Marathon runners frequently slow dramatically in the closing stages after holding an even pace for hours. The causes are physiological and largely set by decisions made much earlier.
How the body fuels a long run
Muscles draw on carbohydrate stored as glycogen and on fat, with the balance depending on intensity. Higher intensities rely disproportionately on carbohydrate.
Glycogen stores are finite and hold roughly enough for a couple of hours of hard running. Fat is abundant but converts to usable energy more slowly.
As glycogen depletes, the body must shift towards fat oxidation, which cannot sustain the same power output. Pace falls as a direct consequence.
Why the decline is sudden rather than gradual
Depletion does not produce a steady decline because the body maintains output until stores approach a critical level. The transition then arrives quickly.
Blood glucose regulation also becomes harder to maintain late in a race, which affects the central nervous system as well as the muscles. Perceived effort rises sharply.
The experience is of a distinct threshold rather than a slow fade. That is why runners describe it as hitting something.
The role of accumulated muscle damage
Each footstrike involves eccentric loading as muscles lengthen under tension, particularly in the quadriceps during downhill sections. Repeated over tens of thousands of steps, this causes microscopic damage.
Damaged fibres produce less force, so maintaining pace requires greater effort from the remainder. Efficiency deteriorates independently of fuel availability.
Course profile matters here, since descents increase eccentric load. A fast downhill early can cost more in the final stages than it gained.
Why early pace determines late pace
Running above the intensity at which fat contributes meaningfully accelerates glycogen use. A few minutes gained early can cost far more later.
Even pacing is generally more efficient than variable pacing, because surges consume carbohydrate disproportionately. Adjusting to competitors carries a metabolic price.
This is why pacing discipline is treated as a skill in itself. The decisive errors occur long before the point at which they become visible.
What preparation actually changes
Training increases glycogen storage capacity and improves the body's ability to use fat at higher intensities. Both extend the point at which the transition occurs.
Practising nutrition during long runs allows a runner to absorb carbohydrate while moving, which partially offsets depletion. Tolerance for feeding at pace is trainable.
Repeated exposure to long efforts also increases resistance to eccentric damage. Adaptation addresses both mechanisms rather than one.
