The main questions of our study are to examine the maximal V̇O₂ achieved during five sets of squat exercises (10 reps per set, 5 sets, 3 min rest interval, 65% of 1RM) in relation to predetermined V̇O₂max and how these values differ according to participants’ training status. Our study showed that the highest V̇O₂ was observed during the 5th set of squat exercises, almost reaching 100% of the participants’ predetermined V̇O₂max. When the highest V̇O₂ values were presented according to training status, participants with higher strength experienced an increase in V̇O₂ during squat exercise up to 108% of their V̇O₂max, while the highest V̇O₂ of participants with lower strength was 93.7% of their V̇O₂max, measured immediately after the final set. When the highest V̇O₂ during five sets were averaged, participants reached over 90% of their V̇O2max. Regardless of participants’ training status, oxygen demand during squat exercise was extremely high.

An increase in V̇O₂ during resistance exercises has been previously reported. However, there are substantial differences in the amount of V̇O₂ between our study and previous studies^9,19. Previous studies reported V̇O₂ during squat exercises ranging from approximately 16 to 31.3 ml/kg/min depending on the length of the rest intervals^9,19. In the present study, we observed values above 40 ml/kg/min, and in some participants, V̇O₂ increased above 50 ml/ kg/min, exceeding their pre-determined V̇O₂max. A significantly greater V̇O₂ observed among our participants could be due to training status and the specific exercise protocol. The 1RM among our participants was 141.4 ± 31.3 kg, whereas the 1RM reported by Ratamess et al.⁹ was 127.9 ± 31.1 kg. Furthermore, Ratamess et al.⁹ employed higher intensity resistance exercise, set at 75% of 1RM, whereas our study employed a lower intensity, 65% of 1RM. Given that all participants in our study successfully completed 10 reps of squats until the fifth set whereas participants from Ratamess et al.⁹ did not, the exercise in the current study elicited a higher demand for aerobic metabolism. Another rationale for the relatively higher V̇O₂ during our squat exercise could be due to different squat techniques. In the present study, all participants were instructed to perform a full squat with a full range of motion. In contrast, other studies either utilized only half squats or did not specify the depth of the squat. Performing full squatting is likely to elicit a higher oxygen demand.

Interestingly, the levels of V̇O₂ relative to V̇O₂max and the highest heart rate relative to maximal heart rate clearly showed that multiple sets of resistance exercise could be considered as vigorous- or high-intensity cardiovascular activity^20,21,22. When viewed from an intensity perspective, squat exercise can be classified as a form of vigorous- or high-intensity activity²². However, since vigorous- or high-intensity aerobic activity is defined as an activity sustained for a prolonged period (e.g., ≥ 10 min)²², squat exercise does not meet this criterion given the rest interval periods and therefore may not be described as such. Furthermore, our findings suggest that aerobic demand of resistance exercise is much greater when individuals could exercise at a higher intensity without sacrificing the volume, represented as number of repetitions. Among the participants with high strength, V̇O₂ exceeded their pre-determined V̇O₂ max at the 4th set of squat exercise, while participants with low strength reached up to 91.69% of their V̇O₂max at the 4th set. One noteworthy implication of our study is that we examined the fluctuations in cardiorespiratory responses and RPE throughout the progression of squatting repetitions and sets. This stands in contrast to merely assessing the average and peak V̇O₂ observed during one bout (i.e., session) of squatting.

During rest intervals, we observed higher CO₂ production than VO₂ consumption, whereas the opposite was observed during the squat exercise periods. Typically, individuals only breathe once at each descending and ascending motion within a repetition during squat exercise, resulting in this distinctive breathing pattern that may cause a difference between pulmonary and cellular metabolic demands. During squatting exercises, participants may not be able to exhale sufficient amounts of CO₂ produced as a result of bicarbonate buffering process. Breathing is modulated by central and peripheral chemoreceptors, which may respond to CO₂ and H^+23,24. Although elevations in CO₂ and H⁺ during squat exercise are the primary precursors to an increase in breathing, breathing is limited to the exercise rhythm during squatting, which may cause hypercapnic acidosis^24,25. When breathing was no longer limited to the exercise rhythm during rest intervals, participants hyperventilated and exhaled CO₂. The increase in V̇CO₂ in relation to V̇O₂ was significant. While we did not measure the partial pressure of arterial CO₂, our results indicated that participants experienced hypercapnia during the five sets of squat exercise. This was demonstrated by the ventilatory efficiency (Supplemental Fig. 2), which showed a continuous increase with successive sets. Diverse breathing techniques employed during squatting may yield varying V̇O₂ and V̇CO₂ responses.

It is unclear whether training proficiency and subsequent muscular strength are determinants of cardiorespiratory fitness^26,27,28,29. Highly trained individuals are accustomed to a higher training intensity and frequency than relatively less-trained individuals, leading to greater neuromuscular output and adaptation³⁰. As such, highly trained individuals can perform a greater volume (load, repetitions, intensity) of squat exercises, which may result in a higher level of V̇O₂ than those with low strength during resistance exercise. Interestingly, we observed that the high strength group showed a higher level of V̇O₂ (relative; normalized to body weight) at the same relative intensity compared with the low strength group. These results suggest that aerobic demand of resistance exercise may be more evident among individuals with certain levels of resistance training status. In addition, the predetermined V̇O₂max level was lower in the high strength group compared to the low strength group, although this difference was not statistically significant. Furthermore, it is crucial to note that all study participants performed the squat exercise at 65% of their individual 1RM. This indicated that the squat load was obviously higher in the high strength group compared to the load used by the low strength group. Therefore, the high strength group may exhibit higher V̇O₂ responses compared to the low strength group due to relatively lower aerobic efficiency and/or the absolute training load during exercise in the high strength group. Individuals, who are not accustomed to resistance exercise, may not have the same cardiorespiratory response as observed in our study.

The effort inherent to the execution of squatting exercises at 65% of 1RM, as performed in our study, is submaximal. This relative intensity corresponds to a margin of repetitions that is less than maximal exertion, influencing the V̇O₂ observed. Previous research^11,31 has established a relationship between the number of repetitions and selected percentages of one repetition maximum in both trained and untrained men. These studies^11,31 indicate that the effort required at 65% of 1RM is substantial but not maximal, which aligns with our findings of significant oxygen demand during the exercise intervals. Our study further highlights that the substantial oxygen demand observed during the squatting exercise is influenced by both the training status of the participants and the submaximal nature of the effort. The high strength group demonstrated a higher V̇O₂ relative to their V̇O₂max compared to the low strength group. This suggests that individuals with higher strength capacity may be able to sustain higher aerobic demands during resistance exercises, even at submaximal intensities.

This study has several limitations. First, the findings of this study are specific to the squat exercise protocol used and cannot be generalized to other resistance exercise protocols, such as chest presses or arm curls. Different volumes (i.e., intensity, repetition, and training load) of squat exercises may result in different outcomes¹¹. Second, nutritional and hydration intakes, which may be potential confounders, were not controlled for in this study. These factors may have impacted the association between squatting and cardiorespiratory outcomes. Lastly, our findings may not be generalizable to wider populations, given that we examined young, healthy, well-trained, male participants only.