Running Gait

For Medical Students

The Principle

Walking always has a foot on the ground. Running has moments when neither foot is.

That single change raises every force, because the body has to be caught after falling rather than handed over.

A higher force applied in a shorter time is the mechanical definition of a harder job.

At the Knee

Running replaces double support with a flight phase, so stance shortens and the force rises. 3.14.1 covers walking.

Peak ground reaction force in running is roughly two to three times body weight, against about one in walking. 3.4.2 covers the forces.

The knee flexes more during stance than in walking, often to around 40 degrees, because there is more to absorb.

Swing flexion is also much greater, often beyond 100 degrees, to bring the heel through quickly.

The quadriceps do more eccentric work per step, and the calf and hip contribute more propulsion.

What Changes It

Speed, which raises forces and shortens contact time together.

Where the foot lands relative to the body, which changes the braking force.

Footwear and surface. 3.4.10 covers shock absorption.

Fatigue, which degrades control before it degrades strength. 3.15 covers sport.

Clinical Relevance

Returning to running after a knee injury is a bigger step than returning to walking, and the force figures explain why.

Knee pain that appears only when running is a loading problem rather than a structural one in most cases. 6.60 covers anterior knee pain.

Running technique is modifiable, unlike most of the knee's mechanics. Section 8 covers the retraining.

4.4.8 covers how running gait is assessed.

MRCS Level

The Principle

The gait cycle's structure survives the transition to running; its proportions do not. Stance falls from about 60 per cent of the cycle to nearer 30.

Everything the knee must do in stance is therefore compressed into half the time at several times the force.

That compression, rather than the peak force alone, is what makes running a different mechanical problem.

At the Knee

Flight phase appears and double support disappears, which is the formal definition of the transition.

Knee stance flexion typically reaches 40 degrees or more, and the quadriceps eccentric demand rises accordingly. 3.4.8 covers energy absorption.

Loading rate rises steeply, and it rises further with a rearfoot strike and an extended knee at contact. 3.4.10 covers shock absorption.

Patellofemoral joint loading rises more than tibiofemoral loading does, because knee flexion increases with the force. 3.12.3 covers the pressures.

Cadence and stride length trade off at a given speed, and raising cadence lowers peak knee load per step.

What Changes It

Speed, which is the dominant variable for force.

Cadence at a fixed speed, which changes load per step without changing the pace.

Foot strike pattern, which moves load between the knee and the ankle rather than removing it.

Overstriding, which lengthens the braking impulse and raises the knee moment.

Fatigue and surface. 3.15 covers sports biomechanics.

How It Is Measured

Instrumented treadmill with motion capture, which gives force and kinematics together and is a treadmill rather than a road. 3.16.7 covers capture.

Overground runway with force plates, which is more representative and gives fewer steps.

Wearables and instrumented insoles, which give many steps in the field at lower accuracy. 3.16.8 covers them.

High speed video for foot strike and knee angle at contact, which is cheap and two dimensional.

Treadmill and overground running differ measurably, so the method belongs with the figure.

Clinical Relevance

Prescribe cadence increase as a load reduction per step, which is a measurable mechanical intervention. Section 8 covers it.

Expect patellofemoral load to rise faster than tibiofemoral when running volume rises. 3.12.3 covers the pressures.

Read a foot strike change as moving load rather than removing it.

State whether a running figure came from a treadmill or from overground.

FRCS Level

At the Knee

Running is where the gap between tissue capacity and applied load is narrowest in ordinary life, which is why it is where overuse presents.

Cumulative load, the product of load per step and step count, is the variable that matters and the one almost never measured. 3.12.9 covers the same argument for pain.

Cadence manipulation is the best evidenced gait retraining intervention at the knee, and its effect is a modest per step load reduction multiplied by a very large step count.

Foot strike retraining redistributes load from the knee to the ankle and Achilles, which is a trade rather than an improvement and should be described as one.

Conventional teaching has moved from prescribing a technique to prescribing a load, which matches where the evidence sits. 3.15 covers sport.

What Changes It

Weekly volume and its rate of change, which is the strongest modifiable risk factor.

Cadence and stride length at a given speed.

Foot strike pattern, which moves the problem.

Body mass, which scales every step. 3.5.7 covers obesity.

Fatigue state, which degrades control and is when late session injuries happen.

How It Is Measured

Laboratory capture, which measures a few steps precisely under observation.

Wearables, which measure very many steps imprecisely and unobserved, and are the better match to a cumulative load question. 3.16.8 covers them.

Training load logs, which are self reported and are what most decisions actually rest on.

Modelled joint contact force, which inherits the error chain described in 3.14.1.

No method measures cumulative knee tissue load in a runner, so the clinical variable remains an estimate built on a proxy.

Clinical Relevance

Manage volume and its rate of change before technique, because that is where the evidence is strongest.

Offer cadence increase as a measurable per step load reduction rather than as a style correction. Section 8 covers it.

Describe foot strike retraining as moving load to the ankle, and say so to the patient.

Avoid presenting a modelled contact force as a measured one.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because adherence to a volume plan decides the outcome more than the plan does.