Skeletal muscle makes up about 36% of an average females body mass (42% for an average male). Skeletal muscle has many functions and is responsible for moving the human body. Each skeletal muscle contains bundles of individual muscle fibres called myocytes. These myocytes contain myofibrils, which are strands of protein that grab onto each other and pull. This shortens the muscle causing the muscle to contract and so movement is created. Most skeletal muscles contain a varying proportion of each fibre type; this includes slow oxidative, fast oxidative and fast glycolytic muscle fibres. Let’s take a look at what characterises each fibre type and how they impact the way we move.
Slow Oxidative Muscle Fibers
This type of muscle fibre is more commonly referred to as slow twitch or type 1 muscle fibres. Type 1 fibres contain high volumes of blood carrying myoglobin as well as mitochondria (an organelle that uses oxygen to create ATP), this gives them a dark red colour. Type 1 fibres produce high levels of ATP through aerobic metabolism allowing them to be extremely fatigue resistant. This gives them great endurance capabilities and as a result they make up the bulk of fibres found in postural muscles like those found in the neck. Type 1 fibres have a low activation threshold so are the first recruited when a muscle contracts, once engaged they can maintain low levels of force for extended periods but once force demands become to great the fast twitch fibres will engage. Endurance athletes such as marathon runners tend to have high volumes of type 1 fibres due to their ability to support prolonged bouts of steady state activity.
Fast Oxidative Muscle Fibres
Fast oxidative fibres are better known as type 2a or intermediate fast twitch fibres. Type 2a fibres are considered to be a hybrid between type 1 and type 2 fibres, this is because they still contain large amounts of myoglobin and mitochondria but unlike type 1 fibres can manufacture and split ATP using both aerobic and anaerobic metabolism. This allows them to produce fast, strong and forceful muscle contractions. Type 2a fibres are more fatigue resistant then fast glycolytic fibres but fatigue quicker then type 1 muscle fibers. Some research suggests these fibres can be converted to type 1 or type 2b fibres to help your body meet specific movement demands.
Fast Glycolytic Muscle Fibres
Type 2b fast twitch muscle fibres rely solely on ATP which is stored in the muscle cells for energy production; they access this ATP through a process called anaerobic (without oxygen) glycolysis. Type 2b fibres are the largest of all muscle fibres but white in colour due to a lack of myoglobin and mitochondria. Considered to be the powerhouse of the three muscle fibres, type 2b fibres have a high activation threshold and will only be engaged if the other fibres can not meet force demands. They reach peak force quickly and produce rapid, forceful contractions that support powerful movements. Type 2b fibres fatigue quickly so can only be utilised for short periods of activity. Regular strength and power training increases the number of fast twitch fibres that are activated for specific movements. Power athletes that need to perform explosive movements or movements under heavy load (ie. sprinters or weight lifters) have greater volumes of type 2b muscle fibres.
The muscles in the human body have a genetically determined combination of each fibre type, this impacts the way we respond to different training techniques and forms of activity. This is partly why some forms of exercise feel more natural then others. Understanding the physiology behind how your body responds to exercise will allow you to be more specific in your goal setting and adopt a training style that best suits your body.
xx Tara
Do you think you have more slow or fast twitch fibres? Let me know in the comments section below.
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