I think being strong is in many ways a mental thing. If you are weak minded no amount of physical strength will help you to get through the day to day difficulties or even the very hard life altering struggles. Strength is mental and spiritual first, at least in my opinion. My body failed me at age 24 with cancer, so I was not physically strong, but I was mentally!
I think that people can be strong in either way...Some people can do physical things that make gasp and say "wow!" and while others who survive incredible circumstances all the while keeping their chin up and not letting life get them down. The emotionally strong can bring you to tears while you listen to their story and you grow admiration for their personal strength and their journey, I have met quite a few of these incredibly strong people and I cry tears of joy for the fact they survived their journey and they are better for it.