CH 7 Energy Transfer during PA

Anaerobic Energy Pathways

energy pathways that do not require 02
• These pathways are extremely fast, but can only supply enough energy for 1-2 minutes of exercise
at best

Aerobic Energy Pathways

energy pathways that do require 02
• These pathways are somewhat slow, but can supply enough energy for hours of exercise
• Cardiorespiratory fitness (CRF) is heavily dependent on the ability to make a lot of energy over a
long period of time, measuring how much 02, we can consume in a given amount of time is
important.
• More 02, consumed means more aerobic energy created and used.

VO2

A measure of how much 02, a person is consuming at a given exercise intensity

VO2max

A measure of how much 02 a person is consuming during maximal intensity aerobic
exercise

• The gold standard for measuring aerobic energy production capability

Sprint Running and
Swimming

Energy Output from
active muscles exceed
resting values by >120x

Marathon Running

The whole body energy
requirement increases
by >20x

Phosphagen System (ATP-PCr)

Energy comes from two intramuscular high-energy
phosphate sources
• ATP and Phosphocreatine (PCr)
• Anaerobic system which does not produce lactate
• Alactic system

Skeletal Muscle

3-8 mmol of ATP per 1kg/2.2lbs of muscle

• Equates to about 4-8 sec of max effort
• We store about 4 – 5x more PCr
• Partly because ATP is such a heavy molecule
• Partly because PCr hydrolysis is such an immediate way to resynthesize
ATP

The quantity of intramuscular phosphagens
substantially influences

“all-out” energy for brief
durations

Anaerobic Glycolysis

• Lactic system
• Intense, short-duration
exercise mainly fueled by
anaerobic muscle
glycogenolysis
• Aerobic takes too long
• O2 in tissue insufficient to work
aerobically
• Result: Pyruvate converted to
Lactate
• Lactate begins to accumulate

Lactate Flux

• Enables glycogenolysis in one cell to supply other
cells with fuel for oxidation
• Muscle is a major site of lactate shuttling
• Highly glycolytic fibers are major sites of lactate
production
• Lactate can accumulate, is shuttled to another cell or blood
• Highly oxidative fibers are major sites for lactate removal
(clearance)
• Lactate from glycolytic fibers received and is oxidized
• Lactate can also be converted to amino acids or
glucose/glycogen (Cori Cycle)
• Continue to be used by muscle or stored
• Lactate accumulation (OBLA) begins when lactate
production exceeds these actions

Light-moderate exercise
intensities (<50% max aerobic
capacity)

Lactate Oxidation (clearance) = Lactate Production, lactate levels remain stable

In untrained individuals at 50 –
55% max aerobic capacity

Lactate Production > Lactate Clearance, Lactate ACCUMULATION
• Quickly & in large quantities for max
efforts lasting 60 – 180 sec
• Especially in cells with high glycolytic
& low oxidative characteristics
• Decreasing intensity to extend duration
will lower lactate production

Lactic Acid Metabolism

• Lactate dehydrogenase (LDH) reduces pyruvate to lactate within the cytosol
• Lactic acid molecule immediately releases a proton (H+) under physiological pH (7.0)
and is termed lactate
• Continued lactate production will cause cellular pH to fall (6.5 after high-intensity
exercise) if H+ are not buffered
Lactic Acid Metabolism
• H+ are removed into the blood
relying on the sodium
bicarbonate system to prevent
pH lowering
• As exercise intensity increases,
proton release rate exceeds
buffering capacity of the cell
resulting in Acidosis
• Acidic environment results in
Fatigue!

Breakpoint Threshold:

The highest VO2 that can be attained before there is rise in BL during incremental
exercise
• Also called: Anaerobic Threshold, Aerobic Threshold

1mM Threshold:

The VO2 during incremental exercise that is associated with a BL concentration of
1mM above the baseline BL concentration

2.5mM BL concentration Threshold:

The VO2 observed during incremental exercise associated with a BL concentration
of 2.5mM

Onset of Blood Lactate Accumulation (OBLA):

The VO2 observed during incremental exercise associated with a BL concentration
of 4mM
• Also called: Anaerobic Threshold

Maximal Steady State (MSS):

• highest VO2 maintained over time without continual increase in BL accumulation
(MSS)
• Also called: Individual Anaerobic Threshold (IAT)

Threshold can be shifted higher (75 –
90% max aerobic capacity)
by:

Training steady-state aerobic
activity at 80 – 90% max (i.e.,
marathoners)
• Genetics
• Local training adaptations to
favor less lactate production
• Higher aerobic capacity
• Increased % oxidative/oxidative-
characteristic fibers

Sprint/Power athletes can achieve _________higher blood lactate
levels than untrained individuals

20 – 30%

Higher degrees of anaerobic training, more reliance on glycolysis
• Result:
• Increased intramuscular glycogen stores → fuel increased glycolysis
• Increased glycolytic enzymes (esp. PFK) → fuel increased glycolysis
• Increased selective recruitment of highly glycolytic muscle fibers → meet increased
glycolytic demand
• Anaerobic training also tends to have higher motivation metrics
• Operating at nearer-max efforts (less aerobic)
• More motivation → push harder → intensities correspond more to glycolytic processes

Lactic acid does not cause fatigue

Fatigue occurs due to the acidosis that
accompanies lactate production

Lactic acid

Benefits muscle metabolism during high-intensity exercise and Lactate becomes an energy source

how does Lactic acid formation benefits muscle metabolism during high-intensity exercise

Pyruvate is converted to lactate when its production exceeds uptake by the mitochondria: Mass
Action Effect
• NADH is oxidised during the conversion of pyruvate to lactate, generating NAD+
• The ratio of NAD+ to NADH is termed Redox Potential
• The Redox Potential must be maintained for anaerobic glycolysis to continue
• NAD+ required for glyceraldehyde-3-
phosphate dehydrogenase reaction (6th
enzymatic reaction)
• Lactate productions slows decline in the
Redox Potential:
• Glycolysis continued and ATP
produced

how does Lactic acid formation becomes an energy source

~70% of lactate produced during exercise is re-oxidised to pyruvate and then
disseminated to CO2 and H2O
• 20% of lactate is taken up by the liver (Cori cycle), turned into glucose which can be
reconverted into glycogen or delivered to the blood
• The muscles use the glucose to restore their glycogen depots
Lactic Acid Metabolism
• As glycogen depletes in the working muscle,
non-working muscles will give up their CHO by
releasing lactate
• Lactate release from the arms increased
during and after 3-3.5h of leg cycling
exercise (Ahlborg and Felig, 1982)
• Concept has been termed the “Lactate
Shuttle”

Aerobic System

Provides nearly all energy transfer for exercise extending beyond
several, continuous minutes

Oxygen Kinetics

O2 uptake initially rises exponentially
(rapid) then plateaus & remains at that
rate (steady-state)
• Reflects a balance of energy required by
muscle vs. ATP produced in muscle
• O2 supply is meeting O2 demand (not
anaerobic)
• Result: Pyruvate remains Pyruvate and is
converted to Acetyl CoA instead of Lactate
• No appreciable blood lactate accumulation
• Duration limited by non-oxygen-related factors
• i.e., dehydration, glycogen depletion,
electrolyte depletion
• Exercise has begun, tissue demand for O2
has also immediately increased
• Problem: there’s a lag between O2
consumption & delivery to mitochondria

Oxygen Deficit

Previously termed “oxygen
debt”
• Difference between O2
consumed & O2 would have
been consumed had steady-
state been achieved
immediately
• So how do we match the
energy demands before
steady state is achieved?
• Alactic & Lactic systems must
compensate until then

Fast-twitch (type IIA/X)

Rapid contraction speed and high capacity
for anaerobic ATP production in glycolysis
• Highly active in change-of-pace and stop-
and-go activities like hockey or soccer
• Type IIa also possesses somewhat high
aerobic capacity

Slow-twitch (type I)

Generates energy through aerobic
pathways
• Slower contraction speed than fast twitch
• Active in continuous activities requiring
steady-rate aerobic energy transfer

How do we anticipate trained
individuals to respond

Reach steady-state sooner
• So lower oxygen deficit values
• Training-related improvements
include
• Bioenergetics
• Overall and/or selective tissue perfusion
(blood flow)

Maximal Oxygen Consumption (VO2max)

Represents the maximal volume of oxygen one
can consume
• AKA “maximal oxygen uptake”, “maximal aerobic
power”, or “aerobic capacity”
• Expressed as L/min, mL/min, or mL/kg/min
• Seen as the region where O2 consumption
plateaus or only increases slightly with
additional increases in exercise intensity
• Provides a quantitative measure of the capacity
for aerobic ATP resynthesis (oxidative
phosphorylation)
• Exercise intensities beyond this point will rely
more heavily on lactic & alactic systems
• Sustained work beyond this point will result in large
amounts of lactate accumulation

Physiology Behind VO2max

Aerobic Capacity relies on a complex
relationship between multiple systems
• Ventilation rate/depth needs to be sufficient for
maximal exchange of O2 & CO2
• Blood hemoglobin levels must be sufficient to carry
said O2
• Cardiac output must be sufficient to quickly circulate
blood
• And blood volume must be sufficient to maintain adequate
pressure
• Tissues must be appropriately perfused to receive
blood & O2
• Perfusion also corresponds to increased O2 demand
• Tissues must have sufficient mitochondria to
adequately use O2
• Mitochondrial activity also dictates tissue’s O2 demand

Excess Post-Exercise
Oxygen Consumption
(EPOC)

Total O2 uptake minus
total O2 theoretically
consumed at rest during
recovery

EPOC reflects two factors

1. Level of anaerobic metabolism in bout
• Short-duration, light-to-moderate intensity
exercise (highly aerobic) → quick recovery
• 50% decrease in ventilation every 30
sec
• Higher intensity exercises (more
anaerobic) → larger deficits, longer
recovery times
• Often larger EPOCs than the deficit
would suggest
• Was rationale for “lactate”
explanation
2. Oxygen cost of adjustments in:
• Ventilation
• Hormones
• Circulation
• Temperature
• Blood oxygenation

Two Components of EPOC

Fast (ATP-PCr) and Slow
(multimodal

Fast (ATP-PCr)
EPOC

Denotes the initial, rapid
decline in O2 consumption
toward rest
• Replenishes high-energy
phosphates (ATP & PCr)
initially spent
• Where short-duration, light-to-
moderate intensity exercise’s
EPOC ends

Slow (multimodal) EPOC

Denotes the later, less-pronounced
declined in O2 consumption toward rest
• Duration dictated by time necessary
to re-balance oxygen costs for
adjusting:
• Body Temperature
• Blood Lactate
• Hormone levels

Duration seems closely tied to
exercise time & intensity

Longer-duration (>60 min),
intense aerobic exercise spends
more time here
• Exhaustive exercise spends
more time still to resynthesize
glycogen from lactate

EPOC’s Implications

Optimal recovery
ultimately depends on
exercise modality
• Steady-state exercise
accumulates little to no
lactate
• Aerobic, smaller O2 deficits
• Passive recovery ideal
• Active recovery would
only elevate total
metabolism and extend
recovery time

Optimal recovery ultimately
depends on exercise modality
• Non-steady-state exercise will
accumulate lactate
• More possibly anaerobic
components, larger O2 deficits
• Active recovery will accelerate
lactate clearance
• At lower intensities, lactate
clearance more readily matches
lactate formation
• Cycling: 30 – 45% aerobic
capacity
• Running: 55 – 60% aerobic
capacity

Intermittent (Interval) Exercise

Applies different work-to-rest
ratios with supramaximal intensity
exercises where continuous
performance for several minutes
would be exhausting
• “Work” intervals target a specific
energy-transfer system
• “Rest” intervals allow some level of
rapid recovery
• Can perform next bout at similar
intensity as before
• Manipulating these ratios “overloads”
the energy-transfer system

Respiratory Quotient (RQ):

CO2 produced compared to O2 consumed
(CO2exp./O2insp.)
Can tell us which “fuel” is being used at rest
and during stead-state exercise

Respiratory Exchange Ratio (RER or R)

Computes like RQ

Measured during non-steady-state exercise
• Can be >1.00 due to hyperventilation and/or
lactate buffering during high-intensity exercise
• Therefore, does not include fuel(s) used

Physical activity requires ATP resynthesis to be sustained

Alactic system fast, but inefficient
• Lactic system more efficient, but lactate accumulates if aerobic environment not
sustained

Energy-transfer systems don’t exist in isolation

Initial oxygen deficit incurred due to anerobic metabolism
• Leads to some lactate buildup
• Low, intensity & steady-state (aerobic) exercise clears this easily corresponding to faster
recovery
• Higher intensity, non-steady-state (anaerobic) exercise leads to accumulation & more
extended recovery times
• Interval exercise allows us to do more of this type work without as large an impact

By measuring and comparing oxygen consumption against carbon dioxide
production, we can easily class exercise as aerobic or anaerobic

Ratios can correspond to preference for utilization of different fuels (and therefore
energy-transfer systems)