Biomechanics in Sport

For Young Learners

The Principle

Sport adds two things to ordinary movement: speed, and changes of direction.

Speed matters because stopping something moving fast takes a bigger force than stopping something moving slowly. Your knee does a lot of that stopping.

Direction changes matter because they twist the joint as well as loading it, and a twist is a different kind of load from a squash.

At the Knee

Landing from a jump can put several times your body weight through a knee in a fraction of a second.

Turning sharply twists the shin against the thigh, and the ligaments inside the knee are what stop the twist going too far.

Doing both at once, landing and turning, is the combination most sporting knee injuries happen in. 3.3.13 and 3.3.14 cover landing and cutting.

For Patients

The Principle

Sporting loads differ from daily ones in size, in speed and in direction. All three matter and the third is the one people forget.

A knee is strongest resisting load straight up and down. It is least able to resist twisting with the foot planted.

That mismatch between what sport asks and what the joint resists well is the mechanical account of sporting knee injury.

At the Knee

Landing, cutting, pivoting and decelerating are the four demands. Each loads the knee in a different direction.

The ligaments inside the knee resist twisting and sliding. The muscles around it control the position the joint is in when the load arrives.

Fatigue changes both. A tired player lands differently and controls the position less well, which is why injuries cluster late in a game.

Clinical Relevance

Returning to sport after a knee injury is a mechanical question as much as a healing one. The tissue has to be healed and the movement has to be controlled.

That is why return to play testing looks at hopping, landing and single leg control rather than only at how the knee feels.

Section 8 covers rehabilitation and Section 6 the individual injuries.

For Medical Students

The Principle

Sporting loads are characterised by magnitude, rate and direction. Daily activity varies mainly in magnitude; sport varies in all three.

High rate loading stiffens tissue and shifts failure towards the ligament rather than its bony attachment, which is why adult sporting injuries are usually ligamentous and paediatric ones often avulsions.

Multiplanar loading is the specific demand. A knee resisting compression, valgus and internal rotation at once has several structures near their limits simultaneously.

At the Knee

Conventional teaching gives peak vertical ground reaction force in landing as three to six times body weight depending on technique and height, developed in well under a fifth of a second.

Non-contact anterior cruciate injury typically occurs within about 50 milliseconds of initial contact, which is faster than a voluntary muscle response can correct.

Position at contact matters more than strength for that reason. The knee is where it is when the load arrives, and pre-activation rather than reaction decides where that is.

Cutting adds an external rotation and valgus moment to the compressive load, which is the combination 3.11.4 describes for the pivot shift.

What Changes It

Technique changes landing force substantially. A stiff landing with little hip and knee flexion transmits more force in less time than a soft one.

Fatigue alters kinematics measurably within a game and is associated with the timing of injuries.

Surface, footwear and the friction between them change the rotational load a planted foot can generate.

Sex differences in landing kinematics are described and are part of why injury rates differ, which Section 9 covers.

How It Is Measured

Video analysis of real injuries gives the mechanism but no forces. Laboratory simulation gives forces but not real injuries. Cadaveric testing gives failure loads with no muscle activity.

No single method gives mechanism, force and failure together, which is why injury mechanics is argued over more than joint load mechanics.

Marker based motion capture during cutting suffers more soft tissue artefact than during walking, so the kinematics are less certain in exactly the task that matters most.

Clinical Relevance

Take a mechanism history in a sporting injury, because the described mechanism predicts the structures involved.

Expect combined injuries from pivoting mechanisms. One load pattern reaches several structures.

Test movement quality as well as strength before return to sport, since position at contact is what the mechanics identifies as decisive.

MRCS Level

The Principle

Frame sporting injury as a problem of control under time pressure. The load arrives faster than a reflex can modify it, so the protective work is done before contact.

That reframing is what moved prevention from strengthening alone to movement training, and it is a mechanical argument rather than a clinical preference.

Multiplanar loading means a sectioning study of one structure understates what a sporting mechanism does.

At the Knee

Give the mechanism precisely: deceleration or landing, knee near extension, valgus collapse with internal tibial rotation, often with the trunk displaced away from the stance limb.

The anterolateral structures load with the cruciate in that pattern, which 2.11.5 covers and which is why combined injury is the rule rather than the exception.

The lateral meniscus and the lateral compartment bone bruise pattern are part of the same mechanical event, which is why they appear together on imaging.

What Changes It

Technique, fatigue, surface and footwear are the modifiable mechanical variables. Sex, notch geometry and tibial slope are not modifiable and are associated with risk.

Previous injury changes both tissue and control, which is why reinjury rates exceed first injury rates.

Programme adherence changes the effect more than programme content, which is a practical point with a mechanical basis: an unperformed movement pattern changes nothing.

How It Is Measured

Prospective cohort studies with baseline biomechanical testing are the design that links mechanics to injury, and they are expensive and uncommon.

Most of the literature is cross sectional comparison of injured and uninjured athletes, which cannot separate cause from consequence.

Keep that distinction when quoting a risk factor, because many quoted factors come from the weaker design.

Clinical Relevance

Document the mechanism in the notes in mechanical terms, since it predicts the injury pattern and informs the imaging request.

Expect anterolateral and meniscal involvement with a pivoting mechanism and look for them.

Discuss prevention programmes with athletes returning to sport, because reinjury risk is the highest risk they carry.

Avoid quoting a cross sectional risk factor as a cause.