Energy Systems and Fatigue

kilocalorie

amount of heat required to raise the temperature of 1 kilogram of water to one degree Celsius - equivalent to 1 Calorie, the “calorie” we refer to in food

what is used to measure energy expenditure

direct calorimetry and indirect calorimetry

direct calorimetry

accurate over time and good for resting metabolic measurements

• not practical or accurate during exercise!

• expensive, slow

• produces excess heat

• sweat creates errors in measurements

indirect calorimetry

combustion of substrate, estimates total body energy expenditure based on O₂ used, CO₂ produced

• measures respiratory gas concentration

• only accurate for steady-state oxidative metabolism

VO₂

volume of O₂ consumed per minute (rate of O₂ consumption)

volume of O₂ inspired - volume O₂ expired

VCO₂

volume of CO₂ produced per minute (rate of production)

volume of expiredCO₂- volume inspired CO₂

respiratory exchange ratio (RER)

VCO₂/VO₂

ranges from 0.7-1.0

• higher in carbohydrates (about 0.92) than fats (about 0.75)

• decreased value if aerobically trained

if the same person runs 1 mile and walks 1 mile on a level surface which activity will burn more calories?

both burn the same # of calories

• same person/weight

• fixed distance

if the same person runs 10 minutes and walks 10 minutes which activity will burn more calories?

running 10 minutes burns more calories

• higher intensity/workload over larger distance

VO_2max

maximal O₂ uptake, point at which which O₂ consumption doesn’t increase with further increase in intensity

best single measurement of aerobic fitness but not best predictor of endurance performance

units for VO_2max

L/min for non weight bearing activities

ml O₂ x kg^-1 x min^-1 for body weight and more accurate comparison for different body sizes

for intensities above 50% VO_2max what is required

carbohydrates

for intensities above 50% VO_2max what is increased

glycolysis

nonlinear increase in blood lactate signifies

VO_2max

to burn more calories you must increase

intensity

lactate threshold

point at which blood lactate accumulation increases markedly, good indicator of potential for endurance exercise

pyruvate production rate > pyruvate clearance rate

lactate threshold is expressed as

percentage of VO_2max

• Untrained: 50-60% VO_2max

• Trained: 70-80% VO_2max

excessive post exercise oxygen consumption (EPOC)

replenishing of the ATP/PCr stores

• continued substrate cycling (glucose and FAs)

• elevated rates of fatty acid oxidation

• increased body temp

• increased levels of catecholamines in blood: epinephrine and norepinephrine

EPOC values

1 min = 50%

3 min = 80%

5 min = 100%

Fatigue

1. Decrements in muscular performance with continued effort, accompanied by sensations of tiredness

2. Inability to maintain required power output to continue muscular work at given intensity

causes of fatigue (short duration)

short duration, high intensity exercise

a. CrP depletion

b. metabolic by products

• increased lactate concentration [HLa] which increases [H+] (1-3 minutes)

  • a. decreased pH

  • b. decreased rate of glycolysis

  • c. interferes with Ca 2+ binding to troponin

causes of fatigue (prolonged duration)

prolonged endurance exercise (70% VO_2max - about 3hrs)

glycogen depletion

1. muscle glycogen levels

a. muscle glycogen declines throughout exercise

b. as muscle glycogen levels decline there is an increased reliance on blood glucose

2. liver glycogen levels

a. liver glycogenolysis attempts to maintain blood glucose levels

b. when liver glycogen levels fall and the rate of liver glycogenolysis is less than the rate of muscle glucose uptake the blood glucose concentration declines

c. time to fatigue due to an inability to provide an adequate supply of carbohydrate to the muscle

hitting the wall

more glycogen/glucose moving out of body than amount of glucose in blood

• out of muscle glycogen

• out of liver glycogen

• can finish race at lower intensities using fat oxidation for ATP production