Effects of a sedentary behaviour intervention in emergency dispatch centre phone operators: a study protocol for the SECODIS randomised controlled cross-over trial oleh Bruno Pereira, Frederic Dutheil, Jeannot Schmidt, Martial Mermillod, Jean-Baptiste Bouillon-Minois, Marion Trousselard, Daniel Pic, Maëlys Clinchamps, Thierry Taillandier