It's impossible to keep up with all the scientific articles about training, but sometimes one experiment leads to a blog post, then a magazine article, then a training fad, then a book and then several years of trying to explain why it's mostly nonsense. Still, I go back and read the original work. Here's why it's usually pointless.
Experiments are generally done at universities, so the people available for the studies are 18-22 years old and generally fit. Decreasing the number of variables helps in getting publishable results, but while the general populace fits a normal bell curve, the studies are looking at only the exceptions at one end of the curve. But maybe you're in that small section because runners are self-selective; most sports require 100% effort for seconds or minutes, but those who are poor at those find themselves doing well in endurance sports.
Studies generally go for 6-12 weeks, because that's as long as you can get volunteers. No one addresses the fact that the results may not be constant, that runners tend to train for months and years. Almost anything that can be tried has been tried, repeatedly, by a large number of people, over the past century of running. If there were something revolutionary in training, everyone would've switched over eventually. How many elite runners do you find doing training significantly different from others (and, if you're going to say "Imagine how much better they'd be if they switched?" I can tell you that it's been at least looked at, and probably tried and quickly abandoned)?
Is it reproducible? Researchers can only get funding for original work, so studies don't get done twice, but that's an important factor - the original results might have been a fluke. Thus, a good study has the participants go through whatever routine they prescribe, then switch to something else, then go back, to see if the results are reproducible. This almost never happens.
A typical study will have one group of runners do one exercise and a second group do another, with their average results compared. Let's say group A improves an average of 5% in some measure and group B improves 8%. You'll see headlines that say that B is better than A. The truth is usually more individualistic. Submit any group of runners to a new exercise and you will have most making a minor improvement, but there will also be super-responders who do unusually well and non-responders who do unusually poorly. Super-responders are the people who tell you that they've "found the secret to training success" and write a book, become coaches and get paid to speak at conventions. Non-responders might simply be over-trained or injured, but also might be predisposed to do poorly at that task.
Sometimes what works at one point in your running career doesn't work in another. A classic example is the soccer midfielder who starts running to improve his endurance and then finds that he's (or she's) a talented runner. Having this person do sprint training will not improve their performance, because they've done so much sprinting in their other sport. Over time, however, that effect diminishes and, their training and racing stagnating, they might need to do sprint work.
Still, sometimes there's an interesting idea involved. That's why I read the articles. Most runners, though, just want to know what's the most efficient use of their training time - or "what's the absolute least I have to do to meet my goals?" which is the question coaches hate the most.
If you really want a simple answer to improved performance, I find most people run best when they've had adequate sleep. If you're only sleeping 4-5 hours per night, there's nothing you can do in training that will help more than sleeping more.
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