At the clinic, the change could happen within ten centimetres.

At one wicket spacing, the runner looked relaxed, coordinated and capable. At the next, the solution began to unravel.

Part One ended with that threshold. The training question is what we should do once we know it exists.

Every runner has a point beyond which their ability to produce speed exceeds their ability to organise it.

For an experienced runner, that threshold may only appear late in a demanding interval session. For a developing runner, it may appear after twenty seconds at a pace a generic plan calls “5K speed.”

The threshold is not determined by pace alone. A runner may move beautifully at a certain speed for ten seconds but lose the pattern after forty.

The speed is within their movement vocabulary. The duration is not.

Fatigue also moves the threshold. A runner may complete two or three good repetitions before the same pace begins to produce reaching, shuffling, tension or loss of structure.

This creates a problem for conventional training plans. The plan may say ten times 400 metres, but a fast 400 is not merely a physiological demand. It requires the runner to organise force, rhythm, posture and elastic recoil for the entire repetition.

There is a common assumption in endurance training that fitness should be built first and mechanics will somehow improve as a consequence.

Often, the opposite happens.

The runner adds mileage, lengthens intervals and accumulates fatigue while repeatedly practising the same inefficient movement solution. They become fitter, but they also become better at surviving their own mechanics.

Layering more volume onto movement that already feels awkward is rarely enjoyable. It can turn running into a prolonged negotiation with the body rather than the flowing movement it ought to become.

That does not mean the runner should wait for perfect form before training seriously. There is no perfect form, and aerobic development remains essential.

Nor does it mean that every tired or imperfect step is harmful. Running is variable, and fatigue is part of training.

But there is a difference between allowing movement to become slightly less polished near the end of a productive session and repeatedly training beyond the movement solution the runner can currently organise.

The first may develop resilience. The second may simply reinforce compensation.

This is not an argument against intervals for developing runners. It is an argument for matching the speed and length of the interval to the movement they can currently sustain.

The athlete must first earn the speed. Their movement skill must then deserve the length of the interval.

Endurance athletes are very good at completing things. Give us a pace, a distance and a number of repetitions and we will usually find a way through.

That quality is valuable. It can also disguise a problem.

A runner can complete ten fast 400s while becoming progressively less effective at running them. Pace may remain stable because effort rises, recovery deteriorates and the athlete becomes more willing to force the movement.

The watch reports success. The movement tells a different story.

The objective of interval training cannot simply be to survive the prescribed distance at the prescribed pace. It is to accumulate useful time at a particular physiological and mechanical demand.

If the desired movement only survives for 150 or 200 metres, shorter repetitions may create more useful practice than insisting on 400s.

That does not necessarily make the session easier. Sixteen well-executed 200s can be more demanding and more productive than eight 400s spent fighting the body.

The interval should be long enough to create the intended stimulus, but not automatically so long that the movement disappears.

This is particularly important for developing runners. Many plans prescribe repetitions according to the metabolic system they are intended to train: 400s for VO2 max, kilometre repetitions for threshold, long runs for endurance.

But the body does not experience an interval as a metabolic label. It experiences forces, contacts, muscular contractions and repeated movement.

The physiology and mechanics arrive together.

The aerobic system does not need perfect mechanics to develop. It needs consistent work at an appropriate intensity.

This gives us considerable freedom.

Easy running can build mitochondrial density, capillary supply, fatigue resistance and general tissue tolerance without forcing the runner to operate near the edge of their movement capacity.

Controlled steady running and sub-threshold intervals can add substantial aerobic stimulus while preserving rhythm and relaxation.

This is what it means to build fitness from below. The runner accumulates aerobic work at speeds they can currently organise.

That does not mean easy running should resemble fast running in miniature. Each pace has its own appropriate movement solution. A shorter, lower-force stride may be exactly right at an easy pace.

The point is not to impose faster-running mechanics on slower running. It is to avoid repeatedly training at intensities where the movement demanded by the task can no longer be organised.

The appropriate pace may initially be slower than the runner’s cardiovascular system seems capable of supporting. This can be frustrating, particularly when the athlete feels fit enough to go faster.

But cardiovascular capacity is only one part of readiness.

If every faster effort immediately produces reaching, spinning, tension or loss of structure, then pushing the aerobic system harder may place the movement system in a situation it is not yet prepared to solve.

The answer is not always more discipline or more suffering.

Sometimes the runner needs better movement options.

While fitness is built below the movement threshold, faster movement can be introduced from above in small doses.

A stride may last only ten to twenty seconds. A hill sprint may last six to ten. A wicket pass may be over almost as soon as the runner finds the rhythm.

These efforts are brief enough to expose the runner to greater force, speed and elastic demand without requiring them to sustain the pattern until it deteriorates.

The aim is not exhaustion. It is quality exposure.

The runner visits a faster and more powerful movement, then stops before it becomes something else.

Repeated consistently, these exposures expand the available movement vocabulary.

The spinner learns that speed can come from projection as well as turnover. The reacher learns to move the body through space rather than searching for distance with the foot. The floater gradually develops the strength and elastic capacity to produce more force, tolerate the quicker cycle of loading and unloading, and preserve control as contact frequency increases.

Strength training can increase the runner’s force-producing capacity and create a larger structural reserve. Hopping and jumping provide controlled exposure to rapid loading and elastic return. Hill sprints develop high force in a movement that still resembles running. Strides connect those capacities to faster locomotion. Wickets provide a physical constraint through which the runner can organise them.

None is a magic drill. Together, they prepare the runner to produce more force, control it and express it within the running movement.

A world-class marathoner or a sub-elite 10K runner may race at around 3:00 minutes per kilometre.

That is clearly not sprinting. Yet at that pace, the runner covers 60 metres in roughly 11 seconds.

Many recreational runners avoid repetitions that short because they appear to train the wrong energy system. Sixty metres is not long enough to create a substantial aerobic stimulus, so it can seem irrelevant to distance running.

That view makes sense only if we treat training as a collection of isolated metabolic systems.

From a movement perspective, the same repetition may be exactly right.

Running at any speed requires force. Faster running requires more force to be produced, accepted and redirected in less time. A short repetition allows the runner to practise that task without demanding that they sustain it after the movement has begun to deteriorate.

The point is not to turn a distance runner into a sprinter.

It is to give the distance runner access to a wider range of movement.

Running is an endurance sport built on a foundation of mechanical power. Every running pace sits somewhere on a continuum of force, speed and duration. Aerobic development determines how long the runner can sustain a given output, but available power and movement skill help determine which outputs exist in the first place.

A runner whose fastest organised running is only slightly quicker than long distance race pace has little mechanical reserve. Every increase in speed places them close to the edge of what they can coordinate.

A runner who can produce relaxed, controlled speed over 60 metres has more room beneath the ceiling. Slower paces occupy a smaller proportion of their available mechanical range.

Short repetitions therefore do not need to reproduce the energy demands of the target race to support it.

Progression should not be defined only by what appears on the calendar.

Ten seconds of good running becomes twenty. Twenty becomes forty. Strides become short repetitions. Short repetitions eventually become 200s or 400s.

The interval grows because the movement has earned it.

That is a more complete definition of progress than simply adding distance or extending repetitions.

The runner is not merely capable of producing the required energy. They are capable of expressing it as true running.

This leads to a different way of thinking about endurance development:

From below, the runner develops the aerobic engine through easy mileage, steady running and controlled sub-threshold work.

From above, they develop speed, force and coordination through brief exposures that end before the movement deteriorates.

One process increases the energy available. The other improves how effectively that energy can be expressed.

As the runner becomes stronger and more skilful, faster movement can be sustained for longer.

The purpose of this argument is not to make runners anxious about form. Nor is it to suggest that every imperfect step must be corrected before meaningful training can begin.

Running is variable. Technique changes with speed, terrain, fatigue, anatomy and experience.

The goal is not to impose one universal stride. It is to expand the runner’s available solutions.

At the clinic, the best spacing was not the widest gap anyone could clear.

It was the widest gap they could still organise.

Training should work the same way.

Build fitness from below.

Extend movement from above.

First earn the speed.

Then earn the duration.