MED213: Aerobic Capacity Notes

Learning Outcomes

• Explain the cardiovascular responses to acute exercise

• Explain blood pressure and blood flow changes during exercise

• Explain the respiratory responses to exercise

• Explain adaptations to aerobic training

• Outline what is known about responders and non- responders to aerobic exercise training

Cardiovascular Responses to Acute Exercise

• Increased blood flow to working muscle

• Involves altered heart function and peripheral circulatory adaptations:

  • Heart rate (HR)

  • Stroke volume (SV)

  • Cardiac output (Q)

  • Blood pressure

  • Blood flow

  • Blood

Resting Heart Rate (RHR)

• Normal ranges

  • Untrained RHR: 60 to 80 beats/min

  • Trained RHR: as low as 30 to 40 beats/min

  • Affected by neural tone, temperature, altitude

• Anticipatory response: HR increases above RHR just before the start of exercise

  • Vagal tone decreases

  • Noradrenaline, adrenaline increases

Heart Rate During Exercise

• Directly proportional to exercise intensity

• Maximum HR (HRmax): highest HR achieved in all-out effort

  • Highly reproducible

  • Declines with age

  • Estimated HRmax = 220 - age in years

  • Better estimated: HRmax = 208 - (0.7 \times age) in years

• Steady-state HR: A plateau at a given intensity. HR for meeting circulatory demands at that intensity.

  • If intensity increases, so does steady-state HR

  • Adjustment to new intensity takes 2 to 3 min

  • Steady-state HR basis for simple tests that estimate aerobic fitness and HRmax.

Stroke Volume (SV)

• Increases with intensity up to 40 to 60% \dot{V}O_{2max}

  • Beyond this, SV plateaus to exhaustion (not always in elite athletes)

• SV during maximal exercise ≈ double standing SV.

• But SV during maximal exercise only slightly higher than supine SV

  • Supine SV much higher versus standing

  • Supine EDV > standing EDV

Factors that Increase Stroke Volume

• Preload: end-diastolic ventricular stretch

  • Increase Stretch (i.e., increase EDV), increase contraction strength

  • Frank-Starling mechanism

• Increase Contractility: inherent ventricle property

  • Increase Noradrenaline or adrenaline, increase contractility

  • Independent of EDV (increase ejection fraction instead)

• Decrease Afterload: aortic resistance (R)

Stroke Volume Changes During Exercise

• Increase Preload at lower intensities increases SV

  • Increase Venous return increases EDV increases preload

  • Muscle and respiratory pumps, venous reserves

• Increase in HR decreases filling time, slight decrease in EDV, decrease SV

• Increase Contractility at higher intensities, increases SV

• Decrease Afterload via vasodilation, increases SV

Cardiac Output (\dot{Q})

• \dot{Q} = HR \times SV

• Increases with intensity, plateaus near \dot{V}O_{2max}

• Normal values

  • Resting \dot{Q} ~5 L/min

  • Untrained \dot{Q}max ~20 L/min

  • Trained \dot{Q}max 40 L/min

• \dot{Q}max a function of body size and aerobic fitness

Fick Principle

• Calculation of tissue O2 consumption depends on blood flow, O2 extraction

• \dot{V}O{2} = \dot{Q} \times (a-v-)O{2} difference

• \dot{V}O{2} = HR \times SV \times (a-v-)O{2} difference

• \dot{V}O_{2max} = the size of the engine

Blood Pressure

• During endurance exercise, mean arterial pressure (MAP) increases

  • Systolic BP increases proportional to exercise intensity

  • Diastolic BP slight decreases or slight increases (at max exercise)

• MAP = \dot{Q} \times total peripheral resistance (TPR)

  • \dot{Q} increases, TPR decreases slightly

  • Muscle vasodilation versus sympatholysis

Blood Flow Redistribution

• Increased Cardiac output increases available blood flow

• Must redirect increased blood flow to areas with greatest metabolic need (exercising muscle)

• Sympathetic vasoconstriction shunts blood away from less-active regions:

  • Splanchnic circulation (liver, pancreas, GI)

  • Kidneys

• Local vasodilation permits additional blood flow in exercising muscle

  • Local vasodilation triggered by metabolic and endothelial products

• As temperature rises, skin vasodilation also occurs

  • Decreases Sympathetic vasoconstriction, increases sympathetic vasodilation

  • Permits heat loss through skin

Cardiovascular Drift

• Associated with increased core temperature and dehydration

• SV drifts downwards:

  • Skin blood flow increases

  • Plasma volume decreases (sweating)

  • Venous return/preload decreases

• HR drifts increases to compensate (\dot{Q} maintained)

Competition for Blood Supply

• Exercise + other demands for blood flow results in competition for \dot{Q}

  • Exercise (muscles) + eating (splanchnic blood flow)

  • Exercise (muscles) + heat (skin)

• Multiple demands may decrease muscle blood flow

Integration of the Exercise Response

• Cardiovascular responses to exercise complex, fast, and finely tuned

• First priority: maintenance of blood pressure

  • Blood flow can be maintained only as long as BP remains stable

  • Prioritized before other needs (exercise, thermoregulatory, etc.)

Ventilation During Exercise

• Immediate increases in ventilation

  • Begins before muscle contractions

  • Anticipatory response from central command

• Gradual second phase increases in ventilation

  • Driven by chemical changes in arterial blood

  • Increases in CO2, H+ sensed by chemoreceptors

  • Right atrial stretch receptors

Ventilation During Exercise (cont.)

• Ventilation increases proportional to metabolic needs of muscle

  • At low-exercise intensity, only tidal volume increases

  • At high-exercise intensity, rate also increases

• Ventilation recovery after exercise delayed

  • Recovery takes several minutes

  • May be regulated by blood pH, PCO2, temperature

Breathing Irregularities

• Dyspnea (shortness of breath)

  • Common with poor aerobic fitness

  • Caused by inability to adjust to high blood PCO2, H+

  • Also, fatigue in respiratory muscles despite drive to increase ventilation

• Hyperventilation (excessive ventilation)

  • Anticipation or anxiety about exercise

  • Increases PCO2 gradient between blood, alveoli

  • Decreases Blood PCO2, increases blood pH, decreases drive to breathe

Ventilation and Energy Metabolism

• Ventilation matches metabolic rate

• Ventilatory equivalent for O2

  • \dot{V}E/\dot{V}O_{2} (L air breathed / L O2 consumed / min)

  • Index of how well control of breathing matched to body’s demand for oxygen

• Ventilatory threshold

  • Point where L air breathed > L O2 consumed

  • Associated with lactate threshold and increases in PCO2

Estimating Lactate Threshold

• Ventilatory threshold as surrogate measure?

  • Excess lactic acid + sodium bicarbonate

  • Result: excess sodium lactate, H2O, CO2

  • Lactic acid, CO2 accumulate simultaneously

• Refined to better estimate lactate threshold

  • Anaerobic threshold

  • Monitor both \dot{V}E/\dot{V}O{2}, \dot{V}E/\dot{V}CO{2}

Limitations to Performance

• Ventilation normally not limiting factor

  • Respiratory muscles account for 10% of \dot{V}O_{2}, 15% of \dot{Q} during heavy exercise

  • Respiratory muscles very fatigue resistant

• Airway resistance and gas diffusion normally not limiting factors at sea level

• Restrictive or obstructive respiratory disorders can be limiting

Elite Athletes Considerations

• Exception: elite endurance-trained athletes exercising at high intensities

  • Ventilation may be limiting

  • Ventilation-perfusion mismatch

  • Exercise-induced arterial hypoxemia (EIAH) i.e. pulmonary gas exchange limitations during exercise

Acid–Base Balance

• Metabolic processes produce H+ which decreases pH

• At rest, body slightly alkaline

  • 7.1 to 7.4

  • Higher pH = alkalosis

• During exercise, body slightly acidic

  • 6.6 to 6.9

  • Lower pH = acidosis

Acid–Base Balance (cont.)

• Physiological mechanisms to control pH

  • Chemical buffers: bicarbonate, phosphates, proteins, hemoglobin

  • Increase Ventilation helps H+ bind to bicarbonate

  • Kidneys remove H+ from buffers, excrete H+

• Active recovery facilitates pH recovery

  • Passive recovery: 60 to 120 min

  • Active recovery: 30 to 60 min

Adaptations to Aerobic Training: Cardiovascular

• O2 transport system and Fick equation

  • \dot{V}O{2} = \dot{Q} \times (a-v-)O{2} difference

  • \dot{V}O{2} = SV \times HR \times (a-v-)O{2} difference

  • Increase \dot{V}O_{2max} = increase max SV x max HR x increase max (a-v-)O2 difference

• Heart size

  • With training, heart mass and LV volume increases

  • Cardiac hypertrophy and increase SV

  • Increase Plasma volume, increase LV volume, increase EDV

Adaptations to Aerobic Training: Cardiovascular (cont.)

• SV increases after training

  • Resting, submax, and max

  • Plasma volume increases with training increases EDV increases preload

  • Resting and submaximal HR decreases with training

  • Increase LV mass with training, increase force of contraction

• SV training adaptations decreases with age

Adaptations to Aerobic Training: Respiratory

• Pulmonary ventilation

  • Decreases At given submaximal intensity

  • Increases At maximal intensity due to increase tidal volume and respiratory frequency

• Pulmonary diffusion

  • Unchanged during rest and at submaximal intensity

  • Increases At maximal intensity due to increase lung perfusion

• Arterial–venous O2 difference

  • Increases Due to increase O2 extraction and active muscle blood flow

  • Increase O2 extraction due to increase oxidative capacity

Adaptations to Aerobic Training: Muscle

• Fiber type

  • Increases Size and number of type I fibers (type II to type I)

  • Type IIx may perform more like type IIa

• Capillary supply

  • Increases capillarization

  • May be key factor in increases \dot{V}O_{2max}

• Myoglobin

  • Increases Myoglobin content by 75 to 80%

  • Supports increases oxidative capacity in muscle

Adaptations to Aerobic Training: Muscle (cont.)

• Mitochondrial function

  • Increases Size and number

  • Magnitude of change depends on training volume

• Oxidative enzymes (SDH, citrate synthase)

  • Increases Activity with training

  • Continue to increase even after \dot{V}O_{2max} plateaus

  • Enhanced glycogen sparing

Adaptations to Aerobic Training: Metabolic

• Lactate threshold

  • Increases To higher percentage of \dot{V}O_{2max}

  • Increases lactate clearance

  • Allows higher intensity without lactate accumulation

• Respiratory exchange ratio (RER)

  • Decreases At both absolute and relative submaximal intensities

  • Increases Dependent on fat, decreases dependent on glucose

High vs. low responders

• HERITAGE family study (n=473)

• VO2max responses to endurance training can be explained by ~30 gene RNA expression signature in muscle (prior to training)

• Explains ~23% of gains in VO2max

How do we solve the problem of non-responders?

• Changes in maximal power (Wmax) after each exercise training period

• Values within the shaded area are within measurement error i.e. non-response

• Non-response = (1) 69%, (2) 40%, (3) 29%, (4) 0% and (5) 0%

• Non-responders respond to a more potent training stimulus!