Detraining: how fast fitness fades when you stop

0:03 · The question

Sooner or later, an injury or a busy month stops your training for a while. Then one question matters: how much fitness will still be there when you come back? The short answer is that each part of your fitness fades on its own clock. In trained runners who stopped for two weeks, VO2max, the most oxygen your body can use, was about four per cent lower. Their VO2max test, which runs until you cannot go on, ended nine per cent sooner. But one situation cost far more than simply stopping, and we come to it in chapter four. Scientists call this detraining: losing part or all of what training built, when you stop or cut back. We follow it through five questions, one per chapter. First, how fast does endurance go?

0:47 · How fast does it go?

In fifteen male endurance athletes, two weeks off cost the same four per cent of VO2max. The American College of Sports Medicine adds that an occasional missed session seems not to lower it. Inside the body, the parts go in order. The blood goes first, and its volume may already fall within two days. Eight trained men stopped for two to four weeks. They had nine per cent less blood, and their heart pumped twelve per cent less with every beat. To keep up, their heart rate at the same effort was eleven per cent higher. The longest study followed seven highly trained people for twelve weeks. Their VO2max was seven per cent lower after twelve days, mostly because the heart pumped less with each beat. By week eight it was fourteen per cent lower, now also because the muscle took up less of the oxygen. At week twelve it stood sixteen per cent lower, a last step small enough to be chance. The enzymes that burn fuel with oxygen in the muscle had made half of their drop within about two weeks. The capillaries, the fine vessels around the muscle fibres, did not clearly decline. After twelve weeks these people still had a higher VO2max than people who had never trained. People who had only recently started training often lost all of their gain after more than four weeks. These were small groups, mostly of men. VO2max is the slow alarm, though. Two studies where people cut back, rather than stopped, show why. Nine well-trained athletes kept one hard session a week for four weeks. Their VO2max stayed where it was. Their time to exhaustion, how long they could hold a steady ride in the lab, fell from seventy nine to sixty two minutes. A test to exhaustion can swing far more than a race does. Ten runners cut their running by two thirds for four weeks, all of it easy, and held their VO2max. Yet they ran five kilometres twelve seconds slower. The runners from the start fit this too: their VO2max test got shorter by more than their VO2max fell. In the other two-week study, both fell by about the same. So endurance starts to fade within about two weeks, and VO2max often shows it later than other tests.

2:57 · Does strength go the same way?

Strength built by lifting mostly went more slowly, though these were other studies, in other people. Twelve power athletes stopped for fourteen days and lost under two per cent on the bench press and squat, small enough to be chance. Only their strength in the lowering phase of a knee extension dropped clearly, by twelve per cent. Nineteen beginners trained one leg for ten weeks, then did nothing for twenty. Their muscle shrank back to its old size. But about sixty per cent of the strength they had gained was still there. The usual explanation is that part of strength is learned: your nervous system keeps knowing how to use the muscle. Not every source agrees: the American College of Sports Medicine writes that strength gains reverse quickly when exercise stops completely. Twenty four trained swimmers cut back or stopped for four weeks. Their strength on a swim bench on land held. Their power in the water fell, by thirteen point six per cent on average. Across a hundred and three studies of people who stopped strength training, strength over many repetitions dropped the most, and explosive power the least. In the studies we read, sprint speed on its own was hardly measured after a break. In endurance athletes, the picture is closer. Fifteen who stopped for two weeks lost five per cent of the strength to straighten the knee, and four per cent of their VO2max. So strength from lifting usually lasted longer, but in endurance athletes, leg strength fell about as fast as VO2max.

4:22 · What kept fitness?

Next, what kept fitness in the studies, and after that, the one situation from the start that cost the most. One lab tested three dials, one at a time. People trained forty minutes a day, six days a week, for ten weeks. Then they turned one dial down for fifteen weeks. Two or four days a week instead of six kept their VO2max. So did thirteen or twenty six minutes instead of forty. At thirteen minutes, though, a long ride to exhaustion got about a tenth shorter. Making the sessions a third lighter did not keep VO2max. Those people had trained for only ten weeks. For athletes, a review found too little data, and points to tapers, the planned cut before a race. For up to four weeks, athletes kept their performance with sixty to ninety per cent less volume. The sessions stayed hard, and they trained nearly as often. Ten well-conditioned runners ran seventy per cent less for three weeks, on five days instead of six, with some hard intervals left. They kept their five kilometre times. Where the number of sessions fell much further, endurance did not hold. Swimmers on three sessions a week instead of six kept their VO2max, and those on one did not. Their power in the water fell in every group. When running is out, other training can stand in. Some trained runners did all of their sessions in the deep end of a pool, running with a flotation belt. They kept their VO2max for six weeks. After four weeks of deep-water running only, other runners' five kilometre time on a treadmill had not changed. None of them was injured, though. They were healthy runners asked to switch. Other people had a forearm in a cast for four weeks, and some of them trained the other arm. They lost two point four per cent of strength in the same muscle of the cast arm. Without that training, they lost twenty one point six per cent. So what held fitness was fewer or shorter sessions, kept hard, and for athletes not many fewer.

6:17 · How still are you?

Every study so far had people who could still use both legs. Now, what happens when you can't. A cast hits the muscle under it hard. In young men who had not lifted weights for six months, one leg went into a full cast. Strength fell nine per cent in one group after five days, and twenty three per cent in another after fourteen. The power athletes who simply stopped for fourteen days lost seven per cent in a held knee test, small enough to be chance. Only in the lowering phase did they lose more, twelve per cent. In one small trial, a boot on one ankle did little to the heart. Two weeks on crutches barely changed VO2max. Their peak power in the test still fell, by about ten watts. The authors add that their study may have been too small to see a drop in VO2max. For your fitness, what an injury still lets you move matters more than its name. Whether it is safe to train with an injury is a question for whoever treats it. And then there is the whole body. In nineteen sixty six, five healthy twenty year old men spent three weeks in bed in Dallas. One of them had been sedentary, another ran up to seventy five miles a week. Their VO2max fell twenty seven per cent. Trained people who simply stopped had lost seven per cent at three weeks. Thirty years later, the researchers tested the same men again. Three weeks of bed rest at twenty had cost them more cardiovascular capacity than thirty years of ageing. That is the situation from the start of this video. In these studies, lying still cost the most.

7:50 · How fast does it come back?

Last, and briefly, the way back. For endurance, it was not faster than the loss, in the few studies that tracked it. Twenty three young people trained one leg for twelve weeks, and nine months later its gene activity showed no clear memory of it. After surgery, a masters cyclist was off for four weeks, the first two mostly lying down. She lost about a quarter of her VO2max. It took six weeks of rebuilding to get it back. Her rebuilding started at about forty per cent of her usual training, so the timeline reflects her plan as much as her body. A triathlete stopped for twelve weeks. When he was measured after twelve weeks of rebuilding, his VO2max was back. His running economy, the oxygen his pace cost, was still seventeen point seven per cent worse, though he had run less while rebuilding. For strength, part of the gain never left, so there was less to win back. So a short break cost a few per cent of VO2max and more on a test to exhaustion,

8:41 · What this leaves you with

and lying still cost far more. Every number in this video, with its source, is on enduranceproof dot com. Still to come on this channel: running economy, how much oxygen your pace costs.