This is a very frequent question among clients at the studio. People also inquire what the spring is equal to in weights.There is no equivalent ratio to convert springs to weights.
Joseph Pilates was well ahead of his time when he chose to use springs for conditioning the body. Unlike a weight, the contraction and release of a spring works the same way your muscles do. The spring stretches and lengthens with you as you move. There is also a resistance created in the opposite direction as the spring moves back together. Springs encourage an eccentric contraction of your muscles and is easier on your joints than weights.
Certified apparatus intstructors study all the different spring tensions and access points to get the best work out for your muscles. A lot goes into determining the resistance of the spring. This includes the attachment point and distance, the tension of the spring, and whether the spring is above or below the body.