Physiology of Endurance Training Part 4 : Skeletal Muscle

Physiology of Endurance Training Part 4 : Skeletal Muscle

August 17, 2019

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Abhay Kedari

Pediatric Nutrition Services
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Skeletal Muscle Fibre Types :

Most textbooks on exercise physiology classify the skeletal muscle fibres into three basic types. However, recent research suggests that there might be several additional muscle fibre types present. For the purpose of this blog, we shall focus only on the prevalent three muscle fibre types, namely :

  • Type I : Slow twitch. These fibres rely on oxidative phosphorylation system for ATP production. They have remarkably high level of endurance, in other words able to contract repeatedly over a period of time without getting fatigued.
    Example : The Soleus muscle predominantly consists of Type I muscle fibres.
  • Type IIA : Fast twitch oxidative glycolytic. These type of fibres consist characteristics of both Type I and Type IIX muscle fibres I.e. they are capable of producing ATP via oxidative phosphorylation as well as glycolytic metabolism. These type of fibres are mostly seen in the diaphragm muscle.
  • Type IIX :Fast twitch glycolytic. These muscle fibres can produce a great amount of force, but they rely on short-term energy systems for AT production and thus have limited endurance capability.
    The gastrocnemius muscle predominantly consists of Type IIX muscle fibres.

Most muscles have all the three muscle types in varying proportion. The soleus muscle, thus consists not just of Type I muscle fibres. Instead, it has Type I as the predominant muscle fibre type but also includes Type IIA and Type IIX in lower proportion.

Type I muscle fibres facilitate endurance activities and are found in abundance in endurance athletes. In contrast, Type IIX fibres are found in relative abundance in sprint and power athletes. Type IIA fibres are essentially a hybrid of Type I and Type IIX fibres.

Training Effects on Skeletal Muscle :

The following three types of training can bring about physiological changes in skeletal muscles :

  • Aerobic (or endurance) training.
  • Anaerobic (or sprint) training.
  • Resistance training.

Aerobic (Endurance)Training :

With endurance training the Type I muscle fibres are challenged predominantly.

Although the percentage of Type I and Type IIX fibres does not change with regular endurance training, but it may cause Type IIX fibres to take on more characteristics of Type IIA fibres.

Other significant effects of endurance training are :

  • Increase in number of capillaries supplying each muscle fibre.
  • Increase in the size and number of mitochondria within the cell, resulting in enhanced oxidative energy production.
  • Increase in oxidative enzymes.
  • Increase in muscle myoglobin count, enhancing oxygen delivery within working muscle.

Anaerobic (Sprint) Training :

Benefits of regular anaerobic training are as follows :

  • Increases ATP – CP and glycolytic enzymes in skeletal muscle.
  • Lactate and H+ ions are byproducts of glycolysis energy system. High H+ concentrations affects skeletal muscle concentration, leading to premature fatigue. With regular Anaerobic training concentration of bicarbonate (HCO3-) in skeletal muscle increases. Bicarbonate acts as a buffer for H+ ions thus improving performance.

Resistance Training

Following are the benefits of regular resistance training (3 to 5 sessions per week for at least 8 weeks) :

  • Increase in size of muscle fibre (hypertrophy). Resistance training in combination with proper nutrition and rest will cause muscle to grow in size.
  • Increase in number of active motor units. A motor unit is made up of several muscle fibres along with the nerve that innervates those fibres. When a motor unit activates it stimulates the muscle fibres it innervates, causing the muscle fibres to contract. Thus more active motor units implies higher muscle fibres stimulated resulting in higher force generation.
  • Increase in muscular strength, usually measured as the maximum weight an individual can lift just once, also known as the one rep max (1RM)
  • Increase in muscular power, the explosive aspect of strength. It depends on how fast the force is generated.
    Example : Athlete A can lift 150kg in 2 seconds for 1RM and Athlete B can lift 150kg in 1 sec, implies Athlete B has twice the muscular power as Athlete A.

In the upcoming blogs we shall briefly checkout the various Endurance tests and assessments.

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