People think an Achilles heel is a weak spot.

But there’s practically no tougher tissue in the body. Think of the forces involved in hopping, alone. The Achilles tendon handles hundreds of pounds as it bridges the bones of the heel and the calf muscle.

Imagine the strain it receives during explosive athletic activities. Much of the time it is as reliable as a steel cable, even during gymnastics, sprints, and dashes to the end zone. But even the toughest material can be pushed to the limit.

The Achilles tendon is no exception, especially at the start of explosive movements, when it is most vulnerable. If it tears, healing can be a difficult process. Injuries sometimes require surgery and can take a year to get better.

The problem is each time the calf muscle contracts or relaxes, the tendon is stretched, compounding the damage. It doesn’t help that the area does not receive nearly as much oxygen-rich blood to promote healing as its neighbor calf muscle.

Now, on top of all that, researchers in Denmark think they have discovered a new reason why it takes so long for Achilles injuries to heal.

They studied the Achilles tendons of people who lived in the early 1960s, when nuclear weapons testing created a spike of a radioactive atom called carbon-14 in the atmosphere … which registered in the tissues of living things.

Decades later, the carbon-14 signatures found in study subjects’ tendons had not changed, almost as if the building blocks of the tendons hibernate in comparison with other body tissues.

The scientists think if there were a way to wake up the tendon’s sleeping regenerative cells, it could speed the recovery process … and healing the Achilles’ heel might not be so tough after all.