Circuit training (CT) is an important type of training that can combine different types of exercises in a single form of training. This training is generally circular with the different exercise stations it contains, so it is known as CT. The initial protocol of this training, developed by researchers at the University of Leeds in 1953, consisted of 9 to 12 moderate intensity exercises (40% to 60% of 1RM) performed at different exercise stations and with a specific repetition and duration. Nowadays, it is seen that this training is applied in different intensities and protocols. Also, CT has gained popularity due to its practicality and effectiveness and is preferred by all age groups in recent years.

CT can be applied in many sport settings and does not require expensive equipment. Also, this training can be adjusted to any level of physical fitness and includes bodyweight exercises and functional movements. This training generally includes aerobic and strengthening exercises and improves musculoskeletal, cardiovascular and pulmonary functions. On the other hand, CT allows individual interaction both between participants and between participants and the sports or health professionals who provide this training. Lastly, the hallmark of CT is that it includes different exercise stations and short rest periods without breaks and the advantage of CT is that it effectively improves several parameters of physical fitness simultaneously.

Current literature investigating the effects of CT on physical fitness parameters in different populations is still emerging. However, it was seen that few studies focused on the healthy women population. Also, it was concluded that these studies reported different results regarding the effects of CT. Accordingly, it has been concluded that 6-week CT improved lower and upper extremity muscle strength and endurance, but had no positive effect on flexibility and body composition in healthy women. However, it has been reported that 12-week CT had no positive effect on body composition, knee flexor and extensor muscle strength in healthy women. Lastly, it has been concluded that12-week of CT had positive effects on body composition, and improved flexibility and lower extremity muscle strength in healthy women. These results in the literature suggested there is no clear consensus on the effects of CT on physical fitness parameters in healthy women.

Moreover, the fact that CT also includes aerobic exercises shows this training may also have an effect on pulmonary function. In this context, previous studies reported the positive effects of this training on pulmonary function in adolescent, professional volleyball players, women with sarcopenia, and people with human immunodeficiency virus. However, there is insufficient evidence regarding the effects of CT on pulmonary function in healthy women. To the best of our knowledge, only one previous study reported the positive effects of CT on some pulmonary function in healthy women. Accordingly, it is clear that studies is needed to examine the effects of CT on pulmonary function on healthy women. This training may also improve pulmonary function in healthy women by increasing the performance of the diaphragm muscle through aerobic exercise. Moreover, these positive effects on pulmonary function may also improve quality of life in healthy women. Lastly, similar to physical fitness parameters, studies investigating the effects of CT on quality of life also included different populations. However, it was concluded there were few studies that included healthy women and the results of these studies differed.

Therefore, the aim of this study is to investigate the effects of 16-week CT on physical fitness parameters, pulmonary function, and quality of life in healthy women. Also, the investigators hypothesized that the 16-week CT would be positive effects on physical fitness parameters, pulmonary function, and quality of life in healthy women.