What can my wife claim as a independent fitness instructor filing a 1099misc? Fitness certifications, membership related to work, workout clothes, protein shakes?

[KrisD]

The IRS says the expenses need to be "Ordinary and Necessary", so use that as a guide. 

Fitness certifications? Yes, but only after she was originally working in this field. (you can deduct training after getting a skill, or to improve your skill, but not the initial cost to get that skill)

Membership related to work? Yes, that would be similar to renting a business space. (but not a family membership, you would need to expense only the difference between a family membership and a self only membership)

Workout clothes? Only if needed and not worn any other time than when working. 

Protein shakes, doubtful. The IRS's stand on that is "you would need to eat regardless"

I hope this helps. Expense the thing she needed to get to work which she wouldn't have paid for if she was not working in this position. 

https://www.irs.gov/businesses/small-businesses-self-employed/deducting-business-expenses