Strong can describe anything with firmness or power. You could have a strong will and resist the distractions of the internet while writing a paper all day, or your paper could have a strong argument, convincing your reader of your wisdom.
Even if you spend hours every day lifting weights, if you don’t eat enough food and drink enough water, you won’t be strong enough to do anything with all that muscle power. If you ran a national bank, you would try to make your currency stronger than that of other countries. Or you might be a woodworker who builds a chair strong enough to hold a three-hundred pound man.