Distance running shoes changed substantially over a short period, and the sport responded with rules limiting their construction. The regulation defines where equipment stops being neutral.
What changed inside the shoe
New midsole foams store and return a greater share of the energy absorbed at each footstrike than earlier materials. Less energy is lost as heat.
Thicker midsoles hold more of that foam, so increasing stack height increases the total energy returned. Height and benefit are directly related.
A stiff plate embedded within the foam changes how the foot bends, reducing energy lost at the toe joint and stabilising the thicker stack. The components work together.
How the benefit is measured
Running economy describes the oxygen cost of running at a given speed, and it is measured in a laboratory rather than inferred from race results.
Improvements in economy translate into faster times at the same effort. The relationship is well established, which is why economy is the standard metric.
Because effects vary between individuals, published findings describe averages across a group. Any given runner may gain more or less.
Why a height limit was chosen
Regulating foam chemistry or plate design directly would be difficult to verify at a race, since it requires destructive testing. Stack height can be measured simply.
The limit therefore acts as a proxy that constrains the amount of responsive material a shoe can contain. It is enforceable at scale.
Separate limits apply to road and track shoes, reflecting the different demands and the shorter distances involved. The rules are event-specific rather than universal.
The availability requirement
Rules also require that a shoe used in competition be available for purchase by any athlete for a defined period beforehand. Prototypes are excluded.
This addresses fairness between athletes sponsored by different manufacturers. A technological advantage must be purchasable rather than exclusive.
Enforcement relies on published lists of approved models, which officials check before major competitions. Compliance is verified administratively.
What the rules leave unresolved
Comparing performances across the transition remains difficult, because times before and after were achieved with different equipment. The sport chose not to annotate records.
Manufacturers continue to improve foams within the height limit, so gains have not stopped. The cap slows the rate rather than fixing it.
The broader question of how much assistance equipment may provide recurs in every sport with technical gear. The height rule is one answer, not a principle.
