The question is: if you exercise with greater intensity, will it pay off with stronger muscles and greater endurance? And the answer, according to Gretchen Reynolds in the New York Times, is: it looks that way. Specifically, it looks that way to researchers at the Scripps Research Institute in Florida, who have been studying the function of catecholamines, biochemicals that, as Reynolds writes, “prime the muscles for getaway or battle,” and a protein called CRTC2, also activated during stress. Reynolds reports that the researchers bred mice that were genetically programmed to produce an abundance of CRTC2, then put them on a strenuous treadmill program. The researchers found that the endurance of the specially bred mice increased by 103 percent after two weeks, while the endurance of normal mice on the same program increased only 8.5 percent. The modified mice also had much greater muscle growth. Reynolds tells us that those differences all were the result of a sequence of events set off by catecholamines, which are only released during exercise that the body perceives as stressful. Hence, with no catecholamines, there is no CRTC2, and with no CRTC2, there is no extraordinary endurance or muscle growth. Yes, more pain, more gain. Read more in the New York Times.