Physiology of Endurance Training Part 4: Skeletal Muscle

Skeletal Muscle Fibre Types :

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

• Type I: Slow-twitch. These fibres rely on the oxidative phosphorylation system for ATP production. They have a remarkably high level of endurance, in other words, able to contract repeatedly over a period without getting fatigued.
  Example: The Soleus muscle predominantly consists of Type I muscle fibres.
• Type IIA: Fast-twitch oxidative glycolytic. These types of fibres consist of 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 types 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 three muscle types in varying proportions. 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 the 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 the working muscle.

Anaerobic (Sprint) Training :

The benefits of regular anaerobic training are as follows :

• Increases ATP — CP and glycolytic enzymes in skeletal muscle.
• Lactate and H+ ions are byproducts of the glycolysis energy system. High H+ concentrations affect 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 the muscle to grow in size.
• Increase in the 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 imply higher muscle fibres stimulated resulting in higher force generation.
• An 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, which 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.

Originally published at https://mindyourfitness.in on August 17, 2019.