A method, system, and program enabling a variable varying with an animal's motions to be accurately measured. First, a reference relational expression representing a relation between myoelectric potentials and motion is set on the basis of the myoelectric potentials and the variable measured by using an inverse dynamics model. Then, the motion variable is measured on the basis of the measured myoelectric potentials and according to the reference relational expression. A reference relational expression is set on the basis of myoelectric potentials measured in a first motion and a motion variable corresponding to the first motion. Then, a motion variable corresponding to a second motion is measured on the basis of the myoelectric potentials measured in the second motion and according to the reference relational expression.