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The overload loop: a distinct reoxygenation pattern above the second ventilatory threshold revealed by a new
Annette Schmidt1,2, Lucas Koch1, Tom Brandt1
1Sports Biology, University of the Bundeswehr Munich, Neubiberg, Germany.
Biology of Sport
|June 1, 2026
Summary
Exercising above the second ventilatory threshold (VT2) causes asynchronous muscle reoxygenation, with inactive muscles deoxygenating during early recovery. A novel metric quantifies this intensity-dependent overload loop, offering insights into vascular regulation during intense exercise.
Area of Science:
- Exercise Physiology
- Sports Science
- Cardiovascular Regulation
Background:
- High-intensity functional training (HIFT) like CrossFit often leads to overpacing, where athletes exceed sustainable exercise intensities.
- This overpacing results in impaired performance recovery within a single exercise bout.
- Understanding the physiological responses to exceeding sustainable intensity is crucial for optimizing training and preventing fatigue.
Purpose of the Study:
- To investigate if exercising above the second ventilatory threshold (VT2) elicits distinct muscle reoxygenation patterns in active versus inactive muscle groups.
- To develop a novel mathematical method for quantifying these intensity-dependent reoxygenation effects.
- To explore the relationship between local muscle oxygenation and systemic cardiovascular responses during high-intensity exercise.
Main Methods:
- Fifty-four healthy men underwent two incremental cycling tests, one below and one above their individual VT2.
- Muscle oxygen saturation (SmO2) in the vastus lateralis (active) and triceps brachii (inactive) was continuously measured using near-infrared spectroscopy.
- A new metric, MUSCLE ΔSmO2/HRrel, was developed to quantify the largest exercise-to-recovery difference in SmO2 relative to heart rate (HRrel).
Main Results:
- Above VT2, the inactive triceps brachii continued to deoxygenate into early recovery, while the active vastus lateralis reoxygenated immediately, creating a distinct "overload loop" pattern.
- This asynchronous response was statistically confirmed by the MUSCLE ΔSmO2/HRrel metric, showing a significant effect in the triceps brachii but not the vastus lateralis.
- Brachial artery diameter changes above VT2 indicated intensity-dependent vascular adjustments during exercise and recovery.
Conclusions:
- Exceeding the VT2 during exercise induces an asynchronous reoxygenation response between active and inactive muscle groups.
- The "overload loop" and the MUSCLE ΔSmO2/HRrel metric provide a quantitative tool to analyze intensity-driven SmO2 dynamics.
- These findings offer new insights into the coordination of local and systemic vascular regulation under unsustainable exercise conditions, relevant for HIFT athletes.
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