Understand How Your Body Works: Muscle Physiology

Alike anatomy, knowledge about body physiology constitutes an important basis for developing better training programs. This post summarizes the basic concepts about muscle physiology covered by Mark Strasser in his article “Muscle Types, Strength Gains, and Energy Systems used in Various Sports”. This article focuses in two aspects of muscle physiology: muscle energy systems & muscle fibers.

Muscle Energy Systems. Energy is supplied to your muscles from the food you eat, which is broken down into usable blocks of energy called ATP (Adenosine Triphosphate). ATP is the primary source of energy for muscle contraction. The energy is derived from removing a phosphate ion from ATP, resulting in ADP (Adenosine Diphosphate). Your body makes ATP available for muscle contraction through three main energy systems:

• Phosphocreatine system: This system comes into play mostly during very intense workloads lasting up to 20-30 seconds. It is very fast, and can supply ATP for more muscular contractions in milliseconds by combining ADP with the phosphate ion from the phosphocreatine molecule.

• Glycolytic system: This system is the primary energy source in activities lasting between 30 seconds and 3 minutes by breaking down muscle and liver glycogen stores. The waste product is lactic acid, which leads to muscular fatigue. This system is anaerobic, as no oxygen is immediately necessary for energy production. However, from the 3rd minute of exercise this system becomes aerobic, since energy production occurs in the presence of oxygen.

• Oxidative system: This system provides the body with energy during exercise of long duration and moderate to low intensity by breaking down the body's fat stores. It is known as the aerobic system, as oxygen is necessary for energy production.

Muscle Fibers. There are two different types of muscle fibers: slow-twitch and fast-twitch muscle fibers. Slow-twitch fibers are predominately used in endurance activities. They are not likely to grow in size as much when trained and rely mainly on the oxidative system (aerobic metabolism) to obtain energy. Fast-twitch fibers are especially activated in explosive movements. They have great potential for growing in size when trained and rely mainly on the glycolytic system (anaerobic metabolism) to obtain energy.

When training with high number of reps you activate mainly slow-twitch fibers, while with low number of reps you activate mainly fast-twitch fibers. However, sets in the 1-5 rep range don't activate as many fast-twitch fibers as sets in the 6-12 rep range. The 1-5 rep range is more appropriated for strength training without increment on size.

You can also find some additional information on muscle energy systems and muscle fibers in the “Anatomy and Physiology” section of Muscle 101 site. Moreover, Derek Charlebois provides a detailed description of the chemical reactions occurred during energy generation in his article “Bioenergetics & Energy Release”.