Running targets growth in slow-twitch muscle fibers. Unlike fast-twitch muscle fibers -- which exert maximum force and tire quickly -- slow-twitch fibers have more endurance for sustaining contractions over extended periods. Your legs might get bigger when running because running involves repetitive contractions that stimulate growth in your leg muscles, which contain a higher proportion of slow-twitch muscle fibers. The most prominent muscle groups in your legs contract continuously with each stride while running.
Running can make your legs bigger because your quadriceps contract continuously while running. Quadriceps muscles -- the most prominent group of muscles at the front of your legs -- form the front of your thighs. Your quadriceps contract for knee extension and hip-flexion joint movements. The hips flex each time you lift your thigh forward while running. Knee extension immediately follows hip flexion, as your leg straightens and hits the ground. Your body continuously alternates between falling forward and regaining stability as you run. Your quadriceps' contractions are particularly critical for the support phase of each stride, which allows you to regain stability.
Your hamstrings are the most prominent group of four muscles that form the back of your thighs. Running can make your legs bigger because your hamstrings contract continuously during this activity. Your hamstrings support 75 percent of the workload for running, according to Olympic coach Dr. Nicholas Romanov, writing for Pose Tech. Hamstrings bend your knees and extend your hips as your thighs move back while running. Hamstrings' contractions pull your feet off the ground and move them under your hips during each stride.
Repetitive glute contractions also help your legs grow when running. Your gluteus maximus -- the largest muscle in your body -- sits above your hamstrings. Glute contractions extend your hips, bringing your thighs back for each stride while running. According to strength and conditioning specialist Bret Contreras, running involves 234 percent more gluteus muscle activity than jumping straight up, which also extends your hips. Your glutes contract to stabilize your hips when your foot hits the ground during the support phase of each stride.
Your lower-leg muscles might get bigger when running because calf-muscle contractions support knee and ankle movements during each stride. Calf muscles help your hamstrings bend your knees while running. Calf-muscle contractions perform ankle movements that drive your body forward while running. Immediately after your feet hit the ground, your calf muscles roll your weight onto the front of your feet by flexing your ankles. Maintaining a strong calf contraction while moving onto the front of feet maximizes your propulsion off the ground, which helps you spring off the ground with each stride.
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This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.