Ch 4: Exercise Metabolism Flashcards
At rest, is ATP produced aerobically or anaerobically?
At rest, ATP is produced almost 100% aerobically
Oxygen consumption (VO2) provides an index of…
aerobic ATP production (aerobic metabolism)
During rest: 3.5 ml/kg/min
Blood lactate levels at rest
Because oxygen is readily available at rest, pyruvate enters the Krebs cycle rather than being converted to lactate… so blood lactate levels are low.
ATP and VO2 at the Onset of Exercise
- ATP demand increases instantaneously
- VO2 does not increase instantaneously
- initial ATP production is through anaerobic pathways
For light to moderate exercise, steady state VO2 occurs within
1-4 minutes
Oxygen deficit
Lag in VO2 (oxygen consumption) at the onset of exercise
Why do trained individuals have lower oxygen deficit?
- better developed aerobic bioenergetic capacity due to cardiovascular or muscular adaptations to exercise training
cardiovascular adaptations= more capillaries
muscular adaptations= greater mitochondrial value
- aerobic metabolism is active earlier and less lactic acid is produced
- reach steady state VO2 earlier
Why does oxidative phosphorylation not achieve full activation instantaneously at exercise onset?
- Inadequate oxygen supply to muscles at exercise onset
- Mitochondria may not have oxygen molecules present and therefore available to participate in the ETC… thus restricting aerobic ATP production.
- This may hold true for high intensity work
- Delay in stimuli (ADP and Pi concentrations) to fully activate oxidative phosphorylation
Oxygen Debt
Elevated VO2 (above resting levels) following exercise
- it takes time for oxygen consumption to return to the levels it was at rest… goes down in curvilinear way
Rapid Phase of O2 debt
- Resynthesis of PC
- Restoration of muscle and blood oxygen stores
Slow Phase of O2 debt
- Elevated HR and breathing
- Elevated body temperature
- Elevated hormones (epinephrine & norepinephrine)
- Conversion of lactic acid to glucose (gluconeogenesis)
EPOC
excess post-exercise oxygen consumption
aka oxygen debt
Why is EPOC greater following High Intensity Exercise?
- higher body temperature
- greater depletion of PC (additional O2 required for resynthesis)
- greater blood concentrations of lactic acid (additional O2 required for greater level of gluconeogenesis)
- higher levels of blood epinephrine and norepinephrine
Bioenergetic Pathways for Short-term high-intensity exercise…
When does ATP come from ATP-PC system or glycolysis?
- ATP comes mostly from anaerobic pathways
Whether ATP provision comes from the ATP-PC system or glycolysis depends on the length of the activity
- First 1-5 sec: ATP-PC system
- > 5 sec: shift to glycolysis
- > 45 sec: anaerobic and aerobic
at 60 seconds: 70% anaerobic/ 30% aerobic
at 2-3 minutes: 50%/50%
At the onset of exercise _____ is the main source of energy
Phosphocreatine
- drops significantly
Because we use all PC up, we shift to
glycolysis
Bioenergetic Pathways for Prolonged Exercise
(How is ATP produced?)
- ATP production primarily from aerobic metabolism
Steady state oxygen uptake can generally be maintained during prolonged submaximal exercise, except for in:
What causes the drift?
- Moderate intensity in a hot and humid environment
- High intensity in a thermoneutral environment
There is a drift in steady state caused by:
- increase in body temperature
- increase in intensity = increase in hormones and metabolic rate
VO2 max
Maximum capacity to deliver and utilize oxygen during exercise
- When an increase in workload no longer evokes an increase in O2 consumption
Affected by genetics and training
Physiological factors that influence VO2 max
- maximum ability of cardiorespiratory system to deliver oxygen to the muscle
- ability of muscles to use oxygen and produce ATP aerobically
Fick Equation
VO2= CO x (a-v O2 diff)
describes the relationship between oxygen consumption, cardiac output and atrial venous oxygen difference
During early incremental exercise (lower intensity) most ATP comes from
aerobic sources
As exercise gets harder (higher intensity) blood lactate concentration
increases
Lactate production occurs in
skeletal muscle- fast twitch
Lactate removal occurs in
liver, heart, muscle
Lactate threshold
aka “Anaerobic Threshold”
OBLA: onset of blood lactate accumulation
the point at which there is a sudden increase in the blood lactate concentration during incremental exercise
- represents an increased reliance on glycolysis
Possible explanations for Lactate Accumulation with Incremental Exercise
- Low muscle oxygen (hypoxia)
- low levels of oxygen in individual muscle cells
- Accelerated glycolysis
- increased epinephrine and norepinephrine at 50-60% VO2 max… stimulates glycolytic rate
- H+ from NADH+H+ that is produced in glycolysis must be shuttled into mitochondria
- During intense exercise, shuttling is not fast enough, so NADH + H+ accumulates
- to regenerate NAD to keep glycolysis going NADH+H donates its H+ ions to pyruvate and lactate is formed
- Recruitment of fast-twitch fibers (harder exercises)
- Type of lactase dehydrogenase present in fast twitch fibers promotes the formation of lactic acid from pyruvic acid
- Reduced lactate removal
- as exercise intensity increases, muscles receive a greater % of cardiac output
- blood is shunted away from the liver (a site of lactate removal)
the liver forms glucose from lactate (gluconeogenesis)
Intracellular Lactate shuttle
Lactate is produced in the cytoplasm of a muscle fiber and can be taken up by the mitochondrion within the same fiber and used as fuel
Intercellular Lactate shuttle
25% is used for gluconeogenesis in the liver
75% is used in muscle aerobic metabolism
Cori Cycle
- lactate in the blood travels to slow-twitch myocytes and cardiac myocytes, where it is then converted back to pyruvate
Lactic acid is removed more rapidly with
light exercise in recovery
optimal intensity: 30-40% of VO2 max
Lactic acid removal post exercise evidence
- 70% of lactic acid is oxidized (used as a substrate by heart and skeletal muscle)
- 20% is converted to glucose
- 10% is converted to amino acids
Where does the lactate threshold appear in untrained individuals vs trained individuals?
untrained: 50%-60% of VO2 max
trained: 65%-80% of VO2 max
Lactate threshold represents an increased reliance on
glycolysis
DOMS
delayed onset muscle soreness
- occurs 24-48 hours after exercise
Reasons it is not true that lactate causes DOMS
- lactic acid removal is rapid and levels return to normal within an hour following exercise
- soreness is rare following routine exercise
What causes DOMS?
microscopic injury to muscle fibers resulting in a slow series of biochemical events leading to inflammation and swelling within the injured muscle
effects of training on lactate threshold
trained individuals can reach higher work rates before LT occurs
RER
Respiratory Exchange Ratio
- estimates contribution of carbohydrate and fat to energy metabolism during exercise
aka Respiratory Quotient (R or RQ)
VCO2/VO2
For RER to be used as an estimate of substrate utilization during exercise…
must be at steady state
RER= 0.70
100% fat
0% carbohydrates
RER= 0.85
50% fat
50% carbohydrates
RER= 1.00
0% fat
100% carbohydrates
Using RER to estimate fuel utilization assumes that…
protein is not used as a fuel during exercise
Protein contribution in fuel
- proteins contribute less than 2% of the substrate used in exercise less than one hour
- may reach 5-10% during the final minutes of prolonged exercise (3-5 hours)
proteins play a minor role as a substrate during exercise
How does diet govern fuel selection during exercise?
high fat, low carbohydrate promote high fat metabolism
How does exercise intensity govern fuel selection during exercise?
- low intensity (<30% VO2 max) exercise relies on fat for fuel
- high intensity (>70% VO2 max) exercise relies on carbohydrates
How does exercise duration govern fuel selection during exercise?
low intensity prolonged exercise increases fat utilization
How does exercise training govern fuel selection during exercise?
endurance trained individuals use more fat and less carbohydrates than less-fit individuals during prolonged exercise at the same intensity
Practical use of the lactate threshold?
- predicting endurance performance
- a marker of training intensity
As exercise intensity increases, how is fat and carbohydrate metabolism affected?
As the exercise intensity increases, there is a progressive increase in carbohydrate metabolism and a decrease in fat metabolism.
The “crossover point”
As the exercise intensity increases, there is a progressive increase in the contribution of carbohydrate as a fuel source.
- as the exercise increases above the crossover point, a progressive shift occurs from fat to carbohydrate metabolism
What causes the “crossover”?
- recruitment of fast fibers
fast fibers have more glycolytic enzymes but less mitochondrial and lipolytic enzymes (enzymes responsible for fat breakdown). Meaning they are more equipped to metabolize carbohydrates.
- increasing blood levels of epinephrine
High levels of epinephrine increase phosphorylase activity, which causes an in-crease in muscle glycogen breakdown; this results in an increased rate of glycolysis and lactate production. Increased production of lactate inhibits fat metabolism by reducing the availability of fat as a substrate.
the absolute amount of fat metabolized during exercise is typically greatest at
higher intensities that are below the lactate threshold
At higher intensities, the total energy expenditure
increases, even if the contribution of fat as fuel decreases
Optimal exercise intensity to use fat as a fuel source depends on
- percentage of energy derived as fat
- total energy expenditure
Carb or fat metabolism during prolonged moderate intensity exercise?
during prolonged moderate intensity exercise there is a shift from CHO to fat metabolism
- lipolysis (breaking down fats) is a slow process and the rate increases only after several minutes of exercise
