Running is a combination of numerous muscles working together. Muscles must contract and relax in a steady rhythm to drive you forward step by step. Running is not just about your lower-body muscles, although they are doing the majority of the weight-bearing work. Your torso, arm and core muscles are also involved.
When a muscle shortens to generate force or tension, it is said to be contracting concentrically. This means that the ends of the bones that make up a joint are being drawn together. For example, as your foot strikes the floor and your knee bends to drive you forward, your hamstrings are contracting concentrically.
Eccentric contractions occur when a muscle generates tension as it lengthens. This can best be thought of as deceleration. As you land on your foot, your quadriceps, the muscles on the front of your thigh, must absorb the force of your landing. This is achieved by allowing your knee to bend slightly in much the same way a spring flexes when compressed. Eccentric muscle contractions result in the extension of a joint.
While your legs are pumping to drive you forward, your torso must remain relatively still and stable. Too much movement in your upper body would result in an ungainly running gait and a lot of wasted energy. To prevent unwanted movement, many of your upper body muscles remain contracted -- this is called an isometric contraction. Isometric contractions result in muscle tension but no actual joint movement. The muscles in your neck must contract isometrically to keep your head up while you run and the muscles of your core or midsection must contract in the same way to stabilize your spine.
Agonists and Antagonists
Activities such as running involve muscles working in pairs. Muscles are commonly arranged on opposite sides of a joint. When one muscle contracts and shortens, the opposing muscle must relax and lengthen to allow movement. The only exception to this is isometric contractions where no movement occurs. Paired muscles involved in running include the quadriceps and hamstrings, gluteus maximus and hip flexors and the hip adductors and abductors.