Kinematics During Squatting

For Medical Students

The Principle

A squat is closed chain flexion under body weight, with the foot fixed and the femur descending. 3.3.3 covers the chain.

It is the task that uses most of the knee's available range, so it exposes mechanics the other activities never reach.

It is also a three joint task. The hip, knee and ankle have to share the descent, and a limitation at any one of them changes the shape of the movement.

At the Knee

Flexion proceeds from full extension towards deep flexion, commonly past 100 degrees and in a full squat beyond 130.

The femoral condyles roll and glide posteriorly throughout, which is what keeps the contact on the plateau as the angle increases. 3.3.2 covers roll and glide.

Beyond roughly 120 degrees the contact moves onto the posterior condyles, where the surfaces are smaller and the load per unit area is higher.

Patellofemoral contact migrates proximally on the patella and the contact area enlarges, which spreads the rising load. 3.12 covers the patellofemoral joint.

What Changes It

Ankle dorsiflexion range, which limits how far the tibia can incline forward. A stiff ankle forces the hip and trunk to compensate.

Hip range and strength, which set how much of the descent the hip can take.

Foot position and stance width, both of which change the rotation reaching the knee.

Trunk inclination, which moves the body weight line relative to the knee and alters the demand.

Clinical Relevance

A patient who cannot squat may have a knee problem, an ankle problem or a hip problem. Watch all three.

Deep flexion symptoms point at the posterior condyles and the posterior meniscus rather than at the structures tested in mid flexion.

Squatting is a practical measure of useful range, since it is the daily task that needs the most of it.

Cultural and occupational need for deep flexion varies, so a range that is adequate for one patient is inadequate for another.

MRCS Level

The Principle

Treat the squat as the clinical test of deep flexion mechanics, and treat its shape as information. Where the movement is achieved from tells you which joint is limiting.

The knee extensor moment rises with flexion in a squat, so the demand increases as the mechanical advantage falls. 3.4.5 covers moments.

Deep flexion is the part of the range least represented in the laxity and apportionment literature, because rigs and clinical tests work in mid flexion.

At the Knee

Posterior femoral rollback continues through the descent and is the mechanism that preserves the extensor lever arm in deep flexion.

Contact area is smallest and contact stress highest at the extremes of the arc, which is part of why deep flexion loading is a cartilage question. 3.5 covers it.

The posterior horns of both menisci are loaded in deep flexion, which is the mechanical account of why deep squatting provokes posterior horn symptoms. 3.6 covers meniscal biomechanics.

Patellofemoral joint reaction force rises steeply with flexion angle and is at its greatest in the deepest part of the squat.

What Changes It

Ankle dorsiflexion, which is the commonest peripheral limit and is often modifiable.

Posterior tibial slope, which changes the resting position the rollback starts from.

Meniscal and cartilage status in the posterior compartment, which sets tolerance rather than range.

Stance width and foot rotation, which change the coronal and transverse demand at the knee.

How It Is Measured

Weight bearing magnetic resonance or computed tomography at chosen angles measures loaded deep flexion position, as a series of static positions.

Fluoroscopy during a squat tracks the contact path, over the range the equipment can follow and usually not to the deepest positions.

Optical capture measures the whole task and loses accuracy at the extremes, where marker occlusion and soft tissue artefact are worst.

Cadaveric deep flexion rigs are technically difficult, which is one reason the deep flexion literature is thinner than the mid flexion one.

Clinical Relevance

Measure ankle dorsiflexion when a squat is limited, before attributing it to the knee.

Expect posterior horn symptoms to appear in deep flexion and to be absent in mid flexion testing.

Read patellofemoral symptoms in squatting as a loading consequence of flexion angle.

Treat the thinness of the deep flexion literature as a limit on confidence rather than as absence of a problem.

FRCS Level

At the Knee

Treat deep flexion as a stated goal with a stated cost. An implant designed for deep flexion loads a small posterior contact area, and the design question is whether the polyethylene and the fixation tolerate it.

High flexion demand is cultural and occupational rather than universal, so the goal has to be set per patient rather than per implant. Section 7 covers technique.

Posterior condylar offset and joint line position determine the achievable flexion after replacement, which is a kinematic consequence of a technical choice.

What Changes It

Posterior condylar offset, which sets how far the knee can flex before posterior impingement.

Insert design and conformity, which decide the contact area available in deep flexion.

Posterior capsular tightness and osteophytes, which limit the achievable angle mechanically.

Patellar height and extensor mechanism length, which set what the deep flexion position costs the extensor mechanism.

How It Is Measured

Lateral radiography measures posterior condylar offset and joint line position, which are the technical variables.

Fluoroscopic deep flexion studies measure whether the designed kinematics occurred, in selected series and prescribed positions.

Retrieval analysis of polyethylene shows where deep flexion loading actually fell, after the fact.

Patient reported ability to kneel or squat is the outcome the measurements are proxies for, and it correlates imperfectly with measured range.

Clinical Relevance

Set the deep flexion goal from the patient's needs rather than from the implant's claim.

Restore posterior condylar offset, since it limits achievable flexion.

Clear posterior osteophytes and release the capsule where they are the mechanical limit.

Counsel that measured range and the ability to kneel correlate imperfectly.