First, how long have you had your tennis shoes? If you've got brand-new shoes, or older shoes, you may run into the numbing sensation more quickly. With brand-new shoes, it may take a week or so to "break them in." If you've got older shoes (tennis shoes should be replaced every 4-6 months if you're working out regularly), your feet may be receiving more impact, again, leading to this sensation. So, break in those shoes or go out and buy a new pair if this applies to you.
Second, make sure you're moving your foot around. I've noticed that especially on ellipticals, where the movement is pretty much pre-determined for you, its easy to avoid flexing and extending your ankle, as you do when you're walking around outside. If you start noticing numbing, take a second to really flex and extend your foot, promoting circulation.
Third, this happens most frequently with extended or intense exercise. If you're dedicating yourself to a really tough workout or a really long workout, figure out a way to break up the monotony, preventing your feet from 'sleeping' on the job. If you're working out for a certain length of time, switch machines every 10 or 15 minutes. If you're trying to do an especially hard workout, after 10 or 15 minutes, hop off your machine & jump rope for 5 minutes
Lastly, I think this sometimes happens because when working out on an elliptical, treadmill, or bike, you're essentially working in 1 plane of motion. Your body's movement is more or less predetermined by the machine. If you walk, run, or bike outside, you have to engage more muscle groups to react to barriers that may be in your path (pot holes, curbs, etc.), which keeps your mind and muscles constantly aware of what's going on around you, preventing this 'foot fatigue.' If you are constantly faced with this problem on a machine, consider taking your workout outside.
[photo credit: Getty Images]
