Introdution: Energy cost is trainditionally defined as the amount of metabolic enegyy spent to cover one unit of distance,with the unit of kJ/m/kg.However,it is difficult to apply this parameter in acyclic sports,e.g.racket sport.In this study,we introcuce a new descprition of energy cost in racket sport,which is the amount of metabolic enegyy spent to cover one stroke,with the unit of J/stroke/kg.