Introduction:
Changes in the concentration of the relaxin hormone are associated with changes in the structure of joints and the musculoskeletal system.

Aim of the study:
Therefore, this study aimed to compare the serum levels of relaxin and flexibility in active and inactive women.

Material and methods:
This research was retrospective and included two groups consisting of active women (n = 8) and inactive women (n = 8) with an age range of 20-35 years, who were selected after a public call according to the inclusion and exclusion criteria of the research. Then, blood sampling and upper and lower limb flexibility measurements were performed. After collecting the data, Stata statistical software version 12 was used to analyze the data at a significant level of p < 0.05.

Results:
The research results showed that the serum levels of relaxin in active women were higher than in inactive women; but this mean difference was not statistically significant (p = 0.069 and t = 1.973). Also, the results indicated that only the relationship between relaxin and lower limb flexibility was statistically direct and significant (p = 0.006). Sports history is probably the cause of an increase in serum levels of the relaxin hormone in active young women. Moreover, the research results showed a significant relationship between the serum concentration of relaxin and the flexibility of the lower limb

Conclusions:
Sports training in active young women increases the relaxin hormone, which may also be associated with increased flexibility, especially in the lower limbs (where it has the greatest effect on the musculoskeletal system).