Part 2 of Thibaudeau, Enjoy!

Principle #3: Keep Your Training Sessions Under 60 Minutes
Cortisol is a stress hormone that's released during bouts of training. Some is needed, but too much cortisol, especially if it stays elevated after the training session, can greatly decrease muscle growth and strength improvements.
Cortisol is catabolic, meaning that it leads to the breakdown of stored substrates. During exercise, this can be useful since it'll breakdown stored glycogen into glucose and stored fat into fatty acids to provide energy for the working muscles. However, post-training it'll continue to breakdown glycogen which slows recovery. It also breaks down muscle tissue into amino acids, making it harder to add muscle mass.
Furthermore, since both cortisol and Testosterone are both made from the same raw material (pregnenolone), constantly elevated cortisol levels will eventually lead to lower Testosterone levels.
Cortisol output during training has been correlated with training volume; the more work being done during a session, the more cortisol is produced. This is especially true when metabolic-type training (high reps, short rest intervals) is used.
To avoid overproducing cortisol, you want to keep your sessions short, around an hour or less.
Another reason to avoid long sessions is related to mental focus. Regardless of how much you love training, at some point your focus will go in the crapper during a long session. The work performed in that state will be unproductive and could even lead to bad habits that'll screw you in the long run.
You can train more than one hour per day, but split your daily volume into two workouts. In fact, splitting your daily workload into several shorter sessions is much more effective, as it leads to both lower cortisol production and higher Testosterone levels. It's been shown that when two daily sessions are used, Testosterone production is higher after the second workout than after the first.
When training twice a day, it's best to train the same body part(s) during both workouts. I like to take this opportunity to train different types of contractions or goals on both occasions. For example:
Option 1: Muscle Building Emphasis
AM: Compound movementsPM: Isolation work
Option 2: Strength and Size Hybrid
AM: Heavy lifting (2 to 6 reps)PM: Moderate loading (8 to 12 reps)
Option 3: Muscle Building or Strength Emphasis (Depending on AM Load)
AM: Concentric/regular liftingPM: Eccentric work
Option 4: Performance Training
AM: Explosive liftingPM: Heavy lifting
Option 5: Powerlifting or Olympic Lifting
AM: Competition movementPM: Assistance work
It'd be a mistake to immediately jump to the maximum amount of training you can do with two-a-days. There should be a progression toward that amount of training.
Week
Session 1
Session 2
1
40 to 50 minutes
20 minutes
2
40 to 50 minutes
20 to 30 minutes
3
40 to 50 minutes
30 to 40 minutes
4
50 to 60 minutes
None
5
50 to 60 minutes
20 to 30 minutes
6
50 to 60 minutes
30 to 40 minutes
7
50 to 60 minutes
40 to 50 minutes
8
50 to 60 minutes
None
To judge if a workout was productive, but not excessive, look for three things:
1. At the end of the workout you're tired but not drained.
2. You feel a pump in the trained muscle. The intensity of the pump will obviously depend on the type of training that you did, but you should feel the muscles that were trained.
3. Two to three hours after the completion of the session you should yearn for more training. If you're still tired or lack motivation to train after this amount of time, chances are the session was excessive.
Principle #4: Contraction Type Depends on the Movement
This goes hand-in-hand with the first principle mentioned. There are basically three ways of executing a movement when it comes to the speed of execution/type of contraction.
1. Constant tension movement: You never release the contraction of the target muscle group during the execution of the exercise. Basically, the muscle you're trying to stimulate must be kept maximally flexed for every inch of every rep of every set. Never let it relax, not even between each rep!
The goal of this type of contraction is to prevent blood from entering the muscle during the set. This creates a hypoxic state because oxygen can't enter the muscle. It also prevents metabolic waste (lactate, hydrogen ions, etc.) from being taken out of the muscle during the set. Both of these factors increase the release of local growth factors like IGF-1, MGF, and growth hormone which will help stimulate growth.
By the way, the use of isometric contractions also falls into this category.
2. Accelerative concentric, controlled eccentric: In this type of contraction, you're trying to accelerate during the actual lifting portion of the movement and lower the weight under control. You go to the exercise's full range of motion, but you briefly pause (around one second) between the stretch position and the following lifting action. This short pause will negate the contribution of the stretch-shortening cycle to the force production.
You see, three things can contribute to producing force when you're lifting a weight: the actual contraction of the muscle, the activation of the reflex known as the stretch-shortening cycle (also called the myotatic stretch reflex), and the fact that muscle tissue is elastic, much like a rubber band.
When trying to maximize the amount of actual work the muscle itself must perform, you want to minimize the action of both the stretch reflex and the elastic contribution of the muscle's structure. By doing a simple one-second pause before lifting the weight, you can accomplish that and thus maximize the amount of force that the muscle must produce.
When you're lifting the weight, try to contract the muscle as fast as you can. This doesn't mean focusing on lifting the bar as fast as you can. Rather, it means that you should attempt to tense the muscle as hard as possible right from the start of the lifting motion. This will maximize the recruitment of the highly trainable fast-twitch fibers.
Finally, when you lower the weight, do so under control. The eccentric portion of the movement is where most of the muscle damage occurs (micro-tears of the muscle fibers) and is a powerful growth stimulus.
3. Using the stretch reflex: With this type of lifting, you want to involve the stretch reflex and elastic component of the muscle. Thus, you want to lift the load as fast as possible. This explosive lifting will improve the capacity, over time, of the nervous system to recruit the fast-twitch fibers.
It isn't effective by itself to stimulate maximum growth in those fibers because you can't fatigue them sufficiently (the time of contraction and duration of the muscle tension per rep is too low). But by lifting this way on some movements, you'll become better and better at activating the fast-twitch fibers. When you're more efficient at doing that, every single other exercise becomes more effective.
So, when do you use each technique?
Every time you do an isolation exercise, use constant tension. Every time! The goal of an isolation exercise is to completely focus the stress on the target muscle. You want a maximal local effect, and to do that you need constant tension. Without constant tension, isolation exercises are pointless. This is actually one of the main reasons why isolation movements get a bad rap. People don't know how to do them properly, and as a result, they end up not being effective at stimulating growth. But when done using constant tension, they're very effective at it.
Don't try to use constant tension lifting with compound movements. Not that it's impossible, but it's a waste of time. The goal of a compound movement is to overload several muscles. By nature, you can't isolate a muscle during a multi-joint exercise, and attempting to do so will make the exercise much less effective than it should be.
With regular compound movements, you want to use the second technique: accelerative lifting, short pause in the stretch position, and a controlled eccentric. This will magnify the hypertrophic effect of the big movements by overloading the involved muscles as much as possible.
Finally, the explosive lifting is best kept for exercises such as the Olympic lifts, plyometrics, and various jumping drills and throws. While these movements won't directly build mass, they'll improve your capacity to stimulate growth by improving your neural efficiency to recruit muscle fibers.
This isn't the end of the line. There are still plenty of principles to cover. Part 3 of Thib's opus magnum will explore ideal training frequency and proper rest intervals.