A study that supports this very well, involved a number of women divided into three groups; the first group would run a set distance of about 40 miles in a week, doing a third of this distance on three different days, running it at their flat out limit. The second group would cover the same distance, but it was spread out over six days, and was run at a more moderate pace. The third group was a control group and did not run at all.

Somewhat surprisingly, only the first group showed strong increases in their HGH bursts or pulse levels. Undoubtedly, the second group would have benefited by resultant strengthening of their cardiovascular system, as well as by gains in muscle tone and by burning off calories. But they did not show the solid HGH gains made by the first group that ran harder and faster.

Of course, as was expected, the control group showed no change in HGH levels.

Other studies have also come up with similar results.
The upshot is, to raise the levels of HGH significantly by means of exercise, strenuous exercise works best.

It is possible that lower body exercises are more effective that upper body workouts for raising HGH levels. If so, that may simply reflect the fact that the quadriceps and other leg muscles are amongst the largest in the body, and for that reason should be more effective in raising HGH levels.

An added benefit to extended, strenuous exercise, is the mild feeling of euphoria that often comes with it, brought on by elevated levels of beta endorphins in the brain. Additionally, these raised levels of endorphins have been shown to promote the secretion of HGH. To what extent this represents the mechanism by which exercise raises the levels of HGH is unknown. In fact, it is little understood at all just why exercise brings about raised levels of HGH. Undoubtedly, this is just one more area currently being researched in this interesting and exciting field.