I'm 55-years old and in relatively good health. I have some risk factors for heart disease so I work out everyday. My favorite aerobic exercise is with the elliptical cross-trainer. But I notice the bottom of my left foot gets numb after about 20 minutes on this machine. What could be causing this?
There are a wide range of causes for numbness in the foot or feet. The elliptical trainer requires foot contact for much longer periods of time than some other forms of exercise. You might not experience this symptom when using a recumbent bike or treadmill.

Numbness, tingling, and loss of sensation in the feet can be a serious condition. Tissue injury leading to ulceration are concerns for anyone with diabetes or other chronic conditions causing nerve damage in the feet.

Since the symptom is only present on one side, it suggests a possible problem with foot alignment. Flat feet or unsupported high arches can cause the heads of the metatarsal bones to drop down. Without the necessary support, this could be enough to put pressure on nerves in the forefoot. The result can be numbness such as you are describing.

Proper shoewear may be important. A shoe that gives good support and fits correctly may be the first step. Too much room in the box of the shoe (where the toes are located) may cause shifting of the foot forward with each stride. This type of pressure can cause problems.

You can also try insoles to support the arch and/or forefoot. If the condition gets worse or is not helped by these measures, then it may be a good idea to have an orthopedist or podiatrist examine your foot. An accurate diagnosis can help in protecting your feet from further injury.