Muscle force/energy relationship
You can calculate the energy consumption using basic physics formulas. 

Work = Force x Distance

and the unit of work is the Joule. Therefore if force is a function of change in length then substituting this function in for force in the above calculation we have that work is a function of length.

If you want to talk about energy consumed in terms of calories rather than joules,

1 calorie = 4.184 joules