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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.

Marathoners

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

where is heart located
medical Reply
thoracic cavity
Ashley
1
Noor
between which line
medical
where is the thorax located at?
angel
heart located between T5-T8
Biplav
thorax is the area between the base of neck to the diaphragm
Biplav
The heart is placed Obliquely behind the body of the sternum and adjoining part of the costal cartilages, so that 1/3 of its lies to the right and 2/3 to the left of the median plane
Manisha
why does a person's complexion change when he relocates to another cold environment & tends to become more light skinned, does this mean the distribution of melanin to the surface of the skin changes ?
Ferrel Reply
what is Aponeurosis
Komolika Reply
aponerosis is modifications of deep fascia of the skin.. it generally covers the muscles Or tendons within it
niyati
A sheet of parly white fibrous tissue that takes the place of tendon in flat muscles having a wide area of attachment
mishra
what is the difference between sex and gender?
Hridya Reply
Those are synonyms
Ferrel
What is Physiology?
Nancy Reply
The study of functions of the human body.
Evans
study of human body
obaidullah
The biochemical study of any organism (eg. different systems) is called Physiology.
Doctor
which category of bone is among the most numerous in the skeleton?
DAMSON Reply
what is a nutrition ?
Jitu Reply
Nutrition is the process of providing or obtaining the food necessary for health and growth.
Gloria
Nutrition is the process of taking food and using it for growth , repair and metabolism ...
Om
how many parts of bone
Amir Reply
know the meaning of both messages
Amir
what is the name of suture that is present between frontal. parietal. occipetal ...
Amir
what is the name of suture that is present between frontal. parietal. occipetal ...
Amir
Coronal suture present between frontal and parietal bone and lambdoid suture present between parietal and occipital bone ...
Om
Squamous suture
Ferrel
& where is the squamous suture ?
Ferrel
what is meant by synovial joint? and what is this role
Fozia
type of joint that is found between bones that move against each other. it's function is to prevent bone friction
Ferrel
epinephrine is the true meaning of epi
Michele Reply
what does the word 'epi' means
Melex Reply
good evening, my question is what are function of metal in human body like aluminum?
Yosef Reply
please want you to help me in understanding digestive system
Hamidu Reply
this is muscular system?
Patel Reply
bruh moment
Viana
It is an organ system consisting of skeletal ,smooth and cardiac muscles
mishra
im too retarded to be here
Viana
naww Viana, I know it's hard to absorb info, I'm having the same issue, but keep practising, chin up.
Katarina
what is the question
Amir
to have a better understanding as to how our human bodies function. the knowledge we gain can improve our quality of life. Without our study of anatomy how could a doctor know what prescription to give us when our bodies is being attacked by a bacteria? We wouldn't!
Robin Reply
discuss nervers system in human being life.
Allen Reply
neuralgia what is other name of it
Muniba

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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