A massage after vigorous exercise unquestionably feels good, and it seems to reduce pain and help muscles recover. Many people — both athletes and health professionals – have long contended it eases inflammation, improves blood flow and reduces muscle tightness. But until now no one has understood why massage has this apparently beneficial effect.

Now researchers have found what happens to muscles when a masseur goes to work on them.

Their experiment required having people exercise to exhaustion and undergo five incisions in their legs in order to obtain muscle tissue for analysis. Despite the hurdles, the scientists still managed to find 11 brave young male volunteers. The study was published in the Feb. 1 issue of Science Translational Medicine.

On a first visit, they biopsied one leg of each subject at rest. At a second session, they had them vigorously exercise on a stationary bicycle for more than an hour until they could go no further. Then they massaged one thigh of each subject for 10 minutes, leaving the other to recover on its own. Immediately after the massage, they biopsied the thigh muscle in each leg again. After allowing another two-and-a-half hours of rest, they did a third biopsy to track the process of muscle injury and repair.