VO₂max is a key indicator of an athlete's aerobic capacity and cardiorespiratory fitness. It plays an important role in supporting sports performance, especially in football, which demands high physical endurance throughout the match (Ansori et al., 2024). Athletes with high VO₂max are better able to maintain game intensity, accelerate recovery, and maintain optimal performance throughout the competition. Therefore, developing a systematic, structured, and sustainable training program is an important factor in improving an athlete's aerobic capacity and physical fitness (Simanjuntak et al., 2022). One approach that can be used is through training activities designed in the form of games or structured physical activities as a stimulus to increase fitness (Firdaus & Irfan, 2023), such as variations in training methods that can improve the physical condition component of agility (Prembanyu & Wiyanto, 2025).

However, an athlete's VO₂max capacity is influenced not only by training method but also by environmental factors, especially the climatic conditions in which training takes place (Apriantono et al., 2020). Indonesia, as a tropical region with high temperatures and humidity, places additional physiological stress on athletes. These conditions can affect the efficiency of the cardiovascular and respiratory systems, thereby altering the body's response to exercise (Pelletier et al., 2023). Athletes who train in hot environments tend to experience increased core body temperature and heart rate (Périard et al., 2021) and greater dehydration than athletes in cold climates (Bassett & Howley, 2023). Long-term exposure to high temperatures can also reduce muscle oxygen efficiency and increase cardiac workload, with implications for aerobic capacity (Nybo et al., 2024).

Empirical conditions in the Bima region, West Nusa Tenggara Province, with average daytime and evening temperatures of 29–30°C, indicate that environmental factors may influence athlete performance. Based on initial interviews with football coaches from the Bima Football Association (PERSEBI), 15-year-old athletes tended to experience a decline in performance after the first 45 minutes of play. This indicates limited aerobic capacity, possibly related to athletes' VO₂max achievement under tropical training conditions.

Given these conditions, effective training management is important to support aerobic capacity and minimize environmental impacts on athletes' VO₂max (Crandall & Johnson, 2023). Factors such as blood volume, heart function, thermoregulation efficiency, and body fluid balance are known to influence VO₂max during physical activity (Sawka et al., 2022). In addition, temperature and humidity are related to thermoregulation. In addition, temperature and humidity conditions are also related to thermoregulation aspects (Maunder et al., 2021), energy metabolism (Melin et al., 2024), respiratory system and oxygen utilization (So et al., 2024), and hydration and electrolyte balance (Luetkens et al., 2023), which indirectly affect the athlete's aerobic performance.

In this context, training time is a relevant variable to study, as differences in environmental conditions between morning and evening in tropical regions can affect VO₂max. Lower air temperatures in the morning than in the afternoon improve cardiorespiratory efficiency. Although various studies have examined improvements in VO₂max through training interventions, research comparing VO₂max based on training time under natural conditions in tropical environments remains limited. This study offers a novel ecological perspective by examining VO₂max differences across naturally occurring training times (morning vs. afternoon) without experimental intervention, in a tropical environment where daily climatic variations may influence aerobic performance.

Thus, this study aims to analyze differences in VO₂max, an indicator of aerobic capacity, among 15-year-old adolescent athletes based on training time in a tropical climate. The results are expected to provide practical contributions to coaches and sports practitioners in designing more optimal training patterns tailored to environmental characteristics.

This study aims to analyze differences in VO₂max values among 15-year-old soccer athletes based on training time in a tropical climate, namely morning and afternoon. The results of the descriptive statistical analysis, which provide a general picture of VO₂max values for 15-year-old soccer athletes after the pretest and posttest, by time group, are shown in Table 1.

An overview of VO₂max values based on exercise groups is presented in Table 1 above. In general, the data show a difference in average VO₂max values between the morning and afternoon exercise groups at the final measurement, with the morning group tending to have higher values than the afternoon group. Before conducting the comparison test, the data were first tested for normality using the Shapiro-Wilk test.

The findings of this study indicate that training time is a significant factor in determining differences in athletes' VO₂max values. The superiority of the morning training group indicates that the relatively more stable environmental conditions in the morning result in a lower thermal load, allowing the cardiovascular system to work more efficiently to meet oxygen demands during physical activity. In this context, circadian rhythms and metabolic efficiency in the morning are thought to contribute to achieving more optimal VO₂max, particularly in tropical climates characterized by high temperatures and humidity. Therefore, the differences found not only reflect variations in VO₂max values but also underscore the importance of synchronizing training time and body condition to support the aerobic performance of adolescent athletes.

These findings align with previous studies that highlight the influence of environmental conditions on VO₂max during physical activity in tropical climates. These bioclimatic conditions involve relatively high temperatures and humidity (Ma et al., 2023). Potentially increased heat load during exercise, particularly in the afternoon, due to accumulated solar radiation. This situation can trigger competition for blood flow between the skin for cooling and skeletal muscles for mechanical activity, ultimately affecting the efficiency of oxygen utilization (Hudain, 2024). Conversely, lower ambient temperatures in the morning provide a more favorable thermal gradient, enabling athletes to maintain physical activity more efficiently.

From a physiological perspective, these findings are also supported by the concept that circadian rhythms influence aerobic metabolism (Bahtra et al., 2023). Although some studies suggest that physical performance can peak in the afternoon, in a tropical climate, higher heat exposure could be a limiting factor (Inzaghi et al., 2025). This condition can increase the risk of dehydration and reduce cardiovascular efficiency, especially in adolescent athletes whose thermoregulatory systems have not yet developed optimally (Plakias et al., 2024). Therefore, training in more moderate environmental conditions, such as early morning, is likely to support VO₂max stability without imposing excessive thermal load (Feng et al., 2024).

In addition, research by Hanief et al. (2025) found that approximately 45% of young soccer athletes exhibit hamstring impulse imbalances, which may increase injury risk. These findings reinforce the notion that adolescent athletes are still in a phase of physiological and neuromuscular development that is vulnerable to training stress. Therefore, timing training sessions that consider environmental conditions is crucial not only to support VO₂max achievement but also to minimize the risk of physical impairment. Other research also shows that pulmonary oxygen diffusion efficiency tends to be higher during early training sessions in high-humidity environments (Clemente et al., 2023), which aligns with this study's findings.

In practice, the results of this study imply that determining training timing should be part of the athlete development program planning in tropical regions. Scheduling training in the morning may be a more adaptive strategy for maintaining VO₂max during training activities. Theoretically, these findings enrich sports science research, particularly regarding the relationship between environmental factors and aerobic performance. This is also relevant to the concept of long-term athlete development (LTAD), which emphasizes adapting training loads to individual and environmental conditions (Kuswoyo, 2023).

This study demonstrates that training time is a contextual factor associated with differences in VO₂max values among adolescent soccer athletes in tropical environments, with morning training tending to provide more favorable conditions than afternoon training, thus resulting in higher VO₂max levels. These findings emphasize the importance of integrating training load and environmental conditions into the planning of athlete development programs. In practice, coaches are advised to prioritize morning aerobic training to optimize VO₂max while minimizing the impact of environmental heat stress. Future research should control for external variables such as sleep patterns, nutritional intake, and hydration. It should also integrate additional physiological measurements and longitudinal designs to gain a more comprehensive understanding of the factors influencing VO₂max in adolescent athletes in tropical regions.

The author would like to thank the coaches, assistant coaches, administrators, and 15-year-old athletes of SSB PERSEBI Bima for their support and participation in this research. Appreciation is also expressed to the Head of STKIP Taman Siswa Bima and LPPM for their financial support through the 2025 Internal Research Grant Program, which enabled this research to be successfully implemented. This article is one of the mandatory outputs of the 2025 STKIP Taman Siswa Bima Internal Research Grant.

The datasets generated and analyzed during this study, including VO₂max measurements and statistical analysis results, are available from the corresponding author upon reasonable request. However, the data are not publicly accessible due to ethical and privacy considerations, as they contain sensitive information about individual adolescent participants' performance.

This research was funded by the 2026 Internal Research Grant Program of STKIP Taman Siswa Bima through funding support distributed by the Institute for Research and Community Service (LPPM) of STKIP Taman Siswa Bima. The funding was used to support research activities, data collection, and the preparation of research outputs in the form of scientific articles.

This research was funded by the STKIP Taman Siswa Bima Internal Research Grant Program (2026). The funding body had no role in the study design, data collection, analysis, or interpretation, or in the writing of the manuscript.