According to the Free Medical Dictionary, muscular endurance is a muscle tissue's "ability to perform repetitive submaximal contractions." The human body has evolved specialized muscle cells to support activities requiring muscular endurance.
Muscle Fiber Types
According to the textbook "Harper's Illustrated Biochemistry," skeletal muscle contains two muscle fiber types. Slow twitch, or type 1, muscle fibers are capable of sustaining long periods of contraction, such as during endurance exercises. Fast twitch fibers, or type 2, are only capable of short, strong contractions; these are commonly used for sprinting exercises.
Form and Function: Slow Twitch
The cellular differences between type 1 and 2 muscle fibers correspond to their function in skeletal muscle contraction. Slow twitch fibers contain mitochondria, organelles capable of generating energy using oxygen. The molecule myoglobin is present in these cell types as well; this specialized protein stores the oxygen necessary for oxidative respiration. This form of energy metabolism provides the greatest amount of energy per molecule of fuel used. It is a longer process, however, and more applicable to endurance muscle cells.
Form and Function: Fast Twitch
Fast twitch fibers, however, do not contain mitochondria or myoglobin. These cell types depend entirely on anaerobic respiration for energy production and are unable to produce ATP through high yield, oxidative processes. Therefore, fast twitch fibers rely on glycolysis for energy; without long-term energy solutions, these cells quickly fatigue and are most useful in sprinting events.
The Biochemistry of Muscular Endurance
Muscular endurance depends on oxygen-consuming aerobic metabolism for the production of ATP. The nutrients that are broken down to yield ATP are blood glucose and free fatty acids. These nutrients are released from storage into the blood stream by epinephrine, or adrenaline. Glucose comes from the liver while free fatty acids originate from adipose tissue, also known as fat. Ultimately, both sources of energy undergo oxidation in order to fuel slow-acting muscle fibers.
Applications
Type 1 and 2 muscle fibers are present throughout skeletal and cardiac muscle. In other words, there are fast and slow twitch fibers present throughout the circulatory system as well as the musculoskeletal system. Functionally, these two cell types give a muscular organ the ability to function in the sprint and the marathon.
The body has the basic building blocks to become an endurance or sprinting athlete. The key is building up the strength and number of the muscle fiber type that most supports your sporting goals.
References
- The Free Medical Dictionary: Muscular Endurance; 2011
- "Harper's Illustrated Biochemistry"; Robert Murray; 2010
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