BackgroundThis study aimed to assess the importance of socioeconomic status (SES) on health-related fitness (H-RF) measurements in young adults and determine the impact of SES over 20 years of substantial social and economic changes in Poland.Material and methodsThe study compared H-RF differences between 2001 (P1) and 2022 (P2) in 252 volunteers aged 18 to 28 years who were grouped into quartiles based on SES and gender. The variables measured included height, weight, body mass index, body fat mass, hand strength (hand grip), abdomen strength (sit-ups), flexibility (sit and reach), and leg power (standing long jump), with a synthetic motor performance index (MPSI) calculated for each participant.ResultsHealth-related differences based on social inequalities included body fat mass and MPSI, and two-way analysis of variance (ANOVA) revealed an interaction between SES and period on motor performance (F = 2.73,p= 0.045). In addition,post-hoctests revealed differences in P1between SES quartiles one and two (p= 0.028). Over the last 20 years, physical fitness decreased and body fat increased. The regression slope showed decreased motor performance with higher amounts of body fat in P2subjects compared to their P1peers.ConclusionThe observed trends may be associated with lifestyle changes shaped by technology development, high-energy and low-quality food access, and increased physical inactivity.