Split routines "work" for bodybuilding
As suggested above, the high volume split workouts are designed to induce a lot of muscle protein breakdown. Zatsiorsky in Science and Practice of Strength Training discusses these types of workouts and says they are designed for hypertrophy. He seems to agree that it is muscle protein breakdown from tension that leads to hypertrophy. Increasing work density maximizes this breakdown and supercompensation process. Obviously, the body can only adapt so fast, so too much volume may cause you to exceed your growth capacity. Another advantage MAY be more sarcomplasmic hypertrophy, if it is true that sarcoplasmic and sarcomere hypertrophy can occur at different relative rates. The other advantage is that high volume, traditional bodybuilding routines, if performed for fatigue as they should be, will allow you to access the highest threshold fibers that are used in lifting 90%+ max loads as in power training. However, the highest threshold fibers will not fire at their maximum frequency and will not develop quite as much power as training with low reps/high weight. They will, however, hypertrophy, which is what bodybuilders want. On the way to accessing the high threshold fibers through high volume work with sets of say 10, the slower fibers also get fired and more protein is broken down which mazimizes hypertrophy. One advantage to the high volume way to these high threshold fibers is that not as much load is used--you don't have to lift such heavy weight.
I think that strength and power training (ie. for sport and powerlifting) and strength and mass (ie. for bodybuilding) overlap and share much in common. However, pure strength training is not designed to maximize hypertrophy, unless a hypertrophy period is included in a special program where weight gain is desired, such as in some football situations. Pure strength training is more designed to TRAIN the existing muscle mass to contract more forcefully at with a greater number of fibers. This is how athletes get much stronger but not much bigger. Hypertrophy will occur with power/strength training, but it's not usually the goal and it may not be maximized if the volume is kept relatively low. Many power athletes that strength train had a large amount of strength potential (ie. muscle mass and neuromuscular connection) before they ever engaged in their power sport. It is usually the body-type / genetic make up that determines a person's sport potential. Strength training explores and enhances that potential. It's the old debate of are you strong because you lift or do you lift because you're strong.
Mass training is designed to increase the muscles POTENTIAL STRENGTH, but not its STRENGTH PERFORMANCE. In other words, in bodybuilding the muscles are trained to get bigger. They get stronger in the sense that they gain more contractile tissue, but the bodybuilding program does not ideally train that contractile tissue to contract with higher force and with higher percentage of fiber recruitment in a given motion. However, the muscle mass of a bodybuilder could be trained to perform a task, such as the bench press, with astonishing power--it's just a matter of "teaching" those large muscles to do something--it's a neurological thing.
In sum, power/strength training is more designed to increase neuromuscular coordination in a given motionits motion training. Hypertropny/mass training is designed to increase muscle mass through muscle breakdown and rebuildits muscle training.
So, traditional bodybuilding routines do what they say they will IMO. They just have to be understood as such. Does that mean you should work each body part once a week only? Not necessarily. Remember, you can only gain so quickly. So simply breaking down a muscle until its liquified is not going to speed up that growth.
Keep in mind that progressive resistance is still important in bodybuilding. Increaseing loads will allow for more total volume in less time and more hypertrophy.
Someone above asked whether reaching the high threshold fibers through volume training versus high weight/low rep (power training) will have the same effect. With regard to hypertrophy: No. The high volume moderate weight (ie. sets of 10) will produce more hypertrophy because the total amount of mechanical work is greater. With regard to power: No. The power training way will fire those fibers at their proper frequency and therefore train more forceful contractions.