How Muscles Work

Muscle and Fitness go hand in hand. Understanding How Muscles Work will help you target Specific Types of Training to achieve your Fitness Goals.

There are different Types of Muscles for slow and fast movements, and different chemical processes used for strength, burst, or Endurance. Both the muscle type and chemical reaction used to produce power will depend on whether your movement requires a slow contraction, a short burst of power, longer sustained movement of low intensity, or high intensity contraction for longer periods of time.

THE TYPES OF MUSCLES: Fast twitch and slow twitch

How Muscles Work Fast Twitch Muscles
The purpose of this Type of Muscles is to provide rapid movement for short periods of time. Fast twitch muscles do not use oxygen - they use glycogen. Reactions using glycogen require anaerobic enzymes to produce power. Glycogen is stored in the muscles and liver, and is synthesized by the body using carbohydrates. Furthermore, fast twitch muscles provides you with strength and speed, and also has two types. The two types of fast twitch muscles function during moderate and maximum muscle effort.

Slow Twitch Muscles
As their name indicates, these fibers have a slower contraction time. Slow twitch muscles use oxygen for power and have a predominance of aerobic enzymes. This Type of Muscles are large muscles found in the legs, thigh, trunk, back, and hips, and are used for holding posture. Adenosine Triphosphate (ATP) is the main source of energy for all muscle contraction. There are several chemical reactions that take place to produce ATP. When a muscle is used, a chemical reaction breaks down ATP to produce energy:

ATP + Actin + Myosin ' Actomyosin + Phosphate + ADP + Energy
This is the chemical reaction that produces energy. However, there is only enough ATP stored in the muscle cell for two or three slow twitch contractions, or one burst of power from a fast twitch contraction. Thus, more ATP must be created. There are three enzyme systems that can create more ATP. The enzyme system that is used depends on whether the type of muscle is a fast twitch or a slow twitch, and whether the muscle is used for strength, burst power, or endurance.

THREE ENZYME SYSTEMS TO CREATE ATP: Strength, Burst Power, and Endurance

The Strength Enzyme System
When muscle strength is required, ATP is created quickly from the following chemical reaction. The enzyme creatine kinase mediates ATP production from the high energy molecule Creatine Phosphate (CP) by an anaerobic reaction:

CP + ADP -> ATP + Creatine
The CP is depleted in just a few seconds. This is why your maximum power can be maintained for only a few seconds. To continue producing high strength power, the speed enzyme system kicks in.

The Burst Power Enzyme System
The enzymes required for this reaction are depleted in less than two minutes. This reaction is called Anaerobic Glycolysis because it uses glucose without oxygen.

Glucose -> 2ATP + 2 Lactate
To continue muscle usage requires the aerobic system to kick in. The aerobic system uses oxygen and sugar for fuel. Your ability to perform well after about two minutes of maximum exertion depends on the aerobic conditioning of your body.

The Endurance Enzyme System
There are three sources of ATP for aerobic muscle to use: carbohydrates, Fats, and amino acid proteins. Among the three, the most efficient in metabolizing are carbohydrates. Thus, carbohydrates are used first. If carbohydrates are not available, your body will then metabolize fat and amino acid proteins. All three of these reactions are called Aerobic Glycolysis because they use glucose and oxygen:

  1. Carbohydrate Metabolism: Glucose + O2 -> 36 ATP + CO2 + H2O
  2. Fat Metabolism: Fatty Acid + O2 -> 130 ATP + C2 + H2O
  3. Amino Acid Protein Metabolism: Amino Acids + O2 -> 15 ATP + CO2 + H2O

Your body stores glucose and fatty acids for these reactions. Your cardiovascular system provides a continuous supply of oxygen. Glycogen is stored in the muscles and liver in sufficient quantities for about two hours of strenuous exercise. You can extend this time by Aerobic Physical Conditioning and high carbohydrate diet. After your stored glycogen are used up, your body obtains its energy from fatty acid metabolism and amino acid protein metabolism. However, these reactions are not efficient, which consequently cause your strength and endurance to drop drastically.

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