External Mechanical Workload and Acute Physiological Responses During Intermittent Normobaric Hypoxia in Elite Cyclists

Background: Intermittent normobaric hypoxia (INH) has been proposed as a strategy to increase physiological stress while maintaining a relatively low external mechanical workload. However, the acute physiological responses elicited during standardized INH combined with low-intensity exercise remain incompletely characterized in elite cyclists. Objective: Our objective is to characterize the acute physiological responses during a standardized intermittent normobaric hypoxia session while maintaining a constant individualized external mechanical workload in elite cyclists. Methods: Ten professional and semi-professional male cyclists (30.4 ± 11.8 years) completed a prospective, single-group experimental protocol with a within-subject repeated-measures design, consisting of a Zone 2 warm-up under normoxia followed by three 10 min intermittent normobaric hypoxia cycles (FiO2 15–16%; simulated altitude 2400–2900 m), each separated by 5 min of normoxic recovery. Blood lactate, muscle oxygen saturation (SmO2), arterial oxygen saturation (SpO2), and heart rate (HR) were assessed across the experimental protocol. Results: External power output remained stable throughout the protocol (~212 W). Blood lactate increased significantly across experimental phases (F(4,28) = 72.70, p < 0.001, η2p = 0.912), while muscle oxygen saturation progressively decreased (p < 0.001). Heart rate also increased progressively across experimental phases (p < 0.001). Arterial oxygen saturation declined during hypoxic exposure and remained stable across successive hypoxic cycles (p = 0.83). Two Cycle 3 blood lactate measurements exceeded the validated analytical range of the analyzer (>25 mmol·L−1) and were treated as right-censored observations. Conclusions: Under the experimental conditions of this exploratory single-group study, a standardized intermittent normobaric hypoxia session combined with constant low-intensity exercise was associated with substantial acute changes in blood lactate concentration, muscle oxygenation, and heart rate despite stable external mechanical workload. These findings characterize the acute physiological responses observed during intermittent normobaric hypoxia and support further randomized controlled studies including matched normoxic conditions, individualized hypoxic titration, and continuous cardiopulmonary monitoring to better understand the physiological mechanisms underlying these responses.