Biomechanics in Everyday Life

For Young Learners

The Principle

You can work out roughly how hard a joint is working by thinking about two things: how bent it is, and how much of your weight is on it.

The more bent the knee, the harder the muscle in front of your thigh has to pull to stop you folding up. And the harder it pulls, the more it squashes the joint.

So a bent knee carrying weight is working much harder than a straight one.

At the Knee

Walking on the flat is the easy one. The knee barely bends under load, so the force through it is a few times your body weight.

Going up stairs bends the knee more and lifts your whole body, so the force goes up.

Squatting right down is the hardest of the three. The knee is very bent and your whole weight is on it, so the force is many times your body weight. 3.3.11 covers it.

For Patients

The Principle

Force through the knee depends on how much weight is on it and how bent it is. Those two together explain most of what people notice about their own knees.

A straight knee transmits load through bone mostly in line. A bent knee needs the quadriceps to hold it, and that muscle pull is added to the load.

That is why the activities that hurt are usually bent knee activities under load.

At the Knee

Walking on level ground is the lightest common load. The knee bends only a little during stance.

Stairs are heavier, and coming down is heavier than going up for the kneecap joint, because the muscle is controlling a descent rather than producing a lift.

Getting out of a chair, squatting and kneeling put the knee in its most bent positions under load, which is why they are usually the first things to become difficult.

Clinical Relevance

Which activities hurt tells a clinician which part of the knee is loaded. Stairs and sitting point to the kneecap joint; walking distance points to the main joint surfaces.

Changing how an activity is done changes the load. Rising from a higher chair, taking stairs one at a time or using a rail all reduce the force without reducing activity.

That is why advice about daily activity is treatment rather than consolation.

For Medical Students

The Principle

Joint reaction force is the sum of the external load and the muscle forces balancing it. Because the quadriceps has a short lever arm relative to the external moment arm, it must generate a force several times larger than the load it balances.

That ratio is why joint reaction force is a multiple of body weight rather than a fraction of it, and why it rises steeply with flexion.

The patellofemoral joint reaction force follows the same logic with its own geometry, and rises faster still with flexion. 3.12.3 covers contact pressures.

At the Knee

Conventional teaching gives tibiofemoral joint reaction force as roughly two to three times body weight in level walking, four to six times on stairs, and higher in deep squatting.

Patellofemoral joint reaction force is roughly half body weight in level walking, three to four times on stairs, and up to seven or eight times in a deep squat.

Stair descent exceeds stair ascent at the patellofemoral joint because the quadriceps is contracting eccentrically and the knee is more flexed through the loaded phase.

Rising from a chair is comparable to stair climbing and is often the activity patients report first.

What Changes It

Flexion angle is the dominant variable in all three activities, because it sets the external moment arm and the required muscle force together.

Speed matters through the ground reaction force, which rises with the rate of loading. 3.4.2 covers it.

Body mass scales the whole system, which is why a given percentage weight change alters joint load by more than that percentage.

Technique changes the moment arm. Chair height, stair strategy and squat depth are mechanical variables rather than preferences.

How It Is Measured

Inverse dynamics from force plate and motion capture data gives the standard figures, calculated rather than measured.

Instrumented implants measure tibiofemoral force directly in a small number of patients with knee replacements, and generally return figures lower than inverse dynamics predicts.

That disagreement is worth knowing. Two credible methods give different numbers for the same activity, and quoting one without the other overstates how settled the figures are.

Clinical Relevance

Map a patient's reported difficulty onto the activity's known load, because the pattern localises the problem better than most examination findings.

Advise on technique as a mechanical intervention. Chair height and stair strategy change the force measurably.

Expect patellofemoral symptoms to present on stairs, slopes and after prolonged sitting, and tibiofemoral symptoms with walking distance and standing.

MRCS Level

The Principle

Work the problem as a free body diagram. External moment equals ground reaction force times its moment arm about the knee; internal moment is muscle force times its moment arm; the two balance in quasi-static conditions.

Joint reaction force is then the vector sum of the external load and the muscle force, which is why it exceeds both.

Keep the distinction between joint reaction force and contact pressure. Pressure is force divided by contact area, and the area changes through the range, so the two do not rise together.

At the Knee

Give the figures with their activity and their method. Two to three times body weight in walking is an inverse dynamics figure; instrumented implant data for the same activity is generally lower.

Contact area rises with flexion at the patellofemoral joint, which partly offsets the rising force. The net pressure still rises, but less steeply than force alone would suggest. 3.12.2 and 3.12.3 carry it.

Meniscal function is what keeps tibiofemoral contact area high. Losing meniscal tissue raises pressure for an unchanged force, which 3.6 covers.

What Changes It

Flexion angle, speed, mass and technique, as at medical student level, with alignment added: a varus knee shifts the load line medially and raises medial compartment force at every step.

Footwear and surface alter the ground reaction force profile, which is a smaller effect than alignment and a real one.

Muscle co-contraction raises joint reaction force while improving stability, so the stabilising strategy has a mechanical cost.

How It Is Measured

Inverse dynamics, instrumented implants, fluoroscopy and pressure sensitive film are the four main methods, and each measures something slightly different.

Pressure sensitive film measures contact area and pressure in cadavers, which is where most meniscectomy pressure figures come from.

Naming the method alongside the figure is the habit that separates a usable quotation from an unusable one.

Clinical Relevance

Localise symptoms by activity before imaging, because the mechanical pattern narrows the compartment more cheaply than a scan.

Discuss activity modification as a load intervention with a size, rather than as general advice.

Assess alignment in any patient whose symptoms are compartment specific and activity related.

Avoid quoting a single joint load figure without the activity and the method attached.