Four important bodily functions are provided by the muscles of our bodies. The forces that lead to motion and athletic activity, of course, are developed by muscles; but even when we are not actively occupied, muscles serve to maintain bodily posture, provide joint stability, and generate body heat.
On a microscopic level, muscles are made out of two types of interleaved molecules laid out in thick and thin filaments along the length of the muscle fibers. When stimulated by nerve action, the thin filaments slides over the thick filaments, causing the overall length of the filament pairs to shorten. Many such pairs of filaments are bundled together into a structure called a fibril, and groups of fibrils make up a single muscle fiber (cell). Groups of muscle fibers are called fascicles, while bulk muscles are made up of groups of fascicles. Within each fascicle, groups of muscle fibers are connected to motor neurons and motor nerves. It is these cell of the nervous system that stimulate muscle fibers into contracting.
Muscles are attached to bones by tough bands of connective tissue called tendons. Muscles are attached to bones in several arrangement, depending on the function of the muscle. Some muscles are formed into continuous circular or oval loops. Such muscles are called sphincters, derived from the Latin word for squeeze. The muscles around the lips, and eye sockets are examples of sphincters. Other muscle arrangements include fan shaped, feather shaped, and parallel bundles.
Fan shaped muscles originate at one point and then spread out like a fan. Many of the muscles of the shoulder are of this type (e.g., the pectorals). Feather shaped muscles attach to tendons with a pattern similar to the one seen in the filaments of a feather attached along its central spine. In some cases muscles are attached to both sides of the tendon, just like the left/right pattern in a feather. In other cases the muscles attach on only one side. The deltoid muscles in the shoulder, and several of the muscles in the arms and legs are of this type. Muscles of the parallel type attach to bone at one point at each end. Muscles of this type include the biceps, and triceps in the arm.
When contracting, muscles can shorten by about 30% of their resting length. The longer a muscle fiber, and the more parallel it is to the long dimension of the muscle, the greater the overall shortening of the muscle. Muscles of this type have the largest range of motion, but tend not to be very powerful. Fan and feather shaped muscles, on the other hand, have shorter fibers packed more closely together. Such muscles have a lesser range of motion put can develop much greater forces.
The total force developed by a muscle depends on the total number of muscle cells in the muscle. On the average, muscles develop about 100 pounds of force for each square inch of cross sectional area. Most muscles are attached to bones on opposite sides of a single joint. When they contract, the bones around the joint are set into motion. A small number of muscles cross over two joints. An example of the latter is the hamstring which crosses over both the hip and knee joints. This attachment of the hamstring is the reason why your knees want to pop up when you do situps.
Depending on where muscles are connected to the bones, a first, second, or third class lever is formed at the joint. In some cases these levers work at a mechanical advantage, but in most cases the opposite is true. This means that in general muscles develop large forces over short distances to move a smaller load (weight) over a large distance. For example, with a typical mechanical disadvantage of 7 a muscle has to generate 700 pounds of force to move a 100 pound load. Such a muscle would have to be about 3 inches in diameter.
Because muscles can only be stimulated to contract, it requires at least one pair of muscles to fully move a joint. In the arm, for example, the biceps are used to bend the elbow, while the triceps are used to straighten it out. Some joints use more than one pair of muscles. The hips joint, for example, is controlled by four muscle groups (two pairs).
Muscle fibers can only exist in a relaxed or contracted state. There is no in between. None the less, muscles on the whole can be controlled to produce a wide range of force and delicate motions. This is because the muscle fibers within a given muscle are stimulated separately by different motor neurons. A single motor neuron is attached to anywhere from 4 to up to several hundred muscle fibers. The typical number is about 150. The muscle fibers stimulated by a single motor neuron is called a motor group. The muscle fibers in a single motor group are spread throughout the muscle in which they are located. The firing of a single motor group causes all the fibers in the group to completely contract, and causes a weak contraction in the muscle as a whole, since each muscle includes many motor groups. As more and more motor groups fire, a stronger and stronger contraction in the entire muscle is produced.
When a motor group fires, the brief contraction of the muscle fibers is called a muscle twitch. Peak tension in the muscle occurs within 0.005 to 0.050 seconds, and then full relaxation is restored within another 0.050 to 0.100 seconds. If the force developed overcomes the load on the muscle, the muscle fibers will shorten, otherwise they will just get rigid. How long this process takes varies for different muscle fibers.
Muscle fibers that respond quickly are called fast twitch muscles. The ones that respond more slowly are called slow twitch. Fast twitch muscles are used for explosive, high power motions, but fatigue easily. Fast twitch muscle is best suited for delivering rapid intense movements for short periods of time (e.g., jumps, and fast footwork). Slow twitch muscles have slower reaction times and develop lesser tension than fast twitch but are more fatigue resistant and are important for activities which require high endurance. (In a chicken, white meat is primarily fast twitch muscle, dark meat is primarily slow twitch!)
The average person has muscles with an even mix of fast and slow twitch muscle fibers. Endurance athletes such as long distance runners and marathon runners have a high percentage of slow twitch muscle, as much as 80% in marathon runners. Sprinters, football linebackers, and the like have a large percentage of fast twitch muscle, as much as 60 to 70%. To a large extent the ratio of fast twitch to slow twitch muscle in a person is genetically determined. In track and field, it is said, great sprinters are born and not made. Recent studies, however, indicate that some muscle fibers can be trained to respond as either fast twitch or slow twitch muscle. Thus, through proper training, it appears the fast to slow twitch ratio can be altered to a limited extent. Skaters who are sluggish in their jumps and spins, and lack explosive power in their skating might consider taking this into account in developing their off-ice training.
Because a muscle twitch occurs so rapidly, less than 1/6 to 1/10 second, repeated stimulation of the motor groups is required to maintain muscle tension for extended periods of time. In order to provide a sustained exertion each motor group is fired by its motor neuron repeatedly. The frequency with which the motor group is fired is variable up to the point where the contraction of the motor group is continuously sustained. This condition can be maintained until muscle fatigue sets in (up to a few seconds for some muscles). In addition to repeatedly firing individual motor groups, muscles activity is controlled by the number of motor groups involved in the contraction.
We will spare you the details of the chemical and electrical reactions that take place in muscles when they contract, but one practical aspect of that which is important to athletic activity is worth mentioning. The force developed by a muscle turns out to be related to its temperature. For a given stimulus, a muscle develops lower tension when its temperature is lower, and produces a higher tension when it is warmed up. The purpose of the warm-up period preceding athletic activity is not only to stretch and get limber, but also to increase the temperature of the muscles so they will produce greater forces during the subsequent athletic activity. Skaters who warm up before their performance and then stand around in the cold shivering before they perform are penalizing themselves by making their muscles significantly less able to produce at the level they are capable of at the start of their program. Since most skaters put some of their most difficult jumps at the start of their programs, it is advantageous that muscles be fully warmed up at the start of the program.
During physical activity, most of the energy used by the muscles of the body goes into producing heat. Only 1/5 to 1/4 of the energy used in muscle contractions goes into actual work (i.e., jumping spinning, moving, etc.). In addition to temperature, the force developed by a muscle is affected by its state prior to the contraction. Specifically, a muscle that contracts from a pre-stretched condition will develop 5 to 10% greater force than a muscle that is contracted from its at rest condition. This characteristic of muscles appears in most forms of athletic activity in some form or another. In baseball, for example, the wind up prior to swinging a bat pre-stretches the muscles of the back used to actually swing the bat when the time comes. Similarly, stretching the arm back prior to throwing a ball increases the force with which the ball can be thrown. The greater the stretch, the greater the force that can be developed.
In skating, this pre-stretching of the muscles is used in jumps, spins and turns. In jumps, reaching back with the arms, and winding up the shoulders and back, stretches the muscles used to generate the rotation when you pull in on the jump. Reaching back with the free leg and bending at the knee prior to the takeoff stretches the muscles used to get the lift in the jump. In spins, pulling back the arm and free leg, and rotating the shoulders and back on the entry edge to the spin, stretches the muscles used when you step into the spin, and when you pull in to get the spin going. In turns, the rotation of the shoulders prior to the turn stretches the muscles used in completing the turn, and allows those muscles to develop a greater force than they would otherwise.
So, when your coach yells at you to stretch, there is a reason!