Stretching And Flexibility PLR Ebook

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Chapter 1: Physiology of Stretching 3

1 Physiology of Stretching

The purpose of this chapter is to introduce you to some of the basic physiological concepts that come into play when a muscle is stretched. Concepts will be introduced initially with a general overview and then (for those who want to know the gory details) will be discussed in further detail. If you aren't all that interested in this aspect of stretching, you can skip this chapter. Other sections will refer to important concepts from this chapter and you can easily look them up on a "need to know" basis.

1.1 The Musculoskeletal System

Together, muscles and bones comprise what is called the musculoskeletal system of the body. The bones provide posture and structural support for the body and the muscles provide the body with the ability to move (by contracting, and thus generating tension). The musculoskeletal system also provides protection for the body's internal organs. In order to serve their function, bones must be joined together by something. The point where bones connect to one another is called a joint, and this connection is made mostly by ligaments (along with the help of muscles). Muscles are attached to the bone by tendons. Bones, tendons, and ligaments do not possess the ability (as muscles do) to make your body move. Muscles are very unique in this respect.

1.2 Muscle Composition

Muscles vary in shape and in size, and serve many di erent purposes. Most large muscles, like the hamstrings and quadriceps, control motion. Other muscles, like the heart, and the muscles of the inner ear, perform other functions. At the microscopic level however, all muscles share the same basic structure.

At the highest level, the (whole) muscle is composed of many strands of tissue called fascicles. These are the strands of muscle that we see when we cut red meat or poultry. Each fascicle is composed of fasciculi which are bundles of muscle bers. The muscle bers are in turn composed of tens of thousands of thread-like myofybrils, which can contract, relax, and elongate (lengthen). The myofybrils are (in turn) composed of up to millions of bands laid end-to-end called sarcomeres. Each sarcomere is made of overlapping thick and thin laments called myolaments. The thick and thin myolaments are made up of contractile proteins, primarily actin and myosin.

1.2.1 How Muscles Contract

The way in which all these various levels of the muscle operate is as follows: Nerves connect the spinal column to the muscle. The place where the nerve and muscle meet is called the neuromuscular junction. When an electrical signal crosses the neuromuscular junction, it is transmitted deep inside the muscle bers. Inside the muscle bers, the signal stimulates the ow of calcium which causes the thick and thin myolaments to slide across one another. When this occurs, it causes the sarcomere to shorten, which generates force.

Chapter 1: Physiology of Stretching 4

When billions of sarcomeres in the muscle shorten all at once it results in a contraction of the entire muscle ber.

When a muscle ber contracts, it contracts completely. There is no such thing as a partially contracted muscle ber. Muscle bers are unable to vary the intensity of their contraction relative to the load against which they are acting. If this is so, then how does the force of a muscle contraction vary in strength from strong to weak? What happens is that more muscle bers are recruited, as they are needed, to perform the job at hand. The more muscle bers that are recruited by the central nervous system, the stronger the force generated by the muscular contraction.

1.2.2 Fast and Slow Muscle Fibers

The energy which produces the calcium ow in the muscle bers comes from mitochon- dria, the part of the muscle cell that converts glucose (blood sugar) into energy. Di erent types of muscle bers have di erent amounts of mitochondria. The more mitochondria in a muscle ber, the more energy it is able to produce. Muscle bers are categorized into slow- twitch bers and fast-twitch bers. Slow-twitch bers (also called Type 1 muscle bers) are slow to contract, but they are also very slow to fatigue. Fast-twitch bers are very quick to contract and come in two varieties: Type 2A muscle bers which fatigue at an intermediate rate, and Type 2B muscle bers which fatigue very quickly. The main rea- son the slow-twitch bers are slow to fatigue is that they contain more mitochondria than fast-twitch bers and hence are able to produce more energy. Slow-twitch bers are also smaller in diameter than fast-twitch bers and have increased capillary blood ow around them. Because they have a smaller diameter and an increased blood ow, the slow-twitch bers are able to deliver more oxygen and remove more waste products from the muscle bers (which decreases their "fatigability").

These three muscle ber types (Types 1, 2A, and 2B) are contained in all muscles in varying amounts. Muscles that need to be contracted much of the time (like the heart) have a greater number of Type 1 (slow) bers. When a muscle rst starts to contract, it is primarily Type 1 bers that are initially activated, then Type 2A and Type 2B bers are activated (if needed) in that order. The fact that muscle bers are recruited in this sequence is what provides the ability to execute brain commands with such ne-tuned tuned muscle responses. It also makes the Type 2B bers dicult to train because they are not activated until most of the Type 1 and Type 2A bers have been recruited.

HFLTA states that the the best way to remember the di erence between muscles with predominantly slow-twitch bers and muscles with predominantly fast-twitch bers is to think of "white meat" and "dark meat". Dark meat is dark because it has a greater number of slow-twitch muscle bers and hence a greater number of mitochondria, which are dark. White meat consists mostly of muscle bers which are at rest much of the time but are frequently called on to engage in brief bouts of intense activity. This muscle tissue can contract quickly but is fast to fatigue and slow to recover. White meat is lighter in color than dark meat because it contains fewer mitochondria.

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