The resistance curve is the point

Lift a dumbbell and the load is the same throughout the movement, even though your leverage changes dramatically. Stretch a spring and resistance rises with extension, which often aligns better with where a joint is actually strong.

This is why spring-loaded footwork feels smooth in a way a weighted equivalent does not. Load is lowest where the joint is weakest and highest where it is strongest.

Springs can assist as well as resist

A weight only ever pulls down. A spring anchored at the far end of a carriage pulls you toward it — which in a roll-up or a supine leg exercise supports the movement rather than opposing it.

This dual role has no equivalent in free weights, and it is the mechanical reason Pilates equipment is so widely used in rehabilitation.

Springs and weights compared
SpringsFree weights
Resistance curveAscending with stretchConstant
Can assist a movementYesNo
Loads at end rangeHighestDepends on leverage
Precision of incrementsCoarse — per springFine — per plate
Best forControl, rehab, range workMaximal strength, hypertrophy

Where weights win

Springs are poor at fine increments. You add a whole spring, which might be a 30% jump, where a weight stack offers 2.5 lb steps. For progressive strength training with precise overload, weights are simply the better tool.

This is why serious practitioners rarely treat Pilates as a replacement for resistance training. They do different jobs.