Bulge Angle Deformer

This is really the one intended to use with a bicep, because it works with only the parent bone ... but you can use either for a bicep.

The joint angle Deformer has the most flexibility, as it works with both parent and child [upper are and forearm, for example.]

Joint Angle Deformer

This is an arm with bones. Set up skin.

Select the UPPER arm bone ENVELOPE -- as deformers need a PARENT bone.

Select vertices you want to affect.

Add gizmo.

A LATTICE will appear over the region of vertices you want to affect.

 

Test animate the object. MOVE the lattice at the target position to adjust the vertices as you wish.

Now it’s a bicep.