The reason HST calls for more frequent training is because the acute anabolic effects of training, such as increased protein synthesis, muscle-specific IGF-1 expression, and other factors involved in modulation of short term protein synthesis, only last for 36-48 hours. There is also mounting evidence of a "summation" effect by exercising while levels of these signals and responses are elevated, as should be expected.
This does not mean that the structural repairs to the tissue have been completed. Research has demonstrated that you can train a muscle before it is fully recovered structurally and not inhibit its ability to continue to recover. So, HST uses this evidence and calls for repeated loading (training) every 48 hours or so to keep the anabolic activity of the muscle high, while trying to stay slightly ahead of the structural recovery curve by constantly increasing the load each workout. Staying ahead of the structural recovery curve is really key to elicit real growth in a person who has lifted for quite a while. Of course, injuries can develop over time if care isn't taken to take time to heal, and prepare the tendons for repeated heavy bouts of lifting (SD and 15s serve this purpose in HST).
"Recovery" can refer to several different things.
1) "Recovery" can refer to the structural repair process of fixing the microtrauma. The damaged proteins can takes several days to be repaired and all evidence of damage removed. Even at the end of seven days after significant muscle damage from eccentric muscle actions, you may still see some small fibers regenerating.
2) Strength - this can be acute recovery as in the necessary time to rest between sets. Or it can mean the days that it usually takes to regain baseline strength after muscle damaging exercise.
So the trick is to have the CNS "recover" just in time to hit the muscle again as the acute anabolic effects are wearing off. That way you can stay anabolic more of the time. Training once every 7 days will still allow you to grow, it just takes longer for the gains to accumulate. Training more frequently is more efficient if your goal is just to get bigger
To understand, you have to consider the total volume over time. A week is easiest to consider, so, over the course of a week, it is the total volume that is important. So 9 total sets for chest can be done in one workout or in several workouts. Both will stimulate growth. However, you will be anabolic more of the time if you can actually create that stimulus more often. In the case of HST, 3 times as often. There is a physiological benefit (acute anabolic effects of training) in doing 9 sets as 3 sets X 3 workouts, as opposed to 9 sets all at once - and then nothing for the next 7 days.
