So let's start with the skeletal and muscular systems. Bone is a dynamic tissue. It responds to chronic stress placed upon it by changing its shape. Where the chronic stress is high, the bone is thickened. And in sections of the bone where the stress is low, it will become thinner and weaker. A strategically targeted training stress on the skeleton can improve an athlete's performance by strengthening the relevant parts. A joint is the connection between two bones, most of which permit movement. Joints are typically either a ball and socket variety, capable of a wide range of rotation, such as the shoulder joint shown here. Or they are of the hinge variety with movement in just two directions. And this is the example of the elbow joint. The shoulder and the hip are examples of a ball and socket joint, and the knee, elbow, and ankle are examples of hinge joints. The knee joint is the workhorse of the body, absorbing millions of shocks over a lifetime. Ligaments holding the knee together have a combined breaking strain of nearly a ton, and that's 907 kilograms. While the smooth cartilage lining the knee joint is only two to four millimeters thick and is roughly 80% water, it can tolerate seven tons, or 6,300 kilograms, before giving way. Attached to the bones and the joints of the skeleton are biological muscular motors capable of producing an infinite variety of mechanical forces and movement. The body weight of most athletes is 40 to 50% muscle. Skeletal muscles are voluntary, meaning that they're under the athlete's control. And the athlete can vary the amount of muscular force produced, start and stop the action at will, and so forth. Involuntary muscles work behind the scenes to keep the heart beating, move food through the digestive system, control the diameter of blood vessels, among the other functions necessary to keep the body working. A connective tissue called fascia is the glue binding every cell with the one next to it, and every muscle together with its neighbor. The fascia connecting the muscle fibers together merge to form a fibrous tendon holding the whole muscle in its correct location. And you'll often see the muscle and its attached tendon referred to as the muscle tendon unit. The fascia is thought to contain a signaling mechanism that provides the reason for training whole body movements, rather than just training isolated muscles. Whole body movements resembling those found in sports, strengthens the fascia in the line of the force, and this permits forces to move more effectively through the line of the connected muscle. Structurally, skeletal muscle resembles a folding telescope. Bundles of tissue are tightly packed inside one another, beginning with bundles of fascicles that make up the entire muscle. In each fascicle is a bundle of muscle cells or fibers. And the muscle fiber is the longest cell in the body, often running the full length of the muscle. It can in fact be up to 30 centimeters long, and that's pretty long for a cell. The muscle fiber is made from bundles or small strains of myofibrils, and these are composed of actin and myosin. The actin and myosin overlap and are ordered into separate units called sarcomeres. And these tiny sarcomeres are only a few micrometers in length, and they lie in series to give these skeletal muscle cells striated appearance. Blood vessels deliver oxygen and fuel to the muscle fiber, and the nerves signal it to contract. In this video, you'll see how the contraction of the muscle occurs. The head of the thick myosin filament pulls the thin actin filaments towards the center of the sarcomere. And as the sarcomere shortens, the entire muscle fiber also shortens up to one-third its normal length. The muscle relaxes when the actin filaments slide back into their original position, lengthening the sarcomeres in the process. And in this way, the whole muscle expands back to its resting length. This is a pretty amazing process. It has been known for over 40 years that muscle fibers come in two basic varieties according to their speed of contraction. They were labeled as either slow twitch, or Type I, or fast twitch fibers, or Type II fibers. Slow fibers are very, very, efficient fibers and therefore important for endurance activities such as long distance running. The maximum contraction velocity of a slow fiber is only around one-tenth that of a fast fiber. The fast fibers produce very high power, very quickly, and therefore are important to power events like sprinting and throwing and jumping. With progress and technology over the past 40 years, we've found two distinct types of fast fibers. And this has expanded our knowledge about how the body adapts to produce force and endurance. Not only do the fast fibers have different speeds of contraction, they have a different mix of energy production capabilities as well. Athletes show a great deal of variation in the proportion of the two fibers, and the two fast twitch fibers and the one slow twitch fiber, depending on their genotype. Successful endurance runners have a very high proportion of slow twitch fibers, whereas successful power athletes have a high number of the two fast twitch fibers.