Muscle Metabolism; Aerobic & Anaerobic Systems

Muscle metabolism and actions relate to the different muscle fiber types. As discussed in the article muscle fibers, for aerobic activity think type I fibers, and for anaerobic activity think of type II fibers.

As a general overview, muscle fibers are made up of sarcomeres containing myosin and actin filaments. These filaments produce muscle contractions. Muscle metabolism requires energy sources from food to convert into ATP (adenosine triphosphate). ATP is generated inside a muscle fiber by different types of cell respiration; aerobic (with oxygen) and anaerobic (without oxygen). It is the substance that fuels muscle contraction by enabling myosin and actin to interact.

AEROBIC RESPIRATION

Aerobic respiration requires glucose, fatty acids, amino acids, and oxygen, producing a lot of ATP. The waste products of this conversion are carbon dioxide and water. Aerobic respiration provides lower, steady power with high endurance. And it is the predominant energy source during rest and light to moderate intensity exercise of longer duration (walking, jogging, etc.). 

ANAEROBIC RESPIRATION

Anaerobic respiration works without oxygen. There are two major systems of anaerobic energy:

• ATP-PC (adenosine triphosphate – phosphocreatine) system, also referred to as the phosphagen system. ATP and PC stores within muscles ready for immediate action. ATP breaks down into ADP (adenosine diphosphate) and a free phosphate molecule (Pi) releasing energy for muscular action. Phosphocreatine breaks down, providing energy to recombine ADP and Pi into ATP, which can be used again for another muscle action.
• Anaerobic glycolysis, also referred to as the lactic acid system, uses only glucose to produce ATP and lactic acid. This system produces much less ATP than aerobic respiration.

SPEED AND POWER vs. ENDURANCE

Anaerobic respiration is much faster than aerobic respiration at producing ATP energy but has low endurance. The ATP-PC system provides the highest amount of power and is immediately available yet also rapidly depleted. It can produce energy more quickly than anaerobic glycolysis but can’t create as much and drains in 30 seconds or less.

Anaerobic glycolysis will reach its limits after a few minutes. With high intensity exercise, the two anaerobic systems provide most of the ATP used for muscle action. This process continues until they reach a point where the production of lactate increases and fatigue quickly sets in.

How muscle metabolism completely works and relates to fatigue isn’t fully understood and is still a keen topic of research and debate.

EPOC

Aerobic respiration replenishes the anaerobic energy system stocks of ATP and PC. The extra oxygen taken in to achieve this is called excess post-exercise oxygen consumption (EPOC). Improving your aerobic fitness helps to improve this replenishment process.

SYSTEM REPLENISHMENT

After intense exercise, the ATP-PC system mostly replenishes in about 2-4 mins.

The majority of accumulated lactate is removed about 75 mins after intense exercise. Interestingly, low intensity exercise hastens this process more than complete rest or higher intensity levels of activity. You can accelerate this process even further if you use the same muscles as those used for the workout activity.

I think that is more than enough on this topic. However, to completely nerd out, you can look more deeply into bioenergetic adaptations and increases in enzyme activity.

SPORTS AND ACTIVITIES

Each sport and activity has uniquely different energy demands. Athletes, exercise physiologists, sports scientists, and the like focus on measuring these different energy demands. Aerobic capacity (VO2max), aerobic power, maximal heart rate, calculated anaerobic threshold, and lactate transition threshold are such measurements. They use these measurements to create a finely tuned training program that suits the particular physiology and requirements of the individual. Engaging the services of an exercise physiologist or sports scientist can help you train to your maximum potential. For the rest of us, understanding these systems helps us to make better use of our time while reaching our goals.