Endurance is not only your heart and lungs

Researchers had 16 men, eight young and eight older, ride a bike three times a week for 12 weeks. They measured the most oxygen each man's body could use in a minute, a common measure of the engine. It had risen within three weeks. By the end it was up 18 percent in the young men and 31 percent in the older ones.

A tendon is a rope of collagen, a tough protein, that ties a muscle to a bone. Every stride, pedal stroke and swim stroke pulls on it.

After a session, the cells in a tendon break down some of its collagen and build new collagen. The breakdown peaks first. The building peaks about a day after the session and stays raised for about three days. So each session adds only a little, and the adding happens in the days after it.

Researchers pooled 27 studies of healthy adults who trained their tendons for eight weeks or more, mostly with heavy work for the calf and the front of the thigh. The tendons got clearly stiffer, which means they lengthened less under the same pull. They grew only a little thicker. Heavier loads made them stiffer than lighter ones.

The middle of the tendon barely changes at all. Researchers in Copenhagen dated the cores of 28 Achilles tendons, taken at autopsies, using a trace of carbon that nuclear bomb tests left in the air in the 1950s and 1960s. Each core dated from roughly its owner's first 17 years of life. Muscle dated the same way had been renewed all along. The core of an adult's Achilles was built while they grew.

Muscle can also outgrow its tendon, and young athletes show it. Researchers compared 19 teenage elite volleyball players with 18 middle-aged players who had once played at the elite level and still competed. Both groups were equally strong. But the teenagers' tendons were thinner, so the same pull was spread over less tendon. Every part of their tendon carried about a quarter more load. Their muscles had grown strong faster than their tendons.

Bone is rebuilt in small patches. When a new load leaves tiny cracks, bone cells clear out the damaged bone within ten to 14 days. Filling the gap with new mineral takes weeks to months, and full strength can take up to a year. In between, the bone has more holes in it than before.

Researchers counted the stress fractures in 701,027 US soldiers after they joined. The fractures climbed steeply from the third week and peaked in weeks five to eight. Those are the weeks when the damaged bone has been cleared away and the new bone is not yet in.

Cartilage is the smooth layer over the ends of the bones in a joint, and it changes with load too. In one trial, middle-aged adults had had surgery on a cartilage pad in the knee, which raises the risk of arthritis. Some of them then did four months of moderate exercise. An MRI measure of the make-up of their knee cartilage improved, compared with people who did not exercise.

So a few weeks into a new sport, your breathing tells you to go further than your shins, knees or shoulders can take.

On the cyclist's first run, his heart, his lungs and his thigh muscles had room to spare. The inside of his shin did not, and that is where the pain came.

Researchers describe a running injury in just those terms: the load on one tissue becomes greater than that tissue's capacity. It is rarely one bad step. It is the same load, repeated, past what one tissue can take.

That gap is a major cause of endurance injuries, and the research on runners shows it. Pooled studies found that novice runners were injured 17.8 times per 1,000 hours of running, and regular recreational runners 7.7, less than half as often. In a review of studies, four of the five most common running injuries were a sore Achilles tendon, shin splints, runner's knee and plantar fasciitis, which is pain in the band of tissue under the foot. Each of those four is a tendon, a bone, a joint or that band under the foot, taking more load than it could.

Riding carries your weight on the saddle, so it builds little bone. Adult road cyclists who train regularly often have low bone density in the spine. Ten years on the bike had given the cyclist a strong engine. They had done little for the bones of his lower leg, which take the shock of every running stride.

Swimming puts its load through the shoulder, stroke after stroke. Researchers studied 80 young elite swimmers, aged 13 to 25, training at the top level of the sport. 91 percent had had shoulder pain, and 69 percent of those scanned had an irritated, thickened supraspinatus, one of the rotator-cuff tendons that hold the shoulder in place. The more hours they swam each week, the more likely they were to have it.

So a frame built in one sport is strong where that sport loads it, and no further. A new sport loads tissues that have not yet had months to change.

Eight elite runners had thicker Achilles tendons than nine elite kayakers, along the whole length of the tendon, for their body size. A kayaker sits in the boat, and his Achilles does little work. Among athletes and inactive people aged 35 to 94, the distance runners had stronger shin bones than the inactive people, and the athletes in the short, fast events had stronger ones still.

The years before a new load count too. In three studies of Israeli infantry recruits, those who had played ball sports regularly for at least two years before they joined had fewer stress fractures in basic training. In two of the studies they had less than half as many.

Even years have a limit. In a footrace across Europe, 4,486 kilometres in 64 stages, about 70 kilometres a stage, researchers scanned 44 runners as they went. These were veterans, with about 18 years of running behind them on average. Of the 14 scanned runners who quit, ten quit with overuse injuries in the soft tissues of their legs, and two had stress fractures.

The long, hard challenges, the mountain marathon, the swim to the island, the week on the bike over the passes, take an engine and a frame. The engine starts to rise within weeks. The frame comes from years of steady training, and how to build it without breaking it comes later in the book.