Exercising is a popular New Year’s resolution, though usually for naught. Various studies have found that nearly one-quarter of people quit their resolutions by the end of the first week and almost half by the end of January. Only 8 percent to 9 percent stick with them for an entire year or to completion.

But if you’re still busily huffing and puffing at the gym or at home, here’s a little insight into what happens inside your body while you wait for gratifying external evidence.

When you push your body physically, your muscles respond by increasing the number of mitochondria — the so-called “power plants” of cells. The key driver is a molecule called peroxisome proliferator-activated receptor gamma coactivator 1 alpha, or PGC-1-alpha, a protein that basically regulates the expression of many genes in skeletal muscles. Besides boosting mitochondrial mass, PGC-1-alpha increases the number of blood vessels in muscles, improving the supply of nutrients and oxygen.

Strength training produces other effects, such as increasing the production of proteins that regulate muscle growth. More proteins, more muscle.