Chapter 4

Question 1

Energy Requirements at Rest

Answer 1

Almost 100% of ATP produced by aerobic metabolism

Blood lactate levels are low (<1.0 mmol/L)

Resting O2 consumption:
– 0.25 L/min
– 3.5 ml/kg/min

Question 2

Rest-to-Exercise Transitions

Answer 2

ATP production increases immediately

Oxygen uptake increases rapidly
– Reaches steady state within 1–4 minutes
– After steady state is reached, ATP requirement is met through aerobic ATP production

Initial ATP production through anaerobic pathways
– ATP-PC system
– Glycolysis

Oxygen deficit: Delay in O2 consumption until steady state VO2 is reached
– Lag in oxygen uptake at the beginning of exercise
– It takes time to active enzyme activity
– Training can reduce amount of O2 deficit

Question 3

Why do trained subjects have a lower oxygen deficit?

Answer 3

Better developed aerobic bioenergetic capacity

Due to cardiovascular and muscular adaptations

Training can increase enzyme activity and number of mitochondria

Question 4

Recovery From Exercise

Answer 4

Oxygen uptake remains elevated above rest during
recovery from exercise

Oxygen debt or Excess post-exercise oxygen consumption (EPOC)
- Rapid and slow component
- Terminology reflects that only ~20% elevated O2
consumption used to “repay” O2 deficit

Question 5

Oxygen Debt or Excess post-exercise oxygen consumption (EPOC)

Answer 5

“Rapid” portion of O2 debt
– Resynthesis of stored PC
– Replenishing muscle and blood O2 stores

“Slow” portion of O2 debt
– Elevated heart rate and breathing = Increased energy need
– Elevated body temperature = Increased ­metabolic rate
– Elevated epinephrine and norepinephrine = Increased metabolic rate
– Conversion of lactic acid to glucose (gluconeogenesis)

Question 6

Why is EPOC greater following high intensity exercise?

Answer 6

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

Question 7

Removal of Lactic Acid Following Exercise

Answer 7

70% of lactic acid is oxidized by cells
- Used as a substrate by heart and skeletal muscle

20% converted to glucose

10% converted to amino acids

Lactic acid is removed more rapidly from the blood if light exercise is performed during recovery
– Optimal intensity is ~30–40% VO2 max
– More converted to glucose in the liver

Question 8

Metabolic Responses to Short-Term, High intensity Exercise

Answer 8

First 1–5 seconds of exercise
– ATP produced via ATP-PC system

Intense exercise longer than 5 seconds
– Shift to ATP production via glycolysis

Events lasting longer than 45 seconds
– ATP production through ATP-PC, glycolysis, and aerobic systems
– 70% anaerobic/30% aerobic at 60 seconds
– 50% anaerobic/50% aerobic at 2 minutes

Question 9

Metabolic Responses to Prolonged Exercise

Answer 9

Prolonged exercise (>10 minutes)
– ATP production primarily from aerobic metabolism
– Steady-state oxygen uptake can generally be maintained during submaximal exercise (below lactate threshold)

Prolonged exercise in a hot/humid environment or
at high intensity
– Results in upward drift in oxygen uptake over time due to increases in body temperature and increasing blood levels of epinephrine and norepinephrine

Question 10

Metabolic Responses to Incremental Exercise

Answer 10

Oxygen uptake increases linearly until maximal
oxygen uptake (VO2 max) is reached
– No further increase in VO2 with increasing work rate

Question 11

VO2 max

Answer 11

“Physiological ceiling” for delivery of O2 to muscle

Affected by genetics and training

Question 12

Physiological factors influencing VO2 max

Answer 12

Maximum ability of cardiorespiratory system to deliver oxygen to the muscle

Ability of muscles to use oxygen and produce ATP aerobically

Question 13

Lactate Threshold

Answer 13

The point at which blood lactic acid rises systematically during incremental exercise

Appears at ~50–60% VO2 max in untrained subjects

Occurs at higher work rates (65–80% VO2 max) in trained subjects

Also called:
Anaerobic threshold

Onset of blood lactate accumulation (OBLA)
- Exercise intensity at which blood lactate levels reach 4 mmol/L

Question 14

Explanations for the Lactate Threshold

Answer 14

Accelerated glycolysis
– NADH produced faster than it is shuttled into
mitochondria
– Excess NADH in cytoplasm converts pyruvic acid to lactic acid

Recruitment of fast-twitch muscle fibers
– LDH isozyme in fast fibers promotes lactic acid formation

Reduced rate of lactate removal from the blood

Question 15

Practical Uses of the Lactate Threshold

Answer 15

Prediction of performance
– Combined with exercise economy

Planning training programs
– Marker of training intensity
– Choose a training HR based on LT

Question 16

Respiratory exchange ratio (RER or R)

Answer 16

The ratio between the volume of CO2 being produced by the body and the amount of O2 being consumed

R = VCO2/VO2

R (glucose) = 1.00

R (Fat) = 0.70

Question 17

Exercise Intensity and Fuel Selection

Answer 17

Low intensity exercise (< 30% VO2 max)
- Fats are primary fuel source

High intensity exercise (> 70% VO2 max)
- Carbs are primary fuel source

Question 18

Crossover concept

Answer 18

Describes the shift from fat to carbs as exercise intensity increases

Due to:
Recruitment of fast muscle fibers
Increasing blood levels of epinephrine

Question 19

Sources of Carbohydrate During Exercise

Answer 19

Muscle glycogen
– Primary source of carbohydrate during high-intensity exercise
– Supplies much of the carbohydrate in the first hour of exercise

Blood glucose
– From liver glycogenolysis
– Primary source of carbohydrate during low-intensity exercise
– Important during long-duration exercise as muscle glycogen levels decline

Question 20

Sources of Fat During Exercise

Answer 20

Intramuscular triglycerides
– Primary source of fat during higher intensity exercise

Plasma FFA
– From adipose tissue lipolysis (Triglycerides -> glycerol + FFA)
– FFA converted to acetyl-CoA and enters Citric acid cycle
– Primary source of fat during low-intensity exercise
– Becomes more important as muscle triglyceride levels decline in long-duration exercise

Question 21

Sources of Protein During Exercise

Answer 21

Proteins broken down into amino acids
– Muscle can directly metabolize branch chain amino acids and alanine
– Liver can convert alanine to glucose

Only a small contribution (~2%) to total energy
production during exercise
– May increase to 5–10% late in prolonged-duration exercise
– Enzymes that degrade proteins (proteases) are activated in long-term exercise

Question 22

Lactate as a Fuel Source During Exercise

Answer 22

Can be used as a fuel source by skeletal muscle and the heart
– Converted to acetyl-CoA and enters Krebs cycle

Can be converted to glucose in the liver
– Cori cycle

Lactate shuttle
– Lactate produced in one tissue and transported to
another

Question 23

The Cori Cycle: Lactate as a Fuel Source

Answer 23

Lactate produced by skeletal muscle is transported to the liver

Liver converts lactate to glucose
– Gluconeogenesis

Glucose is transported back to muscle and used as a fuel