The marvelous human body Part 3


By Elias Abichacra (Phd)

The skeleton of an adult represents between 15 and 20 % of the body weight. Our skeleton, which can totalize up to 350 bones for a newborn baby, gradually gets reduced to 206 different bones when we reach adulthood. Our skeleton plays multiple roles. Firstly, it gives us this particular shape that allows us to stand up in an upright position that differentiates us from other mammals. Secondly the bones serve as fixing points for muscles that can produce tension when they contract. By crossing different joints, muscles that are attached to the bones can produce movement when they contract. Thirdly our red blood cells are produced in the bone marrow. Fourthly bones serve as reserves for minerals like Calcium and Phosphorous. Finally our bones play an important protection role for vital organs like the brain (Skull), lungs and heart (rib cage) and the spinal cord (vertebral column).
In the following chapters I shall talk about muscles. They are the major actors of movement which are visible and invisible. First of all it is important to know that we have 3 types of muscles which are the Cardiac, the Smooth and the Skeleton or Striated muscles. When we talk about Fitness and physical activity (PA) we will only deal with the Cardiac and the Skeleton muscle (SK) that are classified as striated muscles. Only the skeleton muscles are called voluntary muscles whereas the other two are involuntary muscles. The inseparable couple formed by the skeleton and the different muscles works in perfect synergy. Their biological complementarity indicates us that the skeleton needs muscles to stay erect and the muscles need the skeleton to get fixed and produce tension and contract to create movement. This biological pair can go to great lengths of improvement when a person starts to practice a physical activity or sport regularly. Our muscles can be considered as one of the most amazing parts of our body. They are made of 75 % of water. Proteins constitute the most important part. Blood vessels and nerves are inseparable elements that accompany the muscle. The muscle has contradictory properties. The length of the muscle at rest can be extended when stretched or diminish when contracted. This is the reason that classifies the muscle as a plastic organ and not an elastic organ because it can be deformed and change its shape easily.
A muscle is made of fibers that are the contractile elements that produce movement. The muscles of human beings shelter 3 types of fibers. We have Slow Twitch (ST) fibers and Fast Twitch fibers. The latter are divided in two elements where we have the Fast intermediate (FI) and the Fast Fatigable (FF) fibers. The ST fibers are red in color because densely irrigated by blood vessels, are thin and produce a very small contraction force of 2 grams per fiber unit. The FI are a mix of the ST and FF fibers. They have a pink color because less densely irrigated; they are quite thick in dimension with a contraction capacity of 10 grams per fiber unit. The FF fibers are white in color because they are poorly irrigated. Their thickness is the highest of the 3. They can produce 50 grams of contraction force. Through the description of the 3 fibers, it is easy to conclude that force produced depends on fiber diameter. ST fibers are richly irrigated and this gives them the advantage to delay fatigue because they are constantly supplied through blood vessels with Oxygen. These are the most wanted fibers to be an endurance runner. The FI produce 5 times more contractile force but as they possess fewer blood vessels their capacity to withstand fatigue is short. The FF fibers are white because of the rarity of the blood vessels. Their capacity to produce contractile force is tremendous (5 times FI and 25 times ST fibers respectively). Their capacity to resist fatigue is extremely short. They are the fibers of sprinters, jumpers and throwers. Physiology shows us that you can’t be very strong and enduring at the same time. You can’t be Haile Bolt or Usain Gebreselassie! You are either Haile Gebresellasie or Usain Bolt!
The distribution of muscle fibers depends on biological lottery. Each person has a certain percentage of the 3 fiber types. Most of the muscles have a relative equal distribution of the Slow and Fast fibers. Some muscles are exclusively composed (100%) with ST fibers like the 3rd muscle of the calf muscle. The eye muscles are exclusively composed (100%) with FF muscle fibers. The biological heritage influences the fiber distribution in each individual. If the mother and father of an athlete were long distance runners (richly endowed with ST fibers) the child will inherit ST fibers. On the contrary if the parents were sprinters, then the child will inherit FT fibers. The problem appears when each parent has an opposite fiber distribution. To this date, no serious study has elucidated this enigma! That’s why coaches say ‘’if you want to be a sprinter, you have choose your parents that were former sprinters’’!!!
Remember Health is Wealth

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