Junk Miles

On Saturday, Tyler doubled at the BU mini-meet, racing a little less than 3 miles total. He also warmed up about 3 miles for both races, and afterwords, cooled down with another 5 easy miles, for 11 more miles total and a 14-mile day.

On Sunday, I scolded him for all those "junk miles," but he challenged me right back by pointing out that I sometimes did laughably easy 3-mile runs in the morning when I was in the habit of doubling. What about those? Were they also junk miles? What exactly is a "junk mile" and how does it differ from a mile of greater value?

Well, having had a day to think about it, here's my answer: I don't know.

If we were talking about junk food, I'd say that what makes a food "junk" is the high proportion of "empty" calories -- fat or sugar without the other nutrients the body needs. A Big Mac has a lot of fat -- way too much fat for the amount of protein it delivers.

I guess the analogy to junk miles would be that a mile run too slowly provides too much impact stress without the intensity to produce useful training adaptation. Hence, the ubiquitous advice to do long runs at about 70-75% of max heart rate. Hence, also, the famous statement that "long slow distance makes long slow runners."

But missing in this perspective is the idea that running slowly might provide some other benefit, some other training stimulus that justifies the time spent trotting along at a leisurely pace.

Yesterday, I would have argued that the main benefit of a really slow run was that it promoted recovery from all the harder runs where the real training took place. But in searching the web for definitions of junk miles, I ran across the following article in Running Times by Matt Fitzgerald, in which he debunks this line of thinking:

Rethinking Junk Miles

Fitzgerald writes:

"It is widely assumed that the purpose of recovery runs [...] is to facilitate recovery from preceding hard training. You hear coaches talk about how recovery runs increase blood flow to the legs, clearing away lactic acid, and so forth. The truth is that lactic acid levels return to normal within an hour after even the most brutal workouts. Nor does lactic acid cause muscle fatigue in the first place. Nor is there any evidence that the sort of light activity that a recovery run entails promotes muscle tissue repair, glycogen replenishment, or any other physiological response that actually is relevant to muscle recovery. In short, recovery runs do not enhance recovery."

But instead of dismissing recovery runs, or slow miles in general, as useless, Fitzgerald goes on to argue that such runs promote running economy by providing an opportunity to "practice" running when still in a state of fatigue from previous hard efforts. While more intense workouts are necessary for maximizing aerobic and anaerobic capacity, volume (mileage) is useful for improving economy -- the ability to run farther and faster with the same energy expenditure.

Training is thus the art of finding the right balance between intensity and volume -- between the hard workouts and the recovery runs, between capacity and efficiency -- and, not surprisingly, this balance will be different for every runner.

So, back to our original question, what is a "junk mile?"

Well, in truth, we need to judge a mile on what result it produces. In fact, a junk mile might be slow or fast because it's really just a mile that serves no purpose but to pad the training log. Jack Daniels even talks about "junk speed" -- speedwork done at paces that aren't beneficial to improving specific energy systems.

I would argue that a slow run or slow mile can serve many purposes without being junk: it could get you to work or school, it could renew your connection with the outside world, it could be part of your social life. And yes, for some, slow runs can help make you faster.

I still want Tyler to think about all those miles he ran on Saturday and ask himself whether they were necessary or even useful. And I'll try to tell him that my early morning runs at 8:00 pace were useful -- to me, at least -- because they were correlated with some of my best racing in years.

But I'll also try to keep an open mind, and won't be so quick to call his miles "junk." I'll just suggest that he should judge a mile based on where it takes him.