Abstract
troduction and Objectives: Aerobic capacity and muscular strength are two important physical components that influence the performance of middle-distance runners. High- intensity interval Training (HIIT) and unification training have been recognized as effective methods for improving these parameters. However, scientific evidence comprehensively examining the effects of these two methods on middle-distance runners is limited. Therefore, this study aimed to systematically evaluate the impact of HIIT and unification training on the aerobic capacity and muscle strength of middle-distance runners. Methodology: This study was a systematic review that followed the PRISMA guidelines. Data were obtained from PubMed, Scopus, and Google Scholar databases with inclusion criteria including experimental studies published in the last 10 years. A total of 20 articles were selected and analyzed qualitatively. The main parameters evaluated were changes in aerobic capacity (VO₂max) and muscle strength (dynamic and isometric) in middle-distance runners. Results: This review found that HIIT significantly improved aerobic capacity through cardiovascular adaptation and oxygen efficiency. Unification training, which combines aerobic and strength components, showed significant improvements in muscle strength, especially in the lower limb muscles. Several studies have shown that combining HIIT and unification training synergistically affects both parameters. Conclusion: HIIT and unification training improves aerobic capacity and muscular strength in middle-distance runners. The combination of both methods provided more optimal results. The practical implication is that coaches and athletes can adopt an integrated training approach to optimize performance. Further research is needed to explore the most effective exercise duration and intensity. Keywords High intensity interval training, unification training, aerobic capacity, muscular strength, middle distance runners. Resumen Introducción y objetivos: La capacidad aeróbica y la fuerza muscular son esenciales para el rendimiento
The practical implication is that coaches and athletes can adopt an integrated training approach to optimize performance. Further research is needed to explore the most effective exercise duration and intensity. Keywords High intensity interval training, unification training, aerobic capacity, muscular strength, middle distance runners. Resumen Introducción y objetivos: La capacidad aeróbica y la fuerza muscular son esenciales para el rendimiento de los corredores de medio fondo. El entrenamiento en intervalos de alta intensidad (HIIT) y el entrenamiento de unificación se reconocen como métodos eficaces para optimizar estos parámetros. Este estudio evaluó sistemáticamente el impacto de ambos métodos en la capacidad aeróbica y la fuerza muscular. Metodología: Se realizó una revisión sistemática siguiendo las directrices PRISMA. Se consultaron bases de datos como PubMed, Scopus y Google Scholar, seleccionándose 20 estudios experimentales de los últimos 10 años. Los parámetros evaluados incluyeron VO₂max y la fuerza muscular (dinámica e isométrica). Resultados: Los resultados muestran que el HIIT mejora la capacidad aeróbica mediante la adaptación cardiovascular, mientras que el entrenamiento de unificación mejora la fuerza muscular, especialmente en las extremidades inferiores. La combinación de ambos métodos genera efectos sinérgicos en ambos parámetros. Conclusiones: En conclusión, se recomienda la aplicación de un enfoque de entrenamiento integrado para optimizar el rendimiento de los corredores de medio fondo. Futuros estudios deben explorar la duración y la frecuencia óptima de estos métodos. Palabras clave Entrenamiento interválico de alta intensidad, entrenamiento de unificación, capacidad aeróbica, fuerza muscular, corredores de media distancia. Effect of High-Intensity Interval Training and unification training on Aerobic Capacity and Muscle Strength in Intermediate Distance Runners- a systematic review Efecto del entrenamiento en intervalos de alta intensidad y del entrenamiento de unificación sobre la capacidad aeróbica y la fuerza muscular en corredores de distancias intermedias - una revisión sistemática
2025 (febrero), Retos, 63, 698-712 ISSN: 1579-1726, eISSN: 1988-2041 https://recyt.fecyt.es/index.php/retos/index 699 Introduction Aerobic capacity and muscle strength are essential in athletic performance, especially for middle- distance runners who need endurance and strength to maintain speed at various intensities throughout the race. Good aerobic capacity allows the body to transport and utilize oxygen efficiently during long- lasting workouts, while muscle strength plays a role in stability, speed, and injury prevention. Research shows that increased aerobic capacity and muscle strength can improve athlete performance and extend effective training duration, so these two components are often the focus of athletic training programs (Markvardsen et al., 2018). However, one of the challenges in increasing aerobic capacity and muscle strength is choosing an effective exercise method. High-intensity interval Training (HIIT) and unification training emerged as two famous methods in this case. HIIT combines short periods of high-intensity activity with periods of rest or low activity, which has been shown to increase aerobic capacity and cardiovascular endurance in a more time-efficient manner than traditional aerobic exercise (Stöggl & Björklund, 2017). Meanwhile, unification training integrates various physical elements such as strength, flexibility, and endurance in a single program, allowing athletes to develop multiple abilities simultaneously without sacrificing certain aspects (Pol et al., 2020). Another challenge coaches face is combining these two methods in one effective training program for middle-distance runners. This review aims to analyze and evaluate the effects of HIIT and unification exercises on improving aerobic capacity and muscle strength in middle-distance runners. By conducting a systematic review of the existing literature, this article seeks to provide insight into the effectiveness of both training methods and identify the most appropriate training strategies to achieve performance improvement in middle-distance athletes. This systematic research focuses on studies published in the last 10 years to present the latest and relevant data to improve athletes' performance. This research is expected to make an important contribution to coaches and athletes, especially in choosing the most effective exercise methods to increase aerobic capacity and muscle strength. By understanding the benefits of HIIT and unification training, trainers can design training programs that are
in the last 10 years to present the latest and relevant data to improve athletes' performance. This research is expected to make an important contribution to coaches and athletes, especially in choosing the most effective exercise methods to increase aerobic capacity and muscle strength. By understanding the benefits of HIIT and unification training, trainers can design training programs that are time-efficient and deliver optimal results without increasing the risk of injury from overtraining. This potential for performance enhancement can motivate coaches and athletes. In addition, this research can also contribute to the development of more integrated training strategies for middle- distance athletes who need a combination of aerobic endurance and strength. HIIT training has been widely recognized as an effective method of improving athletic performance, mainly because the high intensity provided in a short period triggers optimal cardiovascular adaptation. HIIT has been shown to increase VO₂ max, a key indicator of aerobic capacity, and optimize energy oxidation efficiency, thereby extending athletes' time at high intensity. Research shows that HIIT also affects the ability of muscles to adapt, which ultimately supports improved strength and speed performance in middle-distance running (Caparrós-Manosalva et al., 2023). On the other hand, unification exercises have gained popularity in the last decade for their ability to integrate multiple physical components into a single set of programs. This approach is believed to maximize results more efficiently, especially when used to train athletes who need a balance between strength and endurance. Several studies have found that unification exercises have the potential to significantly reduce fatigue, a reassuring factor for athletes, and increase athlete motivation, due to the variety of exercises provided. Additionally, unification training allows athletes to perform strength training tailored to cardiovascular needs, which makes this method particularly suitable for middle- distance runners. Previous studies have shown a positive effect of HIIT on increasing aerobic capacity, particularly in middle and long-distance runners. HIIT has been shown to significantly improve aerobic capacity parameters in a shorter period than continuous aerobic exercise (de Araujo, Papoti, Dos Reis, de Mello, & Gobatto, 2016). In addition, the combination of HIIT and unification
suitable for middle- distance runners. Previous studies have shown a positive effect of HIIT on increasing aerobic capacity, particularly in middle and long-distance runners. HIIT has been shown to significantly improve aerobic capacity parameters in a shorter period than continuous aerobic exercise (de Araujo, Papoti, Dos Reis, de Mello, & Gobatto, 2016). In addition, the combination of HIIT and unification exercises has been tested in several studies, showing that these two methods have a synergistic effect on improving endurance and muscle strength. However, few studies have directly compared the effects of HIIT and unification training on middle-distance running performance, which is the main reason for this systematic review.
2025 (febrero), Retos, 63, 698-712 ISSN: 1579-1726, eISSN: 1988-2041 https://recyt.fecyt.es/index.php/retos/index 700 Given the significant role of these two training methods in boosting aerobic capacity and muscle strength, this systematic review significantly contributes to the limited literature on the effects of HIIT and unification training on middle-distance athletes. The potential of this research to guide coaches in devising more effective and balanced training programs for middle-distance runners is a reason for optimism and encouragement in the field of athletic training. Method Study Design and Research Approach This study used a systematic review to collect, analyze, and synthesize the results of various studies on the effect of high-intensity interval training (HIIT) and unification training on aerobic capacity and muscle strength in middle-distance runners. This systematic review was selected to provide a comprehensive and reliable evaluation by minimizing bias in data collection. The design of this systematic review focused on experimental and quasi-experimental studies, such as randomized controlled trials (RCTs), which directly tested the effects of HIIT and unification exercises on aerobic capacity and muscle strength variables. The study will adhere to the principles of PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) to ensure clarity and transparency in the selection process and analysis of the study. The study included studies that met specific inclusion criteria: middle-distance runners as the population, HIIT or unification training as the type of intervention, and aerobic capacity and muscle strength as the variables measured. With this systematic approach, the results obtained can provide solid insights to design optimal training programs for middle-distance runners so that this research can make a valuable contribution to sports science and athletic performance. Inclusion and Exclusion Criteria Table 1 below provides the Inclusion and Exclusion criteria that will be used as a reference in the preparation of this article. Table 1. Inclusion and Exclusion Criteria Criterion Inclusion Exclusion Population Middle-distance runners or athletes with similar characteristics. Healthy participants aged 18-35 years. Non-runners or runners with different characteristics. Participants with certain health conditions. Types of Intervention Using HIIT or unification training as the primary intervention. Unification exercises that focus on a combination
a reference in the preparation of this article. Table 1. Inclusion and Exclusion Criteria Criterion Inclusion Exclusion Population Middle-distance runners or athletes with similar characteristics. Healthy participants aged 18-35 years. Non-runners or runners with different characteristics. Participants with certain health conditions. Types of Intervention Using HIIT or unification training as the primary intervention. Unification exercises that focus on a combination of aerobic capacity and muscle strength. Studies that did not use HIIT or unification exercises as the primary method. Studies that measure only one aspect (aerobic capacity or strength only). Variables Measured Studies that measure aerobic capacity (e.g., VO₂ max) and muscle strength (e.g., limb or lower body strength). Studies that only focus on psychological or cognitive variables. The study did not have direct measurements of aerobic capacity or muscle strength. Type of Study Experimental or quasi-experimental studies, such as randomized controlled trials (RCTs). Study with measurements before and after the intervention. Observational, descriptive, or case studies without direct intervention. Studies without measurements before and after or without a control group. Year of Publication Studies published in the last 10 years. The study was published more than 10 years ago. Language The study was published in English. The study was published in a language other than English. Data Sources and Search Strategies The study uses reliable databases, such as PubMed, Scopus, Web of Science, and Google Scholar, to ensure access to relevant and quality studies. This database was chosen because it includes leading scientific journals in the field of sport and health, which has the potential to contain research results related to high-intensity interval training (HIIT), unification training, aerobic capacity, and muscle strength.
2025 (febrero), Retos, 63, 698-712 ISSN: 1579-1726, eISSN: 1988-2041 https://recyt.fecyt.es/index.php/retos/index 701 Search strategies use keywords that have been systematically organized to find specific studies. Critical keywords include "HIIT," "unification exercise," "aerobic capacity," "muscle strength," and "middle- distance runner." These keywords are combined with Boolean operators (AND, OR) to filter search results and optimize relevant findings. For example, a combination of the keywords "HIIT AND aerobic capacity" or "unification exercise AND muscle strength AND middle-distance runners" was used to search results more relevant to the topic of the study. In addition, the study screened articles by title and abstract during the search process to identify studies that met the inclusion criteria. The corresponding articles are then examined in depth through full-text analysis. If necessary, additional searches are performed manually. This involves searching through the reference lists of identified articles and contacting authors for unpublished data. These manual searches are conducted to ensure that all relevant data is included in the study, even if it may not be identified through database searches. This strategy ensures that the data obtained is representative and reliable. Study Selection Procedure The study selection procedure begins with identifying relevant articles from databases such as PubMed, Scopus, and Google Scholar using structured keywords. The articles found then underwent an initial screening process, where the title and abstract of each article were reviewed to exclude studies that did not fit the topic or inclusion criteria, such as studies that did not focus on middle-distance runners or that did not examine HIIT and unification training. Following the screening process, the remaining studies are subjected to a comprehensive full-text evaluation. This meticulous step involves reading the full text of each article to confirm that the study meets all inclusion criteria, such as the type of intervention, the population studied, and the variables measured (aerobic capacity and muscle strength). Studies that do not meet these criteria are rigorously excluded from the analysis. In the data extraction stage, essential data from each study that met the inclusion criteria were systematically collected, including details such as the number of participants, duration of the intervention,
as the type of intervention, the population studied, and the variables measured (aerobic capacity and muscle strength). Studies that do not meet these criteria are rigorously excluded from the analysis. In the data extraction stage, essential data from each study that met the inclusion criteria were systematically collected, including details such as the number of participants, duration of the intervention, measurement method, and the primary outcomes obtained. This information is used to conduct further analysis, either descriptively or, if possible, through meta-analysis. This selection procedure ensures that only relevant, high-quality studies are included in this systematic review. Figure 1. Study Selection Procedure
2025 (febrero), Retos, 63, 698-712 ISSN: 1579-1726, eISSN: 1988-2041 https://recyt.fecyt.es/index.php/retos/index 702 Figure 2. Prisma Results Aerobic Capacity and Muscle Strength in Middle-Distance Runners Aerobic capacity and muscle strength are two essential components that significantly affect the performance of middle-distance runners. AerobicAerobic capacity is defined as the body's ability to efficiently take, transport, and use oxygen during prolonged physical activity. This capacity is often measured through VO2 max, which indicates how much oxygen the body can utilize per minute when the intensity of the workout reaches its maximum. In middle-distance running, the body's ability to use oxygen efficiently allows runners to maintain speed during the race by reducing muscle fatigue. Research shows that high VO2 max strongly correlates with mid-range performance, as the body can make better use of oxygen, which means more energy can be produced through aerobic metabolic pathways (Mccormack, Shoepe, Almstedt, Jennings, & Capel, 2018). In addition, good aerobic capacity also supports the efficiency of the heart and lungs in delivering oxygen to all body tissues, including active muscles, during running. Aerobic exercise done consistently can increase heart stroke volume and the mitochondrial capacity in muscle cells. This not only improves endurance but also reduces the risk of premature fatigue in the muscles that are active during medium- distance running. According to a study by (Lee, Snyder, & Lundstrom, 2020) Runners with a higher aerobic capacity can run at a higher intensity without significantly increasing lactate levels in their blood, which means they can run faster without experiencing excessive fatigue. On the other hand, muscle strength also has a crucial role in supporting the performance of middle- distance runners. Muscle strength is the ability of muscles to produce power or force during physical activity, which is essential in maintaining the speed and efficiency of running steps. Muscle strength, especially in the leg muscles and core of the body, helps runners minimize contact time with the ground, which is the time your foot spends on the ground with each step, and increases the length and frequency of stride. Good muscle strength allows runners to maintain a stable posture, which
maintaining the speed and efficiency of running steps. Muscle strength, especially in the leg muscles and core of the body, helps runners minimize contact time with the ground, which is the time your foot spends on the ground with each step, and increases the length and frequency of stride. Good muscle strength allows runners to maintain a stable posture, which is essential for energy efficiency during running. In a study (Martín-Fuentes, Oliva-Lozano, & Muyor, 2020) found that strength training specifically for the lower leg muscles can improve running economics and help runners maintain higher speeds over intermediate distances. Moreover, muscle strength is a crucial factor in injury prevention, particularly for runners who undergo rigorous training and frequent competitions. Middle-distance runners, in particular, are prone to
2025 (febrero), Retos, 63, 698-712 ISSN: 1579-1726, eISSN: 1988-2041 https://recyt.fecyt.es/index.php/retos/index 703 repetitive stress on their joints and muscles, leading to injuries like tendinitis and stress fractures. Strengthening the main muscles can enhance joint stability and reduce the risk of such activity-related injuries. (Chevidikunnan, Al Saif, Gaowgzeh, & Mamdouh, 2016) Revealed that core and leg muscle strength can significantly reduce stress on joints and muscles during high-intensity activities, helping runners maintain performance with a lower risk of injury. Overall, middle-distance runners need a combination of good aerobic capacity and optimal muscle strength to achieve their best performance. High aerobic capacity supports long-term endurance, while adequate muscle strength allows runners to maximize thrust and energy efficiency. These two components work together to create effective and efficient running movements, which are necessary to maintain speed on the track. Various studies have shown that exercise programs combining aerobic exercise and strength training can effectively improve middle-distance running performance with significant endurance and muscle strength results. High-Intensity Interval Training (HIIT) High-Intensity Interval Training (HIIT) is a training method that combines periods of high-intensity training with short or low-intensity recovery periods. HIIT has become popular in the last decade due to its effectiveness in improving aerobic capacity and cardiovascular adaptation in a relatively short period. These exercises typically include intervals of intensity that are close to or reach 80-95% of maximum heart rate, interspersed with periods of active recovery of lower intensity, which are repeated in several cycles. HIIT is designed to maximize energy expenditure and physiological adaptation through increased intensity that pushes the body out of its comfort zone, which differs from continuous exercise with moderate intensity. The main characteristics of HIIT are high intensity and short recovery time. In a single HIIT session, the body experiences a significant increase in metabolic activity, which encourages more calorie burning, both during the workout and after the workout ends. This condition is called the "afterburn effect", or excess post-exercise oxygen consumption (EPOC), where the body needs more oxygen to restore muscles and balance energy levels, increasing fat burning after the workout. The EPOC resulting from HIIT can
Description
The study systematically reviews the effects of HIIT and unification training on athletic performance.