What can cause numbness in feet while working out?

Compression : Neuropathy. Could be from back, ankle (tarsal tunnel) or neuromas.

Shoes: Check your workout shoes to make sure they are not worn out, and they provide adequate arch support for your foot type. Athletic shoes have a life cycle of about 400-500 miles on average. Also make sure that if you have increased your activity or intensity of it, you have not done so too quickly. Make sure your shoes fit well. If the numbness persists, seek attention of a podiatrist.