1.    Introduction
Resistance training—whether performed with external weights or bodyweight exercises—induces muscle tension and promotes increases in contractile proteins, muscle cross-sectional area, anaerobic capacity, speed, strength, muscular endurance, and motor performance, all of which are essential for enhancing athletic performance (1). The effectiveness of such training depends on variables such as intensity, volume, duration, frequency, and the athlete’s individual capacity (1). Efforts have been made to objectively quantify the balance between training load and the athlete’s tolerance. Athletes often require rapid performance optimization, especially following periods of reduced activity. High-Intensity Interval Training (HIIT), characterized by short bursts of vigorous exercise followed by rest or low-intensity periods, has emerged as an effective strategy under such circumstances (2). HIIT has been shown to enhance athletic performance and improve skeletal muscle metabolism (3). However, when athletes exceed their physiological thresholds, fatigue can ensue, leading to declines in metabolic and neuromuscular performance, diminished muscle contractions, and compromised athletic outcomes. As a result, strategies to delay fatigue—such as the use of anti-fatigue supplements—have become increasingly popular among athletes.
One such supplement is Coenzyme Q10 (CoQ10), a fat-soluble, vitamin-like compound present in all cells (6). CoQ10 plays a vital role in mitochondrial energy production and provides protection against oxidative stress. Previous studies have indicated that CoQ10 supplementation (typically 60–100 mg daily for 4 to 8 weeks) may improve aerobic capacity, anaerobic threshold, muscle strength, and recovery (9). The present study aimed to examine the effects of six weeks of HIIT, with and without CoQ10 supplementation, on muscular strength in male bodybuilders. Muscle strength—particularly in the bench press and squat—was used as a primary indicator of performance. The aim of the present study was to investigate the effect of six weeks of high-intensity interval training with and without Coenzyme Q10 supplementation on bench press and squat strength in competitive male bodybuilders.
2.    Methods
This study employed a semi-experimental design. The statistical population included 200 male bodybuilders from Hamadan. From this population, 30 individuals (mean age: 23.3 ± 0.4 years; height: 172 ± 6 cm; weight: 76 ± 2.6 kg) who had not used coenzyme Q10 supplements or participated in high-intensity interval training (HIIT) during the six months preceding the study were randomly and voluntarily selected. These participants were then assigned to two equal groups of 15 individuals each. Ethical considerations were addressed by administering a health questionnaire and obtaining written informed consent. Participant selection was based on their complete responses to the questionnaire and a signed consent form indicating their full understanding of the research protocol as explained by the investigator.
The study followed a pre-test/post-test design involving two experimental conditions. The first group performed HIIT combined with CoQ10 supplementation, while the second group engaged in the same HIIT protocol but received a placebo instead of the supplement. Inclusion criteria for participation included having no history of chronic illnesses, maintaining a regular exercise routine of at least two sessions per week, and abstaining from the use of coenzyme Q10 or similar supplements during the two months prior to the study. All participants reviewed the research procedures and signed informed consent forms before the intervention began. Muscular strength was assessed using the one-repetition maximum (1RM) test for two fundamental movements: the barbell bench press to evaluate upper body strength, and the barbell back squat to assess lower body strength and endurance. Measurements were conducted using Impulse-brand resistance training machines (manufactured in China). Initially, the heaviest weight a participant could lift once was determined. If the participant could lift the weight for more than one repetition, the following formula was used to estimate their 1RM:
1RM=Lifted Weight (kg)/ (Number of Repetitions × 0.0278) - 1.0278
Each participant selected a weight they believed to be close to their maximum capacity for one lift. If they were able to perform multiple repetitions, the lifted weight and number of repetitions were entered into the formula above to calculate the estimated 1RM. The bench press was performed by lowering the bar to the chest and pressing it back up with both arms simultaneously. The squat involved placing the barbell on the upper back (between the shoulder blades), squatting down fully with the back straight, and then rising to a full standing position. Strength values were recorded in kilograms. The HIIT intervention lasted six weeks and included three sessions per week. Each session comprised short bouts of high-intensity running alternated with low-intensity active recovery. Specifically, the protocol involved 20-meter sprints with 20 to 30 seconds of rest between repetitions. To ensure progressive overload and training effectiveness, participants performed four sprints per session during weeks 1 and 2, five sprints during weeks 3 and 4, and six sprints during weeks 5 and 6. Each training session also included a 5-minute warm-up and a 5-minute cool-down period. For supplementation, participants in the experimental group received 42 pink gelatin capsules, each containing 30 mg of coenzyme Q10, manufactured by Natural Organics (USA). They were instructed to take one capsule daily after a meal. To ensure adherence, participants received daily reminders via text message. The control group received placebo capsules identical in appearance and volume, containing dextrose.
Statistical analysis was conducted using univariate analysis of covariance (ANCOVA) to test the research hypothesis. The independent variables were the six-week HIIT program and CoQ10 supplementation. The dependent variable was post-test muscular strength, with pre-test scores included as a covariate. Prior to analysis, the assumptions of ANCOVA—including homogeneity of regression slopes, linearity, and equality of variances—were tested and confirmed. Strength data for both the bench press and squat were used in the analysis.
3.    Results
The results of the ANCOVA analysis evaluating the effect of six weeks of high-intensity interval training (HIIT) and Coenzyme Q10 (CoQ10) supplementation on muscular strength are presented in Tables 1 and 2.

Table 1 presents the results for bench press strength. The findings show a statistically significant difference between the groups after controlling for pre-test scores (F(1, 27) = 33.58, p < 0.001, η² = 0.554). The observed p-value for muscular strength was less than 0.05, indicating a significant relationship between the group variable and the dependent variable. The effect size (η² = 0.554) suggests that approximately 55% of the variance in post-test bench press strength can be attributed to the HIIT and CoQ10 intervention.
Table 2 presents the results for back squat strength. Similar to the bench press findings, there was a statistically significant difference between the groups (F(1, 27) = 107.222, p < 0.001, η² = 0.799). The effect size indicates that approximately 80% of the variance in squat strength was explained by the intervention.
4.    Conclusion
The present study investigated the effects of six weeks of high-intensity interval training (HIIT), with and without Coenzyme Q10 (CoQ10) supplementation, on muscular strength in male bodybuilders. The results demonstrated significant improvements in upper and lower body strength, with approximately 55% of the variance in bench press performance and 80% in squat performance attributed to the intervention. These findings align with previous research indicating the beneficial role of CoQ10 in muscle function and recovery (15). Multiple studies have reported similar outcomes. For example, Little and Phillips found that CoQ10 supplementation following intense resistance training helped regulate immune markers in professional wrestlers by mitigating muscle damage (16). Similarly, Mehri et al. showed that short-term CoQ10 intake reduced muscle injury markers such as CK and AST in elite swimmers, particularly when combined with pre-cooling strategies (17). Nowotny et al. also reported that chronic CoQ10 supplementation significantly lowered muscle damage biomarkers after exhaustive aerobic activity (18). Moreover, the findings of Saeidi et al. in elite cyclists confirmed that CoQ10 supplementation may modestly improve lipid profiles and reduce muscle damage following intense aerobic exercise (19). Trapp et al. highlighted the antioxidant role of CoQ10, stating that it modulates oxidative stress and inflammatory signaling, thus preserving cell integrity during heavy physical exertion (21). These studies support the physiological rationale behind the current study’s results.
Muscular strength refers to a muscle’s ability to contract against resistance, allowing for increased force generation and repetition capacity. HIIT has been recognized as a powerful method for enhancing muscle mass, strength, and aerobic power. Importantly, it improves VO₂max, which is critical for oxygen delivery and utilization during intense activities. Improved oxygen transport and aerobic capacity result in faster recovery and enhanced performance. Thus, combining HIIT with CoQ10 may lead to superior strength gains by reducing oxidative stress and enhancing oxygen availability to the working muscles, helping athletes achieve performance improvements more rapidly than through training alone.
Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
Funding
This research did not receive any financial support from government, private, or non-profit organizations.
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declare that there are no conflicts of interest associated with this article.