Adaptations of Aerobic and Resistance Training

HMS389 Lecture 1 – Adaptations to Aerobic and Resistance Training

1. What Training Adaptation Means in Physiology

Training adaptation refers to the chronic physiological and structural changes that occur in the body as a result of repeated exposure to exercise stress. Exercise disrupts homeostasis (internal stability), and with repeated exposure, the body remodels itself so that the same workload causes less disturbance in the future.

Adaptations occur across multiple systems:

  • Nervous system (motor control, coordination)

  • Cardiovascular system (heart and blood vessels)

  • Skeletal muscle (fiber structure, mitochondria, enzymes)

  • Metabolic pathways (fuel use, ATP production)

The goal of adaptation is not performance per se, but efficiency and stability under stress.

2. The 3 Fundamental Principles of Training

Overload

Adaptation occurs only when exercise stress exceeds habitual levels. Overload can be applied by manipulating:

  • Intensity

  • Duration

  • Frequency

Without sufficient overload, no adaptive signal is generated.

Specificity

Adaptations are specific to the type of stress applied. The body adapts according to:

  • Muscles used

  • Type of contraction (eccentric, concentric, isometric)

  • Speed of movement

  • Energy system stressed

This principle explains why endurance training improves aerobic capacity, while resistance training improves strength and hypertrophy.

Reversibility

Adaptations are lost when the training stimulus is reduced or removed. Maintaining physiological traits requires ongoing energy investment, so unused adaptations are gradually removed.

3. VO₂max: Definition and Determinants

VO₂max is the maximal rate at which oxygen can be taken in, transported, and utilized by the body during intense exercise.

Physiologically, VO₂max reflects the integrated function of:

  • Lungs (oxygen uptake)

  • Heart (oxygen delivery)

  • Blood vessels (distribution)

  • Skeletal muscle mitochondria (oxygen utilization)

Fick Equation

VO₂max = Cardiac Output × (a–v)O₂ difference

Where:

  • Cardiac Output = Heart Rate × Stroke Volume

  • (a–v)O₂ difference = oxygen extracted by working muscle

4. Central Adaptations to Endurance Training

Stroke Volume as the Primary Driver

Increases in VO₂max with endurance training are driven mainly by increases in maximal stroke volume, not maximal heart rate.

Stroke volume improves through:

Increased Preload

  • Expanded plasma volume

  • Increased venous return

  • Increased end-diastolic volume

This enhances force of contraction via the Frank–Starling mechanism.

Decreased Afterload

  • Reduced total peripheral resistance

  • Improved vasodilation in working muscle

This lowers the resistance against which the heart must eject blood.

Increased Contractility

  • Improved myocardial function allows more forceful ejection of blood

These changes can occur rapidly, even within the first week of training.

5. Peripheral Adaptations to Endurance Training

Increased (a–v)O₂ Difference

Endurance training enhances the muscle’s ability to extract oxygen from the blood through:

  • Increased capillary density

  • Increased mitochondrial number and size

  • Improved oxidative enzyme activity

These adaptations reduce diffusion distance and increase oxygen utilization capacity.

Time Course of Adaptation

  • Early training: central adaptations dominate

  • Long-term training: peripheral adaptations become increasingly important

6. Skeletal Muscle Adaptations to Aerobic Training

Fiber Type Shifts

Endurance training promotes a shift toward slower, more oxidative muscle characteristics, influenced by genetics and training volume.

Mitochondrial Biogenesis

Mitochondrial volume can increase by 50–100% within weeks of training, greatly enhancing aerobic ATP production.

Training also increases mitochondrial turnover, improving removal of damaged mitochondria.

Reduced Lactate Accumulation

Improved mitochondrial function allows faster ATP production aerobically, reducing reliance on glycolysis early in exercise and lowering lactate and hydrogen ion accumulation.

Fuel Utilization Changes

Endurance training shifts metabolism toward greater fat oxidation via:

  • Increased fatty acid transport into muscle

  • Increased mitochondrial fatty acid transport (CPT I)

  • Increased β-oxidation enzymes

This leads to glycogen sparing during submaximal exercise.

Antioxidant Adaptations

Repeated exercise increases endogenous antioxidant defenses, protecting muscle from oxidative damage caused by free radicals.

7. Detraining and Retraining

Aerobic Detraining

  • VO₂max declines rapidly with inactivity

  • Early losses are driven by reduced plasma volume and stroke volume

  • Later losses involve decreased mitochondrial content and oxidative capacity

Mitochondrial Plasticity

  • Mitochondrial gains are rapid but also quickly lost

  • Significant reductions can occur within 1–2 weeks of detraining

  • Retraining restores adaptations faster than initial training

8. Resistance Training Adaptations

Neural Adaptations (Early Phase)

Strength gains in the first 2–8 weeks are primarily neural:

  • Increased motor unit recruitment

  • Increased firing frequency

  • Improved synchronization

  • Reduced antagonist co-activation

Muscle Hypertrophy

Hypertrophy results from increased myofibrillar protein content, leading to larger fiber cross-sectional area and greater force production.

Hyperplasia is not considered a major contributor in humans.

Detraining Effects

Strength declines with detraining largely due to neural regression, while muscle size is relatively preserved. Retraining restores strength rapidly.

9. Molecular Basis of Training Adaptations

Exercise as a Genetic Signal

Muscle contraction activates intracellular signaling pathways that alter gene expression and protein synthesis.

Protein synthesis rises acutely after exercise and returns to baseline within 24–48 hours.

Primary Signals

  • Mechanical stress

  • Calcium flux

  • Energy stress (AMP/ATP ratio)

  • Free radical production

Key Signaling Pathways

  • AMPK: energy sensing, mitochondrial biogenesis

  • PGC-1α: mitochondrial and capillary growth

  • CaMK and calcineurin: fiber type regulation

  • mTOR: muscle protein synthesis

  • NFκB: antioxidant enzyme production

Resistance Training and Protein Balance

Muscle growth occurs when protein synthesis exceeds breakdown consistently over weeks. Adequate amino acid availability is essential.

10. Satellite Cells and Muscle Growth

Satellite cells contribute additional myonuclei to muscle fibers, expanding the fiber’s capacity for protein synthesis.

This process supports long-term hypertrophy and is reduced with aging.

11. Role of Genetics

Genetics strongly influence:

  • Baseline VO₂max

  • Magnitude of training response

  • Hypertrophy potential

Differences in gene expression, particularly within the mTOR pathway, explain high vs low responders to training.

12. Integrated Takeaway

Training adaptations reflect coordinated changes across neural, cardiovascular, muscular, and molecular systems. Endurance and resistance training produce distinct but overlapping adaptations governed by the same fundamental principles of overload, specificity, and reversibility.