See, Zone 2 training helps stimulate Type 1 (“slow twitch”) muscle fibers, which increase the mitochondrial density in our muscles. However, “Women already have a larger amount of slow twitch or oxidative fibers,” Sims notes. “Women have more mitochondria density [and] better mitochondria respiration,” she adds. “When we look at men, yes, they need to do Zone 2 to increase mitochondrial density and the respiratory capacity of mitochondria, but women don’t.”