HSS 486 Energy Expenditure & Fatigue

Calorimetry

Measurement of calorie/energy expenditure

Metabolic rate

Rate of energy expenditure

Byproduct

Something that builds up as a result of a process

Direct calorimetry

A way of measuring energy expenditure that utilizes heat production (not very common); heat production increases with energy production → body temperature increases water temperature in calorimeter

Pros of direct calorimetry

• Accurate over longer periods of time

• Good for resting metabolic measurements

Cons of direct calorimetry

• Expensive

• Slow

• Neither practical nor accurate for exercise

Indirect calorimetry

Way of estimating energy expenditure based on O₂ used and CO₂ produced

• Is accurate only for steady-state oxidative metabolism

• Older methods are accurate but slow, newer methods are faster but expensive

• Used to measure VO₂, RER, basal metabolic rate (BMR)

VO₂

Volume of oxygen consumed per min (L/min or mL/kg/min)

RER

Respiratory exchange ratio; the ratio between rates of CO₂ production and O₂ usage

• Depends on the type of fuel being utilized — more carbon atoms in molecule = more O₂

• Glucose has less carbons (RER = 1.00) than FFAs (RER = 0.71)

Cons of RER

• CO₂ production may not = CO₂ exhalation

• RER is inaccurate for protein oxidation

• RER near 1.0 may be inaccurate when lactate build-up increases CO₂ exhalation

Heart rate monitoring

Way to estimate energy expenditure in the field; based on the assumption that HR is linearly related to VO₂ (can estimate VO₂ from HR)

Limitations for HR monitoring

• Confounded by ambient temperature

• Impacted by upper versus lower body exercises

• Impacted by fitness levels

Self-report measures of energy expenditure

Subjective report from participant in the field (like RPE); easy, efficient, and cost effective

Limitations of self-report measures of energy expenditure

Relies on subjective measures

Basal metabolic rate (BMR)

Rate of energy expenditure at rest; minimum energy requirement for living

• Related to fat-free mass, but also affected by body surface area, age, stress, hormones, & body temperature

• During submaximal exercise metabolic rate increases with exercise intensity

VO₂ max

Maximal O₂ uptake; the point at which oxygen consumption doesn’t increase with further increase in intensity

• Best single measurement for aerobic fitness

• NOT best predictor of endurance exercise

• Plateaus after 8-12 wks of training

EPOC

Excess post-exercise oxygen consumption

• Early exercise — O₂ demand greater than O₂ consumed → O₂ deficit

  • Occurs when anaerobic pathways are used for ATP production

• Early recovery — O₂ consumed greater than O₂ demand → EPOC

  • Replenished ATP/PCr stores, converts lactate to glycogen, replenishes hemo-/myoglobin, clears CO₂

Lactate threshold

Point at which blood lactate accumulation increases markedly; lactate production rate is greater than lactate clearance rate

• Interaction of aerobic & anaerobic systems

• Good indicator of potential for endurance exercise

What is a higher lactate threshold an indicator of?

Better endurance performance (for two athletes with the same VO₂ max, higher lactate threshold predicts better performance)

Characteristics of successful athletes in aerobic endurance activites

• High VO₂ max

• High lactate threshold

• High economy of effort (or low VO₂ for a given absolute exercise intensity

• High percentage of type I muscle fibers

Fatigue

Decrements in performance with continued effort, accompanied by sensations of tired; inability to maintain required power output to continue work at given intensity

• Can be central fatigue or peripheral fatigue

Four major causes of fatigue

• Inadequate energy delivery/metabolism

• Accumulation of metabolic byproducts

• Failure of muscle contractile mechanism

• Altered neural control of muscle contraction

Fatigue due to inadequate energy delivery/metabolism

• PCr depletion (used for short-term, high-intensity effort)

• Glycogen reserves limited & depleted quickly

• Muscle glycogen insufficient for prolonged exercise (+ hypoglycemia = fatigue)

Fatigue and muscle fiber type

Fibers recruited first or most often get depleted fastest; recruitment dependent on exercise intensity

• Activity-specific muscles are also depleted fastest

Fatigue due to accumulation of metabolic byproducts

• P_i from rapid breakdown of PCr, ATP

• Heat retained by body → core temperature increase

• Lactic acid → if not cleared, converts to lactate + H⁺

• H⁺ accumulation causes muscle acidosis → drop in muscle pH → may denature enzymes (unable to make more ATP)

Fatigue due to failure of muscle contractile mechanism

Failure may occur at neuromuscular junction, preventing muscle activation; possible causes include…

• Reduced ACh synthesis & release

• Altered ACh breakdown in synapse

• Increase in muscle fiber stimulus threshold

• Altered muscle resting membrane potential

Fatigue due to altered neural control of muscle contraction

CNS plays role in fatigue…

• Conscious aspect to fiber recruitment

• Subconscious/conscious unwillingness to endure more pain

• Conscious decision to terminate activity

• Interaction between perception of effort & motivation

Acute muscle soreness

Felt during or immediately following strenuous or novel exercise (typically due to fluid retention)

• Disappears in minutes to hours

DOMS

Delayed-onset muscle soreness (24-48 hrs); ranges from stiffness to severe, restrictive pain

• Major cause: eccentric contraction → structural damage

• NOT caused by increased blood lactate concentrations