Foot and Ankle Action
Muscles surrounding your feet and ankles carry a hefty responsibility when you’re running. When your foot makes contact with the ground, the gastrocnemius and soleus muscles in your calf work to cushion the blow and prevent your foot from collapsing upward, or dorsiflexing, too far. Muscles at the bottom of your feet, including the flexor digitorum longus and flexor hallucis longus, contract to flex your toes when you push off the ground so you continue to travel forward. As your foot leaves the ground, your tibialis anterior, peroneus longus and peroneus brevis muscles at the front of your lower leg pull your foot up so it clears the ground as it swings forward for the next step.
The quadriceps -- a collection of four muscles at the front of your thighs that extend your knees -- works to absorb shock as your foot strikes the ground. The quads contract to allow your knee to bend just slightly but prevent collapse. When you’re ready to push off the ground again, they contract to extend, or straighten, your knees. Once your foot is off the ground and your leg is swinging forward, your quads, specifically the rectus femoris muscle, work to pick up and bend your knee. Runners recruit their hamstrings -- three muscles at the back of your thighs that are responsible for flexing your knees -- to keep the knee slightly bent as the foot lands.
Your hamstrings also are responsible for handling movement at your hips. Along with the gluteus maximus at the buttocks, these two major muscle groups provide most of the force and power during running. They contract to extend your hips as you push off the ground. At the front of your hips are a collection of muscles known as the hip flexors, the most notable being the iliopsoas, which are responsible for pulling up your leg after it swings forward so it’s in position to strike the ground again.
Core and Upper Body Action
As you run, your arms swing forward and back at the shoulders in opposition to your legs, which helps maintain balance. Forward swinging is handled by the deltoid muscles in your shoulders, while backward swinging is the responsibility of your latissimus dorsi, which is large muscle group in your back. Your biceps brachii muscles at the front of your upper arm contract to keep your elbows bent to 90 degrees. Your abdominals, obliques and lower back statically contract to keep your torso stable in a slightly forward lean.
Your heart and circulatory system -- responsible for delivering oxygen and nutrients to your working tissues -- are heavily involved when you’re running. Your heart pumps faster, and your circulatory system distributes a greater volume of blood to the muscles in your hips and legs. When you’re running, your working tissues need more oxygen for the process of breaking down and preparing fatty acids and carbohydrates so they can be used for fuel. Runners who are consistent with their workouts typically have lower resting heart rates and a higher maximal oxygen consumption because their hearts are stronger and can pump more blood per beat and their circulatory system is more efficient. These cardiovascular developments that occur over time improve a runner’s performance.