You really don't want to mess with your Achilles tendons. Trust us, injury to these tendons can take months to heal, and even then recovery is often not complete.
A big reason the Achilles is such a foot-dragger at getting better is that the tendon tissue we have as adults is basically the same as we had when we were teenagers.
That finding was published earlier this week in The FASEB Journal.
But how the researchers figured that out is every bit as interesting as the result.
The scientists used fallout from nuclear bomb tests as biological tracers.
Recall that during the Cold War, above-ground nuclear bomb tests carried out by the United States and the old Soviet Union resulted in a dramatic increase in atmospheric levels of radioactive carbon-14.
According to study co-author Jan Heinemeier, director of the AMS 14C Dating Centre at Denmark's Aarhus University, this "bomb pulse" led to a doubling of carbon-14 in the atmosphere from 1955 to 1963.
Heinemeier says other scientists have taken advantage of the pulse to carbon date all sorts of things. "The method is by no means entirely new," he says, but it hasn't been used much for studying human tissue renewal.
Plants take up carbon-14 from the air, animals eat those plants, and we, in turn, eat those plants and animals. In this way, carbon-14 can end up in all of the tissues of our bodies. By measuring how much carbon-14 is present in certain tissues, scientists can get an idea about when those tissues were formed.
Collaborating with scientists at the University of Copenhagen, Heinemeier looked at the carbon-14 content of tendon tissue samples taken from people who were alive during the bomb pulse.
The researchers found that tendon tissue from people who were children or teenagers then contained high levels of carbon-14 attributable to the bomb blasts.
"What we see in the tendons [is] that they actually have a memory of the bomb pulse," says lead author Katja Heinemeier, a senior researcher at the University of Copenhagen and Jan Heinemeier's daughter.
This means that the tendon tissue in those samples was at least several decades old. From these results, the scientists concluded that tendon tissue renewal is almost nonexistent in adults.
Heinemeier speculates that this is because tendon tissue regeneration is sacrificed in favor of tissue strength. "If [the Achilles tendon] needs to be really strong, maybe you can't afford to be reconstructing it all the time," she says.
Looking forward, she thinks that future research should focus on how to provoke dormant tendon tissue cells to grow in the adult body. (Tendon tissue regeneration has already been successfully carried out in culture.)
Jan Heinemeier has a simpler take away message from this study. "The main recommendation is, don't go and injure your tendon!" he says.