A closed roof turns the pitch into a place where sweat stops working

Air handling set for comfort in the stands can leave the playing surface humid and still, which is the worst case for cooling.

Close the roof and the pitch becomes a room. That sounds obvious, and it is, but the consequence for the bodies in the room gets treated as a comfort question when it is actually a thermodynamic one.

An athlete working hard produces a great deal of heat and gets rid of almost all of it by evaporating sweat off the skin. That process depends on two things: how much water vapour the surrounding air can still accept, and how fast the air over the skin is moving. Both collapse under a shut roof. Thousands of spectators exhale moisture. The turf and its irrigation add more. The air sits still because there is no cross breeze and the ventilation is set to move enough air for people sitting down, not for people sprinting.

At that point sweat still pours. It just does not evaporate, so it does not cool. The body responds by sending more blood to the skin in the hope of losing heat by radiation and convection, which means less blood available for the working muscle, which raises heart rate for the same pace. Every one of those steps is invisible to a broadcast. What you see instead is the second half of a closed-roof fixture looking sluggish and the sprint counts falling off a cliff.

Then the injuries. Rising core temperature degrades the fine control of movement before it produces any dramatic symptom, and a tired athlete lands with a slightly less coordinated pattern than a fresh one. That is a slower, sloppier deceleration at the knee and ankle. Cramps show up too, though I think cramp is oversold as a pure fluid story and is better understood as a fatigued nerve and muscle unit misfiring under load.

Where the energy question comes in is the setting. Air handling is one of the largest loads in a big venue, and a stadium chasing an efficiency target has an obvious incentive to run fewer air changes and let the bowl warm up between events. Nobody in the seats complains at twenty six degrees. The people at risk are the two dozen running.

If a venue wants to publish its energy figures, publish the wet bulb reading at pitch level alongside them. That single number tells you whether the building is cooling anyone at all.