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By the end of this section, you will be able to:
  • Describe hypertrophy and atrophy
  • Explain how resistance exercise builds muscle
  • Explain how performance-enhancing substances affect muscle

Physical training alters the appearance of skeletal muscles and can produce changes in muscle performance. Conversely, a lack of use can result in decreased performance and muscle appearance. Although muscle cells can change in size, new cells are not formed when muscles grow. Instead, structural proteins are added to muscle fibers in a process called hypertrophy    , so cell diameter increases. The reverse, when structural proteins are lost and muscle mass decreases, is called atrophy    . Age-related muscle atrophy is called sarcopenia    . Cellular components of muscles can also undergo changes in response to changes in muscle use.

Endurance exercise

Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria.

The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis    , to supply oxygen and remove metabolic waste. To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase in order to maintain a smaller area for the diffusion of nutrients and gases. All of these cellular changes result in the ability to sustain low levels of muscle contractions for greater periods without fatiguing.

The proportion of SO muscle fibers in muscle determines the suitability of that muscle for endurance, and may benefit those participating in endurance activities. Postural muscles have a large number of SO fibers and relatively few FO and FG fibers, to keep the back straight ( [link] ). Endurance athletes, like marathon-runners also would benefit from a larger proportion of SO fibers, but it is unclear if the most-successful marathoners are those with naturally high numbers of SO fibers, or whether the most successful marathon runners develop high numbers of SO fibers with repetitive training. Endurance training can result in overuse injuries such as stress fractures and joint and tendon inflammation.


This photograph shows some runners in a race.
Long-distance runners have a large number of SO fibers and relatively few FO and FG fibers. (credit: “Tseo2”/Wikimedia Commons)

Resistance exercise

Resistance exercises, as opposed to endurance exercise, require large amounts of FG fibers to produce short, powerful movements that are not repeated over long periods. The high rates of ATP hydrolysis and cross-bridge formation in FG fibers result in powerful muscle contractions. Muscles used for power have a higher ratio of FG to SO/FO fibers, and trained athletes possess even higher levels of FG fibers in their muscles. Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the thickness of muscle fibers. This added structure causes hypertrophy, or the enlargement of muscles, exemplified by the large skeletal muscles seen in body builders and other athletes ( [link] ). Because this muscular enlargement is achieved by the addition of structural proteins, athletes trying to build muscle mass often ingest large amounts of protein.

Questions & Answers

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Which plane divides the body into right and left parts
sagittal plane
Thank you
Homestatic regulations usually involves a (n) _ that detects a particular stimulus, and a(n) _that respond to the stimulus by communicating with a (n) _whose activity has an effect on the same stimulus.
A cell is producing proteins to be transported out of that cell. They will be processed on ribosomes that are
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Diabetes insipidus or diabetes mellitus would most likely be indicated by ________.
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study of structure of an object in this case human body.
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This medication is used to treat heavy bleeding during your menstrual period. Tranexamic acid works by slowing the breakdown of blood clots, which helps to prevent prolonged bleeding. It belongs to a class of drugs known as antifibrinolytics
In the body, vitamin K plays a major role in blood clotting. So it is used to reverse the effects of “blood thinning” medications when too much is given; to prevent clotting problems in newborns who don’t have enough vitamin K; and to treat bleeding caused by medications including salicylates, sulfo
Vitamin K plays a key role in helping the blood clot, preventing excessive bleeding. Unlike many other vitamins, vitamin K is not typically used as a dietary supplement. Vitamin K is actually a group of compounds. The most important of these compounds appears to be vitamin K1 and vitamin K2. Vitami
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Blood, fluid that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. ... It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix (plasma), which makes the 
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heart layers
endoceime is the inner layer of heart
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Peer Reply
a buffer is a solution that resists a chemical change.
Yup it also balances the solutions
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How many systems are in the body?
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names the systems pls
name the systems pls
integumentary system, lymphatic system, skeletal system, digestive system, cardiovascular system, reproductive system, respiratory system, urinary system, endocrine system
approximately 11 system
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most common source of phosphoric acid is an 85% aqueous solution; such solutions are colourless, odourless, and non-volatile. The 85% solution is a syrupy liquid, but still pourable. Although phosphoric aciddoes not meet the strict definition of a strongacid, the 85% solution is acidic 
What is osteoblast and what is osteoclast?
osteoblast is the bone forming cells osteoclast is the bone dissolving cells
Which of the following is most likely to be found evenly distributed in water in a homogeneous solution? a. sodium ions and chloride ions b. NaCl molecules c. salt crystals d. red blood cells
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