Dear Editor, I read with great interest the Invited Commentary by Sitko et al. Their discussion of zone 2 definition and expected training adaptations is both timely and necessary, given the ongoing debate about clinical relevance and applications of exercise thresholds.2 While I appreciate their contribution, I believe further discussion is needed on zone 2 definition and its expected physiological adaptations. How Is Zone 2 Training Defined? Sitko et al. provided a valuable summary of alternative variables that can be used to define zone 2. However, no previous literature reviews have systematically assessed intraindividual variations of these parameters nor reported their normative values in athletes.2 Extrapolating these ranges to all athletic populations may introduce bias in training intensity prescriptions and physiological adaptations. Indeed, Meixner et al. demonstrated that zone 2 definition varies widely, depending on the method used to assess the first exercise threshold (eg, maximal fat oxidation rates, first ventilatory threshold, first lactate threshold [LT1]). With at least 8 different methods to identify the LT1 available,4 some place zone 2 below LT1, while others place it closer to zone 3 or 4 (Supplementary Figure S1 [available online]). Thus, specific guidelines on which method should be used to define LT1 are necessary to ensure accurate classification of zone 2. What Key Physiological Adaptations Are Anticipated From Zone 2 Training? Sitko et al. correctly stated that further research is needed to differentiate zone 2-induced adaptations from classic endurance training. They hypothesized that zone 2 training may enhance muscle capillarization, increase mitochondrial enzyme activity in type I muscle fibers, and improve metabolic efficiency, critical power, and VO2max. However, they did not discuss how cardiac efficiency and pulmonary ventilation might also benefit from training in this intensity domain. Beck et al demonstrated that stroke volume plateaus between LT1 and LT2, just before the heart-rate deflection point. Consequently, increased cardiac wall stretching via the Frank-Starling mechanism may promote eccentric myocardial hypertrophy following high-volume zone 2 training. Additionally, prolonged exercise at this intensity may enhance oxygen uptake efficiency, as respiratory rate rises exponentially beyond LT1 due to metaboreflex activation.6 High training volume in zone 2 may strengthen respiratory muscles, improving O2 extraction and diffusion capacity. Fat metabolism may also improve significantly, as maximal fat-oxidation rates align closely with this intensity domain.3 Greater expression of muscle lipases and long-chain fatty-acid transport proteins may support delayed glycogen depletion and its associated fatigue during exercise, complementing the mitochondrial volume density and oxidative phosphorylation adaptations discussed by Sitko et al.1 Investigating these aspects further would provide a more comprehensive understanding of zone 2 training adaptations and their potential benefits. Collaboration between sport scientists and coaches is essential to uncover the fascinating physiological mechanisms behind zone 2 exercise.
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