Breathing is a vital process that occurs automatically, but it can also be voluntarily controlled to achieve specific goals, especially preventive and therapeutic ones. Slow breathing has been reported to have many physical and mental health benefits. The activation of the sympathetic system during inhalation and the increase in heart rate, as well as the activation of the parasympathetic system during exhalation and the resulting decrease in heart rate variability, can be affected by breathing exercises. The physiological effects associated with breathing exercises vary depending on the type of exercise performed. Therefore, investigating and comparing the clinical effects and contributions of different breathing exercises will shed light on which breathing exercises should be preferred in different situations.

This study aims to examine the effects of diaphragmatic breathing, square breathing technique, alternative nostril breathing exercises, and diaphragmatic breathing exercises on respiratory function parameters and heart rate variability, and to compare these different breathing exercises. Healthy individuals aged 18-30 years from the Department of Physiotherapy and Rehabilitation at Bandırma Onyedi Eylül University will be invited to participate in the study. For the study, a sample size of 40 was determined in the "G*Power" analysis system with a moderate effect size according to Cohen, a statistical significance threshold of 5% (alpha level = 0.05), and 80% power (Cohen, J.; 2013), allowing for evaluation of the four groups before the training program, immediately after the first session (acute effect), and after the training program (6 weeks). To account for potential dropouts, the target is to reach 44 participants, which is 10% more than the calculated sample size. Power calculations will be performed during the study process, and revisions regarding sample size will be made if necessary. Participants will be randomized using a sealed envelope randomization method. Respiratory function tests (FVC-FEV1- FEV1/FVC- FEF- PEF) will be performed using a COSMED Pony FX portable spirometer to evaluate participants' respiratory functions. Heart rate variability will be assessed by taking a 5-minute measurement using a Polar H10 device. Four different breathing exercise groups will be created, and participants will undergo diaphragmatic breathing exercises, square breathing technique exercises, alternative nostril breathing exercises, and dynamic neuromuscular stabilization-based breathing exercises for 6 weeks, with sessions lasting 20 minutes, four times a week. The initial session will be conducted face-to-face, while subsequent sessions will be conducted online (via zoom/video calls/Google Meet, etc.). Assessments will be conducted three times: at baseline, immediately after the first session (acute effect), and after the 6-week training program, and will be performed face-to-face.