KartDen

The sustained throttle hold is closer to an isometric contraction than to running or cycling. Blood flow through the working calf is mechanically restricted while the muscle stays under load, so local metabolites accumulate and the muscle spindles become more excitable. That pathway is a more plausible trigger for a sudden cramp than whole-body water balance by itself. The usual explanation that you simply ran out of salt is incomplete. Exercise-associated cramps track more reliably with local fatigue and altered motor-neuron control than with measured electrolyte levels, though heavy sweating can still make the same muscle more irritable. Pedal height and ankle angle matter because a chronically shortened calf (toes pointed on a high pedal) cannot unload between inputs and is the joint position most associated with cramp. Conditioning delays how quickly that pathway activates, and so does giving the ankle a slightly more neutral rest angle on the pedal.

The throttle return spring is usually stiffer than it needs to be and that extra force on your calf the whole time you're out there is what seizes it up. Slide the seat or move the pedals until your heel stays planted and the ankle isn't bent hard. Most people also lock their legs against the sides without noticing so loosen that up and let the seat do the holding.