You feel it, in a more pronounced manner, when you actually get on to the floor. The first rounds are easy, the alternating heats allowing you to catch your breath.

You are not dancing full out yet. The floor is crowded, the music some seconds shorter than what you can expect in the semis or the finals. But you know from extrapolations that you will burn 77 calories in every round of four dances, 19–20 calories in each bout of 1-minute-30-seconds dance, over and above your basal metabolic rate. The Waltz costs you the least, Quickstep the most, Tango and Foxtrot somewhere in the middle.

After the round, you tell yourself to rest. Your tailcoat off, carefully hung up where you can keep an eye on it, you mop your face. You smile at your partner, breathless. Your mind is replaying your last performance, still to obsess over your mistakes.

That can wait till you pay back your oxygen debt. You draw in quick, shallow breaths to make up for the deficit you created with your near-maximal efforts. You know you performed in an anaerobic-aerobic zone, the energy for your activity mostly fuelled by your oxygen intake, but majorly supplemented by the alternate anaerobic metabolism somewhere from the third dance on. Now your system is striving to achieve equilibrium: replenishing adenosine triphosphate and creatine phosphate, the quick-fire energy currencies that got you going at the beginning and kept you on time during the explosive bits in your choreography; and flushing away lactic acid, the by-product of the anaerobic energy production.