Last time I started answering this question:
Q: All the big guys at the gym say you should train only one body part per day. What do you think about that for me as an athlete?
I mentioned that I do not use if for athletes for the following reasons:
Personal experience - it made me less athletic
It is more advanced than what most people need
Too much training time - athletes need that extra time for practicing their sport
Low frequency for each muscle group - sometimes not as ideal for performance training
Local muscle fatigue and excessive muscle soreness - not good when you need your body for your sport
Systematic fatigue - again, not good when training for performance.
Now, here are a few more things to consider...
The steroid factor
It is important to note that this style of training was popular in the steroid era (yes I know there are some very good natural bodybuilders who use it, but even then there is the genetic factor that is hard to account for). As I mentioned last time, I have used this method and it did work for me despite the fact that I have never used steroids and have horrible genetics. However, it is difficult to know if bodybuilders are getting good results because this method is superior to what bodybuilders in the pre-steroid era did or if it is because of the drugs. How is that for a debate topic?
Inefficient exercise selection
The programming problem you run into with this method is finding enough exercises for each muscle group. This often leads to a lot of time spent with less effective exercises. With legs for example, a bodybuilder may start with a superior exercise such as squats but then move to less effective exercises such as leg extensions. Or a great triceps exercise (e.g. dips, close-grip bench) is followed by several versions of triceps kickbacks. In athletic-based training (as for most people), time constraints force you to pick the best exercises.
A high volume of exercises for one body part places a lot of stress on that area. After 100’s of elbow extension repetitions, your elbows will not like you too much. Also, the single joint exercises used heavily in bodybuilding are stressful on the joints. Multi-joint exercises allow the stress to share with several joints. If you are bodybuilder who is healthy enough to get away with it, that may work. However athletes place tremendous stress on their bodies with their sports and cannot afford to place unnecessary stress on their joints with training.
Bodybuilding routines like this often create structural imbalance. Take the one I was doing as an example: I did pushing exercises on Monday (chest day), Thursday (shoulder day) and Friday (arms – close grip bench and dips for triceps). However I only did pulling exercises on Tuesday (back day). This imbalanced pushing and pulling can lead to structural imbalances at the shoulder girdle, poor posture and an increased risk of injury. I'll show you a way around this in a future post.
Again, this is not a slam on bodybuilding. However if you are an athlete, you need to train like one.