Every standard track in the world is run counterclockwise, and athletes train exclusively for left-hand turns. The convention was not derived from the body, and it was contested for a long time before it settled.
Early tracks ran in both directions
Nineteenth-century meetings in different countries used different directions, and some venues alternated depending on the event or the organizer's preference.
Records set in one direction were compared with those set in the other, which caused exactly the confusion you would expect.
Standardization arrived through the governing bodies that formed to organize international competition, and the choice they made became universal by rule rather than by argument.
The physiological explanations came afterward
Various claims are offered about leg dominance, the heart's position or the mechanics of the turn, and none of them are strong enough to have determined the decision.
Athletes adapt to whichever direction they train in, and the asymmetries that develop are consequences of the convention rather than reasons for it.
What is true is that consistency matters enormously, since a runner trained on one direction is measurably disadvantaged on the other.
The convention shaped the body of the athlete
Running only left turns loads the two legs differently, with the inside leg handling a different pattern of force through every bend of every session.
Coaches manage that with deliberate work in the opposite direction, which is a corrective for a problem the rule book created.
Sprinters, who spend proportionally more of a race on the turn, deal with it most, and lane assignment changes the severity for each athlete.
Other sports settled the same question differently
Baseball runners go counterclockwise, speed skaters and horse racing in some countries do too, while horse racing in others runs the other way entirely.
Those divergences show that no universal principle is at work, only a series of independent decisions that hardened locally.
American horse racing and British horse racing running in opposite directions is the clearest evidence that the choice was administrative.
Standardization is what made records possible
Comparable performances require identical conditions, which is why direction, distance, surface, lane width and timing method are all specified.
Each of those specifications ended a period in which results were incomparable, and each was resisted by people whose local practice was displaced.
The direction of the turn is simply the most visible survivor of that process, and it now looks natural because nobody alive has seen the alternative.

