Exercise Physiology Week 5: Lactate and Fatigue

Course Information

• Course name: Exercise Physiology
• Course code: DPT-413
• Instructor: Monira I. Aldhahi
• Week: 5th

Class Objectives

• Understand the lactate buffering system
• Identify gender-related differences in anaerobic exercise characteristics
• Explain laboratory and field assessment techniques for aerobic metabolism during exercise
• Explain major variables used to describe aerobic metabolic response to exercise

The Lactic Acid Myth

• The myth that lactic acid causes muscle fatigue originated with Otto Meyerhoff and Archibald V. Hill
• Their theory is that anaerobic metabolism leads to lactic acid production, which causes acidosis and muscle fatigue
• The 1970s saw a "lactate revolution" changing understanding
• Dr. Brooks and others showed lactate as a muscle and heart fuel, not a cause of fatigue

Hydrogen Ion Production During Exercise

• Acidosis arises from acid accumulation or base loss.
• Alkalosis occurs from acid loss or base accumulation
• Key contributors to exercise-induced muscle acidosis include:
  • Carbon dioxide and carbonic acid production in working skeletal muscles
  • Lactic acid production in working muscles
  • ATP breakdown in working muscles

Pyruvate and H+ Ions

• The body combines excess pyruvate with H+ ions to form lactic acid
• This enables muscles to work longer than their normal capacity

Buffer System

• Lactate and H+ lower blood pH.
• The body uses buffer systems (like sodium bicarbonate) to maintain normal blood pH
• Lactate is transported to the liver and non-exercising tissues for conversion back to pyruvate during exercise or recovery

Lactate Clearance

• Lactate is transported via monocarboxylate transporters (MCTs)
• Lactate is shuttled from the muscle to the heart and other tissues where it can be oxidized or used by the liver

Lactate as a Viable Fuel

• Lactate can be converted back to pyruvate and then glucose
• This process plays a crucial role in energy production and carbohydrate replenishment

Delayed Onset Muscle Soreness (DOMS)

• DOMS is unrelated to lactate
• DOMS is believed to be related to microtrauma within the muscles

Male vs. Female Anaerobic Exercise Characteristics

• Females exhibit generally lower anaerobic characteristics than males in young and middle-aged adults
  • ATP-PC availability and utilization do not differ between the sexes but males have more total energy available due to muscle mass difference
  • Lactate thresholds occur at higher absolute workloads in males, but at a given relative workload above LT, lactate concentrations are the same.

Anaerobic Exercise Characteristics of Older Adults

• Local resting stores of ATP-PC are reduced
• Anaerobic power decreases with age (ex: 32% decline from youth to old age)

Accumulation of Lactate in Older Adults

• Resting blood lactate levels are consistent across different ages
• Lactate accumulation is typically higher in older adults at a given absolute submaximal workload
• Older males have an overall lower lactic acid production than younger males during a 60-second maximal static contraction

Mechanical Power and Capacity in Older Adults

• Peak power in the Margaria-Kalamen Stair Climb test declines with age (20 to 70 years old)
• Males show an approximate 10.3% decline in maximal anaerobic power per decade from age 20 to 88

Aerobic Metabolism During Exercise

• The goal of measuring aerobic metabolism is to quantify energy needed for a given activity

Rationale for Fitness Assessment

• Establish baseline
• Design programs
• Set realistic goals
• Evaluate changes in fitness
• Diagnose further
• Describe pathology
• Understand patient limitations

Measurement of Aerobic Metabolism

• Methods include
  • Direct calorimetry (measures heat)
  • Indirect open-circuit spirometry (measures air; most common in exercise physiology)
    • Open circuit: subject breathes room air
    • Closed circuit: subject breathes in a sealed container containing oxygen

Calorimetry

• Calorimetry measures heat energy released or absorbed in metabolic processes
• Direct calorimetry measures heat production
• It uses specially constructed chambers

Indirect Spirometry

• Indirect calorimetry method for determining heat production by measuring and analyzing expired air
• Measures O2 consumption and CO2 production
  • In a closed system: subject breathes into a container with a designated composition that absorbs exhaled CO2 to measures O2 use
  • In an open system:subject breathes room air, and exhaled air is collected and O2 and carbon dioxide levels measured

Open Circuit Indirect Spirometry

• Most common lab method
• Measurements take place at rest, during submaximal exercise, or at maximal exertion

Aerobic Exercise Responses

• Oxygen consumption is the primary measure of interest, directly related to ATP
• Measuring CO2 helps determine fuel utilization and caloric expenditure

VO2 and VCO2

• VO2: Oxygen consumed (inspired - expired)
• VCO2: Carbon dioxide produced during metabolism, primarily from aerobic cellular respiration

Short-Term, Light to Moderate Intensity Submaximal Exercise

• VO2 levels plateau during short-term light to moderate submaximal exercise
• Time to reach steady state VO2 is shorter in younger adults compared to older adults or at higher intensities
• VO2 is highly correlated with exercise intensity

Long-Term Moderate to Heavy Submaximal Exercise

• A situation where oxygen consumption increases (drifts upward) despite a constant exercise demand. This is likely due to an increase in lactate levels, rising catecholamines, changing energy substrate utilization, increased cost of ventilation, and increased body temperature.

Incremental Aerobic Exercise to Maximum

• VO2 max is the highest oxygen intake an individual can reach
• Max testing can last 8-12 minutes, but for healthy individuals, 7-26 minutes (cycle ergometer). Treadmill 5-26.
• Shorter tests usually require a warm-up.
• Ramp protocols show a linear increase in VO2

Ventilation During Max Tests

• Ventilation (VE), VO2, and VCO2 all increase due to rising energy demands
• For example, the person in Table 4.2 demonstrates reserve: VE rose from 25.21 to 139.04 L/min, meaning a significant reserve capacity

Respiratory Quotient (RER/RQ)

• RER (respiratory exchange ratio) = ratio of CO2 produced to O2 consumed in entire body
• RQ (respiratory quotient) = ratio of CO2 produced to O2 consumed by cells
• Helps determine fuel source
  • Carbohydrates= 1
  • Fat = .71
  • Protein= .80

Static and Dynamic Resistance Exercise

• Physiological responses to static exercise are described relative to maximal voluntary contraction (MVC). Static contractions below 15-25% MVC.
• Dynamic resistance activity has both an aerobic and an anaerobic component. The greater the work (repetitions/duration), the greater the aerobic contribution.