High Altitude Training: Aerobic vs. Anaerobic Systems, Adaptations, and Training Methods

Is high altitude training aerobic or anaerobic?

High altitude training primarily targets and enhances the aerobic energy system by stimulating physiological adaptations that improve oxygen delivery and utilization. While anaerobic efforts still occur and are influenced by hypoxia, the fundamental goal and adaptations revolve around improving the body's capacity for sustained, oxygen-dependent work.

Introduction to High Altitude Training and Hypoxia

High altitude training involves exercising or living in environments with reduced atmospheric pressure, typically above 1,500-2,000 meters (approximately 5,000-6,500 feet). The defining characteristic of such environments is hypoxia, a condition where the partial pressure of oxygen (PO2) in the air is lower than at sea level. This reduction in available oxygen creates a significant physiological challenge, forcing the body to adapt to maintain adequate oxygen supply to tissues. Athletes utilize this stress to induce beneficial changes that can enhance performance upon returning to sea level.

The Aerobic System: Primary Target of Altitude Training

The aerobic energy system is the body's primary mechanism for producing energy (ATP) during sustained, moderate-to-low intensity activities, relying on the presence of oxygen to metabolize carbohydrates, fats, and proteins. At altitude, the reduced PO2 directly impacts this system.

• Oxygen Cascade Disruption: From the air we breathe to the mitochondria in our cells, the entire oxygen transport chain is stressed. Less oxygen enters the lungs, less binds to hemoglobin in the blood, and consequently, less is delivered to working muscles.
• Physiological Adaptations: To compensate for this oxygen scarcity, the body undergoes a series of profound adaptations, primarily aimed at improving oxygen delivery and utilization:
  • Erythropoietin (EPO) Production: The kidneys detect lower blood oxygen levels and release EPO, a hormone that stimulates the bone marrow to produce more red blood cells.
  • Increased Red Blood Cell Mass and Hemoglobin: More red blood cells mean more hemoglobin, which in turn increases the blood's oxygen-carrying capacity.
  • Enhanced Capillary Density: Over time, the body may increase the density of capillaries around muscle fibers, improving the efficiency of oxygen diffusion from blood to muscle.
  • Mitochondrial Biogenesis: Some research suggests improvements in mitochondrial number and efficiency, enhancing the muscles' ability to utilize oxygen for energy production.
  • Improved Ventilatory Efficiency: The respiratory system adapts to process air more efficiently, though this is often a more immediate rather than long-term adaptation.

These adaptations collectively result in a more robust and efficient aerobic system, allowing the athlete to perform at higher intensities with less reliance on anaerobic pathways, especially when they return to sea level.

The Anaerobic System: Its Role and Interaction at Altitude

The anaerobic energy system provides rapid energy for short, high-intensity activities where oxygen supply is insufficient to meet energy demands. This system produces ATP without oxygen, primarily through glycolysis, leading to the accumulation of lactate.

• Initial Reliance and Training Challenges: When first ascending to altitude, athletes often experience a reduced capacity for high-intensity work. The hypoxic environment can lead to a quicker onset of fatigue during maximal efforts, potentially increasing initial reliance on anaerobic pathways for power output. However, this is usually coupled with a reduction in overall maximal power output.
• "Anaerobic Paradox": While aerobic capacity improves, maximal anaerobic power and capacity are not typically the primary targets of altitude training, nor do they consistently show significant improvements. Some studies even suggest a slight impairment or no change in maximal anaerobic performance at altitude, possibly due to:
  • Increased Ventilatory Work: The effort required to breathe in thinner air consumes more energy, diverting resources.
  • Muscle Buffering Capacity: Adaptations related to buffering lactic acid are not directly enhanced by hypoxia in the same way oxygen transport is.
• Anaerobic Efforts Still Occur: Despite these points, athletes at altitude still perform anaerobic efforts during sprints, bursts, or high-intensity intervals. These efforts are simply performed under the added physiological stress of hypoxia, which can alter perceived exertion and recovery. The goal isn't to improve the anaerobic system through altitude, but rather to allow the aerobic system to perform more efficiently, thus pushing the lactate threshold higher and delaying the need for anaerobic contribution.

The Interplay: How Both Systems Are Affected

High altitude training creates a complex physiological environment where both energy systems are engaged, but with a distinct emphasis. The overarching goal is to enhance the aerobic ceiling by optimizing oxygen delivery and utilization.

• Shifting the Lactate Threshold: A key benefit of improved aerobic capacity is an elevated lactate threshold. This means an athlete can sustain a higher intensity of exercise before lactate begins to accumulate rapidly, indicating a greater reliance on the aerobic system.
• Training Intensity Zones: Training at altitude often requires a reduction in absolute training intensity to maintain desired physiological zones (e.g., heart rate zones). An intensity that might be "aerobic" at sea level could push an athlete into a more "anaerobic" zone at altitude due to the reduced oxygen availability. Coaches must adjust training prescriptions accordingly.
• Adaptation to Stress: The body's ability to cope with the stress of hypoxia during both aerobic and anaerobic efforts contributes to overall fitness and resilience.

Practical Implications for Training

Understanding the interaction of energy systems at altitude informs various training methodologies:

• Live High, Train High (LHTH): Athletes live and train at altitude. While it provides a continuous hypoxic stimulus for aerobic adaptations, the reduced oxygen can limit the intensity of training sessions, potentially hindering the maintenance or development of high-speed, anaerobic power.
• Live High, Train Low (LHTL): Athletes live at altitude (to gain aerobic adaptations) but descend to lower altitudes for high-intensity training sessions (to maintain or improve speed and anaerobic power). This "hypoxic tent" or "altitude house" method is often considered the most effective for elite endurance athletes.
• Intermittent Hypoxic Training (IHT): Involves short, repeated exposures to hypoxia (e.g., breathing hypoxic air for minutes at a time) often at rest or during light exercise. The aim is to stimulate similar physiological adaptations without the full commitment of living at altitude.

Conclusion: A Nuanced Perspective

To definitively answer whether high altitude training is aerobic or anaerobic, it's crucial to understand its primary physiological impact. High altitude training overwhelmingly targets and enhances the aerobic energy system. The entire cascade of adaptations—from increased EPO and red blood cell count to improved oxygen utilization at the cellular level—is geared towards making the body more efficient at producing energy with oxygen.

While anaerobic efforts inevitably occur during high-intensity training at altitude, and the anaerobic system is certainly affected by the hypoxic environment, altitude training is not primarily designed to improve anaerobic power or capacity directly. Instead, it aims to make the aerobic system so robust that the reliance on the anaerobic system is delayed and minimized, particularly when the athlete returns to sea level conditions. Therefore, high altitude training is fundamentally an aerobic training modality with a significant influence on overall athletic performance.