knee. 3.1.6 covers what that means for surgery.

Clinical Relevance

Mechanics explains injury patterns. A pivoting injury loads the anterior cruciate ligament because of the direction the forces act, not because of where the ligament happens to be.

It explains examination. Every stability test applies a known force in a known direction at a known angle, which is a mechanical experiment performed by hand. Section 4 covers them.

It explains treatment. Bracing, strengthening, osteotomy and reconstruction are all mechanical interventions and each is judged by whether it restores a measurable quantity.

MRCS Level

The Principle

Define the terms precisely in a viva. Kinematics is motion without reference to force. Kinetics is force and its effects. Statics is the special case where nothing accelerates, and most clinical reasoning uses it.

Stress is force per unit area and strain is deformation as a proportion of original length. The relationship between them is the material's stiffness, and the area under the curve is the energy it absorbs before failing.

Viscoelasticity gives tissue three behaviours worth naming: creep, lengthening under a constant load; stress relaxation, falling tension at a constant length; and rate dependence, greater stiffness at higher loading rates.

At the Knee

Describe the knee as two joints in one capsule with different mechanics. The tibiofemoral joint is a rolling and gliding surface pair constrained by ligaments; the patellofemoral joint is a pulley loaded in compression.

Conventional teaching puts the tibiofemoral joint reaction force at about three times body weight in level walking and considerably higher in stair descent and squatting. 3.4.3 carries the figures.

The instant centre of rotation moves through flexion rather than staying fixed, which 3.3.7 covers. That is the formal statement of why the knee is not a hinge.

Primary and secondary restraints are the organising idea for ligament mechanics. A primary restraint carries most of the load in one direction; a secondary one takes over when the primary has failed, which is why isolated sectioning understates what a combined injury does.

What Changes It

Flexion angle, load rate, alignment and muscle activity are the four variables to name. Each changes the answer to a mechanical question and each is modifiable to a different degree.

Body weight changes joint load more than its own magnitude suggests, because the muscle forces balancing it rise with it.

Surgery changes mechanics deliberately. An osteotomy changes alignment, a reconstruction changes restraint, a meniscectomy changes contact area, and each is a mechanical trade rather than a repair.

Tissue changes with age and with loading history, so the same structure in two patients is not mechanically the same structure.

How It Is Measured

Name the main methods and their limits: optical motion capture with skin markers, which suffers soft tissue artefact; fluoroscopy, accurate but with a radiation cost and a small field; and instrumented implants, which give in vivo force in a small number of patients.

Cadaveric sectioning studies give most of the restraint figures quoted in Sections 2 and 3. Their limits are no muscle activity, an order effect from the sequence of cutting, and a small number of specimens.

Finite element modelling predicts stress inside tissue that cannot be measured directly, and its output is only as good as the material properties fed into it.

Interpretation is the skill rather than the method. A cadaver figure, a laboratory figure and an in vivo figure are three different kinds of number.

Clinical Relevance

Read every examination test as a mechanical experiment: a named force, in a named direction, at a named flexion angle. Knowing the mechanics is how a test is interpreted rather than memorised. Section 4 covers them.

Expect a mechanical explanation for an injury pattern and be suspicious of one that has none.

Describe a surgical plan in mechanical terms when asked to justify it, because that is the language in which the alternatives can be compared.

Avoid quoting a laboratory figure as though it were a clinical one. The limits of the method are part of the finding.