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Lecture 15 Phsyiological Basis of Fatigue
Applied Exercise Physiology (HHP34000EXW)
University of Iowa
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15. Physiological Basis of Fatigue
What is Fatigue? ● Definitions ○ Decrements in muscular performance with continued effort accompanied by sensations of tiredness ○ Inability to maintain required power output to continue muscular work at a given intensity ; “slowing down” ■ Reversible by rest ● Complex phenomenon ○ Depends on type of exercise ○ Muscle fiber type ■ Which muscle fiber type ● Varies rate of fatigue and underlying cause of fatigue ○ Training status, diet ■ Are they a beginner or trained? ○ Can vary from day to day as well as person to person ● Causes may be central or peripheral ○ If peripheral , it can happen at NMJ or level of the skeletal muscle ○ Central fatigue is perception or psychological feeling of tiredness ■ Can also be the inability of the CNS to get motor neurons to fire ○ Synergistic effect ● Major loci/causes ○ Systemic - energy delivery ○ Muscle - (inadequate fuel in the muscle) metabolism; accumulation of by products; failure of contractile mechanisms ○ Nerve conduction/CNS - altered neural control of muscle contraction
Fatigue and Substrate Depletion ● Fuel substrates vary based on energy systems ○ With CHO stores, we get energy quickly and w/o oxygen requirement (glycolysis) ■ The pyruvate is converted to lactate ■ Or we can take the pyruvate (if we do not need ATP replenishment at a faster rate) and we shuttle it to the mitochondria to go through the krebs cycle ● Fat produces a lot of energy, but the higher the intensity the less we rely on fats so in this case we rely on CHO ○ If max intensity there is a short duration on our stored ATP and creatine phosphate ● If we are forced to use CHO as a fuel, it can contribute to fatigue bc we are going through our CHO stores too fast ○ With max intensity exercise, we experience fatigue bc we have PCr depletion ● Phosphagen system ○ PCr depletion is related to fatigue ○ We like to keep ATP/ADP ratio high, so at high intensity we run through these stores quickly in order to maintain our ATP stores
● Glycogen ○ Total glycogen depletion vs rate of glycogen depletion ■ Fatigue does not coincide with the rate of depletion but with the amount of glycogen that is depleted ■ Fastest rate of muscle glycogen depletion occurs early in the exercise ● Ex the fastest rate of depletion will occur over the first hour (out of 3 hrs) but feelings of fatigue will not begin to peak until after that. ■ We feel the most tired when glycogen stores are at their lowest (not the RATE that it is being depleted) ■ CHO will be oxidized (pyruvate going to mitochondria) ○ Effect of intensity ■ At a higher intensity, we experience glycogen depletion in the matter of minutes ○ Effect of exercise time ■ Depletion occurs most rapidly early in the exercise and will slow down as exercise progresses
Substrate Depletion: Effect of Fiber Types ● Type I (slow twitch) are most oxidative ○ We have higher blood flow, higher capillary density, more mitochondria, more myoglobin, use glycogen more efficiently ; will have darker staining in a pic bc they have more blood supply ● Type II (fast twitch) will have higher glycogen stores; have less capillary density, mitochondrial density, greater CHO stores ● Recruitment patterns ○ Fibers that we recruit first and most frequently will fatigue most rapidly ○ The intensity of the exercise will determine how frequently the muscle fibers are being recruited ■ At low intensity, we do not need to recruit type II fibers (we really only need Type I) we only recruit type II when type I is fatigued. ○ Recruitment order ■ Type I recruited earliest but produce least amount of force ■ Type II recruited when more force is needed; we use them for high intensity (therefore we use glycogen ● They favor conversion to lactate rather than oxidatively process the glycogen ○ Recruitment frequency ● Effect of exercise intensity ○ Long duration, exercise of low-moderate (slow twitch)intensity will deplete first
○ High muscle temp may impair muscular function bc it interferes with regulation of certain enzymes in the muscle ○ The hotter the ambient temp , the more fluid we lose to sweat which will compromise our blood fluid volume (blood volume is responsible for fueling muscle and heat dissipation) ■ We use more CHO with blood volume losses which will decrease the time to fatigue ○ As humidity goes up, it takes faster to fatigue bc the primary avenue of heat dissipation, but with high humidity it will impede sweat evaporation so we will not be able to get rid of heat (and sweat evaporation is inhibited)
Fatigue and Metabolic By Products ● Lactate ○ Will accumulate most during high intensity exercise ○ We usually clear it at the same rate that it is created up until we get to the Lactate Threshold. ○ As lactate builds up, acid levels will build up too so we will have a drop in muscle pH ● Buffers and pH ○ pH affects ■ Glycolysis ■ ATP synthesis ○ Will effect our ability to replenish ATP and continue exercise ○ Phosphate and bicarbonate buffers will help prevent pH from dropping too much so we can prolong exercise ○ With max fatigue exercise we do not see muscle pH drop below 6 thanks to the buffers
○ As soon as we stop exercising, we clear the acid ○ If pH drops to 1 or 2 then the cells will die ○ We wanna maintain muscle pH around 7. (lower than blood pH) ■ As pH drops at about 6 it will inhibit ability to use glycogen as a fuel by affecting the glycolysis RLE (PFK) ■ At pH of 6 it will completely shut down ability to use glycogen as a fuel ● Neural factors ○ Failure at NMJ ■ Prevents muscular activation ■ AcH needs to released and bind to cholinergic receptors that will cause depolarization of muscle fiber and then is recycled to create more AcH ○ SR and Calcium release ■ As we fatigue, it may take us longer to bring calcium back into the SR which will take us longer to relax muscle (bc we need ATP to relax the muscle) ■ We may end up with slower release of calcium from the SR to stimulate contraction which will affect our force production of the muscle ● CNS ○ Stress ■ Exercise is a stress to the body so the more outside stress someone has, the more it can affect exercise capacity ○ Subconscious and conscious influence ■ Fiber recruitment has a conscious aspect to it ■ Volitional fatigue: when someone chooses to stop exercising consciously or unconsciously ○ Exercise tolerance ■ Successful athletes are able to block out their discomfort ○ Pacing/efficiency ■ they also learn how to conserve their energy and pace themselves in order to prevent getting to the physiological markers of fatigue
Lecture 15 Phsyiological Basis of Fatigue
Course: Applied Exercise Physiology (HHP34000EXW)
University: University of Iowa
