Essentials of Human Physiology for Pharmacy

148 Essentials of Human Physiology for Pharmacy• Restoration of creatine phosphate reserves• Metabolism of lactic acid• Replacement of glycogen storesDuring the recovery period from exercise, ATP (newly produced by wayof oxidative phosphorylation) is needed to replace the creatine phosphatereserves — a process that may be completed within a few minutes. Next, thelactic acid produced during glycolysis must be metabolized. In the muscle,lactic acid is converted into pyruvic acid, some of which is then used as asubstrate in the oxidative phosphorylation pathway to produce ATP. Theremainder of the pyruvic acid is converted into glucose in the liver that isthen stored in the form of glycogen in the liver and skeletal muscles. Theselater metabolic processes require several hours for completion.11.9 Types of muscle fibersTwo major differences between the types of muscle fibers are:• Speed of contraction• Metabolic pathway used to form ATPAs such, three types of muscle fibers (see Table 11.1) exist:• Slow-twitch oxidative• Fast-twitch oxidative• Fast-twitch glycolyticTable 11.1 Features of Skeletal Muscle Fiber TypesFeatureSlow-twitchoxidativeFast-twitchoxidativeFast-twitchglycolyticMyosin ATPase Slow Fast FastactivitySpeed of contraction Slow Fast FastRemoval of calcium Slow Fast FastDuration ofLong Short ShortcontractionMitochondria Many Many FewCapillaries Many Many FewMyoglobin content High High LowColor of fiber Dark red Red PaleDiameter of fiber Small Intermediate LargeSource of ATP OxidativeOxidativeGlycolysisphosphorylation phosphorylation;glycolysisGlycogen content Low Intermediate HighOnset of fatigue Delayed Intermediate Rapid