You won't see a lot of bodybuilders running — at least, not as their primary exercise. That's because running doesn't build significant muscle mass. You need to hit the weight floor and tackle resistance to develop bulging muscle mass. Think of your typical elite marathoner: He's lean, but light and not especially muscle-bound.
Running is mostly a cardiovascular exercise that strengthens your heart, which is arguably the most important muscle in your body. It can also strengthen muscles in your legs and sometimes cause slight hypertrophy, or an increase in size, especially if you're new to the sport.
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Your calves, which are responsible for the primary push off for every step and catch you when you land, strengthen with regular running. You may also notice that these muscles increase slightly in size or become more firm when you regularly put in miles on the road.
Your calves consist of your gastrocnemius, the larger muscle at the back of your lower leg, and the soleus, a smaller muscle that lies deeper inside.
The gluteus maximus, your large buttock muscle, is also responsible for the acceleration in running. It, along with the hamstrings, pulls your leg backward, so you get appropriate hip extension.
You'll see glutes that are quite developed in sprinters, for whom acceleration is often practiced and essential to success. In long-distance runners, the glutes are often less developed.
Poor glute activation and development can actually lead to injury that runs down the kinetic chain. You may end up with psoas pain, knee issues or Achilles tendinitis as a result of glutes that don't fire properly when you run.
The quadriceps are a set of muscles at the front of your thigh. One of your four quads, the rectus femoris, crosses your hip and knee joint. It's responsible, along with the other quad muscles, for bringing the back leg forward to take another stride.
The quads work to pull the knee up as you take the step forward, thus propelling you forward. To really build the quads while running, choose a downhill course. Downhill emphasizes the eccentric, or lengthening, activation of the quad muscle during which muscle is primarily built.
Your hamstrings are the muscles at the back of the thigh. You may know them as the muscles that feel tight when you fold forward in a stretch. The hamstrings, along with the glutes, work to pull your upper thigh back once your foot hits the ground.
Although running is a fantastic exercise to strengthen your cardiovascular and respiratory system, it alone won't build big muscles. Runners should incorporate resistance exercise two to three times per week to make them stronger, increase running stamina and ward off injury.
Good exercises include weighted hip hinges, squats, lunges, donkey kicks and core work. And, Because running doesn't do much for the upper body in terms of muscle development, you'll also need to incorporate exercises to build muscle in the chest, back, arms and shoulders.
If you see a runner that seems quite muscled, it might be because his running habit has leaned him out so the muscle he has are not hidden under layers of fat. It's likely he hits the weights in addition to running, too.
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