What determines how well we run?

Running is repeated force under a time limit.

Each step asks the body to produce and absorb force, reposition the limbs and maintain balance while energy demand continues. The heart and blood deliver oxygen. Muscle cells use oxygen and fuel to rebuild ATP, the molecule that powers contraction. The nervous system times the movement. Tendons store and return some elastic energy. None of these systems runs alone.

A five-kilometre race and an easy forty-minute run use the same body but ask different questions. The shorter race can be performed at a higher fraction of maximum capacity. A long run makes durability, fuel and heat management more important. Sprinting shifts the problem towards rapid force and coordination. Good training begins by naming the task.

VO₂max: the ceiling, not the complete house.

Oxygen follows a chain. It enters the lungs, crosses into blood, is pumped by the heart, reaches working muscle and is used inside mitochondria to support aerobic ATP production. If any part of the chain cannot meet demand, the maximal rate is constrained.

Cardiac output is the blood pumped each minute: heart rate multiplied by stroke volume, the amount ejected with each beat. The working muscles also need capillaries for exchange and mitochondria for oxygen use. Training can change central delivery and peripheral use. VO₂max therefore reflects an integrated system rather than “lung capacity”.

A higher VO₂max can increase the ceiling for endurance performance. It cannot tell us the speed that the athlete can hold, the cost of that speed or the point at which fatigue changes mechanics. That is why identical VO₂max values do not guarantee identical race times.

Threshold: how much of the ceiling can be used?

As pace rises, the body must turn over ATP faster. Lactate production and removal continue at all intensities, but above certain workloads blood lactate and ventilation begin to rise more sharply. Researchers use terms such as lactate threshold, ventilatory threshold and critical speed. These are related markers, not perfect synonyms.

The practical idea is simpler: a runner may be able to sustain a high fraction of maximum oxygen use before disturbance grows too quickly. Threshold training is work targeted around that transition. It should not be reduced to one universal heart-rate percentage because protocol, measurement and individual response differ.

Economy: the cost of a pace.

Running economy asks how much oxygen or energy is required at a chosen submaximal speed. Imagine two cars with the same fuel tank. One travels farther per litre. In running, better economy means that a familiar pace consumes a smaller share of available energy.

Economy reflects many variables: limb mechanics, tendon behaviour, muscle recruitment, stiffness, footwear, surface, gradient, fatigue and familiarity with the pace. There is no single ideal running form shared by everyone. A visible technique change is useful only if it improves the relevant outcome without creating a cost elsewhere.

Strength and plyometric training can improve economy in some runners. A 2016 meta-analysis by Balsalobre-Fernández and colleagues included five controlled studies with 93 highly trained runners and found improved economy after strength programmes. This is a small, specific evidence base, with varied programmes; it should not be turned into one prescription for every runner. The proposed reasons include better force production, neuromuscular coordination and use of elastic energy. This does not mean every runner should lift maximally or jump immediately. The dose must fit training age, symptoms and the demands already present.

Durability: what remains after the easy first hour?

A runner can look economical and controlled when fresh, then require more oxygen or effort to hold the same speed later. Durability describes resistance to this deterioration. It brings time into the model.

Long-duration performance can be altered by glycogen availability, hydration, heat, muscle damage, pacing and neuromuscular fatigue. Glycogen is stored carbohydrate in muscle and liver. It supports higher rates of energy turnover; when availability falls, the same pace may feel more demanding. Heat adds another demand because blood flow helps both working muscle and temperature regulation.

Durability is specific. A well-trained 5 km runner may still need gradual exposure to the mechanical and nutritional demands of a marathon. A long run is therefore not merely a slower interval session. It teaches the body and the athlete to organise work across time.

Load and capacity must meet gradually.

Training load is the demand created by the session and the wider week. Capacity is the ability to tolerate and perform that demand. Both are multidimensional. Ten kilometres on flat ground is not the same as ten kilometres downhill, fast, in heat or after a long break.

Running-related pain is not proof that one structure has been “worn out”, and pain intensity is not a precise picture of tissue damage. Location, onset, change, swelling, function, previous injury and the response to load matter. Sharp trauma, inability to bear weight, progressive neurological symptoms, chest pain or unexplained severe symptoms require appropriate assessment.

Four common misunderstandings.

“A higher VO₂max always means a faster runner.”

It raises an important ceiling. Economy, threshold, durability, tactics and environment decide how the ceiling is expressed.

“Good runners should make every run hard.”

Hard work provides a strong signal and a large recovery cost. Easier work allows more volume and practice. The right distribution depends on the runner and goal.

“One running form is correct.”

Technique can be assessed and changed for a reason. Visible symmetry or a fashionable foot strike is not automatically more economical or safer.

“Pain means stop forever or push through.”

Both answers ignore context. Adjust the demand, monitor the response and seek assessment when the pattern or severity warrants it.