Slow training builds what fast training cannot

It is Sunday morning, and you ride out along the river for two hours. You never push. You chat with a friend for most of the way, and stop for a coffee in the next village. At home, nothing hurts. Your legs feel better than when you set off.

It felt like nothing. It was not nothing. All morning, you were building roads in your muscles.

01

Your muscles are a city.

Every muscle is made of thousands of long, thin fibres. Each one needs oxygen and fuel delivered to its door, and its waste carried away. Your blood does the delivering. The heart is the pump, and the arteries are the motorways.

The last stretch is the one that counts. Capillaries are the tiny vessels that run between the fibres, the side streets that reach each door. Inside each fibre sit the mitochondria, the small power plants that turn oxygen and fuel into movement. More streets and more power plants mean more work done, for longer, at the same effort.

02

Steady hours build the streets.

In France, eleven unfit adults in their forties rode a bike for eight weeks. They took three months off, then rode for eight more weeks. One block was steady riding at one even effort. The other was intervals: hard, then easy, again and again. Both blocks did the same amount of work.

Both helped. But steady riding grew the capillaries twice as much as the intervals did. The steady effort built more streets.

The power plants follow the hours too. When researchers in Australia reviewed the training studies on mitochondria, they found that the amount of training was what decided how many a muscle grew. How hard you go changes how well each one works. How long you go decides how many you have.

03

A steady hour leaves marks inside the muscle.

Picture the inside of your thigh an hour into that Sunday ride. Blood is streaming through the capillaries, far more than at rest, and it rubs along their thin walls. Each pedal stroke stretches the fibres around them.

That rubbing and stretching is a signal. The muscle fibres keep a growth signal called VEGF packed in tiny pockets, and while they work they release it. VEGF reaches the walls of the capillaries and tells them to grow. New side streets sprout between the fibres. The longer the blood keeps flowing, the longer the signal keeps coming, and base is the effort that keeps it flowing for hours.

The power plants change what the same ride feels like. When the American physiologists John Holloszy and Edward Coyle pulled together what endurance training does to a muscle, one change sat at the centre. A trained muscle has more mitochondria. So at the same speed, each one works less hard, and the muscle is less shaken by the work. It burns its stored sugar more slowly. It burns more fat. It makes less lactate.

That is why the same climb feels gentler after a season of base hours. The hill did not change. The work was shared among more power plants, fed by more streets.

04

Base work runs on fat.

Your body carries two fuels. Sugar is stored in small amounts, and goes fast. Fat is stored all over your body, in far larger amounts.

In England, eighteen cyclists rode at efforts from very gentle to all out. Their muscles burned the most fat at a steady, unhurried effort. Above it, fat burning dropped sharply. Near their hardest efforts, they burned almost none.

Every base hour, then, is practice at burning fat. And the muscle gets better at it. In St Louis, nine men did the same long ride before and after twelve weeks of endurance training. At the same effort, the share of energy that came from fat rose from 35 to 57 percent. They used 41 percent less of their stored sugar. Burning more fat leaves more sugar for the hill at the end.

05

The body was built for the hours.

None of this is new to the human body. The hunter in the Kalahari follows the kudu for three and a half to five hours. Aché men spent almost seven hours a day on foot in the forest. Most of it was not hard. It was steady movement across the land, all day, every day.

That is how people used to get their endurance. Daily life supplied it, in long, steady hours. Base is the only effort you can keep up that long, and come back to the next day. So base is how you pile up the hours, and the hours are what build the roads.

06

The best spend most of their hours at base.

Norway's rowers have won international medals for decades. Researchers asked the medal winners from 1970 to 2001 how they had trained. From the 1970s to the 1990s, their base hours rose by about two thirds. Their hours at race pace and above fell to under a third of what they had been. Over the same years, the largest amount of oxygen they could use rose by 12 percent.

They did not get better by going harder. They got better by doing more, and most of the more was base. Across endurance sports, studies of elite athletes find the same pattern: most of their training is base, with a few hard sessions placed with care.

07

Too fast spoils it.

Between base and hard there is a middle effort. It feels like good work. You come home tired, and tired feels like progress. But it is too hard to keep up for hours, day after day. And it is too easy to build what the hard sessions build.

In Spain, twelve good runners trained for five months. Both groups did the same amount of hard running. One group did more of the rest at base. The other did more of it in the middle. Over a 10.4-kilometre cross-country race, the base group improved by 157 seconds, and the middle group by about 121.

The middle effort also cuts the hours short. It tires you too much to go long, or to come back tomorrow. So the hours are fewer, the growth signal comes for less time, and fewer roads get built. And it burns sugar where base would burn fat, so the muscle gets less practice at the fuel that lasts.

The roads are the ground the harder work stands on. The effort you can sustain, your threshold, is built on top of them, and so is your top speed. Every layer above rests on the one below.

So keep your base run, base ride or base swim gentle enough that you come home better than you left. It is not the day off between the real sessions. It is the morning along the river, building the roads your threshold and top speed will run on.

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