when you just drink the fruit or vegetable juice, you arent getting the fiber from cellulose, hemicellulose, pectin, amylopectin, etc etc.
trust me.....ive taken many classes on it.
the pulp thats left behind is where the fiber is at.
benefits of fiber:
http://www.mayoclinic.com/health/fiber/nu00033
^from same link:
(Refined or processed foods — such as canned fruits and vegetables and pulp-free juice, white bread and pasta, and non-whole-grain cereals — are lower in fiber content.)
i had a nutrition class where the teacher (who was in better shape than 99% of the class) talked about the importance of fiber, and how she recommended fiber one because it met 50% daily requirement of fiber per serving or something like that.
but fiber is better for overall general health, not increasing muscle strength nor muscle size.