Some researchers have suggested that training in a fatigued state may enhance subsequent adaptations. It has been found that training in a glycogen depleted state enhances gene transcription of several markers of training adaptation (Yeo et al. 2008, Hansen et al. 2005). Low muscle glycogen amplifies the activation of signalling proteins (in particular AMPK and MAPK for those who are interested). Both of these proteins help control gene transcription, which ultimately result in adaptations like increased mitochondria.
In two studies on training every day versus training twice every other day, increases in enzyme activity have been more significantly increased in the twice every other day group. In the study by Hansen et al., they used knee extensor exercises with one leg being trained every day and the other twice every other day (2005). The twice every other day leg should significant better time till exhaustion at the end of the training, along with the increased enzyme activity.
In the study by Yeo et al., they compared two different groups using cycling as the means of training (2008). The groups performed either easy or interval training, with the every day group alternating each day between hard and easy. The twice every other day group performed an easy ride early, then the interval session. In their study glycogen content, fat oxidation, and CS and HAD enzyme activity were higher in the twice every other day group, but performance was equally increased in both groups.
What these studies and their findings suggest is that occasional training in a depleted state may lead to increased adaptations. This shouldn't be surprising if you just understand the basics of training and adaptation. What you are doing when training is putting a certain amount of stress on the body. Then during recovery it responds to that stress by increasing its defenses against that stress. For example in weight training, you tear the muscle fibers slightly while training, and then repair them to even stronger levels during recovery so that it can better withstand that stress the next time.
So it should not be surprising that training in a glycogen depleted state produces more stress than normal. The body then adapts. The thing to remember is that the adaptation is specific to the stress and requires recovery. Without recovery, those adaptations aren't taking place. It's a balancing act.
In practical terms, this might help explain why doing doubles is just as beneficial as singles in certain circumstances. It might also explain why Kenyan runners have success with short periods of training 3x a day. By having a 6am run before their main 10am workout they might be enhancing adaptation to a degree. Similarly, the pre-fatigued idea could explain why Special or Specific blocks of training work in Canova's training.
Another practical application is for long runs. Some long runs may need to be done without fuel intake, especially if training for a marathon. Once again, it's a trade off. If you take fuel, your able to have a better quality workout, but you are not able to push into those levels of glycogen depletion to force fuel source adaptations.
On this note, it's important to remember what training adaptation you are looking for. Training with low glycogen for a long run for example may give adaptations that are good for a marathon runner, but perhaps not for some FT 800m runner. Remember, what you are trying to put in crisis to adapt. A good example of this is in the Yeo study mentioned above with cyclists. Performance change was the same in both groups. However, the twice every other day group increased glycogen content,fat oxidation and enzymes related to substrate use. WHy? Because the group was training in a lower glycogen state. Thus, several of the bodies adaptations were aimed at fixing this problem. Over a 1hr time trial, these adaptations didn't matter, thus the performance was the same between the two groups. It is possible, and perhaps likely, that training with low glycogen stores could result in negative consequences for shorter events.
Once again, athletes and coaches have generally figured out all of this stuff for themselves. Back in the 50's-60's Van Aaken was suggesting that his athletes do a hard day of training with minimal food intake, for example. It's just now that science comes along and explains the mechanisms to why it might work.
The bottom line with all this research is to remember it is simply a process of stress and adaptation. Just be aware of what stress you are putting on the body.
Revisiting Singles vs. Doubles: Evidence from Dathan Ritzenhein