Muscle, Fibers and Optimal Muscle Fatigue
Muscle (from Latin musculus, diminutive of mus "mouse") is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads.
There are three types of muscle:
Skeletal muscle or "voluntary muscle" is anchored by tendons (or by aponeuroses at a few places) to bone and is used to effect skeletal movement such as locomotion and in maintaining posture. Though this postural control is generally maintained as an unconscious reflex, the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass).
Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control.
Cardiac muscle is also an "involuntary muscle" but is more akin in structure to skeletal muscle, and is found only in the heart.
Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction. It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorised into Type IIa and Type IIb fibers.
These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types. On average, we have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.
Slow Twitch (Type I)
The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.
Fast Twitch (Type II)
Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.
Type IIa Fibers
These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.
Type IIb Fibers
These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and cannot last as long before it needs rest.
Motor units (a group of identical-twitch fibers) can be contracted either simultaneously, such as when your muscles are made to contract against a very heavy load, or sequentially, such as when the activity you are performing involves a more modest load but is continued over a period during which certain lower-order fibers fatigue out and are replaced by a progressive recruitment of higher-order motor units. Time is important factor in the recruitment process as well, in that as you fatigue through the slow-twitch motor units, you will proceed up to the next-largest motor units, the intermediate-twitch. If you fatigue through those quickly enough so that the slow- and intermediate-twitch motor unit do not have time to recover, then (and only then) you proceed to recruit the fast-twitch motor units, thereby ensuring a sequential recruitment and fatiguing of all of the available motor units. This results in the most thorough involvement (and stimulation) of the muscle or muscle group that you are training.
It is desirable to employ a moderately heavy weight that allows you to progress through all three motor-unit types quickly enough to recruit them all, but not so quickly that only the fast-twitch fibers receive the bulk of the stimulation, and not so slowly that the slow and/or intermediate-twitch motor units can recover and and you end up cycling through the same lower-order motor units again. That would leave the bulk of the fibers in the muscle that are training largely unstimulated.
For exercise to be optimal, as many muscle fibers as possible must be called in to play and fatigued. The fatigue rate should fall anywhere between 40 seconds and 150 seconds. Optimally, we are looking at 45 to 90 seconds for particular set of exercise as the desired time frame for reaching maximum level of fatigue.
You should follow me on Twitter here Follow @DKPostings
There are three types of muscle:
Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control.
Cardiac muscle is also an "involuntary muscle" but is more akin in structure to skeletal muscle, and is found only in the heart.
Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction. It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorised into Type IIa and Type IIb fibers.
These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types. On average, we have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.
Slow Twitch (Type I)
The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.
Fast Twitch (Type II)
Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.
Type IIa Fibers
These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.
Type IIb Fibers
These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and cannot last as long before it needs rest.
It is desirable to employ a moderately heavy weight that allows you to progress through all three motor-unit types quickly enough to recruit them all, but not so quickly that only the fast-twitch fibers receive the bulk of the stimulation, and not so slowly that the slow and/or intermediate-twitch motor units can recover and and you end up cycling through the same lower-order motor units again. That would leave the bulk of the fibers in the muscle that are training largely unstimulated.
For exercise to be optimal, as many muscle fibers as possible must be called in to play and fatigued. The fatigue rate should fall anywhere between 40 seconds and 150 seconds. Optimally, we are looking at 45 to 90 seconds for particular set of exercise as the desired time frame for reaching maximum level of fatigue.
You should follow me on Twitter here Follow @DKPostings
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