The main goal of the present study was to examine the effects of a high-velocity resistance training (HVRT) program on the force–velocity (F–V) profile of different groups of aging adults. An additional aim was to explore whether individual baseline F–V profiles were associated with training-induced improvements in muscle power and functional capacity. Middle-aged and older adults with and without mobility limitations (n = 36, 61 ± 13 years) completed a 12-week HVRT program (2 days/week, 40–60

Introduction & Purpose Poor sleep is a major contributor to mortality and many diseases. Epidemiological data suggests that sleep disturbances affect overall health and physical function [1] and growing evidence points to an association of sleep quality with muscle health and body composition across the lifespan [2]. Effective prevention and treatment programs rely on understanding the role of sleep in the development of sarcopenia and obesity. This project investigated the association of sleep quality with BMI and muscle function in older adults in Austria. Methods For this cross-sectional study, older adults aged 65+ were recruited across Austria to complete an online survey and self-measurements. We collected personal information, sleep quality via the Pittsburgh Sleep Quality Index (PSQI), habitual physical activity with the International Physical Activity Questionnaire: Short Form (IPAQ), and the risk of sarcopenia via SARC-F score. BMI was derived from self-measured body height and mass. Muscle function was assessed using a self-administered 30s chair rise test (CRT30) with detailed video-based instructions. Multiple linear regression models were calculated in a hierarchical manner to predict the number of repetitions in the CRT30, the SARC-F score and the BMI. Confounding variables were added to the unadjusted models (only PSQI) in blocks. Results 203 participants (96 female; age: 74.0±8.3years; BMI:26.0±3.4kg*m2) fulfilled the inclusion criteria. They performed 9.40±4.07 repetitions in the CRT30, had a mean PSQI of 6.47±3.17, and a median SARC-F score of 2 [IQR: 3]. To predict CRT30, the final model included PSQI, age and sex (F(3,198) = 38.68, p < .001; R2adj. = 0.360). Adding further confounders did not significantly improve the model. PSQI (β = –0.244, p < 0.001) and age (β = –0.492, p < 0.001) were significant predictors of CRT30, while sex was not (β = –0.075, p = 0.188). The same variables were included in the final model to predict the SARC-F score (F(3,198) = 94.11, p < 0.001; R2adj. = 0.582), with no improvement when adding more confounders. PSQI (β = –0.231, p < 0.001) and age (β = –0.671, p < 0.001) were significant predictors of SARC-F score, but sex was not (β = –0.050, p < 0.279). No significant association was found between PSQI and BMI. No significant predictive power of physical activity on CRT30 was found in a subsample. Discussion Low self-reported sleep quality was independently associated with low objective physical function (30CRT) and low self-reported function (SARC-F). This is in agreement with previous results, although sex differences reported earlier [3] were not observed. No association of sleep quality with BMI was found. As only a small fraction of participants was obese or underweight, our sample may not have been diverse enough to detect the previously reported association. If improving sleep hygiene in older adults can mitigate the risk of sarcopenia needs to be investigated. Conclusion Poor sleep quality is independently associated with low muscle function but not BMI in a sample of predominantly non-obese Austrian older adults. The potential role of sleep health in sarcopenia prevention warrants further investigation. References [1] Stone, K. L., & Xiao, Q. (2018). Impact of Poor Sleep on Physical and Mental Health in Older Women. Sleep Medicine Clinics, 13(3), 457–465. https://doi.org/10.1016/j.jsmc.2018.04.012 [2] Chien, M.-Y., Wang, L.-Y., & Chen, H.-C. (2015). The Relationship of Sleep Duration with Obesity and Sarcopenia in Community-Dwelling Older Adults. Gerontology, 61(5), 399–406. https://doi.org/10.1159/000371847 [3] Denison, H. J., Jameson, K. A., Sayer, A. A., Patel, H. P., Edwards, M. H., Arora, T., Dennison, E. M., Cooper, C., & Baird, J. (2021). Poor sleep quality and physical performance in older adults. Sleep Health, 7(2), 205–211. https://doi.org/10.1016/j.sleh.2020.10.002

Prior evidence suggests that individuals with higher fitness exhibit attenuated endocrine and autonomic responses to acute psychological and physical stressors. However, it remains unclear whether this stress-buffering effect depends on the type of stressor. This study examined whether aerobic capacity modulates psychophysiological responses to two stress-induction paradigms (Trier Social Stress Tests, TSST; Maastricht Acute Stress Test, MAST). We hypothesized that fitter participants show lower stress reactivity, with stronger attenuation expected for the MAST, which includes a physical stress component. Healthy participants (n = 59; 28 women; age: 27.1 ± 5.5 years) completed the TSST and MAST on separate days under standardized conditions. We assessed self-reported stress, salivary hormones (cortisol, cortisone, DHEAS), and autonomic cardiac regulation (heart rate and root mean square of successive differences of heart periods). Aerobic fitness was determined via graded exercise testing (peak oxygen uptake; V̇O2peak: 50.4 ± 8.6 mL/kg/min). Multilevel regression models were used to examine the associations between V̇O2peak and stress responses. Higher V̇O2peak was significantly associated with lower heart rate across both stress-induction paradigms (b = 0.99 bpm, p = 0.002). For other outcomes, no significant association with V̇O2peak was found. The associations did not differ between TSST and MAST. Aerobic fitness was linked to lower heart rate during the experimental sessions, indicating a modest cardiovascular advantage under acute stress. Contrary to our hypothesis, this effect did not differ between the two stress paradigms, suggesting that the influence of fitness was not specific to the type of stressor. Aerobic capacity, therefore, seems to reduce cardiovascular load during stressful situations in a general manner.

PURPOSE:Breathing practices are increasingly adopted by athletes due to the potential benefits for recovery, performance, and well-being. Slow-paced breathing is believed to increase cardiac vagal activity and relaxation, potentially supporting adaptive processes. Sprint interval training, by contrast, elicits a strong stress response. Therefore, we hypothesized that adding slow-paced breathing to highly demanding sprint interval training would enhance recovery, as indicated by cardiac autonomic control and sleep metrics, ultimately contributing to greater performance gains. METHODS:Thirty-four participants were randomized into an intervention or control group. While both groups completed 4 weeks of supervised sprint interval training, only the intervention group practiced slow-paced breathing after each training and every evening before bedtime. Running performance, vagally mediated heart-rate variability (root mean square of successive differences), and sleep metrics were assessed throughout the intervention to evaluate the potential benefits of the breathing exercise. RESULTS:Sprint interval training led to significant improvements in maximal power decrements (-8.26%, 95% CI, -11.48 to -5.04, P < .001) and overall work (8.84 kJ/kg, 95% CI, 4.58 to 13.1, P < .001), but performance gains did not differ between groups (P > .05). Unexpectedly, nocturnal heart rate showed a slight increase over time in the intervention group (+3.1 beats·min-1, 95% CI, 0.47 to 5.72, P < .05) compared with the control group. CONCLUSION:Despite positive effects reported in other training and performance contexts, 4 weeks of daily slow-paced breathing did not confer additional performance improvements or physiological benefits when combined with sprint interval training. These findings suggest that 10-minute sessions of slow-paced breathing following highly demanding sprint interval training offer limited value for enhancing performance or supporting recovery.

Age-related declines in muscle mass and strength result from intrinsic ageing and reduced mechanical loading. Masticatory muscles, being continuously used for feeding, may be less affected. We cross-sectionally compared ageing effects on locomotor and masticatory muscles in 30 women: young (26±1.8 y), middle-aged (44.1±3.4 y), and older (69.5±3.1 y). Cross-sectional area (CSA) of the quadriceps femoris and masseter was assessed by panoramic ultrasound, and knee extension torque (KE) and bite force (BF) were measured via isokinetic dynamometry and a bite force sensor. Both muscles showed progressive age-related reductions. Quadriceps CSA declined to ~75% in middle-aged and 56 ± 13% (older) of young values, while masseter CSA declined to ~94% and 79%, respectively. Strength also declined (KE: ~81% and 62%; BF: ~75% and ~68% of young values; all p<0.05), with no difference between middle-aged and older adults. Effect sizes for CSA were larger in the quadriceps (d = 1.56-2.95) than masseter (d = 0.29-1.09). For strength, effect sizes from young to middle-aged were similar for KE and BF (d = 1.20 vs. 1.33) but greater for KE from young to older adults (d = 2.28 vs. 1.16). Both muscles were smaller and weaker in older groups, but while quadriceps strength roughly mirrored CSA reductions, masseter strength declined disproportionately, suggesting neuromuscular contributions beyond muscle size.

Previous powerlifting research has largely examined correlates of performance in single competitions or over short observation periods, leaving long-term patterns of athletic development among sustained competitors insufficiently described. The present investigation addresses this gap by examining performance progression, observed peak performance timing, and correlates of peak career success in classic powerlifting. This retrospective observational study analyzed IPF-affiliated classic powerlifters with at least 10 completed competitions. Population-level age-performance patterns were described using LOESS smoothing with a lifter-level cluster bootstrap. Between-group differences in annual performance progression were examined using multivariable ordinary least squares models with lifter-clustered standard errors. Associations with observed peak timing were examined using multivariable ordinary least squares models, complemented by discrete-time event-history robustness analyses, and correlates of peak career performance were examined using multivariable ordinary least squares models. Within this selected cohort, annual gains generally declined with increasing years in sport. Peak performance occurred at approximately 27 years of age and observed individual peak performance was typically reached after 4–6 years of competition participation, around the 10th competition. Higher competition frequency was associated with earlier observed peak timing when indexed by year in sport, but with later observed peak timing when indexed by competition number and was not associated with greater peak performance. Age and IPF GL Points at the first competition were the strongest multivariable correlates of observed peak career performance, whereas attempt success rate, sex, and weight-class change patterns showed statistically significant but comparatively small effects. Early competitive performance and age at entry were strong multivariable correlates of observed peak career performance, while higher competition frequency appeared to relate primarily to the pacing and competitive exposure of development rather than to peak magnitude. These findings may inform early benchmarking and individualized long-term competition planning.

This study examined acute neuromuscular, biochemical, and subjective fatigue responses to two strength training protocols-back squats with calf raises vs. back squats with plyometric exercises-in elite youth soccer players. The aim was to track the progression and resolution of fatigue over a 72 h recovery period and evaluate the practical feasibility of both protocols within high-performance training schedules. Thirty-two male athletes from a national youth elite training center participated in a crossover-controlled study. Each player completed both interventions, separated by an 8-day washout period. A comprehensive test battery assessed neuromuscular performance via the countermovement jump (CMJ), drop jump, isometric mid-thigh pull, adductor squeeze, and sit-and-reach test; biochemical markers via creatine kinase (CK); and subjective markers using the Hooper Index, visual analog scale (VAS), and session rating of perceived exertion (sRPE). Measurements were taken at baseline and 24, 48, and 72 h post-exercise. No significant interaction effects (training modality × time) were found (p > 0.05), indicating comparable fatigue and recovery trajectories for both protocols, whereas significant main effects of time were observed across all parameters, including CMJ [F(3,26) = 29.373, p < 0.001, η 2 = 0.772] and CK [F(3,26) = 51.504, p < 0.001, η 2 = 0.856]. Fatigue peaked between 24 and 48 h post-exercise and returned to baseline by 72 h. Subjective fatigue (Hooper, VAS) mirrored objective markers. Both traditional and reactive strength training induced short-term fatigue that resolved within 72 h, supporting their safe implementation in elite youth training programs, provided adequate recovery is allowed between sessions.

This study aimed to investigate the effect of training intensity on contralateral adaptations following unilateral power-oriented resistance training (PT) in older adults. This secondary analysis of a within-person randomized controlled trial included 45 older adults (≥ 65 years; 25 women). After an 8-week control period (n = 45), part of the participants completed 12 weeks of a volume load-matched PT on one lower limb using either light loads (40% 1-RM, n = 9) or heavy loads (80% 1-RM, n = 10), while the contralateral limb did not exercise. Unilateral performance was assessed before and after each period through isometric tests (maximal isometric force, rate of force development, and muscle excitation) and dynamic tests (1-RM, maximal muscle power and maximal unloaded velocity determined through a force-velocity relationship test). Additionally, mid-thigh muscle mass and whole-body physical function were assessed. Linear mixed models were conducted to compare the effect (Cohen's d) of training intensity on the efficacy and magnitude of the contralateral adaptations and their impact on physical function. No contralateral adaptations were observed in any of the isometric tests. In contrast, untrained limbs showed comparable improvements in 1-RM (d = 0.35-0.42; p ≤ 0.001) and maximal muscle power (d = 0.49-0.54; p ≤ 0.001) regardless of PT intensity. Only heavy-load PT improved maximal unloaded velocity in the contralateral untrained limb (d = 0.68; p ≤ 0.001), while mid-thigh muscle mass (d = 0.24; p ≤ 0.013) and whole-body physical function assessed through the timed up-and-go test improved only after light-load PT (d = -0.74; p < 0.001). In conclusion, unilateral PT induced significant contralateral gains in dynamic muscle strength and power, regardless of training intensity, with heavier loads enhancing velocity-specific adaptations. Notably, light-load PT induced mild hypertrophy in the contralateral untrained limb and whole-body physical function, which might be particularly relevant for enhancing the intrinsic capacity of older adults. Trial Registration: ClinicalTrials.gov ID: NCT03724461.

Background: Regular exercise was previously shown to reduce glucocorticoid and cardiac-autonomic responses to psychosocial stressors. Specifically, laboratory-based stress induction procedures are recognized as valid experimental manipulations of the physiological stress response. Nevertheless, comparative research between different types of stressors is limited. This study was designed to examine the multi-system psychophysiological response to two stress-induction procedures—psychosocial (Trier Social Stress Test; TSST) and psychophysical (Maastricht Acute Stress Test; MAST)—in male and female athletes. Methods: In a crossover pilot study, 12 athletes (6 female) underwent a TSST and a MAST, one month apart. Saliva hormones and cardiac-autonomic response (heart rate and HRV) were analyzed, besides an untargeted proteomics analysis. Results: The MAST revealed a lower elevation of heart rate (SMD = −1.47 [−2.51, −0.43]) and reduction in RMSSD (SMD = 0.98 [0.01, 1.95]) compared to the TSST. No statistically significant differences were found for hormones or subjective stress (all p < 0.05). Sex comparisons of the area under the curve exposed overall lower responses in women for aldosterone (SMD = −1.50, [−2.45, −0.51]), cortisol (SMD = −1.35, [−2.28, −0.39]), cortisone (SMD = −1.43, [−2.38, −0.46]), overall glucocorticoids (SMD = −1.44, [−2.38, −0.46]), and stronger reduction in testosterone-to-cortisone (SMD = 1.41, [0.44, 2.35]). Interestingly, sex differences were more evident in response to the TSST. Conclusions: Found sex differences underscore the importance of sex sensitive research in stress and exercise science. Our data support the presented methodological approach and encourage properly powered research on stressor comparison in relation to sports and physical fitness.

Introduction: Prior evidence suggests that individuals with higher fitness exhibit attenuated endocrine and autonomic responses to acute stress. However, it remains unclear whether this stress-buffering effect depends on the type of stressor. This study examined whether aerobic capacity modulates psychophysiological responses to two stress induction paradigms (Trier Social Stress Tests, TSST; Maastricht Acute Stress Test, MAST). We hypothesized that fitter participants show lower stress reactivity, with stronger attenuation expected for the MAST, which includes a physical stress component. Methods: Healthy participants (n = 59; 28 women; age: 27.1 ± 5.5 yrs) completed the TSST and MAST on separate days under standardized conditions. We assessed self-reported stress, salivary hormones (cortisol, cortisone, DHEAS), and autonomic cardiac regulation (heart rate, RMSSD). Aerobic fitness was determined via graded exercise testing (VO2peak: 50.4 ± 8.6 ml/kg/min). We used multilevel regression models to examine the associations between VO2peak and stress responses. Results: Higher VO2peak was significantly associated with lower heart rate across both stress induction paradigms (b = 0.99 bpm, p = 0.002). For other outcomes, no significant association with VO2peak was found. The associations did not differ between TSST and MAST. Discussion: Aerobic fitness was linked to lower heart rate during the experimental sessions, indicating a modest cardiovascular advantage under acute stress. Contrary to our hypothesis, this effect did not differ between the two stress paradigms, suggesting that the influence of fitness was not specific to the type of stressor. Aerobic capacity, therefore, seems to reduce cardiovascular load during stressful situations in a general manner.

The present study aimed at determining the test–retest reliability of commonly used measures in a sample of middle-aged and older participants who presented with a wide range of functional abilities. Participants were middle-aged (40–55 years) and older adults (> 60 years) with- and without mobility limitations, tested both before and after a 4-week period (n = 43). Measures included maximal dynamic (1-RM) and isometric strength, surface EMG (sEMG), peak power, ultrasound-derived muscle size and quality, as well as the performance on the 30-s sit-to-stand, habitual and maximal gait speed, timed up-and-go, stair climb, and 6-min walk tests. Reliability was assessed by means of the intraclass correlation coefficient (ICC), the standard error of measurement (SEM), the minimal detectable change (MDC), and the coefficient of variation (CV). Both dynamic and isometric strength measures displayed excellent reliability (ICCs = 0.96–0.99; CV = 2.2%-7%), whereas muscle size and quality (ICCs = 0.88–0.98; CV = 3.3%-8.7%), functional performance (ICCs = 0.78–0.92; CV = 4.2%-6.8%) and peak power (ICCs = 0.76–0.98; CV = 6.6%-12.8%) results showed good to excellent reliability. Peak power at 80–90% 1‑RM and sEMG measures, on the other hand, showed larger absolute error (CV = 14.4%-18.3% and CV = 14.3%-19.8%, respectively), despite good relative reliability (ICCs = 0.85–0.86). Further results include the comparison between the three subsets of participants included. Our main analysis suggests that most of these measures are sufficiently reliable, even when the two tests are performed a month apart from each other. Our study also supports the notion that, provided that the same equipment and procedures are used, the test–retest reliability of the measures is mostly comparable between the subsets of participants investigated.

Publications from recent years suggest that the biological developmental stage of underage athletes has a significant impact on their athletic performance. This scientific study is the first quantitative attempt in Austria to investigate the impact of the relative age effect and biological maturity on the physical performance of young elite soccer players. Birth data from a total of 98 male players in the U13 to U18 age groups of a first-division Austrian soccer club were collected. By measuring height, sitting height and body mass, the individual biological age was calculated using the Mirwald equation. The athletes underwent a standardized battery of tests, which included assessments of speed, strength and endurance. The results indicate a significant effect of the relative age effect on player selection in the U14 and U15 teams (p < .05), which diminishes with increasing player age. Compared to typical Austrian adolescents of similar age, U13, U14 and U15 players demonstrate a higher level of biological maturity (p < .05). Correlational analyses revealed that the maturity offset, reflecting the time before or after the greatest individual growth spurt, was positively related to eccentric hamstring strength (r = 0.82) and vertical jumping ability (r = 0.61) and positively related to sprint performance over 5, 10 and 20 m (0.62 < r < 0.69; all p ≤ .001). Biological maturity and the associated anthropometric adaptations are decisive for athletic performance. The study supports the notion that biologically more mature players achieve better athletic performance than less mature players, especially before the onset of puberty.

Kadlubowski, B, Keiner, M, Wirth, K, and Csapo, R. Effects of traditional strength vs. combined strength and plyometric training on sprint, jump, and maximum strength performance in elite youth soccer players-A 6-month controlled trial . J Strength Cond Res 39(7): e878-e889, 2025-Soccer relies on dynamic movements like sprinting, jumping, and directional changes, with explosive actions crucial for scoring. Despite numerous studies on enhancing speed and jumping ability, the effectiveness of isolated plantar flexor muscle strength training remains unclear. This study compared the effects of combined training interventions-back squats with either calf raises or plyometric training-on sprint, jump, and maximum strength performance in youth soccer players. Over 6 months, 89 male players from 2 elite training centers in Germany were divided into 3 groups: plyometric exercises plus back squats (PLY-BS), calf raises plus back squats (CR-BS), and a control group with conventional soccer-specific training. Outcome measures included back squat and calf raise 1 repetition maxima (1RM), linear sprints (5, 10, 30 m), and countermovement (CMJ) and drop jump (DJ) performance from varying heights. Except for the 5-m sprint and BS 1RM, both intervention groups showed significant improvements compared with the control. Significant "time x group" interactions were found for 10 and 30 m (p < 0.001, 0.181 < eta(2)(p) < 0.226), CMJ (F(2,81) = 7.089, p < 0.005, eta(2)(p) = 0.149), and all DJ heights (p < 0.001, 0.190 < eta(2)(p) < 0.295). Effect sizes for 10, 30 m, and CMJ were similar, but DJ performance showed slightly greater gains in the CR-BS group than in the PLY-BS group. Our findings suggest that combining back squats with calf raises or plyometrics can significantly enhance sprint and jump performance in youth soccer players, with calf raises offering superior improvements in drop jump performance.

Background: This study aimed to assess the effects of age and maturity status on the knee extension torque–velocity (T-V) relationship, muscle power, and physical performance in prepubertal boys. Methods: This cross-sectional study included 53 prepubertal boys (9–13 years old) who underwent a physical and maturity status examination. Knee extension T-V relationship was assessed using an isokinetic device, and maximal isometric torque (T0), maximal unloaded velocity, maximum muscle power, and T-V slope (T-Vslope) were derived. Physical performance was assessed using the countermovement jump (CMJ) height. Linear regression models were used, and the level of significance was set at p ≤ 0.05. Results: All torque and power parameters were significantly correlated with age (r = 0.49–0.58) and maturity status (r = 0.74–0.78) (all p < 0.05). No associations were found for velocity-related parameters (r = 0.20) or T-Vslope relative to T0 (r = 0.18) (both p > 0.05). Interestingly, the associations disappeared when muscle function parameters were normalized to body mass (all p > 0.05), whereas normalization to height squared only weakened the associations, which remained statistically significant (p < 0.05). Finally, CMJ height was most strongly associated with T-V and power parameters normalized to body mass, especially at higher contraction velocities. Conclusions: Torque and muscle power derived from the knee extension T-V relationship increased as a function of age and maturity status in prepubertal boys. These associations vanished upon adjustment for body mass, while normalization to height squared only weakened the correlations. Muscle function produced at faster contraction velocities and normalized to body mass emerged as the best indicator of functional muscle performance in prepubertal children among the studied neuromuscular parameters.

Short Term Body Mass Manipulation (SBM) is frequently used in powerlifting by athletes to qualify for lower weight classes and improve relative competitiveness. The three primary physiological pathways that SBM leverages are gastrointestinal content reduction, glycogen storage and body water manipulation, in addition to post-weigh-in refueling. Despite its high prevalence among athletes competing in the International Powerlifting Federation (IPF), the scientific literature on SBM remains limited, and sport-specific guidelines are currently lacking. This narrative review summarizes the current evidence on SBM in powerlifting, with a focus on physiological mechanisms, practical implementation, and associated risks. The specific demands of IPF competition, consisting of maximal strength performance after only a two-hour window between weigh-in and competition, necessitate uniquely tailored SBM strategies. SBM should not be regarded as a standard preparation method. Instead, it should be seen as a targeted intervention to be applied with caution and strategic intent. The decision to implement SBM must be based on individual assessment, physiological plausibility, and a well-considered cost-benefit rationale. Ensuring effective rehydration and refueling between weigh-in and competition is critical to support both safety and performance. This review provides sport specific, evidence-based recommendations to assist practitioners in applying SBM responsibly within the context of powerlifting.

Soccer is a complex sports discipline that requires players to engage in diverse high-intensity and multidirectional activities. The optimization of strength and conditioning programs requires a comprehensive understanding of the physical attributes influencing player performance. While previous research has demonstrated the influence of knee and hip extensor muscles on the performance in sprints and other explosive movements, this study aimed to establish the relationship between plantar flexor muscle strength and high-intensity actions. Back squat (BS) and calf raise (CR) one-repetition maxima as well as linear sprint (5-, 10-, 30 m) and drop jump performance from different heights (15, 30, 45 and 60 cm) were measured in 45 elite youth players (age: 16.62 ± 1.1 years). Results showed significant negative correlations between BS strength and sprint times (r = −0.60 to −0.61), confirming the importance of lower limb extensor muscle strength in short-distance sprints. While no significant correlations were found with sprint performances, CR strength was significantly associated with drop jump test results from 45 and 60 cm drop height (r = 0.36 to 0.46). These findings demonstrate that isolated CR strength positively influences the performance in actions involving rapid stretch-shortening cycles, which suggests that current strength and conditioning programs for youth soccer players should be extended to also include exercises specifically targeting the plantar flexor muscles. While this cross-sectional study provides novel insights into the complex interplay between muscle strength and soccer-specific performance, its findings need to be corroborated in longitudinal studies directly testing the impact of plantar flexor strength training.

Abstract Background Daily nutrition plays an important role in supporting training adaptions and endurance performance. The objective of this 10-week study was to investigate the consequences of varying carbohydrate consumption and the glycaemic index (GI) together with an endurance training regimen on substrate oxidation, muscle energy storage and endurance performance under free-living conditions. Sixty-five moderately trained healthy men (29 ± 4 years; VO2 peak 55 ± 8 mL min−1 kg−1) were randomized to one of three different nutritional regimes (LOW-GI: 50–60% CHO with ≥ 65% of these CHO with GI < 50 per day, n = 24; HIGH-GI: 50–60% CHO with ≥ 65% CHO with GI > 70 per day, n = 20; LCHF: ≤ 50 g CHO daily, n = 21). Metabolic alterations and performance were assessed at baseline (T0) and after 10 weeks (T10) during a graded exercise treadmill test. Additionally, a 5 km time trial on a 400-m outdoor track was performed and muscle glycogen was measured by magnet resonance spectroscopy. Results Total fat oxidation expressed as area under the curve (AUC) during the graded exercise test increased in LCHF (1.3 ± 2.4 g min−1 × km h−1, p < 0.001), remained unchanged in LOW-GI (p > 0.05) and decreased in HIGH-GI (− 1.7 ± 1.5 g min−1 × km h−1, p < 0.001). After the intervention, LOW-GI (− 0.4 ± 0.5 mmol L−1 × km h−1, p < 0.001) and LCHF (− 0.8 ± 0.7 mmol L−1 × km h−1, p < 0.001) showed significantly lower AUC of blood lactate concentrations. Peak running speed increased in LOW-GI (T0: 4.3 ± 0.4 vs. T10: 4.5 ± 0.3 m s−1, p < 0.001) and HIGH-GI (T0: 4.4 ± 0.5 vs. T10: 4.6 ± 0.4 m s−1), while no improvement was observed in LCHF. Yet, time trial performance improved significantly in all groups. Muscle glycogen content increased for participants in HIGH-GI (T0: 97.3 ± 18.5 vs. T10: 144.5 ± 39.8 mmol L wet-tissue−1, p = 0.027) and remained unchanged in the LOW-GI and the LCHF group. At the last examination, muscle glycogen concentration was significantly higher in LOW-GI compared to LCHF (p = 0.014). Conclusion Changes in fat oxidation were only present in LCHF, however, lower lactate concentrations in LOW-GI resulted in changes indicating an improved substrate metabolism. Compared to a LCHF diet, changes in peak running speed, and muscle glycogen stores were superior in LOW- and HIGH-GI diets. The low GI diet seems to have an influence on substrate metabolism without compromising performance at higher intensities, suggesting that a high-carbohydrate diet with a low GI is a viable alternative to a LCHF or a high GI diet. Trial registration: Clinical Trials, NCT05241730. https://clinicaltrials.gov/study/NCT05241730. Registered 25 January 2021.