How Muscles Work

Understanding how muscles work will help you target specific types of training to achieve your climbing goals. There are different types of muscles for slow or fast movement, and different chemical processes used for strength, burst or endurance. Depending on whether your movement requires a slow contraction, short burst of power, longer sustained movement of low intensity, or high intensity contraction for longer periods of time, will determine the muscle type and chemical reaction used to produce power.

THE TYPES OF MUSCLES: Fast twitch and slow twitch

Fast Twitch Muscles. The purpose of this type of muscle 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. There are also two types of fast twitch muscles. These two types of fast twitch muscles will function during moderate and maximum muscle effort. Fast twitch muscles provide you strength and speed.

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. These types of muscles are large muscles found in the legs, thigh, trunk, back, hips and are used for holding posture.

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. 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 fast twitch or 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 form the following chemical reaction. The enzyme creatine kinase mediates ATP production from the high energy molecule creatine phosphate by an anaerobic reaction:

CP + ADP → ATP + Creatine

The CP (Creatine Phosphate) is depleted in just a few seconds. This is the reason 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. Carbohydrates metabolize the most efficiently and are therefore used first. If carbohydrates are not available, your body metabolizes fat and amino acid proteins. All three of these reactions are called Aerobic Glycolysis because they use glucose and oxygen:

1. Carbohydrate Metabolism: Glucose + 02 → 36ATP + C02 + H20
2. Fat Metabolism: Fatty Acid + 02 → 130 ATP + C02 + H20
3. Amino Acid Protein Metabolism: Amino Acids + 02 → 15 ATP + C02 + H20

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 glycogen stores are used up your body obtains its energy from fatty acid metabolism and amino acid protein metabolism. These reactions are not efficient, which consequently cause your strength and endurance to drop drastically.


Muscles change and develop with regular exercise but the effects differ, depending on whether you engage in strength, speed, or endurance training.

Strength and burst training cause the muscle fibers to enlarge. Individual muscle fibers increase in diameter as a result of an increase in intracellular protein fibrils.

Endurance training causes more blood vessel formation than does speed or strength training, which produces an increased capacity for aerobic metabolism within the muscle cell. This change is seen after a few weeks of training and is maximized in about three months. The aerobic enzymes that metabolize carbohydrates, fats, and proteins double.

It is important to develop your strength and speed systems, but if you want to continue past about two minutes of high intensity workouts, you need to have your aerobic systems developed.


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