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“The two main muscles you’re trying to build when doing calf exercises are the soleus and the gastrocnemius,” Braun says. The gastrocnemius is the main muscle you see when looking at someone’s calves — it will be thick and well-defined if it’s highly developed. The soleus, in contrast, isn’t outwardly visible — it’s situated deep beneath the gastrocnemius. Both muscles work together to help flex your foot, but only the gastrocnemius plays a role in knee flexion (bending your knee).
Though the gastrocnemius is primarily responsible for the outward appearance and size of your calves, it’s important to engage and exercise both parts of the muscles, since each has a unique function. “The gastrocnemius contributes to jumping, acceleration, and explosive speed and power,” says William P. Kelley, C.S.C.S., ACT, while the soleus contributes to walking and running endurance. That’s because the soleus has a very high density of Type 1, or “slow-twitch,” muscle fibers, he explains. Type 1 muscle fibers have slow contraction times and a high resistance to fatigue, which is helpful since your legs get a lot of action carrying your body from once place to another.