You can actually look at it as a robot arm. The foot would be the base, the knee the base joint, and the hip another joint on the arm.
As such, the knee would be experiencing the highest torque.
So to calculate, its just like this image (M1 is knee torque and M2 is hip torque):
so the equation at 90 degrees for the knee joint is (assuming L2 is 0):
force * length = knee torque
weight = (1 ft-lb)/(1 ft) = 1 lb
this is only static torque, however. this does not account for dynamic torque (moving a mass). what you want to do is assume the knee joint is the center of a wheel, and the upper leg length is the radius of a wheel. then use my dynamics tutorial
to select the appropriate motor. this way you can also choose your leg velocity and acceleration.
hopefully i didnt over complicate it for you!