Quote[/b] ]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).