Hello there, 
My question: which muscle is stronger - the biceps or the triceps? 
I specifically want to know which muscle is stronger in terms of being
able to work against moving a heavier load.
I was taught that anti-gravity muscles (muscles that work against
gravity such as the flexor action brought about the contraction of the
biceps muscle) are stronger. Whereas muscles that don't work against
gravity (such as the contraction of extensor muscles such as the
triceps) are weaker.
This of course is no longer true if a person specifically exercises
the triceps muscle to increase its strength.
A colleague of mine and her personal trainer believe that the triceps
is stronger than the biceps because:
- the triceps is a physically larger muscle with 3 heads, as opposed
to the biceps 2 (this is correct, but I don't think this alters the
fact that the triceps is still weaker than its antagonist)
- when lifting weights in the gym, more weight can be lifted using a
machine that isolates the triceps muscle than when a machine is used
that isolates the biceps muscle. Other gym users have also reported
this.
Many thanks, 
Laura

Just my 2 cents:

I think that there is a perception that the triceps are stronger based
on weightlifting exercises only because people actually cheat on the
triceps pushdowns.  They lean into it or otherwise fail to isolate the
triceps.  Try this experiment: load up a curl bar with enough weight
to cause failure after ten reps of a strict biceps curl.  Use a
preacher rack or other device to ensure you aren't cheating.  Now lie
down on a bench and use that same weight to do the same number of reps
in a strict French curl, isolating the triceps.  I doubt you can do
ten reps.  Let me know if I am wrong.