In the Celtic tradition it would depend on the individual you asked. Here are some tattoo or craft ideas a Celtic individual might consider when asking oneself this question.
1.) Zoomorphic figures are often used to symbolize an animal which has given them knowledge or strength.
2.) Coats of arms also show strength... being involved in a Clan.
3.) One can use Ogham letters to spell out the word strength. Even draw knotwork around it or whatever you want.
4.) There is a symbol (with symbol meaning) for "inner strength" displayed at celtic-tattoo.net