Jumping

For Medical Students

The Principle

To leave the ground you have to push against it harder than your own weight.

That push comes from muscles shortening while under tension, which is the opposite of what they do when landing.

Jumping and landing are therefore two different jobs, even though they look like two halves of one movement.

At the Knee

A jump begins with a countermovement, a quick dip, which stretches the muscles before they shorten.

That stretch makes the following contraction stronger, which is why almost nobody jumps from a standing start without dipping first.

The knee extends powerfully through takeoff, with the hip and ankle extending at the same time.

The patellar tendon carries very high loads during takeoff, which is why jumping sports produce tendon problems. 6.64 covers patellar tendinopathy.

3.14.8 covers the landing, which is the other half and the riskier one.

What Changes It

Muscle strength and the speed it can be produced at.

The depth and speed of the countermovement.

Arm swing, which contributes more than people expect.

Fatigue, which reduces jump height before the athlete notices. 3.15 covers sport.

Clinical Relevance

Jump height is a simple and sensitive measure of lower limb function, which is why it is used in return to sport testing.

Comparing one leg against the other on a single leg hop finds deficits that a two legged test hides.

Patellar tendon pain in a jumping athlete is a loading problem. 6.64 covers it.

Section 8 covers the rehabilitation.

MRCS Level

The Principle

Jumping is the chapter's only concentric task at the knee. Everything else here is the knee braking.

A concentric task is limited by power, which is force multiplied by velocity, rather than by eccentric control.

That difference is why jump testing and landing testing measure different things and both are used.

At the Knee

The countermovement exploits the stretch shortening cycle, raising the force available in the subsequent concentric phase.

Knee extensor moment peaks during the propulsive phase, with the hip and ankle contributing in a proximal to distal sequence.

Patellar tendon force in jumping is among the highest the extensor mechanism sees, and it rises with countermovement depth.

Vertical jump height depends on takeoff velocity alone, so everything else is a means to that.

Single leg and double leg jumps load the knee differently, and the limb symmetry index from single leg hops is the clinically used quantity.

What Changes It

Peak power and rate of force development.

Countermovement depth and speed, which trade force against time.

Joint range, since a knee that cannot flex cannot use the countermovement.

Pain and quadriceps inhibition, which cap the propulsive phase.

Arm swing and trunk contribution, which raise height without changing the knee.

How It Is Measured

Force plate, which gives takeoff velocity, power and asymmetry directly and is the best value measurement in this chapter.

Jump mat or optical timing, which gives height from flight time and nothing about how it was produced.

Motion capture with a force plate for joint moments and power.

Hop test batteries in clinic, which give distance and symmetry without kinetics. 4.4 covers the examination.

Flight time methods overestimate height whenever landing posture differs from takeoff posture, which is common in an injured limb.

Clinical Relevance

Measure single leg symmetry rather than absolute height, since the uninjured limb is the useful comparator.

Prefer a force plate to a jump mat where available, because how the height was produced is the clinical information.

Expect flight time methods to overestimate in an injured limb that lands differently.

Treat jump testing as a measure of power and landing testing as a measure of control. 3.14.8 covers landing.

FRCS Level

At the Knee

Jump testing occupies a particular place in knee practice: it is the most widely used functional measurement and one of the least standardised.

Limb symmetry index above 90 per cent is the conventional return to sport threshold, and it is a convention rather than a validated cutoff. 6.1 covers the condition.

Its known weakness is that the uninjured limb also deconditions, so symmetry can be achieved by both limbs being poor.

Preinjury or population normative data addresses that and is rarely available, which is the practical limitation.

Conventional teaching has moved from a single hop test to a battery, and from symmetry alone to symmetry plus absolute performance plus movement quality.

What Changes It

Which test in the battery is used, since they do not agree with each other.

Whether the comparator is the other limb or a normative value.

Deconditioning of the uninjured limb, which inflates symmetry.

Pain and confidence on the day, which affect effort and are not separable from capacity.

Quadriceps strength, which underlies all of it and can be measured directly.

How It Is Measured

Hop test batteries with limb symmetry index, which is the standard and is poorly standardised between units.

Instrumented force plate jumps, which separate power, asymmetry and strategy. 3.16.7 covers instrumentation.

Isokinetic quadriceps testing, which measures the underlying capacity more directly than a hop does.

Movement quality scoring on landing, which measures a different construct. 3.14.8 covers it.

No single test predicts reinjury well, which is the finding the batteries were a response to.

Clinical Relevance

Use a battery rather than a single hop, and state which tests it contained.

Interpret limb symmetry against absolute performance, because the uninjured limb deconditions too.

Measure quadriceps strength directly as well, since it underlies the hop and is less confounded.

State that the 90 per cent threshold is a convention rather than a validated cutoff.

Weigh the athlete's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because a passed battery and a ready athlete are different things.