Influence of 4-week lower extremity high-intensity interval training on energy metabolism and maximal oxygen uptake of elite swimmers

Introduction

Competitive swimming performance results from the complex interplay of multiple factors, including energy metabolism, technical execution, psychological resilience, and physiological conditioning (Dalamitros et al., 2016; Sousa et al., 2018; Zacca et al., 2020; Zamparo, Cortesi & Gatta, 2020; Sammoud et al., 2021; Song & Sheykhlouvand, 2024). Among various training methods aimed at enhancing these components, high-intensity interval training (HIIT) has gained considerable attention for its efficiency and physiological benefits. HIIT alternates periods of near-maximal exertion (typically 80–100% VO₂max or HRmax) with intervals of rest or low-intensity activity, thereby stimulating both aerobic and anaerobic systems (Sperlich et al., 2010; Gillen & Gibala, 2014; Moniz, Islam & Hazell, 2020). Numerous studies have demonstrated that HIIT is effective in improving swimming performance, particularly through enhancing aerobic capacity, anaerobic power, and energy efficiency (Sperlich et al., 2010; Dalamitros et al., 2016; Clemente-Suárez & Arroyo-Toledo, 2018; Weng & Hou, 2020; Massini et al., 2023). Sprint interval training (SIT) has also gained popularity due to its similar effectiveness and reduced time demands (Eigendorf et al., 2021). For swimmers, HIIT is a preferred training modality not only for aerobic enhancement but also for its ability to complement technical training such as stroke mechanics, starts, and turns.

From a bioenergetic perspective, HIIT imposes demands on all three primary energy systems: the phosphagen system, anaerobic glycolysis, and aerobic metabolism (Gastin, 2001). Training protocols such as 90/30 and 30/30 manipulate the work-to-rest ratio to modulate these energy pathways, influencing both lactate accumulation and oxygen uptake (Gosselin et al., 2012; Almeida et al., 2021). Compared with moderate-intensity training, HIIT more effectively improves repeated sprint ability (RSA), enhances metabolic recovery, and reduces fatigue-inducing byproducts (Edge et al., 2005; Norberto et al., 2021). Such adaptations are especially important for events ranging from 50 m to 400 m, where energy system interplay is critical (Gonjo et al., 2018). Furthermore, tailoring HIIT programs to emphasize aerobic, anaerobic, or mixed metabolic responses offers coaches a powerful tool for targeted performance development (Ponimasov & Bolotin, 2019).

Despite consistent findings supporting HIIT’s physiological benefits, results regarding its influence on VO₂max in elite athletes remain mixed. Some studies suggest that when total training volume is significantly reduced, as in Kilen et al. (2014), HIIT may not elicit significant VO₂max gains, possibly due to the athletes’ already optimized cardiovascular capacity. Others, however, report VO₂max improvements of 3–4% even within short durations (Caputo & Denadai, 2008; Menz et al., 2015; Weng & Hou, 2020). Training mode also plays a role; for example, Twist and Wist (Twist, Bott & Highton, 2023) found that running-based HIIT induced greater cardiovascular load than cycling-based HIIT. Moreover, these improvements appear to be mediated by peripheral adaptations, such as increased stroke volume, rather than changes in hemoglobin mass or blood volume (Menz et al., 2015). The effectiveness of HIIT and SIT depends on factors such as protocol design, training status, and individual variability (Sloth et al., 2013; Del Giudice et al., 2020; Rosenblat, Granata & Thomas, 2022).

However, a significant gap remains in the literature regarding HIIT’s localized effects on swimmers’ lower-limb energy metabolism and neuromuscular adaptation. While upper-body propulsion dominates in swimming, lower-limb contributions to starts, turns, and kicking patterns are critical for competitive success. Most existing studies have focused on systemic responses, overlooking the specific adaptive changes in lower-limb musculature. Thus, this study aims to investigate the effects of a 4-week HIIT intervention targeting the lower limbs of elite swimmers. Specifically, we examine how variations in training intensity and interval structure affect the activation of phosphagen, glycolytic, and aerobic energy systems, as well as VO₂max. We hypothesize that a 4-week lower-limb HIIT intervention will significantly enhance the energy metabolism—specifically phosphagen, glycolytic, and aerobic energy systems—as well as maximal oxygen uptake (VO₂max) in elite swimmers, compared to a control group maintaining regular training.

Materials and Methods

Experimental approach to the problem

The testing and intervention at the Physical Training Research Center of Shanghai University of Sport. All participants completed the lower-limb strength cycling tests within the same week, with at least 24 h between the two tests (Funai et al., 2025). The ideal follow-up testing time for each participant was the same as the initial test time. Before all tests were conducted, height and weight measurements were taken to assess the athletes’ height, weight, and BMI, ensuring the accuracy of the VO₂max values. This study was approved by the Ethics Committee of Shanghai University of Sport (102772021RT031), and all participants signed informed consent forms. The research roadmap is as follows Fig. 1.

HIIT energy metabolism test

Test instrument

The following equipment were used in this study: a Lode lower-limb cycle ergometer (Lode Excalibur Sport; Lode BV, Groningen, Netherlands; Fig. 2) for aerobic/anaerobic power testing, a K5 portable metabolic system (Cosmed K5, Cosmed, Rome, Italy) for gas exchange analysis, a Polar Accurex Plus heart rate monitor (Polar Electro Oy, Kempele, Finland), and an EKF BIOSEN S-line lactate analyzer (EKF Diagnostic, Barleben, Germany). Additional materials included a Borg RPE scale, sterile blood collection needles, alcohol swabs, EKF sample tubes, medical gloves, a stopwatch, and markers.

Test mode

Before the lower-limb power cycle test was conducted, the seat and handlebar positions of the power cycle were adjusted on the basis of the participant’s subjective comfort, ensuring that the knee joint remained slightly bent when the leg was fully extended. The training load on the Lode lower-limb power cycle was set at 4.5% of the participant’s body weight. For example: T = body weight × 4.5% × 9.8 × torque = @N.M. This study modeled the HIIT test after the Tabata protocol, with continuous testing and verbal encouragement provided throughout the process to maximize the athlete’s enthusiasm.

Procedure

The K5 device was preheated for 30 min and calibrated in accordance with the manufacturer’s requirements, including pressure, gas composition (standard gas concentrations: 15.00% O₂, 5.00% CO₂, and 80.0% N₂), and gas volume (3 L gas tank). Each participant selected an appropriate mask size prior to the first test, and the same type of mask was used for all four tests. Gas collection, storage, and analysis were conducted using standard software (Mate Soft Cosmed Italy). The treadmill was calibrated prior to the test. Upon arrival at the testing site, the participants rested for 5 min while their resting blood lactate levels were collected (20 µL from the earlobe) and the experiment procedures, precautions, and RPE scale were explained. A warmup of 5 min at 10 km/h was performed on the treadmill, followed by 5 min rest, during which the calibrated gas analyzer was fitted. Before the test, the participants were instructed on how to use the RPE scale, which includes 15 levels representing fatigue from mild to severe. The RPE scores were recorded before and after the test. The test began with participants performing 20 s of all-out pedaling, followed by 10 s rest. Each participant completed six sets within 170 s. Blood lactate samples (20 µL from the earlobe) were collected and analyzed using a lactate analyzer at 3, 5, 7, and 10 min before and after the test. After the test, the K5 device was removed 10 min post-exercise, during which the participants sat on a bench for passive recovery and were reminded to minimize speaking.

Data operation

The aerobic, glycolytic, and phosphagen energy supplies during all-out exercise were calculated using accumulated oxygen uptake, accumulated blood lactate, and the rapid phase of oxygen debt post-exercise. Phosphagen energy supply was determined as the rapid phase of oxygen debt (mL) multiplied by the energy equivalent (J/mL). Glycolytic energy supply was calculated using accumulated blood lactate (mmol), the oxygen-lactate conversion factor (3.0 mL/mmol/kg), body weight (kg), and the energy equivalent (J/mL). Aerobic energy supply was derived from accumulated oxygen uptake (mL) multiplied by the energy equivalent (J/mL). The rapid phase of oxygen debt was computed as the actual oxygen uptake during the first 3 min post-exercise minus the inverted oxygen uptake curve (slow phase) during the subsequent 3 min: Rapid phase of oxygen debt = actual oxygen uptake (first 3 min)—slow phase (first 3 min). When the respiratory exchange ratio (RER) exceeded 1.0, the energy equivalent was set at 21.131 J/mL of oxygen consumed (Stegmann, 1977). In this study, resting oxygen uptake values were standardized as 3.5 mL/min/kg for females and 4.0 mL/min/kg for males.

Results

Discussion

This study systematically analyzed the responses of elite swimmers to 4 weeks of lower-limb HIIT intervention, focusing on changes across different energy metabolism pathways and VO₂max. The results showed that lower-limb HIIT significantly enhanced phosphagen, glycolytic, and aerobic energy supplies while improving VO₂max. Compared with the control group, the experimental group demonstrated significant advantages in energy metabolism and aerobic adaptability, confirming the effectiveness of lower-limb HIIT in promoting metabolic adaptation and aerobic capacity. VO₂max, a core indicator of aerobic metabolic capacity, not only enhances the efficiency of aerobic energy supply but also aids in lactate clearance, accelerates phosphocreatine recovery, and delays fatigue. For endurance and speed-strength athletes, a high VO₂max contributes to optimizing the balance between energy systems, thereby improving overall athletic performance. The findings underscore the importance of simultaneously developing aerobic and anaerobic metabolic systems in training programs (Archacki et al., 2024). In terms of the energy metabolism system, this study found that after the HIIT intervention, the experimental group showed significant improvements in energy supply. Following the intervention, all key components of energy metabolism showed significant improvements. The phosphagen and glycolytic systems exhibited marked increases, reflecting enhanced anaerobic energy contribution during high-intensity efforts. Aerobic energy supply also improved notably, indicating a greater capacity for sustained oxidative metabolism. Overall, total energy supply demonstrated a substantial increase, highlighting the comprehensive enhancement of metabolic output induced by the HIIT protocol. These results indicate that HIIT effectively activates the phosphagen system to meet the energy demands of short-duration, high-intensity exercise while enhancing the adaptability of glycolytic and aerobic metabolism, thereby improving overall energy supply efficiency (Sloth et al., 2013). Consistent with the roles of different energy systems mentioned in the introduction, the phosphagen system plays a critical role in short-duration explosive actions such as starts, turns, and sprints. By contrast, the glycolytic and aerobic systems provide essential energy support during prolonged, sustained efforts in swimming (García-Ramos et al., 2016). HIIT, through the strategic alternation of high- and low-intensity phases, activates multiple energy systems and achieves efficient energy utilization. This mechanism is one of the key reasons for its significant effects observed in this study.

The lower-limb HIIT intervention led to significant enhancements across all energy systems. Specifically, the phosphagen, glycolytic, and aerobic pathways each exhibited notable increases in energy supply, demonstrating improved anaerobic and aerobic metabolic function. The overall rise in total energy supply was particularly prominent, aligning with previous research by Eigendorf et al. (2021) and further supporting the efficacy of HIIT in enhancing metabolic performance. This finding indicates that lower-limb HIIT effectively activated different energy supply pathways and enhanced the metabolic efficiency of the athletes. In the experimental group, VO₂max showed a significant improvement following the HIIT intervention, reflecting enhanced aerobic capacity. In contrast, the control group did not exhibit any notable changes, highlighting the specific effectiveness of the training program.These findings demonstrate that HIIT has a significant effect on improving maximal aerobic capacity (Song & Sheykhlouvand, 2024). Significant interaction effects between time and group were observed across all energy supply pathways, including phosphagen, glycolytic, aerobic, and total energy metabolism. The effect was especially pronounced in the total energy supply, indicating that the training intervention had a substantial and differentiated impact on the experimental group compared to the control group. This finding indicates that the training effects became fully evident over time in the experimental group, confirming the effectiveness of lower-limb HIIT in optimizing energy metabolism and enhancing aerobic capacity (Sousa et al., 2018). The present study provides empirical support for the scientific training of swimmers (Nugent et al., 2019).

However, the results also revealed the interactions between different energy systems. While HIIT significantly increased the absolute energy supply across various metabolic pathways, the relative contribution percentages of each system showed no significant changes. This finding suggests that the primary effect of HIIT lies in enhancing the total energy supply rather than altering the relative contributions of individual energy systems (Gastin, 2001). This result is closely related to the contributions of different energy systems mentioned in the introduction: during short-duration, high-intensity exercise, the phosphagen system plays a dominant role, and the aerobic system contributes more significantly during prolonged, sustained activities (Pyne & Sharp, 2014). HIIT effectively harnesses the potential of different energy systems by repeatedly applying high-intensity stimuli within a short duration, combined with brief recovery periods, achieving efficient integration of energy metabolism (Edge et al., 2005). The design of this study highlights the application value of HIIT in lower-limb-specific training. In swimming, lower-limb power output and technical movements play a critical role in overall performance (Zhou et al., 2024). As mentioned in the introduction, lower-limb strength and metabolic adaptation are essential for supporting the start, turn, and acceleration phases in swimming (Zamparo, Cortesi & Gatta, 2020). Therefore, this study effectively enhanced energy supply and VO₂max through targeted lower-limb HIIT training, providing robust support for lower-limb performance in competitive swimming scenarios.

The present findings support and expand upon prior research demonstrating that high-intensity interval training (HIIT) enhances both aerobic and anaerobic energy systems in competitive athletes. For example, Sperlich et al. (2010) and Dalamitros et al. (2016) reported that HIIT led to significant improvements in VO₂max and metabolic capacity in swimmers, consistent with our findings. Notably, our results also revealed significant increases in glycolytic and phosphagen energy contributions, in line with findings from Gosselin et al. (2012) and Almeida et al. (2021), suggesting that short-term HIIT can elicit robust adaptations in fast-twitch muscle fibers and anaerobic metabolism.

The physiological mechanisms behind these adaptations may involve increased mitochondrial density, enhanced oxidative enzyme activity, and improved muscle oxygen utilization. HIIT has also been shown to stimulate rapid glycogen resynthesis and lactate clearance capacity, which may explain the improved glycolytic and phosphagen system performance observed in our study (Edge et al., 2005). In addition, neuromuscular efficiency, particularly in the lower limbs, may have contributed to the observed increase in energy system efficiency and VO₂max.

This study has several limitations. First, the sample size was relatively small and limited to elite swimmers from a single institution, which may affect the generalizability of the findings. Second, although we observed significant improvements in VO₂max and energy metabolism, we did not measure neuromuscular activity or lactate clearance directly, which could provide additional insights into the underlying mechanisms. Third, the menstrual cycle phases of female participants were not controlled, which may have influenced physiological responses. Future research should include larger and more diverse populations, incorporate neuromuscular and hormonal markers, and stratify data by sex and training status to better understand the full range of HIIT-induced adaptations.

In summary, this study demonstrated the significant effects of 4 weeks of lower-limb HIIT intervention in improving energy metabolism and VO₂max. HIIT not only significantly enhanced the energy supply efficiency of the phosphagen, glycolytic, and aerobic metabolic systems but also showed remarkable advantages in increasing VO₂max. These findings provide empirical support for the scientific and systematic training of competitive swimmers, further emphasizing the application value of HIIT in lower-limb-specific training. Coaches and researchers should fully leverage the benefits of HIIT, tailoring training programs to individual differences to maximize athletic performance.

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