Sub-Threshold Training: Definition, Benefits, and Application

What is Sub Threshold Training?

Sub-threshold training refers to exercising at an intensity below an individual's key physiological thresholds, such as the lactate or ventilatory threshold, primarily to enhance aerobic capacity, improve fat utilization, and build a robust endurance base.

Defining the "Threshold"

To understand sub-threshold training, it's crucial to first define what "threshold" refers to in exercise physiology. In the context of endurance exercise, the most commonly referenced thresholds are:

• Lactate Threshold (LT): This is the exercise intensity at which lactate begins to accumulate in the blood at a rate faster than it can be cleared. Below this point, lactate production and clearance are balanced. Exceeding the LT signifies a greater reliance on anaerobic metabolism, leading to increased fatigue.
• Ventilatory Threshold (VT1 or Aerobic Threshold): This is the point during exercise where ventilation (breathing rate) begins to increase disproportionately to oxygen consumption. It often correlates closely with the lactate threshold and signifies the point where the body starts to rely more on carbohydrate metabolism and less on fat.

These thresholds represent a shift in the body's primary energy system and physiological stress. Training below these points means operating predominantly within the aerobic energy system.

The Concept of Sub-Threshold Training

Sub-threshold training, often synonymous with "Zone 2 training" in many heart rate zone models, involves sustained exercise at an intensity where the body primarily uses fat as fuel, lactate production is low and stable, and the effort feels sustainable for long durations. It is characterized by:

• Low to Moderate Intensity: The effort level is comfortable, allowing for conversation.
• Aerobic Dominance: Energy is primarily supplied by the aerobic system, utilizing oxygen to break down fats and carbohydrates.
• Metabolic Stability: Lactate levels remain low, and the body efficiently clears any produced.

This contrasts sharply with high-intensity interval training (HIIT) or supra-threshold work, which push the body beyond these thresholds into anaerobic zones, leading to rapid fatigue and higher physiological stress.

Physiological Adaptations and Benefits

Engaging consistently in sub-threshold training elicits a range of profound physiological adaptations that are foundational for endurance performance and overall health:

• Enhanced Aerobic Base: This is the cornerstone benefit. Sub-threshold work significantly improves the efficiency of the aerobic energy system.
  • Increased Mitochondrial Density: Mitochondria are the "powerhouses" of cells, where aerobic energy production occurs. More mitochondria mean greater aerobic capacity.
  • Improved Capillary Density: More capillaries around muscle fibers enhance oxygen and nutrient delivery to working muscles and waste product removal.
  • Increased Aerobic Enzyme Activity: Enzymes involved in the Krebs cycle and electron transport chain become more active, improving the efficiency of aerobic metabolism.
• Improved Fat Utilization: By training at lower intensities, the body becomes more efficient at burning fat for fuel. This "fat adaptation" spares glycogen stores, which is crucial for prolonged exercise and can delay fatigue during longer events.
• Reduced Training Stress & Enhanced Recovery: Sub-threshold sessions impose less stress on the central nervous system and musculoskeletal system compared to high-intensity work. This allows for higher training volumes with reduced risk of overtraining and faster recovery between sessions.
• Injury Prevention: The lower intensity and impact associated with sub-threshold training reduce the strain on joints, tendons, and muscles, thereby lowering the risk of overuse injuries.
• Sustainable Progression: Building a strong aerobic base allows athletes to absorb more high-intensity work later in a training cycle, as the body's foundational fitness is robust.
• Cardiovascular Health: Regular sub-threshold training strengthens the heart, improves its pumping efficiency (increased stroke volume), and enhances overall cardiovascular function, contributing to long-term heart health.

Practical Application: How to Implement Sub-Threshold Training

Implementing sub-threshold training effectively requires monitoring intensity to ensure you stay within the desired zone.

• Intensity Zones: The goal is to stay below your ventilatory or lactate threshold. This typically corresponds to:
  • Zone 2 Heart Rate: For most individuals, this is approximately 60-70% of maximum heart rate (MHR) or 70-80% of heart rate reserve (HRR). Precise zones can be determined via laboratory testing (e.g., lactate threshold test, VO2 max test).
  • Rate of Perceived Exertion (RPE): An RPE of 3-5 on a 10-point scale (where 1 is very light and 10 is maximal effort). You should feel comfortable and able to maintain the effort for an extended period.
  • Talk Test: You should be able to hold a full conversation without gasping for breath, but you should not be able to sing. You should feel comfortable enough to speak in complete sentences.
• Monitoring Methods:
  • Heart Rate Monitor: The most objective and common method. Set heart rate alerts to keep you in Zone 2.
  • Rate of Perceived Exertion (RPE): A subjective but effective method, especially for those sensitive to their body's signals.
  • Talk Test: A simple, practical method that doesn't require equipment.
• Duration and Frequency: Sub-threshold training sessions are typically longer in duration (e.g., 30 minutes to several hours, depending on fitness level and goals) and can be performed frequently throughout the week.
• Examples:
  • Long Slow Distance (LSD) Runs: Steady-paced runs where you maintain a conversational pace.
  • Steady-State Cycling: Consistent effort on a bicycle, indoors or outdoors, without significant surges.
  • Aerobic Swims: Continuous swimming at a comfortable, sustainable pace.
  • Brisk Walking/Hiking: For beginners or those focusing on general health.

Who Benefits Most from Sub-Threshold Training?

While beneficial for almost anyone, certain populations derive particular advantages from prioritizing sub-threshold training:

• Endurance Athletes: Runners, cyclists, triathletes, and swimmers use it as the bulk of their training volume to build a robust aerobic engine, which is the foundation for all higher-intensity work.
• Beginners: It's an excellent starting point for individuals new to exercise, allowing them to build fitness safely without excessive stress.
• Individuals Focusing on Health and Longevity: The cardiovascular and metabolic benefits are profound for general health, weight management, and reducing the risk of chronic diseases.
• Those Recovering from Injury or Overtraining: Its low-impact, low-stress nature makes it ideal for active recovery and rebuilding fitness without exacerbating issues.

Integrating Sub-Threshold Training into a Comprehensive Program

Sub-threshold training is not meant to be the only type of training; rather, it is the fundamental base upon which other, higher-intensity work is built. A well-rounded training program often follows a periodized approach:

• Base Building Phase: Dominated by sub-threshold training to build aerobic capacity.
• Build Phase: Introduces more intensity (threshold work, intervals) on top of the established aerobic base.
• Peak Phase: Focuses on race-specific intensity and reduced volume.

By dedicating a significant portion of training volume to sub-threshold work, athletes can improve their ability to sustain higher intensities for longer periods during races or challenging workouts, as their aerobic engine becomes more powerful and efficient.

Conclusion

Sub-threshold training is a cornerstone of effective endurance development and overall physiological health. By consistently operating below your physiological thresholds, you systematically enhance your body's aerobic machinery, improve fat metabolism, and build a resilient foundation that supports both high-performance goals and long-term well-being. It is a testament to the principle that sometimes, training smarter and steadier yields the most profound and sustainable results.