After a good run, the burn in your legs sure makes it feel like you're building muscles, but common knowledge pegs running as a strictly cardio workout. On top of improving your heart and lung health and melting away about 105 calories per mile, according to research from Syracuse University and Runner's World magazine, running does have an effect on muscular strength, but the relationship between running and muscle development is a little more complicated than you might think.
Starting with the basics, the act of running engages specific muscle groups. Running squarely targets the quads and the calves, which are primarily tasked with keeping your body upright and fighting forces of gravity. Running also engages the hamstrings and, on a secondary level, the muscles of the “core,” which include the trunk, hips and all-important abs.
Endurance vs. Strength
While running does build leg strength, it only does so to support the needs of your body in regards to your running routine. So while your legs get stronger as you progress from jogging a mile to running a half-marathon, you will see progress. However, personal trainer Holly Perkins points out that this increase falls in the “muscular endurance” category -- your legs develop the muscular endurance needed to support your regular run. If you've reached a plateau, turn to strength training to shift the focus from building muscular endurance to building muscular strength.
Rather than relying on running to build muscular strength, you'll likely find greater success by building muscular strength to improve your run. Strength-training exercises, despite popular opinion, will not turn you into a glistening hulk of a bodybuilder. In addition to promoting a healthy metabolism and encouraging lean muscle development, strength training boosts your oxygen efficiency by up to 8 percent, according to Runner's World magazine. Of course, leg-focused exercises give you the most bang for your buck when it comes to bolstering your running skills. Particularly effective workouts include squats, lunges, leg presses and other exercises that focus on the thighs, hams and calves. Pairing strength training with regular running is a two-way street; you'll improve the quality of your run by training your legs, and the increased endurance you gain from running helps you train harder in the gym.
Although running can't replace strength training when it comes to muscle development, you can encourage greater muscle engagement during your run with a few simple tweaks. To engage both slow-twitch and fast-twitch muscle fibers, spice up your regular routine with interval training about twice per week, and throw in hill repeats, sprints or strides once every two weeks. These exercises stimulate fast-twitch fibers, while regular-tempo running focuses on slow-twitch fibers.
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