The start of a new lifting program leaves your quads quivering, shoulders shaking, and pretty much any other muscle group super sore. The same goes for your first time back in the gym after a short- or long-term hiatus. It’s a sign of progress and growth. But after a few sessions, you’re not getting sore any more. Why?
Researchers from Brigham Young University call the reduced-soreness phenomenon the “repeated bout effect.” And while they don’t know precisely why you feel less sore the second go-around, they do have a pretty good idea.
Our immune system is at play, helping our muscles repair themselves and protect against further damage. But more specifically, it’s our T-cells that flock to mend our muscles. T-cells are a type of white blood cell produced in your thymus gland that are absolutely essential to your immune response.
"You think of T-cells as responding to infections, not repairing muscles—but we found a significant accumulation of T-cells infiltrating damaged muscle fibers," study author Robert Hyldahl said in a press release. "Our study is the first to show T-cells present in human muscle in response to exercise-induced damage."
In the study, published in Frontiers in Physiology, researchers put 14 men and women through two vigorous rounds of exercise on an isokinetic dynamometer machine (a positioning chair attached to an adjustable strength machine and computer control unit that measures the performance of various muscle groups) four weeks apart. In short: "All of them got really sore," Hyldahl said.
Before and after each bout of exercise, the researchers took muscle biopsies to analyze the tissue. They discovered (as they expected) an increase in certain white blood cells after the second bout of exercise. What they didn’t expect to find were T-cells, which an undergraduate coauthor—enrolled in an immunology course at the time—suggested they look for.
"T-cells, up until recently, were not thought to enter healthy skeletal muscle," lead study author Michael Deyhle said. "We hadn't planned on measuring them because there's no evidence that T-cells play a role in infiltrating damaged muscle tissue. It's very exciting."
Their presence suggests muscles become more effective at recruiting immune cells following a second go-around of exercise, and these cells may facilitate accelerated repair. In other words, the muscle seems to remember the damage and react as if it were responding to toxins, bacteria, or viruses.
The team also found an increase in inflammation after the second round of exercise. In the past, researchers believed inflammation goes down after the second bout of exercise, contributing to that "less sore" effect; but the enhanced inflammatory response suggests inflammation probably doesn’t worsen exercise-induced muscle damage.
"Some people take anti-inflammatory drugs such as Ibuprofen and Aspirin after a workout, but our study shows it may not actually be effective,” Hyldahl adds. “The inflammation may not be directly causing the pain, since we see that muscle soreness is reduced concurrent with increases in inflammation."
Basically, this is just a really science-y way of saying you need to change your workouts every six to eight weeks. Read more about how to do that here.