You’ve taken a vacation from the gym, but now you’re back and feeling great. However, the next day you are in agony. Not to be defeated, you return two days later, and this time, the after-pain isn't nearly as fierce. Why are your muscles less sore in the days following your second trip to the gym compared to the first?

Brigham Young University researchers say the reason has everything to do with the white blood cells known as T-cells, immune system responders, which are present in your sore muscles following exercise.

Repetition

The “repeated bout effect” has long been known to scientists and intuited by gym goers. Essentially, this term refers to how repeated exercise seems to weaken the impact — both positive and negative — as your body becomes accustomed to it. Though exercise scientists have studied "repeated effects" for years, they’ve never understood exactly why it occurs. Dr. Robert Hyldahl, assistant professor at BYU, designed a study to investigate whether the immune system is involved.

The team began by enlisting seven men and seven women, all healthy and young. These 14 participants, who had not participated in consistent, structured physical activity in the past six months, performed a vigorous workout focused on particular muscle groups and then performed the same routine 28 days later. During the study period, participants didn’t change their activity levels and refrained from using pain or anti-inflammatory drugs.

Before and after each bout of exercise, the team took muscle biopsies from the participants for analysis. The BYU group found an expected increase in certain white blood cells after the second bout of exercise.

Yet they also found other unexpected guests at the muscle party: T-cells.

Until recently, T-cells were not thought to enter healthy skeletal muscle. T-cells act as the intelligence service of your immune system. They basically roam your body scanning for irregular or abnormal developments and then attack. T-cells are remarkable in that they can “remember” a germ or virus first encountered in infancy and continue to detect and fight it throughout your life.

According to Hyldahl and his colleagues, the presence of T-cells suggests muscles become more effective at recruiting immune cells following a second bout of exercise. Your muscles “remember” what damaged them the same way your immune system remembers toxins, bacteria, or viruses. These cells even facilitate accelerated repair, the team explains, as inflammation increases following a second round of exercise.

In the past, physiologists believed muscle soreness dwindled as inflammation decreased, but the new results suggest inflammation is a normal, healthy process the body uses to heal itself. Bottom line: if you’re taking Ibuprofen or aspirin after the gym, it may be working against you.

Source: Deyhle MR, Gier AM, Evans KC et al. Skeletal Muscle Inflammation Following Repeated Bouts of Lengthening Contractions in Humans. Frontiers in Physiology. 2016.