Understanding Thresholds in Endurance Sports: A Key to Performance

In endurance sports, performance isn’t determined only by how high your VO₂ max is, but by how efficiently you can operate near your physiological limits. Central to this is the concept of thresholds. These points define the boundaries between intensities your body can sustain for long periods and those that lead to rapid fatigue.

For cyclists and endurance athletes, thresholds explain why some efforts feel controlled while others spiral out of reach. They also clarify why structured training produces results where “just riding harder” does not.

What We Mean by “Threshold”

At low intensities, energy is produced primarily through oxidative metabolism. This oxygen-driven process occurs inside the mitochondria, using fat and carbohydrates to generate ATP efficiently. As exercise intensity increases, glycolysis accelerates, producing more lactate. Lactate itself is not “waste.” It is an important fuel but signals an increasing reliance on anaerobic metabolism.

When lactate production begins to outpace clearance and oxidation, it accumulates in the blood. The points along this curve, where clearance can no longer perfectly match production, are what we call thresholds.

• First Threshold (LT1, Aerobic Threshold): This is the lowest intensity at which blood lactate begins to rise measurably above baseline. Below LT1, the body relies heavily on fat metabolism. Lactate is stable, and effort feels sustainable for hours.

• Second Threshold (LT2, Lactate Threshold, or Maximal Lactate Steady State): This is the intensity where lactate increases more sharply, making it difficult to maintain homeostasis. Around LT2, lactate is elevated but can remain temporarily stable. Above it, lactate rises continuously, leading to fatigue.

  
  

These thresholds mark the transitions between easy endurance, controlled but demanding work, and unsustainable effort.

Why Thresholds Matter for Performance

Thresholds are not arbitrary markers. They represent fundamental physiological limits.

• Below LT1: Energy comes primarily from fat oxidation with stable lactate. Training here builds mitochondrial density, capillarization, and efficiency in fat metabolism. These adaptations support endurance and recovery.

• Between LT1 and LT2: Both fat and carbohydrate are used, with lactate rising but manageable. Training in this range improves lactate transporters (MCT1, MCT4), buffering capacity, and the ability to oxidize lactate in neighboring fibers or organs. This is where athletes build the capacity to ride “comfortably hard” for long periods.

• At LT2/MLSS: This is the highest workload where lactate is elevated but steady. It represents the maximal sustainable oxidative rate, fueled mainly by carbohydrate metabolism. Racing and time trials often hinge on the ability to sustain power near this level.

• Above LT2: Lactate production exceeds clearance, acidity rises, glycogen is consumed rapidly, and fatigue accelerates. These efforts can only be sustained for minutes, not hours.

  
  

For performance, LT2/MLSS is often a stronger predictor of endurance ability than VO₂ max. While VO₂ max defines the size of the aerobic engine, thresholds define how much of that engine you can actually use in competition.

How Thresholds Are Measured

There are several ways to identify thresholds, each grounded in physiology but with different applications for athletes.

1. Blood Lactate Testing

During an incremental step test, blood lactate is sampled at each workload.

• LT1 is seen as the first measurable rise above baseline.

• LT2 is identified where lactate begins to rise sharply.

• The maximal lactate steady state (MLSS) is determined through repeated 20–30 min trials at constant workload. This is the highest power where lactate remains stable (≤1 mmol rise over the final 20 minutes).

  
  

2. Ventilatory Thresholds

With gas analysis, thresholds are identified from changes in ventilation. As acidity increases, bicarbonate buffers hydrogen ions, producing more CO₂, which alters breathing.

• VT1 corresponds to the first non-linear rise in ventilation, often near LT1.

• VT2 reflects a steeper rise, often near LT2.

  
  

3. Field Tests

For practical use, athletes often estimate thresholds through performance tests:

• A 30–60 min maximal effort closely approximates LT2/MLSS.

• Critical Power models, based on several shorter efforts, estimate the highest sustainable oxidative workload and correlate well with lab-derived thresholds.

  
  

Each method has limitations, but the key is consistency. Using the same approach over time helps monitor progress.

What Happens in the Body at Threshold

As workloads rise toward LT2, more fast-twitch muscle fibers are recruited. These fibers are powerful but less efficient, producing more lactate and hydrogen ions. While lactate can be oxidized in other fibers or organs, the increasing acid load must be buffered. This leads to extra CO₂ production and heavier breathing.

At LT2 or MLSS, the system is at its limit. Carbohydrate oxidation dominates, lactate is elevated but stable, and oxygen uptake is near maximal steady state. Push harder, and clearance pathways are overwhelmed. Lactate climbs, pH drops, glycogen stores are rapidly drained, and fatigue forces a reduction in intensity.

This balance explains why threshold efforts feel “hard but manageable,” while going slightly above threshold feels unsustainable within minutes.

Misconceptions About Thresholds

• “Lactate equals soreness”: The burn during hard work is mainly due to H⁺ ion accumulation (acidosis), not lactate. Delayed onset muscle soreness (DOMS) is caused by microscopic muscle damage and inflammation, not leftover lactate.

• “Threshold is always 4 mmol.”: This was a historical convention, but lactate levels at LT2/MLSS vary widely between individuals and testing protocols. Some athletes reach threshold at 3 mmol, others at 5 mmol. Fixed points are misleading; individualized assessment is more accurate.

• “Ventilatory and lactate thresholds are identical.”: They often align but are not interchangeable. Differences in physiology, modality, and testing method mean they can diverge. Using both methods together gives more confidence.

• “Threshold training is all you need.”: Threshold training is valuable, but endurance performance relies on a mix. Long endurance below LT1, threshold intervals around LT2, and high-intensity sessions above LT2 all stimulate different adaptations that complement each other.

• "Lactate is bad / a waste product”: Lactate is a fuel. Muscles, the heart, and even the brain can utilize lactate as an energy source. What hurts performance is the accumulation of hydrogen ions (H⁺) that lowers pH, not lactate itself. Training near threshold improves your ability to clear and reuse lactate effectively.

• "Threshold = anaerobic metabolism”: Even well above threshold, the majority of energy still comes from aerobic metabolism. Threshold simply marks the point where anaerobic contribution rises disproportionately, and where lactate starts accumulating faster than it can be cleared.

  
  

Applying Thresholds in Training

• Endurance rides: Stay below LT1 to build aerobic capacity and promote recovery.

• Threshold intervals: Workouts like 2 × 20 min or 3 × 12 min near LT2 develop the ability to sustain high power outputs.

• Over-unders: Alternating just below and above LT2 trains the body to tolerate and clear lactate more effectively.

• Pacing: Knowing LT2 helps set realistic pacing in time trials, climbs, and breakaways. This prevents overpacing that leads to early fatigue.

• Monitoring progress: Repeated testing or field trials every 8–12 weeks reveal whether thresholds are shifting, guiding training adjustments.

  
  

Conclusion: The Importance of Understanding Thresholds

Thresholds represent the body’s dividing lines between sustainable and unsustainable effort. LT1 marks the transition from easy endurance to controlled work, while LT2/MLSS marks the upper boundary of sustainable oxidative metabolism. Training around these points drives specific adaptations that improve endurance performance more effectively than unfocused hard riding.

Understanding thresholds allows athletes to train smarter, pace better, and ultimately perform at a higher level. They are not just numbers from a lab test but practical tools for building a stronger, more efficient engine.

References

• Faude O, Kindermann W, Meyer T. (2009). Lactate threshold concepts: how valid are they? Sports Med, 39(6):469–90.

• Beneke R. (2003). Maximal lactate steady state concentration (MLSS). Sports Med, 33(6):407–26.

• Jones AM, Burnley M, Black MI, Poole DC, Vanhatalo A. (2019). The maximal metabolic steady state: redefining the ‘gold standard’. Physiol Rep, 7(10):e14098.

• Beaver WL, Wasserman K, Whipp BJ. (1986). A new method for detecting anaerobic threshold by gas exchange (V-slope). J Appl Physiol, 60(6):2020–2027.

• Pallarés JG, et al. (2016). Validity and reliability of ventilatory and blood lactate thresholds in well-trained cyclists. PLoS One, 11(9):e0163389.