lec 5

Lecture objectives

• Define the aerobic–anaerobic transition (AAT) and analyze the different methods for identifying it.
• Compare lactate threshold (LT), maximal lactate steady state (MLSS) and critical power (CP) tests for use with athletes.
• Describe what is meant by the term steady state and the concept of steady-state exercise capacity.
• Summarise excess post-exercise oxygen consumption (EPOC) and the oxygen deficit that precedes aerobic metabolism after exercise begins.
• Discuss exercise economy and its practical importance for performance and fatigue resistance.
• Describe the likely mechanisms of fatigue in high‑intensity aerobic endurance sports.
• Highlight the metabolic and cardiovascular adaptations to aerobic endurance training.
• Understand role of skeletal muscle fibre adaptations and the time course of VO₂max, LT and economy improvements.
• Recognise potential dietary and supplement strategies to augment LT adaptations and performance (e.g., caffeine, beta‑alanine, sodium bicarbonate, creatine, citrulline malate).

Aerobic endurance: definition and measurement

• Aerobic endurance definition: the body's ability to produce energy for exercise involving the whole body during sustained activity.
• How to measure aerobic endurance:
  • Primary variables:
  • V̇O₂max (maximal oxygen uptake) as the traditional standard.
  • Lactate threshold (LT) and related lactate measures (e.g., OBLA, MLSS).
  • CP (critical power) as a related performance predictor.
  • Exercise economy (oxygen cost at a given workload).
  • Macronutrient energy sources:
  • Carbohydrates and fats are the dominant macronutrients for energy production during endurance tasks; protein contribution is minimal except in prolonged or glycogen-depleted states.
• Transition between aerobic and anaerobic energy systems is now used as a measure of aerobic functional capacity.

Aerobic–anaerobic transition (AAT) and Lactate Threshold (LT)

• AAT and lactate:
  • AAT is closely associated with the formation of lactate (lactate accumulation reflects the balance of lactate production vs removal).
• Identification of AAT/LT:
  • Traditionally identified via blood lactate analysis techniques.
  • Alternatively identified via non-invasive gas analysis (respiratory gas measures).
• Lactate threshold (LT) concept:
  • LT represents the exercise intensity at which lactate begins to accumulate above baseline steadily.
  • The V̇O₂max test has historically been the gold standard measure of aerobic endurance, but LT provides crucial information about the sustainable intensity for performance.
  • As exercise intensity increases, fast glycolysis accelerates, producing lactate faster than it can be cleared, leading to rising lactate concentrations.
• LT-related measures:
  • Onset of blood lactate accumulation (OBLA) and LT itself.
• Practical LT testing considerations:
  • Protocols are incremental tests typically with 3–4 minute stages.
  • Measurements taken at the end of each stage include blood lactate, HR, and sometimes RPE.
  • LT identification methods:
  • Visual inspection (simple approach).
  • Mathematical approaches: log–log transformation, the D‑max method, or the individual anaerobic threshold.

Lactate Threshold (LT) and OBLA

• OBLA (onset of blood lactate accumulation):
  • Defined as the exercise intensity at which blood lactate concentration reaches 4 mM: $ext{lactate concentration} = 4 ext{ mM}.$
• LT testing protocol details:
  • Incremental protocol with stages typically 3–4 minutes long.
  • Measurements at the end of each stage include blood lactate, HR, and sometimes RPE.
  • Specificity in LT testing: the LT test should reflect the athlete’s sport and recruitment patterns.
• LT identification approaches:
  • Visual inspection is common in practice.
  • Mathematical methods include log–log transformation, D‑max, and the individual anaerobic threshold method.

Maximal lactate steady state (MLSS)

• MLSS definition:
  • The highest exercise intensity that can be maintained for 30 minutes with a lactate concentration change of less than 1 mM during the final 20 minutes:
    ext{change in lactate during last 20 min} < 1 ext{ mM}.
• Typical MLSS testing protocol:
  • First trial (T30 MLSS 30 min) conducted at an intensity around 75% of V̇O₂max: $ext{Intensity}_{T1} <br /> ightarrow 0.75 imes V̇O₂max.$
  • On a separate day, a second T30 is performed with intensity guided by the first result.
  • If MLSS is reached or lactate falls during the final 20 minutes, the second test is performed at an intensity ~5% higher than the first: $ext{Intensity}<em>{T2} = 1.05 imes ext{Intensity}</em>{T1}.$
• Practical significance:
  • MLSS and LT are better predictors of success in certain race distances than V̇O₂max alone.
  • HR at which AAT occurs is often used to regulate training intensity just below, at, or just above the AAT.
• Key points:
  • The most commonly used methods to identify AAT are LT and MLSS.
  • LT testing uses incremental workloads; MLSS uses a constant-workload test.

Critical Power (CP) and W′

• CP concept:
  • CP is an exercise intensity between LT and V̇O₂max, approximately equivalent to MLSS in many contexts.
  • CP is highly individual and generally occurs around 70 ext{–}90 ext{%} imes V̇O₂max.
• CP and performance relationships:
  • CP correlates with performance across multiple sports:
  • 10 km running: $r = -0.85$
  • Half marathon: $r = -0.79$
  • 17 km cycling: $r = -0.71$
  • 40 km cycling: $r = -0.91$
  • 400 m swimming (critical velocity, CV): $r = 0.86$
  • Negative correlations with time indicate higher CP is associated with faster performances; CP is thus a strong predictor of endurance performance.
• CP and W′ (pronounced W prime):
  • CP represents the maximal power/velocity sustainable for a prolonged period.
  • W′ (or AWC) represents a finite store of work that can be done above CP using anaerobic energy sources.
• CP testing protocols:
  • Originally developed by Scherrer and Monod (1965).
  • Testing can use between 2 and 10 exhaustive trials depending on the protocol.
  • CP protocol basics:
  • A typical protocol uses three tests to exhaustion, ideally on separate days, but can be on the same day with adequate rest.
  • The intensity for each test should aim for exhaustion within roughly 1 min, 6 min, and 10 min respectively for the three trials.
  • Sport-specific protocols exist to establish an athlete’s CP.

CP testing protocol details

• Three-test approach (typical):
  • Exhaustion times targeted at: 1 min (test 1), ~6 min (test 2), ~10 min (test 3).
• Purpose:
  • Define CP (the sustainable, high-intensity ceiling).
  • Define W′ as a finite energy store to be expended above CP during efforts.

Steady-state exercise and steady-state max

• Steady-state exercise definition:
  • A steady state is achieved when energy supply matches energy demand for the exercise task.
  • Time to reach steady state increases with higher workload; typically around $4 ext{ minutes}$.
• Steady-state max:
  • The maximal intensity that can be sustained for a period of time.
  • Related to MLSS and CP: improvements in steady-state capacity come with training adaptations.
• Physiological basis of improved steady-state capacity:
  • Increases in mitochondrial content and function, capillary density, and enzyme activity contribute to improved oxidative metabolism, enabling higher sustainable intensities.

Exercise economy and energy bookkeeping

• Exercise economy definition:
  • The energy cost of performing a given workload; reflects the relative efficiency of an athlete.
  • Economy influences response to a given workload and fatigue resistance.
• Measurement:
  • Typically assessed by plotting oxygen consumption ($V̇O₂$) across a range of workloads and calculating the slope or area under the curve.
• Practical significance:
  • Better economy delays fatigue and improves performance at submaximal intensities.
• Oxygen deficit and EPOC:
  • Oxygen deficit: delay in the rise of aerobic metabolism at the start of exercise, meaning energy must be supplied by anaerobic pathways initially.
  • EPOC (Excess Post-Exercise Oxygen Consumption): post-exercise elevation in oxygen consumption above resting levels, reflecting the recovery processes (replenishment of PCr and glycogen, lactate clearance, tissue repair, etc.).

Fatigue in endurance sports

• Fatigue mechanisms in high‑intensity endurance tasks include:
  • Increasing acidosis (accumulation of hydrogen ions affecting enzyme activity and contractile function).
  • Accumulation of inorganic phosphate (Pi) and potassium (K+) perturbing excitation–contraction coupling.
• Short-duration events:
  • Glycogen depletion becomes a limiting factor.
  • Central fatigue (central nervous system drive reduction) may contribute.
• Long-duration events:
  • Progressive metabolic perturbations and dehydration may contribute to fatigue.

Physiological adaptations to aerobic endurance training

• Aerobic power adaptation (VO₂max):
  • VO₂max improvements depend on training intensity, duration, mode, specificity, prior training status, and individual response.
  • Increases in VO₂max are most pronounced in the first 6–12 months of training.
  • After this initial period, performance gains tend to align more with improvements in LT and exercise economy rather than further large VO₂max gains.
• Representative research (examples mentioned in slides):
  • Impellizzeri, Rampinini, & Marcora (2005) – high‑intensity interval training (HIIT) impacts on young athletes.
  • Engel et al. (2018) – systematic review and meta‑analysis on HIIT in young athletes.

Adaptations of skeletal muscle fibres: size, type and metabolic capacity

• Muscle fibre size and type responses to endurance training:
  • Type I (slow-twitch) fibre size may decrease slightly with endurance training.
  • Hormonal influences (cortisol and testosterone) affect protein degradation in fibres.
  • Type IIx fibres adapt toward Type IIa, with IIa shifting toward properties of Type I fibres; i.e., a shift toward more oxidative, fatigue-resistant profiles.
• Training structure: block periodization of high‑intensity intervals may provide superior training effects in trained cyclists.
  • Cited study: Rønnestad, Hansen, & Ellefsen (2014).

Muscle fibre structure and metabolism with endurance training

• Training-induced structural/metabolic changes include:
  • Increased capillary density around each muscle fibre: about $10 ext{–}15 ext{ extpercent}.$
  • Myoglobin concentration: increases up to $75 ext{ extpercent}$–$80 ext{ extpercent}.$
  • Mitochondrial adaptations: increased number by about $15 ext{ extpercent}$ and size by about $35 ext{ extpercent}.$
  • Enzyme activity increases:
  • Krebs cycle enzymes (e.g., citrate synthase).
  • Succinate dehydrogenase (SDH).
  • Enzymes involved in β‑oxidation respond to endurance training.
  • Lipid stores: greater triglyceride storage in muscles allows immediate lipid fueling for oxidation.
  • Glycogen storage: endurance training appears to enhance glycogen synthesis and storage.

Training-induced LT adaptations and practical implications

• LT adaptations:
  • LT improvements tend to be specific to the exercise mode (mode-specific adaptations).
• Supplements and buffering strategies (debate and practical considerations):
  • Caffeine may influence perception of fatigue and performance.
  • Beta‑alanine may support buffering capacity and delay acidity-related fatigue.
  • Creatine monohydrate may support high-intensity performance and possibly LT adaptations.
  • Sodium bicarbonate and citrate buffers may enhance buffering capacity and performance in events with high lactate production.
  • Citrulline malate as a supplement discussed as a potential ergogenic aid.
• Practical takeaway:
  • Nutrition and supplementation strategies should be considered in context with training status, goals, sport, and safety considerations.

Knowledge integration: LT, MLSS, CP – similarities and differences

• Similarities:
  • All three metrics relate to sustainable or near-sustainable intensities that support endurance performance.
  • They provide practical benchmarks for training prescription and race pacing.
• Differences:
  • LT is the intensity where lactate begins to accumulate above baseline during incremental exercise; reflects metabolic shift.
  • MLSS is the highest constant intensity that can be sustained for 30 minutes with minimal lactate drift; emphasizes endurance pace durability.
  • CP is a power/velocity threshold between LT and V̇O₂max, serving as a practical pump‑primer for pacing and W′ energy store considerations.
• Practical use comparison:
  • LT tests are useful for prescribing intensities just under LT for tempo work.
  • MLSS provides a target for sustained interval training near the anaerobic threshold.
  • CP informs training zones across a broader duration spectrum and helps define the W′ reserve for repeated efforts.

Knowledge integration prompts (exam-style considerations)

• Compare LT, MLSS, and CP in terms of:
  • What each measures, how it is assessed, and what athlete this best serves.
  • How each relates to real-world race performance across different event durations.
  • The typical training implications and how they guide pacing strategies.

Resources referenced

• YouTube and online reviews mentioned as supplementary materials for broader context:
  • Aerobic endurance reviews and LT identification methods.
  • Discussions on AAT transitions and practical LT testing.

End-of-lecture recap and next steps

• Remember the core concepts:
  • AAT is tied to lactate production vs clearance; LT/OBLA provide practical threshold markers.
  • MLSS defines a sustainable race pace for longer efforts; CP/W′ define the high-intensity boundary.
  • Steady state and economy determine how efficiently the body uses energy at submaximal intensities.
  • Training adaptations include cardiovascular improvements (VO₂max), metabolic shifts (LT), and muscle fiber and capillary/mitochondrial remodeling that improve endurance performance.
• For exam preparation, be able to:
  • Define LT, OBLA, MLSS, CP, and W′; describe how each is measured and used.
  • Explain the physiological basis of steady-state, oxygen deficit, and EPOC.
  • Describe typical adaptation timelines and which traits improve first with endurance training.
  • Discuss practical considerations for training prescription using LT, MLSS, and CP.