The second session of the day is fought with a different fuel

Glycogen refills at a fixed rate, so an afternoon session on a depleted muscle changes both what is trained and what can be damaged.

Morning session at eight, afternoon session at three. Seven hours between them, which sounds generous, and is not enough for a muscle that emptied itself properly in the first one.

Glycogen resynthesis has a ceiling. Even with ideal feeding started immediately, muscle refills over many hours, and full restoration after a genuinely depleting session takes the better part of a day and sometimes longer. Nothing accelerates that ceiling much. The transporters and the enzymes work at the rate they work at.

So the athlete arrives at the second session with a partially fuelled muscle, and several things change at once. Fat oxidation covers more of the energy cost, which is fine at low intensity and inadequate at high. Available power at the top end drops. Perceived effort rises for the same pace. And, the part that matters mechanically, muscles low on glycogen behave differently under load, because glycogen sits alongside the machinery handling calcium release inside the fibre. Depleted fibres release calcium less effectively, which reduces force and blunts the muscle's ability to control lengthening contractions.

Controlling lengthening is the protective function. A muscle that cannot brake well takes more strain per fibre during every deceleration, every landing, every stride at speed. That is a plausible mechanism for something coaches observe constantly, which is that the injuries cluster in the back half of the second session on a double day.

There is a second layer in the nervous system. Low carbohydrate availability affects central drive and, in longer efforts, the perception of effort in ways that shift movement quality before the athlete registers any specific tiredness.

None of this is an argument against double sessions, which are unavoidable for anyone doing serious volume. It is an argument about ordering. Put the high speed and high force work in the fuelled session and the aerobic or technical work in the depleted one, rather than the other way round, which is how it often ends up for reasons of facility availability rather than physiology.

Training low has genuine uses for the aerobic machinery, and I do not dispute them.

Just not on a day that involves sprinting, because that is the session where the fuel gauge and the injury list are reading the same dial.