Because of the way the foot roll is rigged, legs can’t be animated using FK if they’ve been rigged with the Auto-Rig, as you rarely need FK on legs but very often need the foot roll controls.

To be able to animate a leg using FK (but without foot roll controls), there are two ways:

• Rig it as a hominoid front leg (arm).
• Create a custom rig, with a standard IK.