Every muscle cell holds a long DNA molecule coiled into a tight ball. Sections of that ball can be open or closed. Open sections work like buttons on a remote control: they let the cell switch on the genes it needs. With age, some of those buttons close, and the muscle stops working properly.
Scientists took muscle samples from 287 people aged 20 to 79 and looked at which DNA sections close with age. It turned out that the sections that catch hormone signals are the ones that close most often. Hormones are substances that tell a muscle to grow and to repair itself after exercise. If their "receivers" are closed, the muscle can't hear the commands and grows weaker.
Until now it was unclear which DNA sections change as muscles age. Now it's clear that it's not the genes themselves that close, but their switches. It's as if the batteries were pulled out of the remote: the buttons are still there, but the signal doesn't get through. These findings will help doctors better understand why muscles weaken with age and, perhaps, find ways to slow it down.
The chromatin regions that close with age are most often far from genes (more than 5 thousand base pairs from the transcription start site), meaning aging hits the 'control panels,' not the genes themselves.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.