Oliver Opatz

Objective: Ultra-endurance cycling offers a natural laboratory for studying physiological responses under sustained extreme load. Continuous in-race monitoring is rarely reported. The aim of this study was to investigate the feasibility of a multimodal framework of physiological parameters including metabolic, cardiovascular, and muscle-mechanical patterns during an ultra-endurance event. Approach: This study stress-tests a multimodal framework of physiological parameters of a 58-year-old male athlete during the Race Across America (RAAM) 2024, covering 4,933 km in 11 days from Oceanside, California, to Atlantic City, New Jersey. Parameters included energy expenditure, continuous blood glucose levels, heart rate, power output, passive muscle stiffness and resting tone, as well as sleep times. Main Results: The multimodal monitoring toolkit proved feasible and provided continuous, physiological measurements throughout the RAAM, enabling the observation of the following physiological changes: The athlete lost 2.3 kg of total weight and had an estimated energy deficit of 21,169 kcal. Blood glucose levels decreased over the course of the RAAM (0.92 mg/dl/d, p < 0.001), with an increased time spent below 100 mg/dl (p < 0.001). Heart rate during cycling progressively decreased, stabilising at a plateau of 94 bpm. Power output-to-heart rate ratio initially dropped until day 7 before peaking on day 11. Mean passive muscle stiffness and resting tone increased during the race compared to baseline levels, with distinct response patterns observed between two leg muscles and one lower back muscle. The total sleep deficit was 65 hours during the RAAM. Significance: Continuous, multimodal in-race physiological monitoring during the RAAM proved feasible and operationally useful, enabling real-time adjustments to pacing, fuelling and recovery. This framework offers a field-deployable template for ultra-endurance events. Future research should focus on larger, multi-participant studies and long-term follow-up to characterise the physiological responses to extreme endurance.

Optimal neuroendocrine responses are essential during hypergravity (+Gz) exposure. Peripheral skin cooling (PSC) may enhance neuroendocrine function, potentially improving +Gz resiliency and influencing leukocyte and hematologic factors. This study investigated whether PSC augments the cumulative +Gz stress index (CGSI) and shifts it toward noradrenergic dependency. Eighteen men underwent a graded +Gz profile in a crossover design, with PSC applied using Arctic Sun cooling pads. Neuroendocrine and blood profiles were assessed pre- and post-+Gz. CGSI did not differ between groups, but serum osmolality increased only in PSC (p = 0.03). In PSC, CGSI correlated with norepinephrine (p < 0.01, r = 0.71) and other markers, suggesting enhanced norepinephrine responsiveness despite similar serum levels. This response may be cardio-protective for space missions and ICU patients. Additionally, baseline serum metanephrine emerged as a potential marker for +Gz resilience, with PSC showing potential leukocytic and hematologic involvement in CGSI.

Introduction: Optimal neuroendocrine responses, particularly noradrenergic responses, are critical during hypergravity (+ Gz) exposure. Peripheral skin cooling (PSC) has the potential to enhance noradrenergic function, which could improve + Gz resiliency and may also affect leukocyte and hematologic factors. We predicted that PSC would augment cumulative + Gz stress index (CGSI) neuroendocrine, leukocyte and hematologic activity compared to controls, and that CGSI would shift to a noradrenergic dependency in PSC compared to controls. Methods 18 men in a cross-over design underwent a graded + Gz profile. PSC was applied using Arctic Sun cooling pads around the thighs with 8°C water infusion. Neuroendocrine parameters and a complete blood profile with differential were obtained before and after + Gz. CGSI was calculated as + Gz * seconds. Pre / post comparisons were made within the groups with paired t-tests and between groups with independent t-tests. Pearson's correlation analysis was performed between CGSI and neuroendocrine/leukocyte/hematological parameters. Results CSGI did not differ between groups. In both groups, all neuroendocrine, leukocyte and hematological factors changed significantly from pre to post + Gz, except for serum osmolality, which increased only in PSC (p = 0.03). In PSC, CGSI correlated with noradrenaline (p < 0.01, r = 0.71), metanephrine (p = 0.02, r = -0.54), neutrophiles (p = 0.03, r = 0.51), and thrombocytes (p = 0.04, r = 0.48), while in controls, CGSI correlated with metanephrine (p = 0.02, r = -0.54). Pre + Gz metanephrine correlated with CGSI in both groups. Discussion PSC did not confer a higher CGSI nor induce a greater neuroendocrine, leukocytic or hematologic response compared to controls. However, the increase in serum osmolality may indicate increased vasopressin activity. The strong correlations between CGSI and norepinephrine in PSC suggest that despite equal average serum levels, the responsiveness of norepinephrine is enhanced. This may serve as an optimal cardio-protective countermeasure for manned space missions and possible ICU patients. The correlations between neutrophiles and thrombocytes in PSC suggest a leukocytic and hematologic involvement in CSGI. In addition, this is the first study to demonstrate that baseline serum metanephrine may serve as a potential neuroendocrine parameter to assess + Gz resilience.