Hi Ernesto, there are a few solutions to your problem. The first is to create IK constraints Inverse Kinematics - Spine User Guide creating an IK for the feet too, or parenting the foot bone to the IK target bone.
The second option, if you're on Spine ESS instead of PRO, is to copy and paste the foot bone transforms during the animation so that it stays in place.
Tools - Spine User Guide: Transform copy
You can also use the pose tool and select all the bones of the leg, then drag the foot bone to act as something similar to an IK constraint, but not quite one.
You could also parent the feet bones to the root or something similar, but this requires being desperate and on ESS, if I were you I'd try the other options first 😃