Hi Mariu. My suggestion would be to have this checked by a physician, since if it’s due to a circulatory issue that needs to be checked-out medically.
I agree with Calvin that it could be caused by less circulation of blood to that area since it is constant pressure on the foot. One thing that may help is to try putting more weight through the heel to alleviate the sensation.
Another issue could be shoes that do not fit properly, so make sure your clients check this as well.
All the time. When you use an elliptical, the feet typically doesn’t leave the footpads. So the feet doesn’t actually do any active movement. On top of that, you are standing on your feet for an extended period of time. As a result, there’s less circulation of blood to the area than to any other parts of the body. This leads to the sensations mentioned in your question.
The sensations usually resolve when you start moving the foot around or taking weight off one or both feet. If there is one foot that experiences this, but not the other, then that’s usually a sign of which foot is supporting the weight/pressure.