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Muscle Fuel: Energy Source For Muscle Contraction
By Lee E. Brown, EdD, CSCS,*D
In order for a muscle to contract, it must have a ready fuel supply. This is best accomplished through the transfer of fuel from oxygen that is transported to the muscle through the bloodstream. The primary fuel for all muscle contraction is adenosine triphosphate or simply ATP. 1. There are three main ways that fuel is transported to the muscle and they are derived through the use of a timing or intensity system. How hard and how quickly you ask the muscle to contract will determine which fuel source is utilized.
Phosphagen System
This system provides fuel for primarily short duration, high intensity exercise 4. However, this system is active at the beginning of all exercise regardless of intensity. It works by combining ATP and creatine, which are stored in the muscle, to produce energy for muscle contraction. Yet, since the stored amounts are so low in the muscle (generally highest in fast twitch muscle fibers) this fuel source may only last as long as 10 – 20 seconds. 2. Requiring no oxygen for delivery, this is the primary fuel source for heavy resistance training. 5.
Glycolysis System
This is the breakdown of carbohydrates for fuel which is either stored in the muscle as glycogen or is delivered to the muscle in the blood stream as glucose 1. There are two main forms of glycolysis, fast and slow. 3. Fast glycolysis is used for times when oxygen is in short supply. Fast glycolysis results in the formation of lactic acid, the byproduct of energy supply to the muscle. 4. An increase in lactic acid in the muscle can involve muscular fatigue and ultimately cessation of exercise. Slow glycolysis is used if there is enough oxygen to allow a continuous supply of fuel. The byproduct of this form of glycolysis is pyruvate, which is not converted to lactic acid but is transported elsewhere. The end result of these two systems is that glycolysis can produce fuel for 30 seconds to a minute for moderate heavy resistance training. If continued, lactic acid would result in muscular fatigue and ultimate stoppage of the exercise. 5.
Oxidative System
This system is often referred to as the aerobic system. As you might expect, this fuel supply offers energy to the muscle through the use of continuous oxygen transport. This system works at rest and during very low intensity exercise such as repeated repetitions during resistance training for walking or running. 1. This form of energy primarily utilizes fats (70%) and carbohydrates (30%) as fuel sources, but as intensity is increased there is a switch in substrate majority from fats to carbohydrates. 3.
Glucose and glycogen are used when oxygen is present in large quantities. Enzymes can break down fats stored in cells, which can be used as a fuel source. Protein is not a significant source of fuel for muscle contraction but it can be broken down into branched chain amino acids and converted to energy. The oxidative system usually supplies energy for low intensity exercise lasting up to one and a half hours.
Summary
While all three energy systems (phosphagen, glycolysis, and oxidative) are active all the time, which system provides the majority of energy will be determined by a time based and intensity based process. Phosphagen is primarily used for 10 – 20 seconds, glycolysis for up to a minute, and oxidative is the primary system for all long-term exercise. For the most part, each system uses a different fuel and is specific to the relative intensity of the exercise session, with phosphagen being high intensity, glycolysis being moderate intensity, and oxidative being low intensity.
References:
- Baechle TR, Earle RW. (Eds). (2000). Essentials of strength training and conditioning, 2nd Ed. Champaign, IL: Human Kinetics.
- Fleck SJ, Kraemer WJ. (1997). Designing resistance training programs, 2nd Ed. Champaign, IL: Human Kinetics.
- Jones NL, McCartney N, McComas AJ. (Eds). (1986). Human muscle power. Champaign, IL: Human Kinetics.
- McComas AJ. (1996). Skeletal muscle. Champaign, IL: Human Kinetics.
- Stone MH, O'Bryant H, Garhammer J. (1981). A hypothetical model for strength training. Journal of Sports Medicine, 21:344.
About the Author
Lee E. Brown, EdD, EPC, CSCS,*D, is Assistant Professor and Director of the Human Performance Laboratory at Arkansas State University. He received his Doctorate at Florida Atlantic University, where he was Health Sciences Lab Coordinator. Dr. Brown is a Fellow of the American College of Sports Medicine, a USAW Certified Club Coach and a Certified Strength and Conditioning Specialist with Distinction (CSCS,*D) with the NSCA.
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