The movements that take place during activities such a running are due to the contraction of skeletal muscles pulling on bones which move at flexible pivot points, or joints. The several types of contractions that occur within skeletal muscles during running include isotonic concentric and isotonic eccentric contractions. Contractions also occur within your slow twitch and fast twitch muscle fibers.
Muscle contraction involves the interaction of microscopic filaments of actin and myosin within each muscle cell, or fiber, in which finger-like projections on the myosin filament grasp and pull each actin filament. When this contraction results in shortening of the muscle, as in the lifting motion of a biceps curl, it is called a concentric contraction. Contraction that occurs as your muscle lengthens under stress, for example, lowering the weight in a biceps curl, is referred to as an eccentric contraction.
The most typical muscle contractions are those that result in movement such as the shortening movements of muscle in concentric contractions or lengthening in eccentric contractions. Contraction associated with such movement is called isotonic contraction. Imagine trying lo lift an immovable object such as a barbell with 1,000 lbs. of weight. Your muscles would contract, tension would develop but nothing would move and your muscle wouldn't shorten. This type of contraction, tension without shortening, is called an isometric contraction.
Eccentric isotonic contractions occur most notably during downhill running. During a normal, flat run, the knee extensors or quadriceps muscles contract to straighten your leg. When running downhill, the quadriceps will contract eccentrically as your knee begins to bend after contacting the ground in an effort to prevent your knee from bending too far and too fast. In addition, the tibialis anterior muscle on the front of your shin also contracts eccentrically to control the downward motion of your foot after your heel strikes the running surface.
Each of your muscles is made of a combination of two types of muscle cells, or fibers, known as fast-and slow-twitch fibers. Fast-twitch fibers are just that, they contract rapidly but are not capable of long endurance. Slow-twitch fibers, while they contract more slowly, can handle endurance events quite easily. When you begin running at a relatively slow pace, you do so using predominantly slow-twitch fiber contractions. As you speed up your pace, your recruit more of the fast-twitch fibers to contract.