Anaerobic Respiration

During bursts of strenuous exercise, when muscles contract to maximum possible effect, blood vessels supplying oxygen to muscle fibers are squeezes, limiting the delivery of oxygen.  Under these circumstances muscle fibers switch to anaerobic respiration, which does not require oxygen, to meet their energy needs.  It frees far less energy than aerobic respiration, but happens much more rapidly.

Aerobic Respiration

When a person is resting, or performing light exercise, aerobic respiration provides most of the ATP for muscle contraction.  During aerobic respiration glucose, and other fuels like fatty acids and amino acids, are broken down completely to water and carbon dioxide by a sequence of reactions that take place inside mitochondria.  This process requires the input of oxygen.

Muscle Metabolism

Energy rich fuels like glucose cant be used directly for muscle contraction.  They must be converted to ATP (adenosine triphosphate) first.  ATP is a substance that stores, carries, and releases energy.  During contraction ATP enables myosin and actin to interact.  ATP is created inside a muscle fiber by two types of cell respiration: aerobic or anaerobic.  A muscle fiber contains enough ATP to power a few seconds of contraction.  Afterward, contractions of ATP need to be maintained at a steady level.