That is absolute garbage
There are so many factors that come into play when it comes to bodybuilding. a 3 day split might work well for one person, but limit anothers growth. Everyone is an individual.
Ronnie Coleman trains twice a day! If thats not considered overtraining, then I dont know what is. 290lbs ripped at olympia. Granted he has enough juice coursing through his veins to make an elephant fall over, but its evidence leaning in the other direction.
The easist way to determine whats best for you is to try different approaches.
Take the following into consideration:
Genetics - Which one are you?
Endomorph: Wide shoulders and hips. Put on fat easily, but have the ability to retain muscle mass. Athlete Type: Prop in Rugby
EctoMorph: Skinny guys with no muscle or fat. Also called hard gainers - Athelete type: marathon runner
Mesomorph:Lose and gain fat easily. Wide shoulders, retain muscle easily, slim waist, barrel chests. Athelete type: Sprinter
You need to determine which genetic type matches yours, and adjust your diet and routine accoringly
Endomorphs need a mixture of cardio and weight training with a reduced calorie intake. 3 day weights, 5 days cardio. 50% of food from lean protein.
Ectomorphs need higher calorie intake than maintence. 4-5 days, one bodypart at a time on the weights. 60% of diet from low GI carbs.
Mesomorph can use variations of both depending on goals. Mesomorph for men is the hourglass for women. Everyone wished they had one.
Simply put, muscle grows when muscle is ripped to pieces everytime you push weights. Protein synthesis repairs muscle, which grows as a result to the damage you have caused it. Your muscle fibers increase, meaning you can lift more than before. So you overload your muscles again, causing more repair work to be required
Muscle growth slows when cortisol levels rise. Cortisol is a chemical the body releases in times of stress. Train for more than an hour straight, and cortisol levels skyrocket. Do your cardio on an empty stomach in the morning, and cortisol levels skyrocket. Dont take in enough carbs during the day to support gylcocegen replacement, cortisol skyrockets.
CORTISOL FORCES THE BODIES METABOLISM TO USE MUSCLE AS YOUR PRIMARY SOURCE OF FUEL.
This is why people believe you should not train more than 3 days a week. Next time you are in the gym, speak toa few of the larger chaps there, and ask them if they are training 3 days a week. Unless they are maintaining, I bet you most are doing 4-5 days a week. Schedule your program so each bodypart gets 2 days to recover. Its not going to be counter productive.
My program
Moday: Chest and calves
4 x Bench Press
4 x Incline Dumbell Press
3 x Pullovers
3 x Dips
3 x cable crossovers
4 x seated calf raises
4 x standing calf raises
Tuesday: Back and Abs
3 x Chins
6 x Deadlifts
3 x Barbell Rows
3 x Cable Pulley Rows
3 x Close grip pulldows
8 x various abs exercises
Wednesday: Legs
5 x Squats
5 x Leg Presses
3 x Leg Extensions
5 x Leg Curls
Thursday: Shoulders and Calves
4 x Shoulder Press
4 x Clean and Jerk
4 x Side raises
4 x Rear Delts
6 x Shrugs
Friday: Arms and Abs
6 x Barbell Curls
3 x Hammer Curls
3 x Preacher curls
4x Close grip bench press
4 x triceps push dows
2 x reverse grip pull downs
3 x kickbacks
8 x Abs
Saturday and Sunday OFF. Obviously vary your workout and change it once every 6-8 weeks. No one likes routine.
If you keep your training session under 60 mins, you are not going to raise cortisol levels.
Now like I said, everyone is different, and everyone has different aims. But dont believe that anything more than 3 days a week is counter productive. It depends on what you are trying to achieve, the intentisty of your training and your diet. 80% of this game is diet. Bodybuilding is a 24 hour job. Not a 3 hour a week pass time.