Hemingway has a great line in the novel: "...the world breaks everyone and afterward many are strong in the broken places." In my book I use this line as a metaphor for a theory of health. I didn't intend to cheapen or dilute a classic quote from a great piece of writing by making it relate to a "fitness strategy."
I wanted to help people understand what hormesis means (and you certainly don't have to remember the latter term at all to stay fit).The concept is that if you have undergone a little pain or hard effort but not too much, the cells of the body will respond adaptively and that this mechanism can make you fitter and more immune to physical or infectious threats later on. Chapter 11 of my book Fitness For Geeks goes into more detail on this notion.
Another more scientific way to put it is "activate an adaptive stress response that raises the resistance of the organism against high doses of the same agent." These actions include:
- cold water swimming or cold thermogenesis, sometimes combined with heat stress like a sauna or thermal bath
- high intensity exercise such as sprinting or lifting heavy weights
- Fasting or caloric restriction
- one or two glasses of wine (one shouldn't conclude however that having a glass of wine inoculates you from the effect of numerous wine glasses!)
The idea is that there is a sweet spot for this kind of fitness pursuit, a preferable dose/response. Too much, and you get weaker, more broken at the broken places so to speak. This is why moderate duration exercise, short-term lifting or sprinting, is generally better than ulta-level exercise, at least from a practical fitness standpoint. I think difficult physical ordeals like an ultra race might be hormesis if you only do them a few times with very long durations between them, but I can't prove that.
Certainly, over the milennia of humanity, societies have concluded that hard physical training to an extent, and mild discomfort such as cold ocean or river swims "harden" the body or organism against infection and premature physical decline.