During aerobic exercise, most of the energy your muscle cells need is supplied by the mitochondria, a cellular component often referred to as the "powerhouse" of the cell. That’s why one of the key benefits of training is that you develop more and better-functioning mitochondria to fuel your efforts.
At last week’s American College of Sports Medicine annual meeting, one of the most interesting sessions I went to focused on how mitochondria respond to training. The central question posed in a talk by David Bishop, of Victoria University in Australia, was: Is there an optimal training stimulus to trigger the formation of new and more powerful mitochondria? (The slides from the presentation are available here.)
As Bishop and his colleagues explained in a review a few years ago, there are two linked questions. First, how much new mitochondria can your cells grow? And second, how efficiently can each unit of mitochondria supply the energy needed for muscular contractions?
The safest answer is that we don’t know for sure at this point. But several studies in both rats and humans, including one published by Bishop and his colleagues earlier this year, suggest the following rule of thumb: How much you train determines how much mitochondria your cells will contain; and how intensely you train determines how powerful each unit of mitochondria is.
In the recent study, neither continuous sub-threshold training nor long interval workouts of 4-7 x 4:00 at 90 percent of peak power (the highest power reached in a VO2max test on a stationary bike) changed mitochondrial function, but sprint workouts of 4-10 x 30 seconds at 200 percent of peak power of did boost mitochondrial function. You needed that very high intensity to boost the output of each mitochondria. On the other hand, none of the workouts, done three times a week, increased the overall amount of mitochondria after four weeks.
In contrast, a more recent unpublished study by the same group put subjects through interval workouts twice a day, seven days a week (!), for three weeks. This time, a marker of the amount of mitochondria increased by 40 percent. To get more mitochondria, in other words, you need to accumulate volume.
As Bishop pointed out during the talk, these results fit nicely with the “polarized training” paradigm observed by Stephen Seiler—the idea that top endurance athletes generally accumulate high volumes of training at low intensities, but also include a small amount, perhaps 10 to 20 percent of the total, at very high intensities. That combination of high volume and high intensity (but not both at once) presumably gives athletes huge amounts of very potent mitochondria.
Of course, when you hear a nice, tidy story like this that seems to make perfect sense, warning bells should be ringing. And sure enough, not everyone agrees with this picture.
I had a chance to chat later in the conference with Martin MacInnis, a postdoc in Martin Gibala’s lab at McMaster University. MacInnis was presenting some new data at the conference that compared 30 minutes of continuous training with an interval workout of 4 x 5:00 hard at 65 percent of peak power with 2:30 recovery, with the intensities chosen so that both groups did the same amount of total work.
One of the cool things about this study is that the workouts weren’t assigned to different people—instead, they were assigned to different legs on the same people! By using single-leg cycling, all the other extraneous factors, from genetic variation to stress and diet, were held constant in each comparison.
After six workouts over two weeks, the interval leg saw bigger improvements in both the amount of mitochondria and how well each mitochondria functioned compared to the continuous-cycling leg. That fits with the idea that intensity is what triggers improvements in function, but it doesn’t fit with the idea that the amount of mitochondria depends on the volume of training, since both legs completed the same amount of work.
How do we reconcile this seeming contradiction? More studies, I guess. Ultimately, as I’m sure Bishop would agree, the picture is never going to be quite as simple as “intensity does only one thing and volume does only another,” but it may still turn out that the general trend holds true.
One final postscript: While I was waiting to chat to MacInnis at his poster, a Japanese researcher was asking him lots of questions. Eventually, the researcher pulled out his card, and I saw MacInnis’s eyebrows rise by an inch. I peeked over his shoulder, and mine did the same when I saw the name on the card: Izumi Tabata. Yes, that Tabata, he of the 20-seconds-hard, 10-seconds-easy Tabata Protocol. It was a cool moment, sort of like going to a car show and bumping into Henry Ford, and suddenly remembering that behind the famous name is an actual person.
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