By Gavin Laird:

Single factor training.
Probably 99% of ordinary people in gyms are currently training according to single factor training theory, or the principle of supercompensation. Probably 5% of elite strength athletes are training this way and they are all bodybuilders. Now I know most people are not even aware of what dual factor theory is so here is a brief explanation.

Single factor theory treats fitness and fatigue as existing to the exclusion of each other. For example if you are tired and have sore muscles following a training session you should wait until you feel better and have fully recovered before training again. This fits in with supercompensation theory, which dictates that after training your fitness decreases slightly (because you are tired) and then rises back up again to a point just above where it was prior to the workout. At this point you train again with a slightly greater load and push up your fitness a little further and so on.

Dual factor theory looks at fitness, fatigue and preparedness as being separate but not exclusive to one another. Fitness is your long-term ability; it changes slowly and is not related to fatigue. Preparedness is your immediate ability i.e. what can you do RIGHT NOW and it is influenced by fatigue. According to dual factor theory you can train to the point of extreme fatigue, and have a terrible state of preparedness but still be making improvements in long-term fitness. In other words you DO NOT have to fully recover between workouts all the time and nor should you.

Dual factor training requires periods of stimulating (high) loads, retaining (moderate) and detraining (low) loads in the long term but it removes the need for an athlete to time each individual workout in accordance with fatigue levels. The reason that dual factor training is so unused by bodybuilders is number 3 on my list....

3. Intensity intensity...
Bodybuilders like to train "hard". They boast of training to "failure", doing "triple drop sets", "forced reps" and all kinds of other extremely fatiguing techniques. The problem with this is that although their musculature may recover from this onslaught in a few days their central nervous systems are absolutely fried. The CNS can take a week or more to recover from this kind of repeated efforts to failure training, which makes repeating the workouts with a similar or greater (stimulating) load impossible for quite some time. Why oh why oh why would anyone want to do this? Your muscles recover from almost any stimulus within 72 hours but if you have stressed the CNS so greatly that it can no longer apply any force then you will become detrained as the CNS recovers. By the time your preparedness is back up to a high level the fitness gain from training has almost completely gone. This is OK in the short term but to train like this week in week out whilst attempting to increase poundage's or total load in a linear manner is a lunacy that literally forces you to reduce training frequency and total load to a minimal level. Frequency and total load are the key determinants of successful training for size and strength! Why would anyone deliberately minimise both of them?