THis CT article explains it quite well:
Chronic adaptations to training simply mean that the structural adaptations to your training regimen will be relatively stable. At this point your body will be perfectly adapted to the training stress you are used to present to your body and thus it will not require further adaptation (read no further progress). At this point most peoples assume that their body has adapted to the exercises they are using and that’s why they change them around. Initially this will bring new strength gains but more often than not this is not correlated with added muscle gains. Why? Well the gains you get from switching exercises are mostly due to and increased neural efficiency at performing the new movement. In other words you initially get stronger in the new movement because you learn to be better at it! This has led peoples to believe that when they stagnate they must change the exercises around. This is erroneous in most cases (sports where relative strength and neuromuscular efficiency is the goal are another animal altogether!).
As it was stated your body adapts to stress - in our case physiological stress in the form of strength training. Your body will adapt itself to the stress placed upon it, not to the means which provide that stress. Your body doesn’t know if you are doing barbell curls or preacher curls, nor does it care to know about it! All that your body “knows” and need to know is that there is a physical stress x placed upon the elbow flexors (biceps brachii, brachioradialis, brachialis and the various forearm muscles). To make an oversimplification out of it, your body only needs the following information to start and modulate the adaptive response:
1) What structures are affected by the stressor?
2) What is the magnitude, or importance, of the stressor on each of the structures?
3) What's the nature of the stress?
Obviously the structures affected by the stressor will depend on the exercise you use (chances are that a squat will cause more stress in the lower body than a bench press!) however because of the structure of your muscles, changing the exercises you perform for a specific body part will not lead to a great variation in the structure affected by the stressor (some recent research indicate that it might be possible to recruit different fibres with different exercises though). The only things that you can vary when you change your exercises around are the muscles involved. For example preachers curls are good to develop peaking biceps because this exercise will place most of the stress on the brachialis (placed under the biceps and which give the impression of a peaked biceps when overdeveloped) not because this exercise recruit different parts of the biceps which lead to the development of a peak.
Thus changing your exercises around can help resume your progress by working previously under worked muscles or by improving the neural factors involved in weight lifting performance. Both of which can be of benefit to athletes and bodybuilders. But once you have changed your exercises so many times that no muscles are under worked in relation to each other and that your nervous system is efficient in all the exercises you do you will not be able to kick start your progress simply by changing the exercises you use.
Probably the most important factor in triggering the adaptive response is the magnitude of the stressor. For easy comprehension's sake we will define the factors involved in the magnitude of the training stress:
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The tension produced (intensity)
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The total duration, or workload at which tension is produced during your workout (volume, either in it's time under tension form, or tonnage form
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The total load of tension (intensity x volume) per unit of time (density)
Muscle strength is exhibited by creating muscular tension. The harder a muscle needs to contract the more tension it must produce. So basically the heavier the resistance, the greater is the required muscular tension. For you scientific minds out there the higher is the muscle tension produced, the greater is the rate of protein degradation (which is one of the factor triggering growth stimulation).
If maximum muscle tension were the only factor involved in developing size and strength we would simply have to do singles in every exercises we do and grow like crazy! Unfortunately (or fortunately for some!) it's not the only factor involved.
The amount of growth stimulated is dependant upon the amount of muscle protein degraded during training. The more muscle protein are degraded the more your body will need to mobilise it's resources and the more it will "rebuild" the muscles to avoid such a stressful (pun intended) situation in the future!
The amount of degraded protein is a function of the rate at which protein is degraded (if you degrade 10x proteins per second you will degrade more protein than if you were to degrade 5x proteins per second all else being equal). And as it was stated the rate of protein degradation is determined by the importance of the muscular tension created. The other important factor involved in determining the total amount of degraded proteins is the duration of the degradation process. Obviously the more time you spend degrading proteins the more proteins will be degraded! This second factor is determined by the volume of training.
A third factor in modulating the adaptive response is the density of training. The more work you perform per unit of time, the more important will be the growth stimulation (this is mostly due to an increase in growth hormone production).
So to resume. To stimulate muscle growth you need:
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High tension contractions
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High total time under tension
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High density of training
As we stated, tension is what is required to produce force. The more force needs to be produced, the more tension your muscles have to create. Now, force is defined as such:
F = MA
In which F means force, M means mass and A means acceleration. In other words you can either increase the force output by:
a) Increasing the weight lifted (lifting heavy loads relatively slowly)
b) Increasing the acceleration/speed (lifting light loads very rapidly)
c) Using an optimal combination of weight and acceleration (moderate loads lifted as fast as possible)
In regular bodybuilding training method a) is the only one currently used. Which means that bodybuilders are only stimulating 33% of the growth they could trigger if they used all three methods!
Increasing the weight lifted (lifting heavy loads relatively slowly)
The first method is already well known of most bodybuilders and powerlifters. It involves increasing the weight that one lifts. Basically there is two ways of making this work.
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Keeping the reps relatively high and trying to increase the weight as often as possible
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Using low reps and very heavy loads
It is generally accepted that point 1. Is the approach to use. We disagree. More weights = more force to be produced = more tension = more stimulation.
I know what you are thinking: “I’ve used low reps in the past, I got stronger but not bigger”. Maybe, but that’s because you forgot that muscle mass is stimulated via 3 factors (tension, total time under tension, density). So if you kept doing the same number of sets when using low reps as you did when you were using high reps you greatly diminished the total time under tension factor which probably negated the benefits of using very heavy weights.
Let us illustrate our point:
If one keep using the same number of sets
High reps: 3 sets of 10 reps with 120lbs for the chest, each rep lasts 4 seconds, the “workout” lasts 12 minutes (total volume: 30 reps, total tonnage 3600lbs, total time under tension 120 seconds, density: 300lbs/min)
Low reps: 3 sets of 3 reps for the chest with 200lbs, each rep lasts 4 seconds, the “workout” lasts 12 minutes (total volume: 9 reps, total tonnage: 1800lbs, total time under tension: 36 seconds, density 150lbs/min)
Difference:
Total volume: -21reps
Total tonnage: -1800lbs
Total time under tension: -84 seconds
Density: -150lbs/min
So in this case using heavier weights will lead to less gain. However if we were to adjust the sets to keep the same volume:
If one adjust the sets
High reps: 3 sets of 10 reps with 120lbs for the chest, each rep lasts 4 seconds, the “workout” lasts 12 minutes (total volume: 30 reps, total tonnage 3600lbs, total time under tension 120 seconds, density: 300lbs/min)
Low reps: 10 sets of 3 reps for the chest with 200lbs, each rep lasts 4 seconds, the “workout” lasts 20 minutes (total volume: 30 reps, total tonnage: 6000lbs, total time under tension: 120 seconds, density: 300lbs/min)
Difference:
Total volume: equal
Total tonnage: +2400lbs
Total time under tension: equal
Density: equal
In this case the second workout will obviously be more effective. When all other things are equal, the workout with the heaviest average weight will always stimulate more growth. Why? Simply because heavier weights increase the “M” in F = MA compared to lighter sets. So it is the premise of the HTT program that the first type of training to include is heavy lifting then adjusting the sets to have a high enough total time under tension to stimulate muscle growth.
Using an optimal combination of weight and acceleration (moderate loads lifted as fast as possible)
As we stated, Force can be increased many ways. One of the best ways to produce a high level of force and tension is to lift moderate loads in an explosive manner. The only movements that allow that type of training are the olympic lifts and their variations. These movements are by far the most powerful lifting movements that one can do and as an added benefit they involve most of the muscles in the body at the same time, with a special emphasis on the legs, lower back, upper back, traps and shoulder muscles. In fact look at elite weightlifters and for the same bodyweight they have the most muscular legs, back and traps or all athletes who train with weights (even bodybuilders). So it really is a wonder why bodybuilders have not picked up on it!
Well to be fair they DID pick up on it WAY back. When elite bodybuilders were still natural, the olympic lifts were a major part of their training. John Grimek who was considered to be the best-developed man before steroids was also a competitive olympic weightlifter (even went to the World Championships), Steve Stanko who won the Mr. Universe when the title still meant something was also a world class olympic lifter. Bodybuilding legend Reg Park (Arnold’s hero and model) regularly used power cleans in his training and I could go on and on.
When you look at the physique of these guys you’ll agree that it’s an example to follow for natural trainee.
. BTW Sorry I missed your call last night, I 'm in tonight…