bob,
Intentional discrimination on the basis of real, measurable, ability difference is permitted.
In the US, there are specific anti-discrimination laws which say that you cannot discriminate on certain explicit basis. For example, it is illegal to hire only whites and ignore qualified black applicants. But outside of the specific 'protected classes', you most certainly can discriminate on just about any basis. In most of the US, for example, if you simply don't like gay people, you can simply refuse to hire gays, with no legal penalty. (Massachusetts does make it illegal to discriminate on sexual orientation.) Don't like that someone doesn't exercise? Don't hire them.
Interpretation by the courts has resulted in some strange rules that are hard to reconcile with common sense; which goes back to the original question; you can require employees to be strong enough to lift a particular load, unless they are so weak as to be 'disabled', at which point the law requires you to 'provide reasonable accommodation'. If you selectively hire 'healthy looking' people, you are probably okay discrimination wise, except that someone who is 'morbidly obese' may be considered disabled by the courts, thus making not hiring them illegal discrimination.
Note: I am not a lawyer, and the above is just a lay reading. And while it is probably legal to discriminate against lawyers, its probably a bad idea.
Massachusetts list of 'protected classes' for various situations:
http://www.mass.gov/mcad/services.html -Jon
