One common side effect of exercise is tingling or numbness in the extremities, including the feet. Although the cause is usually a minor problem such as tight-fitting shoes, this sensation could indicate a serious problem. If your foot falls asleep during exercise, don't panic. Check to see whether improper posture or an ill-fitting shoe is to blame before you assume a serious medical condition.
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Exercises that commonly cause numbness in the feet include bicycling, running and using devices such as an elliptical or stair-climbing machine or a treadmill. These exercises place pressure on the nerves underneath the balls of the feet, causing numbness. Another potential cause of numbness is a neuroma, or swelling on the nerve. Ultrasound imaging will determine whether your numbness is caused by a neuroma.
Improper footwear is the most common cause of foot numbness during exercise. Shoes that are too tight in length, width or height can compress your nerves, particularly the nerve in the metatarsal arch at the ball of your feet, and make your feet fall asleep. Make sure your shoes are tall enough to accommodate your arches if you have high arches. Loosen your shoelaces and avoid using toe clips if you experience a numb foot while cycling.
To decrease pressure on the balls of the feet, try using an orthotic device such as an arch-supporting insole, especially if the numbness results from cycling. Choose one that closely conforms to the curve of your arch to provide optimal support. Avoid those that extend under the toes since they may increase the pressure on your toes and contribute to tingling or numbness.
If your foot continues to fall asleep during exercise after you've ensured that your footwear is not the case, you may suffer from a more serious medical condition. Problems in the lumbar region, or lower spine, such as herniated discs, misalignment of the spine and disc degeneration can cause discomfort, tingling and numbness in the feet. Numbness accompanied by leg pain might indicate acute compartment syndrome, a serious condition requiring immediate medical attention.