Muscle structure, Muscle Fibres and Muscle Contraction.

Updated: Apr 3


In order to get the most from our training it is important to understand muscle structures , fibre types relevant to your goals and what is involved in contraction Muscles are like elastic bands, although much more complex, they attach onto bones via Tendons or Aponeurosis, the bones act as levers for movement and the muscles shorten and create a pulling force on bones creating movement. The structure of a muscle can be described as a series of structures within structures, much like nesting dolls, the first structure is the muscle or muscle belly, this is protected by a sheath (like clingfilm protecting food) that is called the Epimysium , the muscle belly is the largest part of the muscle and is made up of a series of other structures, the next layer is a bundle of structures called fascicles, the fascicle is also protected by a sheath called the Perimysium, then within the fascicles lies a bundle of muscle fibres each muscle fibre is protected by a sheath called Endomysuim.


Each muscle fibre can be characterized by it’s colour. There are actually 3 types of muscle fibres, type 1, slow twitch fibres which are red in colour and smaller than type 2 fibres. Type 1 fibres are used for low intensity exercise, like moving around the house or walking jogging or running, these are red in colour due to a rich blood supply, they use the aerobic energy system and fatigue slowly as the rich blood supply enables them to dispose of waste products like carbon dioxide and Lactic acid efficiently. Then there are type 2b, fast twitch fibres, these are relatively large with the ability to grow in size, type 2b fibres are used for high intensity exercise near maximal intensity, for powerful/explosive activities such as weight lifting or sprinting, these are white in colour due to a minimal blood supply meaning that they fatigue quickly as there is less contact with the bodies transportation system to quickly remove waste products, type 2b fibres use the anaerobic energy system. When someone wants to train endurance e.g if they are getting ready for a marathon then they should focus on training their slow twitch fibres using a combination of continuous cardio and low weight, high rep resistance training and when someone wants to train power and strength for sports involving power or are a bodybuilder or someone looking for muscle tone then they should be focussing on training their type 2 fibres using a combination of HIIT Cardio and Resistance training with high to moderate weight between the 1-12 rep range. Everyone is born with a different percentage of ratio of slow and fast twitch fibres creating different body types like ectomorph (slow twitch dominant), mesomorph (fast twitch dominant) and endomorph (fast twitch dominant with a tendency to store extra body fat), this is why some people like Usian Bolt who has a high percentage of fast twitch fibres are genetically gifted for powerful activities like sprinting and Mo Farrah who has a high percentage of Slow twitch fibres are genetically gifted for endurance activities like long distance running. That being said however there is the option to out rule your genetic pathways through training as there are intermediate fibres called type 2a which are between fast and slow twitch and are pink in colour, with regular training you have the ability to transform these fibres into fast or slow twitch fibres dependent on the type of training you do. So just because you were born with the genetic code to be naturally swayed towards endurance doesn’t mean you can’t build your power up and vise versa. This does however explain how those with the mesomorph body type find it easier to pack on muscle than Ectomorphs.


Going back to muscle structures the muscle fibre can be broken down into units of Sarcomeres which can be further broken down into contractile protein Myofilaments called Myosin and Actin. Strength training between the 1-6 rep range increases the amount of myofilaments, making the muscle Fuller and harder. Myosin and Actin are responsible for muscle contraction and slide along each other, when this happens inside multiple muscle fibres the force created is strong enough to contract the entire muscle and create movement at a joint. Although the Myosin and actin are described as sliding along each other they actually grab hold of one another and pull one across the other, when energized with ATP.

(Adrenosinetriphosphate) the Myosin heads expand and grab hold of the actin and pull it along, for this to happen calcium must be present as Calcium opens up the Binding sites on the actin, enabling the Myosin heads to grab hold of the binding site, this motion of contraction is known as the Sliding Fillament theory

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