It is very frustrating for aspiring muscle builders to see that some people can build and maintain muscles with little to no effort, while others have to work twice as hard for half the results. What's responsible for this difference in response to exercise?

A new study, in the journal Nature Communications, offers a molecular explanation for this.The new research also explains why some people seem to respond better to endurance training and aerobic exercise rather than muscle training.

The molecule in question is a protein called c-Jun N-terminal kinase (JNK). According to Dr. Lessard of the Joslin Diabetes Center, Boston, "It's like a switch.. if the switch is on, you'll have muscle growth. If it's turned off, you have endurance adaptation in the muscle. We've identified an exercise-activated biological pathway that hasn't been studied at all," she adds.

The new study builds on previous work that identified that JNK molecular pathway was responsible for how well a mouse would perform on the treadmill. In the new study, the researchers investigated more about the reasons why JNK would have such a vital role. So, Lessard and team knocked out the JNK gene in a group of mice and compared their behavior with that of normal mice.The knockout rodents remained healthy and continued to run on the treadmill for long periods of time. Next, the researchers trained both groups of rodents to run. This was when the scientists remarked that the aerobic capacity of the JNK-free mice had increased drastically, along with their blood vessels and a muscle fiber that builds endurance. By contrast, in a muscle growth experiment, JNK-free rodents failed to increase their muscle mass, whereas regular mice doubled theirs.

The new research also included tests in humans. Lessard and colleagues asked healthy volunteers to engage in either weight-lifting exercise or cycling, a type of endurance workout. The tests found that JNK was highly active in the former kind of exercise but not active in the latter. Because studies also found JNK to be implicated in metabolic inflammation, the researchers hope that inhibiting this pathway will prevent metabolic conditions such as type 2 diabetes.

Also, the findings may help those who want to build muscle but can't because of various diseases. "We've begun to figure out how muscle decides whether it will grow or adapt for endurance, which really hasn't been known. And we're finding that this process is directly linked to the risk of type 2 diabetes."

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