Exercise physiology

From Iusmphysiology

• started here on 04/11/11.

Contents 1 Exercise physiology 1.1 Objectives 1.2 Metabolic aspects of exercise 1.2.1 Energy conversion in skeletal muscle 1.2.2 Energy suply to muscle during exercise 1.2.3 Oxygen consumption during exercise 1.2.4 VO2 max: Maxiumum O2 consumption 1.2.5 Oxygen debt 1.2.6 Exercise intensity, duration, and fuel availability dictate exercise nutrient use 1.2.7 Diet can affect how glycogen recovers after exercise 1.3 Respiration during exercise 1.3.1 Control of ventilation during exercise 1.3.2 Respiratory stimuli 1.4 Cardiovascular response to exercise 1.4.1 Blood flow distribution during rest and exercise 1.4.2 Relationship between CO, VO2, and work 1.4.3 Relationship between SV and HR 1.4.4 SV, HR, and CO at varying levels of training 1.5 Body temperature during exercise 1.5.1 Exercise hyperthermia 1.5.2 Effect of training on various cardiovascular parameters 1.5.3 Enzyme adaptation during endurance exercise training 1.5.4 Training changes muscle fiber type 1.5.5 Exercise increases insulin sensitivity 1.6 Additional benefits of daily exercise

Exercise physiology

Objectives

• The student will be able to describe the 3 metabolic systems that supply energy during exercise and relate exercise conditions with nutrient fuel use.
• The student will understand how oxygen consumption varies with exercise intensity.
• The student will be able to describe the 2 stages of oxygen recovery.
• The student will be able to describe respiratory changes during exercise.
• The student will be able to describe chemical and neural mechanisms stimulating ventilation during exercise.
• The student will be able to describe the dynamic relationship between changes in stroke volume and heart rate during exercise.
• The student will be able to describe the redistribution of blood flow to muscles and other organs during exercise.
• The student will understand the unique regulation of temperature during exercise.
• The student will understand the effect of training on cardiovascular, respiratory and metabolic function.

Metabolic aspects of exercise

• Exercise requires lots of ATP for all the work the muscle is doing.
• ATP can be generated in three ways: the phosphagen system, the glycogen-lactic acid system, or the aerobic system.
  • The phosphagen system uses creatine kinase to move the phosphate group off of creatine to ADP, generating ATP.
    • The phosphagen system (and the resident ATP) covers the energy for the 0-60 seconds of vigorous exercise.
  • The glycogen-lactic acid system runs glycogen through glycolysis to generate lactic acid.
    • The glycogen-lactic acid system covers the energy for the 1-4 minutes of vigorous activity.
  • The aerobic system uses the electron transport chain to generate ATP from glucose, fatty acids, and amino acids.
    • Aerobic oxidation of muscle glycogen, plasma glucose, and liver glycogen cover the energy for minutes 4-200 (and then tapers off).
    • Aerobic oxidation of plasma FFA (free fatty acids) and adipose tissue TAGs (triacylglycerides) cover the energy for minutes 45 and beyond.

Energy conversion in skeletal muscle

• Recall that glycolysis takes glucose to two pyruvate molecules, generating 6 ATP and 2 NADH.
• Recall that bursts of heavy activity utilize the phosphagen system and the glycogen-lactic acid systems for production of ATP.

Energy suply to muscle during exercise

• During exercise, epinephrine is elevated which signals to the liver, skeletal muscle, and adipose tissue.

• Epinephrine at the liver:
  • Epinephrine causes the liver to increase glycogenolysis and gluconeogenesis.
  • Epi--like glucagon--binds to a receptor that elevates cAMP levels and thus triggers activation of appropriate enzymes.
  • Note that gluconeogenesis can use lactic acid as a precursor to be converted into glucose.

• Epinephrine at the muscle:
  • Epinephrine at the skeletal muscle signals for the use of glycolysis (the anaerobic burning of glucose).
  • Epinephrine binds to a cAMP elevating receptor on skeletal muscle which leads to activation of appropriate enzymes for converting glucose into ATP and aerobic intermiediates (think NADH and pyruvate).
  • Glucose converted to pyruvate too quickly to be used in the (limited capacity citric acid cycle--oxphos) can be converted to lactic acid and secreted into the blood to be used in gluconeogenesis at the liver.
    • Recall that this loop (glucose -> pyruvate (to get the ATP and NADH) -> lactic acid -> liver -> glucose -> muscle -> pyruvate...) is called the cori cycle.

• Epinephrine at the adipose tissue:
  • Epinephrine at the adipose tissue causes TAG breakdown into FFAs for secretion into the blood.
  • Epinephrine binds to a receptor that activates the hormone-sensitive lipase.

Oxygen consumption during exercise

• The basal rate of oxygen consumption is about 0.25 L / minute.
• Light exercise can elevate oxygen consumption 3-fold to about 1 liter.
• Heavy exercise can elevate oxygen consumption 8-10 fold to nearly 3.5 liters.

V_O2 max: Maxiumum O₂ consumption

• The V_O2 max is the point at which oxygen uptake and transport are at their maximum capacity.
• The V_O2 max can be reached upon heavy exercise of large muscle groups for over three minutes.
• At V_O2 max, the blood lactic acid levels have reached over 8 mM.
  • Recall that pH has an effect on Hb's oxygen carrying ability so these high levels of lactic acid inhibit continued elevation of oxygen transport.
• O2 consumption at V_O2 max is at its maximum even if exercise continues at a higher rate; that is, one can consume energy at a higher rate than the V_O2 max allows for production of energy, but the difference in will come from anaerobic energy sources.

• Factors affecting V_O2 max include age, gender, and training level.
  • The younger the higher the V_O2 max.
  • Men have higher V_O2 max points than women.
  • Training can increase the V_O2 max.

How?

• Physiological limitations to V_O2 max:
  • You have to get the oxygen to the muscle, so the cardiovascular system can limit the V_O2 max by how well blood flows to the muscle.
  • Efficiency in O2 use determines how much O2 is needed for a given output; the more efficient the mitochondrial enzymes of the muscle, the higher the V_O2 max can reach.
  • You have to get the oxygen into the blood, so pulmonary diffusion can limit the V_O2 max; the faster / better the pulmonary diffusion, the higher the V_O2 max can reach.

Oxygen debt

• Oxygen debt is the idea that when oxygen demand increases rapidly (as in the case of exercise), too little oxygen is consumed during the initial phase as the body compensates (i.e. increased respiration and circulation).
• The debt from this initial phase is "paid off" by a lingering oxygen demand beyond the period of exercise.
  • The "pay off" has two components: the fast component and the slow component.
    • The fast component restores oxygen stores.
    • The slow component finishes metabolism of lactic acid into waste or glucose.

Exercise intensity, duration, and fuel availability dictate exercise nutrient use

• We previously saw that different energy sources were used as exercise moved from short to prolonged, and thus the duration can affect the energy use.
• Intensity affects energy source: light exercise energy needs are met by lipid metabolism while high-intensity exercise energy needs are met by metabolism of carbohydrates and protein.
  • Lipids are burned for light exercise.
  • Carbs and protein are burned for high intensity and long duration exercise.
• The diet, too, can affect which fuels are used during exercise.
  • A high carbohydrate diet dictates a higher pecentage of the energy used during exercise be from carbs; similarly, a high fat diet dictates that more energy come from fat..
  • Note that a high-fat diet has the earliest exhaustion (as in exhaustion of available energy sources) point, then a mixed diet, and then a high-carbohydrate diet.

What is that steady state point all about?

Diet can affect how glycogen recovers after exercise

• Diet can affect how glycogen stores are recovered ‘’’after’’’ exercise, too.
• Muscle glycogen replacement after depletion via exercise and best with a high carbohydrate post-exercise diet.
• Fat and protein diets are comparable to eating no food at all in terms of glycogen store restoration.

Respiration during exercise

• Exercise increases the oxygen demand of the body; the lungs meet the elevated demand by ‘’increasing the rate of respiration’’’, ‘’’increasing the volume of respiration’’’, and ‘’’increasing the diffusion capacity of the lung’’’.
  • The respiratory rate can increase 4-fold to 50 breaths / min.
  • The respiratory volume can increase 6-fold to 3000 mL / breath.
  • The diffusion capacity can incrnease 4-fold to 80 mL / mmHg.

How does the diffusion capacity increase?  Vasodilation, right?  Anything else?

• Through increased rate and volume, total ventilation (TV) can be elevated nearly 25-fold!
  • TV = mL / min
  • TV = rate * volume
  • TV = breaths / min * mL / breath
  • TV (rest) ~= 12 breaths / min * 500 ml / breath = 6000 mL / min = 6 L / min
  • TV (work) ~= 50 breaths / min * 3000 ml / breath = 150000 / min = 150 L / min

Control of ventilation during exercise

• Ventilation elevation (‘’’hyperpnea’’’) during exercise occurs in ‘’’three stages’’’: fast, slow, and steady state.
  • Fast stage: ventilation is immediately and rapidly elevated.
  • Slow stage: ventilation continues to be elevated but at a slower, attenuated rate.
    • Within 2 minutes of commencing exercise, the slow stage is nearly half-way complete.
  • Steady state stage: ventilation is maintained as needed.
    • Within about 4 minutes of beginning constant exercise, the steady state has been met.

Respiratory stimuli

• Recall that there are chemoreceptors that measure blood pH, PCO2, and PO2 and provide stimulation to the CNS.
  • However ‘’’these chemoreceptors do not account for the respiratory response to exercise’’’.
• It is posited that ‘’’feed foward regulators of ventilation may activate neural reflexes’’’ to cause elevated respiration in response to exercise.

• Beyond these chemoreceptors, there are several neural stimuli that may contribute to respiration elevation during exercise.
  • motor cortex efferent (that is, the signals ‘’’from the locomotive forebrain’’’ that are leaving the brain telling the body to move) may contribute to respiration control
  • proprioceptors (which are also called ‘’’ergoreceptors’’’) send afferent signals to the brain as perception of where in space the limbs reside and how much tension is on the muscles; these afferent signals may be a source of respiratory control
  • lung stretch receptors and vascular stretch receptors (which would be activated during exercise) send afferent signals to the brain and may therefore affect exercise respiration

**elevated sensitivity to respiratory center neurons may cause …
**elevated body temperature may cause an increase in respiratory response

• stopped here on 04/11/11.
• started here on 04/12/11.

Cardiovascular response to exercise

• The cardiovascular system responds to exercise by increasing blood delivery to the muscle in three ways: vasodilation, elevated heart rate, and elevated strove volume.
  • These changes can increase muscle blood flow 25-fold.
  • Elevated blood supply helps to deliver nutrients and remove waste at the necessary elevated rate.
• Recall that cardiac output is a function of heart rate and stroke volume.
  • As heart rate and stroke volume both increase, cardiac output increases, also.
  • Heart rate increases linearly but stroke volume plateaus.
    • Stroke volume may plateau because of the mechanical limitations of filling time and ejection time.
  • With long term training, the heart can adapt to reach even higher heart rates and larger stroke volumes.
• Note that there is a linear relationship between cardiac output, V_O2, and workload.
  • Recall that V_O2 is a measure of how much oxygen is used.

**However, at VO2 max, the relationship...

Blood flow distribution during rest and exercise

• There are blood flow changes throughout the body during exercise; in general, the heart and skeletal muscle are given more blood, the internal organs are given less blood, and the brain is held steady.
  • Brain continues to receive the same amount of blood.
  • Cardiac blood flow increases 4-fold.
  • Skeletal muscle blood flow increases nearly 20-fold.
• The total cardiac output can increase nearly 5-fold.

Relationship between CO, V_O2, and work

• These three variables that describe the delivery of oxygen, the use of oxygen, and the work of the skeletal muscle tissue are linearly related.

*Note that as one reaches VO2 max, oxygen consumption plateaus.
**As mentioned  before, work performed beyond one's VO2 max is fueled by anaerobic energy sources.

Relationship between SV and HR

• Recall that cardiac output is a function of heart rate and stroke volume.
• Heart rate and stroke volume are not increased linearly as the demand for oxygen increases.
• Recall that stroke volume plateaus before heart rate; that is, the heart rate is elevated relatively slower than the stroke volume.
  • On a graph, this means that stroke volume plays a more significant role in the early phase of increased cardiac output (CO) than does heart rate.
  • At the latest phases of increasing cardiac output, the elevation of the heart rate will play a more prominent role in generating increased CO (cardiac output).

SV, HR, and CO at varying levels of training

• Cardiac output of athletes and non-athletes are compared in resting and maximum effort conditions.
• Training directly affects the stroke volume, directly affects the oxygen demand of tissue, and indirectly affects the heart rate.
  • Training elevates the stroke volume of the heart and decreases the oxygen demand of the body.
  • Therefore, as stroke volume (even resting stroke volume) goes up (causing CO to go up) and oxygen demand (even resting oxygen demand) goes down (decreasing the CO demand), the heart rate will reach lower levels in athletes as it need not contribute as much tot he CO.
  • Similarly, as SV is elevated (and therefore CO is elevated) and oxygen demand is lowered (and therefore CO demand is decreased), the heart rate will maintain lower exercise levels in athletes.
• Training causes a spread in the CO at rest and maximum effort conditions; that is, training causes decreased CO required for rest and an increased ability to generate high CO at maximum effort.
  • The spread of CO is primarily due to an increase in SV and a less-significant decrease in HR (at both resting and maximum effort conditions).

Body temperature during exercise

• Recall that the body is only about 20% efficient at converting food energy to work, the rest is lost as heat.
• In exercise dates, about 75% of metabolic energy burned is lost as heat.
• During exercise, the hypothalamus may set a new (regulated) set point.
  • Note that this is a regulated set point which means that it is not pathological because it will be reset when the offending stimuli (exercise) is removed.
  • A new set point can be demonstrated by the elevation of rectal temperature with elevated oxygen use (that is, exercise).
  • Note that an elevated hypothalamic set point reduces stress on thermoregulatory mechanisms (think sweating and cutaneous blood flow).
• Recall that one thermoregulatory mechanism is the shifting of blood flow to cutaneous regions as a vent for heat carried by the circulatory system.
  • During exercise, the body maintains blood flow to skeletal muscle in spite of a shift toward cutaneous circulation.

Exercise hyperthermia

• Recall that hyperthermia is the excessive storage of heat in the body.
• During exercise, heat production elevates rapidly but heat dissipation mechanisms lag behind.
  • This lag of heat loss is the reason that core body temperatures are elevated in the initial stages of exercise.
  • As heat dissipation mechanisms catch up to heat generation, a new equilibrium is met and heat is not stored.

Effect of training on various cardiovascular parameters

• As mentioned before, training causes an increase in SV (and therefore an increase in CO) but does not cause an increase in HR.
• Training also causes a slight increase in oxygen extraction ability of muscle tissue (and therefore a larger A-V PO2 gradient).

Enzyme adaptation during endurance exercise training

• Long term endurance training can lead to enzyme efficiency gains in the citric acid cycle enzymes, and the converstion of glycogen phosphorylase B to A.
  • Recall that glycogen phosphorylase breaks glycogen down into glucose.
  • Recall that glycogen phosphorylase B is inactive and glycogen phosphorylase A is the active form.
  • So to be more efficient at converting GP-B to GP-A is to be more efficient at converting glycogen into glucose.

• Other benefits from long term endurance training include:
  • increased number of capillaries
  • elevated maximum oxygen uptake
  • increased size of muscle fibers

• Note that these benefits develop over 24 weeks of training and degrade in just 6 weeks lacking training.

Training changes muscle fiber type

• Recall that two adaptations to endurance training are increased capillary production and changes in muscle fiber type; these changes are mediated by phosphatases and kinases.
  • Endurance training activates various phosphatases and kinases which act as transcriptional regulators.
  • Contractile protein genes important for determining the fiber type are elevated.
  • Mitochondrial genes important for mitochondrial biogenesis are elevated.
  • Angiogenic growth factor genes are elevated.

Do we need to know any of the specifics from the images?

Exercise increases insulin sensitivity

• The section header says it all: daily exercise increases sensitivity to insulin.

Additional benefits of daily exercise

• Increased HDL, decreased LDL.
• Increased bone mineral density; improved coordination.
• Increased immune function.
  • Need long term study evidence.
• Healthier pregnancy:
  • Prevents excess weight gain in mother.
  • Prevents excess weight gain in fetus.
  • Prevents gestational diabetes.

• stopped here on 04/12/11.