Abstract: BACKGROUND: Core stability tests are regarded as an important indicator for evaluating the function of core muscles. They not only reflect the functional state of the human trunk but also have a close connection with the structure and function of the diaphragm. 
OBJECTIVE: To investigate the relationship between diaphragmatic ultrasound parameters and core stability in young adults. 
METHODS: From January to April 2024, diaphragmatic ultrasound parameters and core stability indicators were assessed in 163 young participants. Subjects were categorized into a sedentary group (n=83) and an exercise group (n=80). Intergroup differences were evaluated using independent samples t-tests or Mann–Whitney U tests. Simple correlation and canonical correlation analyses were conducted to examine the relationships between diaphragmatic ultrasound measures and core stability parameters. 
RESULTS AND CONCLUSION: (1) The sedentary group showed lower values than the exercise group in diaphragmatic thickness at the end of quiet inspiration, diaphragmatic thickness at the end of quiet expiration, diaphragmatic thickness at the end of deep inspiration, and diaphragmatic thickening fraction during deep breathing (P < 0.001); the sedentary group showed lower values than the exercise group in prone pressure, supine pressure, dominant side single-leg hop, sit-ups, and trunk extensor endurance (P < 0.01); the sedentary group showed higher values than the exercise group in prone trunk proprioception and supine trunk proprioception (P < 0.01). (2) Most diaphragmatic ultrasound parameters and core stability parameters showed statistically significant correlations among subjects. However, dominant side single-leg standing was not statistically correlated with most diaphragmatic ultrasound indicators. The correlation between deep inspiratory diaphragmatic slope and various core stability indicators was not statistically significant. The first two canonical correlation coefficients of diaphragmatic ultrasound parameters and core stability parameters were 0.900 and 0.572, accounting for 81.60% and 9.29% of the variance, respectively. (3) The proportion of variance in diaphragmatic ultrasound parameters explained by their own first canonical variable and the opposing first canonical variable was 33.30% and 27.00%, respectively. The proportion of variance in core stability variables explained by their own first canonical variable and the opposing first canonical variable was 64.40% and 52.20%, respectively. These findings indicate that significant differences exist in diaphragmatic ultrasound parameters and core stability between sedentary and exercising young adults, and there is a strong canonical correlation between diaphragmatic ultrasound parameters and core stability. 

Key words: diaphragmatic ultrasound, core stability, young adults, diaphragm thickness, diaphragm thickening fraction, proprioception, muscle endurance