After last year's hot, sunny Boston Marathon, a team of researchers at Massachusetts General Hospital asked 22 finishers to undertake an even more daunting challenge: Rest.
The goal was to track the cardiovascular changes that occur when trained runners undergo "exercise abstinence" for a period of eight weeks. "This area is difficult to study," lead author Charles Pedlar explained, "since most runners don't want to take a long break."
While most of us focus on the need to exercise more, there's a growing realization that rest – more than just a day or two off, in some cases – also plays a crucial role in maintaining good health and optimal performance. But figuring out how much rest is enough (or too much) isn't easy, since different parts of the body and mind recover at different rates.
Encouragingly, the results of Pedlar's study, which were presented earlier this month at the annual meeting of the American College of Sports Medicine in Denver, suggest that fears of totally losing fitness after a few weeks are overblown.
All the runners in the study had followed a structured 18-week training program, running for seven to eight hours each week. Then came the supposedly easy part: For the following eight weeks, they exercised for no more than two hours a week.
During the rest period, they underwent a series of performance tests, blood analyses and heart imaging in the fourth and eighth weeks. One of the key parameters the researchers measured was hemoglobin, the protein in red blood cells that transports oxygen from the lungs to the muscles. Under normal circumstances, your bone marrow creates about two million new red blood cells a second to balance out the removal and destruction of old ones. This balance, Pedlar said, is crucial to maintaining aerobic fitness – and the fitness-boosting flow of new red blood cells is augmented by exercise.
The biggest surprise in the new results was that total hemoglobin stayed roughly constant throughout the eight weeks, despite the enforced rest. This suggests that the runners' bodies responded to the lack of training stimulus by allowing older blood cells to stay in circulation longer, in order to "defend" their hard-earned fitness levels.
That doesn't mean detraining had no effects. By four weeks postmarathon, the volume of blood plasma, the liquid that carries red blood cells around, had declined. So too had heart-wall thickness and performance in a time-to-exhaustion test – but the declines were "only relatively modest" considering the length of the break, Pedlar said: "Runners can be encouraged by our findings."
Why might a break lasting weeks rather than days be beneficial? Most of us gauge our recovery by assessing how our muscles feel – but that's not necessarily the right indicator.
Guillaume Millet, a researcher at the University of Calgary, has led a series of studies on "neuromuscular" recovery after some of the toughest ultramarathons in the world. He and his colleagues use electric stimulation to evoke involuntary muscle twitches, in order to measure the lingering effects of fatigue in the central nervous system as well as the muscles themselves.
In one study of finishers of the Ultra-Trail du Mont-Blanc, a 166-kilometre "extreme mountain ultramarathon," Millet found that most measures of neuromuscular fatigue had returned to normal within nine days of the race.
"Does that mean we are ready to run another UTMB nine days after the first one?" he asked. "Certainly not."
Other factors, such as body-wide inflammation, a response that varies widely among individuals, could mean that some people require much longer before they're ready to resume full training.
And beyond the physiological benefits, occasional extended breaks have equally powerful psychological effects, said Brad Stulberg, the co-author, with Steve Magness, of the new book Peak Performance: Elevate Your Game, Avoid Burnout, and Thrive with the New Science of Success.
Stulberg cited the example of American runner Bernard Lagat, a two-time Olympic medalist who, in 2016, at the age of 41, competed in his fifth Olympics. One of the secrets to his longevity? Every fall since his professional debut in 1999, Lagat has taken five full weeks off running to recharge physically and mentally.
Not everyone needs a five-week festival of sloth, of course. "Context matters," Stulberg said. "The longer and harder your season, the longer the break."
Less appreciated, Stulberg said, is the fact that the brain also "recovers" from hard work during periods of rest, sorting through, connecting and consolidating information and emotions. In a workplace context, he said, studies have found that breaks of seven to 10 days can ward off symptoms of burnout.
In theory, then, an annual or semi-annual break of at least one to two weeks from your usual training or work routine is a no-brainer. But in practice, one thing marathoners – and executives – have in common is their unwillingness to step off the treadmill.
Pedlar's results offer a timely reminder that if you've put in a period of intense work, those gains won't evaporate in a few weeks. Instead, by taking a break, then resuming training with renewed energy and vigour, you'll be ready to push past your previous limits.
Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience