Muscle fibres work with the “all or nothing” principle. Either a muscle fibres contracts, or it doesn’t.
Muscle fibres can only contract with full strength. It’s the type of muscle fibres which makes the difference.

The order of activation of muscle fibres is pre-defined.

At first fibres type 1 are consulted.
If the tension is insufficient, type 2A is consulted.
If the maximum tension is not enough anymore, type 2X is consulted. Type 2B will be consulted at the end for ultimate support.

Scheme (order of activation):

1 -> 2A -> 2X -> 2B