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Anaerobic Threshold. Does it exist? How is it determined?. Anaerobic Threshold. How is anaerobic threshold defined? power just before onset of metabolic acidosis &  RER (Wasserman et al., 1973) What was/is theoretical basis for anaerobic threshold?

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anaerobic threshold

Anaerobic Threshold

Does it exist?

How is it determined?

anaerobic threshold2
Anaerobic Threshold
  • How is anaerobic threshold defined?
    • power just before onset of metabolic acidosis &  RER (Wasserman et al., 1973)
  • What was/is theoretical basis for anaerobic threshold?
    • effects on R (RER), lactate formation, VCO2, blood [bicarbonate]
  • How is anaerobic threshold determined?
    • changes in blood [lactate]
    • ventilatory measures
      • RER, VCO2, VE, PETO2 and PETCO2, VE/VO2 and VE/VCO2
anaerobic threshold does it exist comments by ga brooks 1985
Anaerobic threshold: does it existComments by GA Brooks (1985)
  • Key component of AnT hypothesis is that muscle becomes hypoxic during submaximal exercise
    • femoral PvO2 did not fall <10 Torr when at 50% of VO2max(Pirnay et al., JAP, 1972)
    • muscle produced La at 10% of VO2max; La production linearly related to work rate;  blood flow did not affect La production (Connett et al., Am J Physiol, 1984)
    • muscle produces La before blood LT or VT (Green et al., JAP, 1983)
anaerobic threshold does it exist comments by ga brooks
Anaerobic threshold: does it existComments by GA Brooks
  • VT and AnT occur at same point  AnT causes VT
    • this relationship does not always hold, therefore must be considered invalid
    • disassociation of AnT and VT in glycogen-depleted subjects (Segal & Brooks, JAP, 1979)
    • McArdles patients cannot produce La, but still exhibit VT (Hagberg et al., JAP, 1982)
    • measuring LT is cheaper (and more accurate) than VT
control of ventilation
Control of Ventilation
  • Ventilatory control is by:
    • feedback (central and carotid chemoreceptors)
    • feed forward (central command, muscle feedback)
  • Redundancy mechanisms control VE
  • VE responds more closely to demands for CO2 clearance than O2 uptake
    • ventilation below lactate threshold regulates PaCO2 keeping it at resting levels
buffering of blood ph
Buffering of blood pH
  • Primary blood buffer is bicarbonate

H+ + lactate- + Na+ + HCO3- Na+ + lactate- + H2CO3  H2O + CO2

new method for detecting ant by gas exchange beaver wasserman whipp jap 1986
New method for detecting AnT by gas exchange(Beaver, Wasserman & Whipp, JAP, 1986)

V-slope method criteria

  • break in linearity of VCO2–VO2
  • break in linearity of VE–VO2
  • VE/VO2breaks with linearity while VE/VCO2remains constant
  • PETO2–VO2 begins to rise while PETCO2–VO2is slowly rising or constant
  • RER–VO2having been flat or rising slowly changes to more positive slope
slide9

Respiratory compensation point

stimulated by pH

Ventilatory threshold

stimulated by CO2 production

slide10

Most difficult subject. AnT was detected by only two investigators.

AnT points detected by six investigators (multiple vertical lines).

Beaver, Wasserman, & Whipp, JAP, 1986)

blood lactate accumulation and removal

Blood Lactate Accumulation and Removal

Effects on Blood Lactate Concentration

lactate response to prolonged exercise 70 of vo 2max
Lactate Response to Prolonged Exercise(70% of VO2max)

(Kolkhorst & Buono, Virtual Exercise Physiology Lab, 2004)

lactate response to incremental exercise endurance trained athlete
Lactate Response to Incremental Exercise(endurance-trained athlete)

(Kolkhorst & Buono, Virtual Exercise Physiology Lab, 2004)

slide15

Mitochondrial PO2 during exercise

Muscle intracellular PO2 and net lactate release. Note that PO2 remains above critical mitochondrial O2 tension (1 torr). (Richardson et al., 1998)

Relationship between mitochondrial VO2 and PO2. Critical mitochondrial PO2 is around 1.0 torr. (Rumsey et al., 1990)

slide16

Why does blood lactate increase during heavy exercise?

  • lactate appearance exceeds lactate removal
  • evidence does not point to muscle hypoxia
  • FT recruitment
    • FT fibers have M-LDH
    • ST fibers have H-LDH
  • epinephrine release
slide17

Effects of epinephrine (EPI) on metabolism

  •  glycogenolysis
  •  glycolysis
  • inhibits lipolysis
effect of altitude on la response
Effect of Altitude on La Response

At altitude:

  • LT occurs at same relative intensity
  • blood [La] higher at same absolute workloads
  • muscle blood flow similar at same absolute workloads
  • EPI threshold occurs earlier at altitude
  • Lactate paradox – peak [La] is less under hypoxic conditions than at normoxia
reading assignment for tue oct 9
Reading Assignmentfor Tue, Oct 9

Holden, S-MacRae, SC Dennis, AN Bosch, and TD Noakes. Effects of training on lactate production and removal during progressive exercise in humans. J Appl Physiol 72: 1649-1656, 1992.

-- or --

Stanley, WC, EW Gertz, JA Wisneski, DL Morris, R Neese, and GA Brooks. Systemic lactate turnover during graded exercise in man. Am J Physiol 249 (Endocrinol Metab 12): E595-E602, 1985.

metabolic fate of lactate25
Metabolic Fate of Lactate
  • During exercise:
    • ~75% oxidized by heart, liver, and ST fibers
  • During recovery:
    • oxidized by heart, ST fibers, and liver (1 fate)
    • converted to glycogen
    • incorporated into amino acids
    • La metabolism depends on metabolic state
slide26

Fate of lactate 4 hr after injection under three recovery conditions. Note that oxidation is 1 pathway of removal.

determining lactate turnover during exercise tracer methodology
Determining lactate turnover during exercise: tracer methodology
  • use naturally occurring isotopes
    • 13C and 2H isotopes most commonly used
  • pulse injection tracer technique
    • labeled La added to blood in single bolus
    • concentration measurements taken over time
    • rate of concentration decline represents turnover rate
  • continuous-infusion technique
    • labeled La added at increasing rate until equilibrium point is reached, i.e., La appearance = La removal
slide30

WHEN YOU ARE IN DEEP TROUBLE,

LOOK STRAIGHT AHEAD, KEEP YOUR MOUTH SHUT and SAY NOTHING.

primed continuous infusion technique used by stanley et al and macrae et al
Primed continuous-infusion technique(used by Stanley et al. and MacRae et al.)
  • turnover rate = appearance - disappearance
  • Ra dependent on:
    • volume of distribution
    • arterial [La]
  • Rd = Ra minus arterial [La]
  • metabolic clearance rate (MCR) = Rd / [La]
    • calculates La clearance rate relative to arterial [La]
    • increasing MCR indicates Rd is dependent on arterial [La]
lactate response to graded exercise stanley et al jap 1985
Lactate response to graded exercise(Stanley et al., JAP, 1985)
  • Ra and Rd exponentially related to VO2
  • “linear” relationship between Ra and arterial [La]
  • curvilinear relationship between Rd and arterial [La]
  • MCR decreased at higher work rates
    • Rd was slowed as blood [La] increased
    • Rd is dependent on blood [La]
    • Rd is function of blood [La] and Ra
slide33
Rates of blood lactate appearance (Ra) and disappearance (Rd) during graded exercise before and after training

Holden et al., JAP, 1992

training adaptations to lactate kinetics holden et al jap 1992
Training adaptations to lactate kinetics(Holden et al., JAP, 1992)
  • submaximal Ra  by training
  • peak Ra similar regardless of training status
  • at same relative intensities, Ra was  at <60% and similar at >60%
  • submaximal Rd  by training
  • peak Rd  regardless of training status
  • at same relative intensities, Rd was similar at <60% and  at >60%
  • at same relative intensities,  [La]
    • at <60% was due primarily to  Ra ( EPI and CHO metabolism)
    • at >60% was due primarily to  Rd (lactate shuttle)
effect of training on blood lactate response
Effect of training on blood lactate response

65% pretraining

65% posttraining (same relative workload)

65% posttraining (same absolute workload as 45% pretraining)

45% pretraining

Bergman et al., Am. J. Physiol., 1999

lactate clearance

Lactate clearance

Monocarboxylate transporters (MCTs)

monocarboxylate transporters
Monocarboxylate transporters
  • facilitated diffusion transport of lactate and pyruvate in and out of cells
    • located on plasma and mitochondrial membrane
    • reversible transporter
    • involves H+ transport
  • MCT1 and MCT4 are major MCT isoforms
    • MCT1 found more in oxidative fibers
    • MCT4 found more in glycolytic fibers
    • at least 8 isoforms of MCTs known in humans
slide40

Prevalence of MCT1 and MCT4 in muscles of different fiber type composition

Semimembranosus soleus

MCT1

MCT4

monocarboxylate transporters41
Monocarboxylate transporters
  • La transport is essential for muscle pH regulation
  • MCT activity regulated mostly by La gradient
    • pH gradients can also increase transport rate
  • exercise training  MCT1, but not MCT4