The muscles around your waist, including your abdominals, obliques and lower back, all work to provide stability while running. Running does not work these muscles directly, but they do receive indirect work helping you balance and remain upright. If you wish to work them specifically, you must train them specifically. Cardio exercises such as running, when combined with a healthy diet, can also play a role in a tight, flat midsection.
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Strong Abs for a Six-Pack Look
The main muscle of your abdominals, the rectus abdominis, is the long, flat, paired muscle running up and down that most people refer to as "the abs." This muscle contracts slightly to help keep you from leaning forward when you run. If you wish to work it more when you run, wear a backpack and keep the backpack high on your back. Your rectus abdominis is heavily worked during crunches and sit-ups.
Strengthen Your Sides
Your obliques are the muscles at the sides of your waist. Your obliques work to keep you from wobbling from side to side while running. These muscles are slightly more active than your rectus abdominis when running. If you wish to work your obliques more while running, carrying dumbbells in your hands will accomplish this. These muscles also are worked through any exercise where you lean to the side or rotate.
A Strong Lower Back
While not a muscle near your stomach, your lower back works in conjunction with your abdominals and obliques to provide stability and maintain posture while running. This muscle is more active if you lean forward, but you should avoid this and maintain efficient running technique. Carrying something in front of you or wearing a backpack on your chest will work your spinal erectors, the muscles of your lower back, more.
More Than Meets the Eye
Smaller, or hidden muscles around your stomach also work while running. Your transverse abdominus, a muscle located below your obliques, also contracts while running. This muscles works to provide stability to your torso, pelvis and spine. Unlike many muscles, your transverse abdominus activates by contracting inwardly and does not contract in conjunction with other abdominal muscles, according to a 1999 study.