#isolation curl

The space in between those two vectors, which is where your forearm travels, is made up of the various combinations of those two forces. This means the resultant force which you have to resist to execute a repetition is stronger than either of the borderline forces on their own.

And whilst it fluctuates as your forearm travels up and down, it’s multidimensional and remains sufficiently strong throughout the range of motion. Thanks to this, your bicep is kept under tension until the set is completed. There is no way it can rest, even partially, between reps.