From The Economist
The researchers got 12 members of Oxford ’s heavyweight squad to row on machines in four 45-minute sessions over two weeks. In two sessions they rowed in complete isolation and in the others in groups of six, perfectly synchronised. Immediately following each session their endorphin levels were tested. Because endorphin levels can only be measured directly through an invasive lumbar puncture—unfeasible, even for notoriously pain-hungry oarsmen—the researchers used a readily accepted proxy: they deduced pain tolerance, and hence endorphin levels, by gradually tightening a cuff around each rower’s arm. When he said “now” they stopped squeezing and noted the pressure. As expected, the rowers’ pain thresholds were significantly higher following the group sessions. This was despite nearly identical power outputs in all four tests and efforts to control for possible confounding variables, such as the time of day.