If you've ever took some time away from the gym only to be slammed with muscle soreness after your big comeback, you're not alone. Scientists have been studying this phenomenon for years (they call it the "repeated bout effect"), but they haven't been able to explain exactly why your muscles become less sore the more you work out.
According to the most recent study on the topic, this type of muscle soreness actually has more to do with your immune system and how it repairs your muscles, than does with you getting stronger between workouts. Researchers at Brigham Young University pinpointed the exact immune response that helps to fight soreness: T-cells, as in the little immune army that fights off infections. (Bonus: We have 10 easy ways to boost your immune system.)
The Frontiers in Physiology study is the first to find that T-cells repair human muscles. The researchers looked at a group of 14 men and women going through two rounds of vigorous exercise, 28 days apart. Before and after each workout, researchers took muscle biopsies to measure the chemistry of what was going on in the exercisers' bodies. They found a serious uptick in T-cells after round two.
According to the study authors, the increase in the presence of T-cells in your muscles post-workout may indicate that your muscles become better at recruiting these immune repair workers the more you exercise. In other words, your muscles remember the damage done during that first trip to the gym and are better prepared the second time around.
What's more, up until now, the assumption was that muscles become less inflamed the more you exercise, which contributes to the less-sore effect. But this study actually found that people's muscles were more inflamed after the second round of exercising. Since muscle inflammation probably has nothing to do with your exercise-induced soreness, popping an ibuprofen after a particularly challenging core class may not do much. Instead, keep an eye out for these 5 Times Sore Muscles Are a Bad Thing and try these 6 Ways to Relieve Sore Muscles After Overtraining.