As a teenager, I started working out because I wanted to look stronger. Not be stronger, look stronger. What does it mean to be strong anyway?
We think having a six-pack is the epitome of strength. The main protagonist of Fight Club jokes about how art directors and advertisers define masculinity. A toned body and visible abs are the marks of beauty and everybody should look like a sculpted David.
The archetype of the fighter is also how most people imagine strength. A fighter doesn't want a six-pack, a thin skin would leave an opening for the enemy. In the French army, the ideal soldier is depicted as a "thin cat" (chat maigre), strong yet agile.
More than physical strength, mental strength is what makes someone strong. Physical strength is acquired by developing mental strength, but the contrary isn't always true.
Mental strength originates from stress, pain, loneliness, rejection, frustration, adversity. Once you accept what's happened to you and you manage to go past self-destruction, you can focus on growth.
Strength is the ability to overcome any external stress. When a Christian says, "Lord, give me strength", he probably doesn't want a bigger triceps. In a sense, strength, for human beings, is Darwin's definition of adaptability: the capacity to go through change, unharmed.