Strength, power and endurance of muscles are important parameters of muscular function. Strength depends on size as larger muscle mass allows greater contractile force to be exerted. Power refers to the amount of work that can be performed by muscles in a particular time frame. Chemical energy obtained by breaking down adenosine triphosphate (ATP) is used for muscle contraction. There are 3 major metabolic pathways of ATP generation in muscles.
Table 1. Profiles of three major ATP-generating pathways in muscles.
|Metabolic Pathway||Source of Energy||Endurance
(length of time of
|Products of Reaction||Moles of ATP/min|
|Phosphagen system||Creatine phosphate||8-10 seconds||Creatine, ATP||4|
|Glycogen (anaerobic)||1.3 – 1.6 minutes||Pyruvic acid, Lactic Acid,
|Aerobic system||Glucose (aerobic),
(as long as nutrients last)
|CO2, water, ATP||1|
After strenuous exercise glycogen-lactic acid system can replenish both ATP and phosphocreatine while the aerobic system can replenish all systems. Recovery of aerobic system involves two processes. First is the repayment of oxygen debt; oxygen uptake remains high after exercise to replenish phosphocreatine system and stored oxygen and continues for another 40 minutes at a slightly lower level until lactic acid is removed. Second is the recovery of muscle glycogen; this occurs faster with carbohydrate-rich food and much slower with protein or fat-rich food.
In humans, all muscles have different proportions of slow twitch or Type I and fast twitch or Type II fibers depending on the type of myosin molecules expressed. Fast twitch fibers provide extreme power for few seconds to a minute or two. Slow twitch fibers provide endurance to enable muscle contraction for minutes to hours.
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