January 2012 Journal of Physiology.
The article primarily discusses two different modes of sprinting, and their positive effects on the body: 1) Thirty seconds of all-out cycling (or sprinting), 4 to 6 repetitions, with four minutes of rest in between reps. This is called the Wingate Test; 2) 10 times 60-second bursts at about 90 percent of maximum heart rate, with one minute rests between each rep.
The authors describe the first one, the Wingate Test, as "extremely demanding and [possibly] not ... safe, tolerable or appealing for some individuals." Therefore, they designed the second protocol to be easier for people who are just getting back into exercise or coming off a disease.
Personally, the first protocol almost seems easier because of fewer reps and longer rests. It, in turn, is easier than the Tabata protocol described in the book and many other places: 8 20-second sprints all-out with just 10-second rests. As I get into Springtime sprinting, I'll have to put these two protocols (other than Tabata, which has kicked my butt before) to the test.
At any rate, the training gains from these protocols are remarkable, as they usually are when they show up in these scientific journals. When compared with endurance training that typically takes many hours per week, the HIT sessions produced "an increased capacity for whole-body and skeletal muscle lipid oxidation, enhanced peripheral vascular structure and function, improved exercise performance as measured by time-to-exhaustion tests or time trials and increased maximal oxygen uptake."
Translation: you can burn fats and utilize oxygen better, and you can go longer and harder than before.
serve as an effective alternate to traditional endurance training, inducing similar or even
superior changes in a range of physiological, performance and health-related markers in
both healthy individuals and diseased populations.
Among many other health benefits, the article says that HIT also activates a protein in your muscle cells called PGC-1a. This protein or protein complex helps control fat metabolism and the building of more mitochondria in your muscle cells (the scientific term being "mitochondrial biogenesis").
Interestingly, to those of us who do cold outdoor exercise (and are looking for some benefits from it!), PGC-1a is also induced by cold exposure. It plays a key role in adaptive thermogenesis, by which you burn calories to stay warm.
A typically unmentioned benefit of sprint intervals in these articles is that they help make you faster, particularly compared with slow endurance training. Sprinting helps you regain your former speed, or the legacy that birth bequeaths most of us as humans.
You don't need to be faster, you say? It's true, modern life does coddle us. As Laurence Gonzales put it in his book Deep Survival, we're like fish in an aquarium. The oxygen bubbles up to us, the food comes down. Having more speed, however, might save your or a loved one's, or a stranger's, life some day. Less fatally, foot speed might help you catch up to that person you have a crush on, who's way ahead of you in the subway station, and that would change your life!