Energy Absorption

For Medical Students

The Principle

Energy absorption is what happens when a muscle lengthens while resisting a load. The muscle takes energy out of the movement rather than putting it in.

It is called eccentric work, or negative work. Lowering a heavy object is eccentric; lifting it is concentric.

Eccentric work produces more force for less effort and causes more muscle soreness, which is why walking downhill is easier on the lungs and harder on the legs.

At the Knee

The knee absorbs energy in early stance of walking, through most of a landing, and throughout stair descent. 3.3.13 covers landing.

It does it by flexing against quadriceps control. The flexion is the distance over which the energy is taken out.

More flexion over more time means a lower force for the same energy, which is why a soft landing is gentler than a stiff one.

The hip and ankle absorb energy too, and the share each takes is trainable. A limb that absorbs more at the hip loads the knee less.

What Changes It

The energy to be absorbed, set by height, speed and body mass.

The flexion achieved and the time taken, which together set the force.

Quadriceps capacity, which is the brake.

Fatigue, which reduces the flexion achieved and so raises the force.

Clinical Relevance

Eccentric demand is why descending stairs is harder than ascending, and patients report it in that order. 3.3.12 covers stairs.

Eccentric strength is trainable and is a specific rehabilitation target rather than general strengthening. Section 6 covers management.

Teaching a softer landing reduces force without reducing activity, which is the mechanical basis of technique programmes. 3.1.5 covers injury prevention.

A knee that cannot absorb passes the energy on, to the hip, the ankle or the joint surfaces.

MRCS Level

The Principle

Joint power is the moment multiplied by the angular velocity, and its sign tells you the direction of energy flow. Negative power is absorption.

Integrating power over time gives work, so a power curve through a cycle shows when each joint is absorbing and when it is generating.

Not all absorbed energy is dissipated. Tendon stores part of it elastically and returns it, which is why running is more efficient than its muscle work alone suggests.

At the Knee

The knee is net absorbing in walking's loading response and net generating in pre-swing, which is visible as a sign change in the power curve. 3.3.9 covers walking.

In landing the knee is almost entirely absorbing, and conventional teaching gives it the largest share of the three joints in a typical drop landing.

In stair descent the knee absorbs through most of stance, which is the highest sustained eccentric demand in daily life.

Absorption and joint force rise together, because the absorbing moment is produced by muscle and muscle compresses the joint. 3.4.3 covers joint force.

What Changes It

Movement strategy, which redistributes absorption between the joints and is the modifiable term.

Speed and height, which set the energy.

Tendon properties, which set how much is returned rather than dissipated.

Fatigue and pain, which reduce the knee's share and shift it elsewhere.

How It Is Measured

Inverse dynamics with a force plate gives joint power, and integrating it gives work per joint.

The limb's total absorption is measured reliably; the division between joints inherits the error of the moment calculation.

Ultrasound and modelling estimate the tendon's elastic contribution, which is not separable by inverse dynamics alone.

Loading rate from the force plate is the practical surrogate for whether absorption was adequate.

Clinical Relevance

Target eccentric capacity specifically in rehabilitation, since it is the function the knee uses most in daily life.

Read a reduced knee contribution in a power curve as a strategy change rather than necessarily as weakness.

Use loading rate as the practical measure of whether a landing is being absorbed.

Expect absorption and joint force to rise together, so a well absorbed landing is not an unloaded one.

FRCS Level

At the Knee

Treat eccentric capacity as a return to activity criterion rather than a rehabilitation stage. A limb that cannot absorb will either avoid the task or pass the energy to the joint surface.

Asymmetrical absorption persists after cruciate reconstruction in many patients with normal laxity, which is a kinetic deficit behind a normal clinical examination. 3.3.13 covers landing.

The cartilage consequence follows from the loading rate and the cumulative cycles rather than from a single peak, which is the argument for graded exposure over avoidance. 3.5 covers contact mechanics.

What Changes It

Quadriceps capacity and its symmetry, which set the brake.

Graft donor site, which can remove part of the absorbing musculature.

Movement strategy and confidence, which redistribute the work.

Cartilage and meniscal state, which set what poorly absorbed loading costs.

How It Is Measured

Drop landing analysis with force plates measures limb and joint absorption and asymmetry, in a prescribed task.

Single leg hop and triple hop tests measure performance and are what return to sport criteria use, with a recognised ceiling effect.

Isokinetic eccentric testing measures capacity directly and is not universally available.

No measurement predicts an individual's tolerance of repeated absorption, which is worth stating in counselling.

Clinical Relevance

Include eccentric capacity and landing symmetry in return to activity criteria.

Expect asymmetry to persist behind a normal examination after reconstruction.

Prefer graded exposure to avoidance, since cumulative cycles rather than peaks drive the cartilage consequence.

State the uncertainty about individual tolerance rather than offering a figure.