Sprinting

For Medical Students

The Principle

Pushing a movement to its maximum does not scale every part of it equally.

Some structures reach their limit long before others, and those are the ones that fail.

So the hardest version of an activity is not simply the ordinary version with bigger numbers.

At the Knee

Sprinting shortens ground contact further and raises forces again above running. 3.14.4 covers running.

Knee swing flexion is extreme, bringing the heel close to the buttock to shorten the limb's swing.

The hamstrings reach their greatest length while under high tension in late swing, which is where they tear.

The knee itself is rarely the structure that fails in sprinting. The hamstring usually is. 6.69 covers tendinopathy around the knee.

What Changes It

Speed, which is the whole variable here.

Hamstring strength at long muscle lengths, which is specific and trainable.

Fatigue, which is when most sprint injuries happen. 3.15 covers sport.

Previous hamstring injury, which is the strongest predictor of the next one.

Clinical Relevance

A knee complaint in a sprinter deserves the hamstring and the back checked as well.

Returning to sprinting is the last step of a rehabilitation, not an early one. Section 8 covers the programmes.

The injuries here are usually muscle rather than joint, which changes what to look for.

4.4.8 covers running gait assessment.

MRCS Level

The Principle

In sprinting the knee is a transmission rather than a source. The hip extensors generate and the knee passes it on.

A joint acting as a transmission is loaded in a different pattern from one acting as a spring or a brake.

That is why the knee's injury profile in sprinting looks nothing like its profile in landing. 3.14.8 covers landing.

At the Knee

Ground contact falls to around 0.1 seconds or less, and peak vertical force rises above the running range. 3.4.2 covers the forces.

Stance knee flexion is relatively limited in sprinting, because a compliant knee would waste the contact time.

Swing flexion is at its maximum for the gait family, often beyond 120 degrees.

Peak hamstring musculotendon length and force coincide in late swing, which is the injury window.

Patellofemoral loading does not rise as steeply as in distance running, because stance flexion is lower. 3.12.3 covers the pressures.

What Changes It

Running speed, which is the dominant term.

Hip extensor power, which drives the stride.

Hamstring strength at long lengths, specifically eccentric capacity.

Fatigue within a session, which shifts the injury risk to the later repetitions.

Surface and footwear, which alter the contact time.

How It Is Measured

Instrumented track with embedded force plates, which captures a very small number of steps at full speed.

High speed video with marker tracking, which gives kinematics without force.

Musculoskeletal modelling for musculotendon length and force, which is how the hamstring injury window was described. 3.16.4 covers modelling.

Wearables for speed and step count in training, which is what sports medicine actually uses. 3.16.8 covers them.

Full speed sprinting is the hardest gait to capture, so its literature rests on fewer trials than the rest of this chapter.

Clinical Relevance

Assess the hamstring at long lengths in a sprinter, since that is where the failure occurs.

Expect knee complaints in sprinters to be referred or secondary more often than in distance runners.

Stage sprinting last in a return to sport progression. Section 8 covers the stages.

Treat fatigue as a risk variable rather than a performance one.

FRCS Level

At the Knee

The instructive point about sprinting is negative. It is the fastest, highest force gait in the chapter and the knee is not usually the structure at risk.

That is because stance knee flexion stays low, so the quadriceps eccentric demand and the patellofemoral pressure do not scale with speed the way they do in distance running. 3.14.4 covers that.

The demand moves to the two joint muscles, and the hamstring's late swing position is the specific vulnerability.

For the cruciate, sprinting in a straight line is a low risk activity; the risk appears when the sprint changes direction. 3.14.6 covers cutting.

Conventional teaching groups running and sprinting together in return to sport criteria, and their knee demands differ enough that the grouping is questionable.

What Changes It

Whether the sprint is linear or involves a change of direction, which is the largest single factor for the knee.

Hamstring eccentric capacity, which is the modifiable risk factor with the best evidence.

Prior hamstring injury, which is the strongest predictor and is not modifiable.

Fatigue and session position.

Acceleration against maximal velocity phase, which load the limb differently.

How It Is Measured

Modelled musculotendon dynamics, which is how the injury window is described and is not a direct measurement. 3.16.4 covers it.

Isokinetic and Nordic testing for eccentric hamstring capacity, which is clinically available and measures a proxy.

Sprint velocity and acceleration from wearables, which is the volume and intensity record. 3.16.8 covers them.

Force plate data at full speed, which exists for few athletes and fewer trials.

There is no measurement of knee tissue load in sprinting, so claims about it are inferred from kinematics and a model.

Clinical Relevance

Separate linear sprinting from change of direction in a return to sport plan, because their knee demands differ. 3.14.6 covers cutting.

Target eccentric hamstring capacity, which is where the evidence for prevention sits.

Expect a knee complaint in a sprinter to be referred, patellofemoral or secondary to another structure.

State that knee load in sprinting is inferred rather than measured.

Weigh the athlete's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, since readiness to sprint is as much confidence as capacity.