What do you think the difference between the old muscle slider and the new one will be? Definition maybe?
Maybe you can choose if the sim is dry in musculature. Sorry, I don't know the real translation, but in the gym I train myself, the more serious people (including me) are training and eating a lot in wintertime but our diet changes to loose more fat and change that into muscle when the summer is knocking on the door. So if my translation about a dry trainer is correct it means a person where you can see the muscle lines more deeply and he has a six- or maybe an eight-pack. Also his upper veins are more viable. And believe me, this is totally something different than the fat- and musclebar we have ingame so far.
Or it could be simply a more specialized bar where some muscles are better trained than others. Like sims who use the bike more often will have better trained leg muscles. Or bodybuilders where you can see the upper body (triceps, biceps, shoulders, chest) has had the best training.
I don't know if they ad that in the game but it's sure more realistic. Besides that, I would know no other function to a specialized muscle bar.
Anyway, what I do hope is EA will add some more realistic things to the fitness sims ingame. Nobody will ever wish to train 24 hours straight and even if hey do, they will not get any stronger from it, they will ruin their bodies. The best muscle training can be done within one hour a day, four days a week. That's my personal experience and my gym buddies agree. Also, muscle growth happens during our resting period, not during the training. So your fitness bar will raise a little while you are sleeping. Besides that, food is an tremendously important issue. I don't see anything back about that ingame. What happens to the protein shakes of The Sims 2?
It's all in the magical triangle of training, resting and feeding people. And that should be based on ones personal bodytype.