Muscle Fibres


Muscles are contractile tissue controlled via the nervous system. The role of skeletal muscle is to produce force and cause motion to propel the body forward through movement or functioning within internal organs. Most muscle contractions occur without conscious thought and is necessary for survival, like the contraction of the heart or peristalsis, which pushes food through the digestive system. Other forms of muscle movement are deliberate and focused such as exercise and training to induce muscle growth and strength.


In mammals there are 3 types of muscle: skeletal or striated muscle; smooth or non-striated muscle; and cardiac muscle, which is sometimes known as semi-striated. The muscle tissue we focus on for exercise is striated or skeletal muscle.

Skeletal muscle cells are elongated cells ranging from several millimetres to about 10 centimetres in length and from 10 to 100 micrometres in width. These cells are joined together in tissues. They are under voluntary control, and anchored to bones via tendons, used for skeletal movement in exercise and maintaining posture. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass).

Skeletal muscle is made up of highly regular arrangements of bundles. This allows the muscle to contract and relax in short, intense bursts. These bundles are made up of myofibres. A single myofibre is made up of myofibrils – a muscle cell packed full of sarcomeres – the functional unit of contraction. Under a microscope, it’s the overlapping of actin and myosin filaments within a Sarcomere that creates the contraction (see diagram 1). When a muscle contracts, the actin and mosyin filaments slide towards each other until they touch. Upon relaxation they slide back away from each other to relax.

Skeletal muscle can be further divided into two subtypes, with each person being made up of a completely different array of both types of muscle fibres.

  • Slow twitch muscle – dense with capillaries and rich in mitochondria and myoglobin. It can therefore carry more oxygen and thus sustain aerobic activity. Also known as ‘oxidative’ muscle and makes them ideal for endurance type exercise.
  • Fast twitch muscle – there are 3 major kinds, but to keep it brief, these fibres are lower in mitochondria and myoglobin density, with the ability to contract very quickly and with a greater amount of force but can sustain only short anaerobic bursts of activity before muscle contraction becomes painful – often incorrectly attributed to a build-up of lactic acid). This is simply lack of oxygen in these muscle fibres. This makes these muscle fibres ideal for short, powerful movements. In the limbs, most people have predominantly fast-twitch fibres, since you need more power for movement.


Skeletal Muscle Structure

Exercise and The Impact on Muscle

After exercise that may be either particularly difficult or new for the body, muscle soreness can occur. The discomfort felt in muscles during and straight after strenuous exercise is caused by lactic acid accumulation. Lactic acid is a normal by-product of glucose metabolism in the muscle for energy production (see Appendix 1 below for chemical explanation). However, it can irritate the muscle causing discomfort or soreness. Lactic acid isn’t the only culprit for muscle soreness. In fact lactic acid is removed from muscle approximately 1 hour after exercise– so it doesn’t explain soreness experienced days after a workout. This is called Delayed Onset Muscle Soreness (DOMS).

Delayed Onset Muscle Soreness (DOMS)

It is the pain and stiffness felt in muscles several hours to days after unaccustomed or strenuous exercise. This can be caused by eccentric (lengthening) of a muscle under tension (such as the lengthening of the quadricep in a squat movement) which causes small scale damage (micro trauma) to muscle fibres, and more specifically the microfibrils. The healing of these micro tears in the muscle fibres occurs rapidly to prevent muscle damage if the same movement was to be performed in the future. When micro trauma occurs in the muscle fibres there is swelling in the muscle compartment caused by an influx of white blood cells, prostaglandins (inflammatory factors), and other nutrients and fluids that are required to repair the damage. In addition, the second theory is that the micro tears are thought to occur through the centre of the Sarcomere, which may create tension and stiffness along the muscle fibres. The natural sliding motion of the actin and myosin filaments when a muscle contracts is also reduced adding to stiffness. The combination of theories leaves the muscle starting to feel sore and tight anywhere from 24-72 hours after the activity was carried out.

How can this be applied to my training?

The Human Body can adjust to provide a better outcome for the future at any time. When we perform the same eccentric movement a week after the first, we are left feeling much less stiff and sore. This is because the muscle has the ability to rapidly adapt to the damage from eccentric exercise, to prevent further damage in the future. This is called the ‘adaptation process’. The muscle develops many new traits to prevent the same micro trauma from occurring, such as the replication of more sarcomeres within a single microfilament, as well as more capillaries which deliver more oxygen and nutrients to a muscle for longer periods of support – the list goes on (see future blogs for more info on exercise induced muscular adaptation processes). As each week passes, and the eccentric movement of a muscle is repeated, the discomfort in the muscle is reduced….until NEW ROUND WEEK 1! This is where prevention and treatments come in.

Preventing and Treating DOMS

To help prevent or reduce severity of DOMS, gradually increasing the intensity of a new exercise program can help to reduce the extent of micro trauma and thus inflammatory response in the muscle after. The 8-week programs are designed to do specifically this. Starting low and increasing weights and intensity of movements in a measured deliberate process is key. A level of soreness will be expected at each increase, or inclusion of new movement – yet the intensity of muscle pain can be reduced.

Lifestyle plays a big part in how your muscles may feel through the ‘adaptation process’. Always start with natural aids first. Any measure of movement that can gently increase blood flow to the muscle, such as low intensity activity, massage, hot baths or an Infrared sauna can help. Making sure your diet is supporting your level of activity is also essential. Hydration, inclusion of good fats and nutrient dense foods will allow the body to deliver the necessary elements required to the muscle for faster recovery. The use of supplements can also aid in muscle recovery and reduction in stiffness. Magnesium, Glutamine, Turmeric and Fish Oil can all help to reduce stiffness in muscles and joints, as well as speeding up muscle regeneration. Magnesium specifically helps to relax the actin and myosin filaments that may not have relaxed into normal formation. Low levels of magnesium also lead to faster build-up of Lactic Acid. Turmeric and Fish Oil acting as natural anti-inflammatory agents to reduce tension along the microfilaments. Glutamine helps by speeding up muscle regeneration through the adaptation process by helping heal the micro trauma or micro tears more efficiently.

Lastly the use of oral anti-inflammatories has been proven to help reduce the inflammation that occurs within the microfilaments leading to less discomfort, however natural and non-medicinal approaches are preferred first.

Appendix 1 – Lactic Acid production cycle in mammals

During power exercises involved in HIIT workouts or repetitive weight lifting the body recruits the use of fast-twitch muscle fibres. During this intense exercise, the circulatory system cannot keep up with the muscles demand for oxygen. To maintain a steady supply of energy, muscles shift from aerobic metabolism (which requires oxygen) to anaerobic metabolism, which does not. At this point the body will then break down glucose from carbohydrates to provide energy resulting in a compound called Pyruvate. When sufficient oxygen is not available – pyruvate is then converted to Lactic Acid. Lactic acid is rapidly broken down into a compound called lactate, resulting in the release of hydrogen ions. Your body can clear Lactate by metabolising it for energy but when Lactate production exceeds the clearance rate, it accumulates in your muscles and bloodstream. It is not the Lactate that causes muscle fatigue but in fact it is the acidity in your tissues due to the build-up of Hydrogen ions formed during Lactic Acid metabolism. This process is known as the ‘ Cori Cycle’ – see Diagram 2 below. The concentration of blood lactate is usually 1–2 mmol/L at rest, but can rise to over 20 mmol/L during intense exertion and as high as 25 mmol/L afterward.


The Cori Cycle