Cardiopulmonary exercise testing with elastic resistance for determining ventilatory thresholds and maximal oxygen uptake in middle-aged adults.

Metabolic insights into the 3xTg-AD Alzheimer model mice: Unraveling the hypothalamic-pituitary-thyroid axis and beyond.

Exercise while under energy deficit alters metabolic responses, such as promoting fat oxidation; however, its effects on the neuromuscular system, which consumes fuel, remain unclear. This study investigated motor unit (MU) firing characteristics during low- and high-intensity exercise with or without energy intake following overnight fasting. Eighteen healthy adults participated. After an 8-10 hour fast, participants ingested either placebo jelly containing 0 kcal (Non-caloric feeding [NCF]) or high-carbohydrate jelly containing 7 kcal/kg body mass (Caloric feeding [CF]). Two hours later, the respiratory exchange ratio (RER) during dynamic knee extension at ~20% of peak oxygen consumption and high-density surface electromyography (HDsEMG) from the vastus lateralis during isometric knee extensions at 30% and 70% of maximal voluntary contraction (MVC) were measured. HDsEMG signals were decomposed into individual MU action potentials, which were tracked across the conditions, and the MU firing rate and recruitment threshold were calculated. RER was lower under the NCF than CF (p<0.001), indicating enhanced fat oxidation. In both 30 and 70% MVC, the MU firing rate and recruitment threshold did not differ between conditions (both p > 0.05). During 30% MVC, the difference in the MU recruitment threshold from the NCF to CF condition was positively associated with recruitment threshold (p=0.002), whereas no recruitment threshold-dependent changes were observed during 70% MVC (p = 0.110). These findings suggest that overnight fasting may modulate MU recruitment patterns during low-intensity exercise. These results may highlight a potential neuromuscular strategy to optimize energy metabolism under fasting conditions.

The primary aim of this study was to examine the validity of energy expenditure (EE) estimated via branched chain modeling (BCM) during treadmill walking with a weighted vest. Thirteen sedentary adults with overweight/mild obesity [M = 11, F = 2; 29.0 (5.5) yr; maximal oxygen uptake (V̇O2max): 27.4 (2.0) mL·kg-1·min-1; body mass index (BMI) = 29.6 (3.0)] were recruited. Participants completed a ramp incremental and steady-state walking (4.0, 4.5, 5.0, and 5.5km·h-1) whilst wearing weighted vest (10% body mass; VEST) and unweighted control (CON). EE was estimated via indirect calorimetry (EEIC) and BCM (EEBCM), using a combined accelerometry/heart rate (HR) sensor (ActiHeart 5), and cardiorespiratory responses were determined from gas analysis. Limits of agreement (LoA) and concordance correlation coefficient (CCC) were used to assess validity of EEBCM. Repeated-measures ANOVA (condition*velocity) was used to examine physiological responses. Significant main effects of VEST included elevated oxygen consumption (V̇O2; ml.kg-1.min-1;p < 0.001), ventilation (p < 0.001), respiratory exchange ratio (RER; p < 0.001), HR (p < 0.001), and both EEIC (p < 0.001) and EEBCM (p < 0.001). There was a strong correlation between EEIC and EEBCM in both CON (CCC = 0.80) and VEST (CCC = 0.82). Mean bias (95% LoA) between EEIC and EEBCM was - 6.7 (-71.5,58.2) J·kg-1·min-1 (CON) and 3.6 (-50.3,57.6) J·kg-1·min-1 (VEST). Adults with overweight/mild obesity demonstrated an increase in EE during treadmill walking with a weighted vest. BCM was sensitive to this increase and also to treadmill velocity., However the agreement in estimated EE between indirect calorimetry and BCM was marginal for both CON and VEST conditions, and thus not interchangeable.

We previously showed that nocturnal fat oxidation is reduced in older individuals with overweight/obesity and impaired glucose tolerance and insulin sensitivity compared with young lean individuals. Here, we compared nocturnal energy expenditure and substrate oxidation across groups varying in age, body composition and metabolic status to unravel factors underlying variations in nocturnal substrate metabolism. Data were collected from 18 previously conducted human clinical studies (N=187), all performed under conditions of energy balance with similar diet composition and meal timing. Individuals were categorised into four groups: young lean (YL); older lean (OL); older with overweight/obesity (OBE); and older with overweight/obesity and type 2 diabetes (T2D). Nocturnal energy expenditure and substrate oxidation were determined by whole-room indirect calorimetry, body composition was assessed by air-displacement plethysmography or dual energy x-ray absorptiometry, and glucose, insulin, HOMA-IR, NEFA and triglycerides were measured from fasted blood samples. Group comparisons for nocturnal energy expenditure and substrate oxidation were performed using Kruskal-Wallis tests, and over time using linear mixed models including group × time interactions, with Bonferroni correction applied to both analyses. Multivariate linear regression analysis was applied to identify whether age, sex, HOMA-IR, fasting NEFA, fasting triglycerides, fat mass and fat-free mass were independent factors of nocturnal energy expenditure and substrate oxidation. Nocturnal energy expenditure, adjusted for fat-free mass, was higher in OBE compared with YL and OL (p<0.01 for both); it was also higher in T2D compared with OL (p<0.01). Nocturnal fat oxidation, expressed as a percentage of energy expenditure, was lower in OBE (median: 46.28%, IQR: 37.74-53.05) and T2D (median: 46.48%, IQR: 41.05-53.65) compared with YL (median: 52.95%, IQR: 47.82-57.61; p<0.01 for both) and OL (median: 55.21%, IQR: 54.15-58.89; p<0.01 for both). Standardised linear regression models revealed that fasting triglycerides were positively associated with nocturnal respiratory exchange ratio (β=0.337; 95% CI 0.165, 0.508) and nocturnal carbohydrate oxidation (% of energy expenditure; β=0.337; 95% CI 0.166, 0.509), and inversely associated with nocturnal fat oxidation (% of energy expenditure; β=-0.352; 95% CI -0.520, -0.813). Nocturnal fat oxidation (% of energy expenditure) is diminished in older individuals with overweight/obesity, irrespective of diabetes status. No differences in nocturnal energy expenditure (adjusted for fat-free mass) or substrate oxidation were observed between young and older lean individuals, suggesting that age per se may not strongly influence nocturnal substrate metabolism. Fasting triglyceride level was the strongest associated factor of nocturnal substrate oxidation.

This study examined the durability of physiological and biomechanical responses in trained trail runners during prolonged submaximal running combined with repeated uphill efforts. Twenty-three trail runners (11 females) completed a 180-minute treadmill run at 85% of the speed associated with their lactate threshold +0.5 mmol·L-1 (ie, steady state). Every 60 minutes, participants performed a 12-minute uphill time trial (+12%). Physiological variables (oxygen uptake [V˙O2], respiratory exchange ratio [RER], blood lactate [BLa], substrate oxidation, heart rate, and perceived exertion) and gait parameters (stride length, cadence, ground contact time, duty factor, vertical oscillation, and leg stiffness) were continuously recorded. By the end of steady-state running, heart rate and perceived exertion increased (P < .001), whereas V˙O2, BLa, and energy cost remained stable. Carbohydrate oxidation decreased (P < .001), while fat oxidation increased (P < .001). Across uphill time trials, distance (-6.6%, P < .001) and stride length declined, while ground contact time and duty factor increased; RER and BLa decreased (both P < .001), indicating a progressive metabolic shift toward greater fat reliance. Despite these changes, oxygen cost and leg stiffness were preserved. Prolonged submaximal and repeated uphill running induced small but coordinated physiological and mechanical drifts without compromising overall energetic cost. These findings reveal high durability in trained trail runners, reflected in their ability to preserve biomechanical stability and running efficiency despite increasing perceived fatigue.

The present study had three main objectives: (a) to evaluate the in-field validity of different commercially available cardiopulmonary exercise testing (CPET) systems when used by end-users following typical calibration procedures, (b) to measure the variability in accuracy among identical CPET units, and (c) to explore the relationship between the age of the units, as well as the maintenance practices, and their measurement accuracy. Fifty-seven CPET systems, calibrated and operated by end-users in clinical practice, research, or sports settings, were assessed against a metabolic simulator that simulates breath-by-breath gas exchange. The values measured by each system [minute ventilation (V̇E), oxygen uptake (V̇O2), carbon dioxide production (V̇CO2), and respiratory exchange ratio (RER)] were compared to the simulated values to evaluate the accuracy. Absolute percentage errors during the simulations ranged from 1.41% to 24.6% for V̇E, 3.29%-10.6% for V̇O2, 2.86%-13.3% for V̇CO2, and 1.90%-10.0% for RER. Inter-unit variability (%) ranged from 1.98% to 12.7% for V̇O2, 1.49%-8.10% for V̇CO2, and 1.93%-4.24% for RER. No consistent relationship between system age and accuracy was observed, nor between annual maintenance and accuracy. The validity of metabolic carts for measuring respiratory gas variables varied significantly even between identical systems, despite passing manufacturers' calibration checks. Furthermore, inter-unit variability of most systems exceeded intra-unit test-retest variability, thus necessitating caution when using devices interchangeably, as this may increase measurement noise, even within the same laboratory. Most inaccuracies seemed related to technological errors, although some user errors were also identified, indicating the need for a holistic approach to identify errors.

Endurance performance determinants are typically assessed under fresh conditions, although physiological and mechanical characteristics deteriorate during prolonged exercise, a phenomenon recently termed durability or resilience. However, it is unclear whether durability and mechanisms underpinning it differ between sexes. Eleven females and 11 males matched for performance completed three laboratory visits including: a graded exercise test, a 12-min uphill time trial (TT), and a 180-min treadmill run at standardized moderate intensity, interspersed with repeated TTs every 60 min (total distance: 36 ± 3 and 42 ± 3 km, respectively). Physiological, biomechanical, and neuromuscular variables were assessed throughout the steady-state run and TTs and analyzed with linear mixed models. Time-trial performance declined during the run, with females displaying smaller speed decrements than males after 3 h (-1 vs. -10%, p < 0.01) and smaller reductions in carbohydrate oxidation and respiratory exchange ratio during steady-state (p < 0.05) and TTs (p < 0.01). Males displayed larger reductions in peak blood lactate during TTs (p < 0.001), while peak heart rate (HR) and oxygen uptake remained unchanged in both groups. During the prolonged run, females showed greater resilience for maximal isometric knee extensor force, HR, and perceived exertion (p < 0.05), whereas running economy deteriorated similarly between sexes. Biomechanical adjustments occurred in both sexes, with larger alterations observed in males for TTs (stride length; p < 0.05) and in females during steady-state (contact time and stiffness; p < 0.05). In conclusion, highly trained female runners demonstrate greater durability than performance-matched males. Sex differences are characterized by superior metabolic and neuromuscular resilience in females, whereas biomechanical changes appear similar between sexes. Finally, whether these findings persist under distance-matched conditions warrants investigation.

Background: In patients with left ventricular hypertrophy, resting structural parameters alone may not explain exertional symptoms. Hence, we investigate whether combined Cardiopulmonary Exercise Testing- Exercise Stress Echocardiography (CPET-ESE) can provide an integrated functional characterisation of hypertrophic phenotypes. Methods: As a preliminary investigation, this prospective single-centre pilot study enrols 43 patients, categorised into: obstructive hypertrophic cardiomyopathy (n = 19), transthyretin cardiac amyloidosis (n = 15), or preserved-ejection-fraction hypertrophic phenotypes (n = 9). Patients undergo symptom-limited semi-supine CPET-ESE on an electronically braked cycle ergometer with an individualised ramp protocol. Peak effort is defined by symptom limitation and respiratory exchange ratio criteria (), while peak VO2 is defined as the highest 30 s averaged value. Results: Exercise responses differ across phenotypes. Patients with obstructive hypertrophic cardiomyopathy have higher peak VO2 than the other groups, despite their lower chronotropic reserve. The preserved-ejection-fraction hypertrophic group shows lower peripheral oxygen extraction, whereas transthyretin amyloidosis shows a mixed central and peripheral limitation pattern. Right ventricle–pulmonary artery uncoupling is observed in the latter two groups. Conclusions: The use of CPET-ESE may help describe distinct physiological exercise profiles in hypertrophic phenotypes, but these findings should be considered exploratory. The small, heterogeneous and single-centre cohort precludes definitive mechanistic or predictive conclusions and supports the need for larger validation studies.

Acute metabolic responses to breaking up sedentary time in older adults.

Recently the influence of the menstrual cycle and more importantly, fluctuations in key sex hormones across the follicular (FP), ovulation (OP) and luteal (LP) phases have on during exercise substrate oxidation has been questioned. In addition to a significant amount of work not following best practices when determining menstrual phase, the inter- and intra-individual variability in hormonal concentrations across cycles may be affecting our understanding. To examine the effect of three hormonally distinct phases of the menstrual cycle (FP, OP, LP) on during and post-exercise substrate oxidation following an acute bout of submaximal aerobic exercise in the fed state while utilizing best practices to determine menstrual phase. Thirteen females (age: 24±4 y, BMI: 24.8±5.4 kg·m -2, V̇O2max: 41.02±5.30 mL·kg·min -1) completed 30 min of submaximal aerobic exercise in the FP, OP, and LP where menstrual phase was tracked using cycle counting, oral body temperature, ovulation strips, and blood sampling. Between-phase differences in respiratory exchange ratio (RER), substrate oxidation, oxygen consumption (V̇O2), overall session O2, and post-exercise metabolism were compared using a series of linear mixed models (LMMs; timepoint x phase). There were no between-phase differences in V̇O2, overall session O2, post-exercise metabolism, or RER (P>0.119, R 2<0.993). There were also no between-phase differences in fat (P=0.417, R 2=0.920) or carbohydrate oxidation (P=0.235, R 2=0.845) during or post-exercise. These findings demonstrate no differences across menstrual phase in during and post-exercise metabolism following an acute bout of submaximal exercise.

Purpose Maximal oxygen uptake (VO2max) is a strong, independent predictor of both all-cause and disease-specific mortality, making it a key health and performance marker for individuals with heart disease. While cardiopulmonary exercise testing is the gold standard for determining VO2max, its use in primary healthcare settings remains underexplored. This study aims to evaluate standard criteria for assessing VO2max in individuals enrolled in cardiac rehabilitation. Methods A cross-sectional study was conducted in a primary healthcare setting. Participants who referred to exercise-based cardiac rehabilitation underwent a cycle ergometer exercise test to determine VO2max. The primary validation criterion was the presence of a VO2 plateau, defined by cut-offs of ≤50 and ≤100 mL O2/min. Secondary criteria included respiratory exchange ratio, rate of perceived exertion, and maximal heart rate. Results Among 93 participants, 88% reached a VO2 plateau using the ≤100 mL cut-off. Additionally, 61% met at least two secondary criteria, indicating maximal effort. Conclusion In primary healthcare cardiac rehabilitation, a cut-off of ≤100 mL may be suitable for identifying a VO2 plateau. For those not reaching a plateau, maximal effort can be assessed using RER ≥1.15 and RPE ≥17 as criteria.

Introduction: Strength training (ST) influences metabolic demand and training efficiency. Oxygen consumption (VO2) represents a central physiological marker linking exercise structure to energy expenditure, yet the effects of simultaneously engaging upper and lower body musculature remain underexplored. Objective: To compare the acute physiological responses to simultaneous versus isolated execution of ST exercises, focusing on VO2. Methodology: Ten recreationally trained men (25.0±3.97 years) participated in a randomized crossover design. Participants performed two protocols: simultaneous execution (SS) of incline dumbbell press and 45º leg press, and isolated execution (SI) of the same exercises in separate sets, both at 70% of one-repetition maximum. VO2 (absolute and relative), heart rate (HR), and respiratory exchange ratio (RER) were continuously measure. Linear mixed-effects models were applied to compared conditions. Results: SS elicited significantly higher VO2 than SI (VO2A: 1.58±0.06 l.min-1 vs 1.23±0.06 l.min-1; VO2R: 20.91±0.42 ml.kg-1.min-1 vs 16.38±0.55 ml.kg-1.min-1; p&lt;0.001), with very large effect sizes. HR. was also higher in SS (p&lt;0.001), whereas RER was significantly higher in SI (p&lt;0.001). Discussion: The elevated VO₂ during SS likely reflects accelerated VO₂ on kinetics driven by greater concurrent muscle mass activation and reduced metabolic recovery periods, increasing systemic ATP turnover and cardiorespiratory demand. These findings highlight exercise configuration as a key modulator of physiological load beyond traditional variables such as intensity and volume. Conclusions: Simultaneous upper and lower body ST execution enhances VO2 per unit of time and represents a time-efficient strategy to increase metabolic stress, with practical implications for optimizing ST prescription in health and performance settings.

Skeletal muscle mass and force decline with age, and the loss of muscle force precedes muscle atrophy. However, the underlying mechanisms remain unclear. Here, we investigated the role of the myosin co-chaperone, uncoordinated mutant number-45 myosin chaperone B (UNC45B), in regulating muscle mass and force. UNC45B expression decreased in mouse gastrocnemius muscle with age, particularly at 24 months old, and adeno-associated virus vector-mediated knockdown of Unc45b in 3-month-old mouse triceps surae muscle first reduced plantar flexor torque and then decreased gastrocnemius muscle mass. In addition, Unc45b knockdown in the triceps surae muscle resulted in lower bone mineral density. While maximum Ca2+-activated force in mechanically skinned fibers was not affected by Unc45b knockdown, Unc45b knockdown decreased the ratio of depolarization-induced force to the maximum Ca2+-activated force. We established tamoxifen-inducible skeletal muscle-specific Unc45b knockout (Unc45b imKO) mice to investigate whether the muscle atrophy and weakness due to the loss of Unc45b impacts metabolism and behavior. We found that Unc45b imKO reduced muscle mass and force at a whole-body level, but did not influence systemic glucose tolerance, insulin sensitivity, or the respiratory exchange ratio. However, Unc45b imKO mice reduced the amount of deeper non-rapid eye movement sleep, locomotor activity, and body temperature during the sleep phase. We conclude that UNC45B is essential for maintaining fast-twitch muscle mass and muscle force. In addition, Unc45b deficiency-mediated muscle loss is also associated with bone fragility, decreased body temperature, and impaired sleep quality.

Intraoperative respiratory exchange ratio to predict complications after cardiopulmonary bypass surgery: a retrospective cohort study.

The circadian clock system has been demonstrated to modulate immune function, thereby resulting in a pronounced diurnal rhythm in the host's response to infection. However, the role of the circadian clock throughout the entire course of sepsis remains unclear. This study aims to investigate whether the endogenous circadian clock mediates differential effects on sepsis at different stages of the disease. We induced sepsis in mice by intraperitoneal injection of lipopolysaccharide (LPS) during the rest phase (Zeitgeber Time 6, ZT6) and the active phase (ZT18). We found a significant time-dependent dual effect: Mice in the ZT18 group exhibited a more intense inflammatory response in the acute phase within 24 h, characterized by more severe hypothermia and elevated serum and tissue levels of tumor necrosis factor (TNF-α), leading to higher acute-phase mortality. However, upon extending the observation period to 5 days, it was observed that ZT18 group had a higher survival rate. The long-term survival advantage of the ZT18 group was associated with stronger metabolic resilience, characterized by faster recovery of the respiratory exchange ratio (RER) and energy expenditure, as well as earlier restoration of daily activity capacity. Lesioning the central clock, the suprachiasmatic nucleus (SCN), eliminated the diurnal differences in LPS-induced mortality and metabolic disturbances, indicating that the phenomenon is driven by the central clock. These findings provide a new theoretical framework for developing stage-specific chronotherapies for sepsis.

BackgroundExercise physiology is a specialized field of physiology that focuses on examining the effects of exercise and physical activity on the structure and function of living organisms. Knowledge of exercise physiology is essential for enhancing athletic performance, enabling individuals to adapt to extreme conditions, and managing chronic diseases. The objective of this research was to assess the level of knowledge in exercise physiology among medical students in Syria, who are future healthcare professionals.MethodsThe data was collected from a cohort of 704 undergraduate medical students who were actively following clinical internships. To evaluate their understanding of exercise physiology, a web‐based survey was utilized as a research tool. The survey is aimed at assessing participants′ foundational knowledge about diverse physiological concepts linked to physical exercise, encompassing topics such as respiratory exchange ratio, physical fitness, V̇O2 max (maximum oxygen uptake), and exercise at high altitudes.ResultsA total of 55.5% of the participants exhibited a poor level of knowledge in exercise physiology. Misconceptions were evident regarding various aspects, including the definition (75.7%), application (64.4%), significance (55.3%), and related physiological factors (58.2%) of V̇O2 max. Furthermore, misconceptions were identified in the assessment of physical fitness, with proportions of 51.6%, 42.6%, and 38.8%, respectively, for specific evaluation questions. Similar misconceptions were prevalent in areas such as muscle groups and energy systems during training (34.9%), training prescription variables (42.8%), respiratory exchange ratio (64.4%), and high‐altitude performance (69.9%). Additionally, statistically significant differences in knowledge levels were observed based on respondent characteristics (p < 0.05).ConclusionsSyrian medical students demonstrated limited knowledge and moderate awareness of exercise physiology. These findings emphasize the need to enhance medical curricula by integrating exercise physiology more effectively and promoting active, applied learning strategies to strengthen future physicians′ competence in exercise‐based patient care.

Exposure to per- and polyfluoroalkyl substances (PFAS) elicits changes in various metabolic responses but few studies have evaluated whole-body metabolic changes. This study compared whole-body bioenergetics in adult female C57BL/6 mice orally dosed with 0 or 7.5 mg/kg perfluorooctanoic acid (PFOA) or fed a 60% high fat diet (HFD) for 24h or 15d. After 24h, HFD mice showed increased oxygen consumption (VO2) and reduced active-phase respiratory exchange ratio (RER), indicating early lipid utilization. After 15 d, PFOA-exposed mice exhibited decreased resting RER and suppressed active VO2, suggesting impaired circadian metabolic activation. In contrast, HFD mice maintained elevated VO2 across both resting and active cycles and showed reduced RER variation between phases, indicating metabolic inflexibility. PFOA exposure also increased relative liver weights and ACOX-1 activity. HFD resulted in an increase in body weight and increase in fat mass without hepatomegaly. These results indicate that both exposure paradigms disrupt whole-body metabolism and circadian bioenergetics. PFOA induced pronounced peroxisomal proliferation and suppressed resting-cycle energy expenditure without weight gain, whereas HFD drove obesogenic effects with impaired substrate switching. These findings provide mechanistic insight into how environmental and dietary stressors alter metabolic rhythms.

Cold-induced thermogenesis (CIT) mediated by brown adipose tissue (BAT) and skeletal muscle holds therapeutic utility for obesity, but relative tissue contribution to CIT-driven metabolic flexibility (MetF) and its correlation to insulin resistance remain undefined across diverse ethnicities. This study aimed to investigate ethnic differences in cold-induced MetF and its link to insulin sensitivity in Chinese and Asian Indian individuals with overweight. Forty-one participants with pre-metabolic or metabolic syndrome of two ethnic groups (31 Chinese, 10 Indian), with an average body mass index (BMI) of 27.5 kg/m2 underwent whole-body calorimetry following 1-h cold exposure (~14.5 °C) and oral glucose tolerance test (OGTT) in a crossover design. Resting metabolic rate (RMR), respiratory exchange ratio (RER), carbohydrate oxidation (COX), and fat oxidation (FOX) were assessed. Blood glucose and insulin were measured to assess insulin sensitivity. Asian Indians had significantly higher fasting insulin levels and lower insulin sensitivity than Chinese participants. Chinese participants had greater increase in energy expenditure and a more pronounced fuel switch (RER, COX and FOX) with cold. For Chinese individuals, better cold-induced MetF was independently associated with lower fasting insulin and insulin resistance, even after adjusting for age, sex, and body fat. This association was not observed in Asian Indians. Significant ethnic differences exist in the association of CIT-driven MetF to insulin resistance. Chinese individuals exhibited robust MetF-insulin sensitivity correlation, whereas Asian Indians showed a blunted response without such independent association. This suggests that ethnicity-specific strategies targeting MetF are crucial for addressing metabolic health disparities.

Background and Study Aim. Ultramarathon running represents a demanding form of endurance activity characterized by prolonged duration and substantial physiological load. Despite the application of various approaches to performance assessment, the relative contribution of different physiological characteristics to race performance across standard ultramarathon distances remains a subject of practical interest. The aim of this study was to identify and directly compare the functional fitness characteristics associated with race performance at 50 km and 100 km distances. Materials and Methods. Thirty-one experienced ultramarathon runners performed an incremental treadmill test to volitional exhaustion to determine maximal oxygen uptake (VO₂max), aerobic threshold (AeT), anaerobic threshold (AnT), and the corresponding running speeds and respiratory exchange ratio (RER). Body mass index (BMI) was also assessed. Correlation and linear regression analyses were used to examine the relationships between physiological variables and race performance. Results. The relationships between physiological variables and race performance differed between the two distances. At 50 km, performance was primarily associated with aerobic power–related variables, including VO₂max (r = 0.79), running speed at VO₂max (r = 0.82), and body mass index (r = -0.82). At 100 km, stronger associations were observed for threshold-related variables, particularly running speeds at AeT (r = 0.78) and AnT (r = 0.76), while the association with BMI was weaker (r = -0.52). Average race speed corresponded to 71.7% of vVO₂max at 50 km and 62.5% at 100 km. Conclusions. The findings indicate a distance-dependent shift in physiological characteristics associated with ultramarathon performance. Shorter ultramarathon distances are more closely associated with aerobic power–related variables, whereas longer distances show stronger associations with threshold-related characteristics. These results support the concept of distance-specific physiological profiles in ultramarathon running. From a practical perspective, training approaches may benefit from accounting for these differences, with relatively greater emphasis on higher-intensity variables at shorter distances and threshold-related characteristics at longer distances.