Background and Objectives: This study aimed to translate the Hip disability and Osteoarthritis Outcome Score (HOOS) into the Polish language, to determine its validity and reliability, and to assess its main psychometric properties. Materials and Methods: A total of 332 hip osteoarthritis (OA) subjects were recruited to the study group and 90 healthy subjects to the control group. The study consisted of the HOOS translation and the assessment of the discriminative power, internal consistency, and the potential floor and ceiling effects followed by the determination of the construct validity and test-retest reliability. The analysis was performed using Western Ontario and McMaster Universities osteoarthritis index (WOMAC) and SF-36 questionnaires. Results: The translation process consisted of forward translation, reconciliation, backward translation, review, harmonization, and proofreading. The hip OA patients reported a reduced HOOS score when compared to the control subjects. The discriminant validity of the questionnaire was confirmed. A Cronbach’s alpha of 0.97 was found, indicating a high internal consistency. The HOOS showed a significant correlation with the SF-36 and WOMAC, which ranged from r = −0.93, p-value < 0.05 for WOMAC total score to r = 0.92, and p-value < 0.05 for WOMAC daily living. No floor or ceiling effects were found. A very high intraclass correlation coefficient (ICC) was found (0.93–0.97) for the total score and the individual domains of the HOOS. Conclusions: The Polish HOOS is valid and reliable for evaluating the outcomes of hip OA patients in Poland. This questionnaire may be used with confidence for clinical and research purposes.

Postural stability is an important component of functional health in older adults; however, longitudinal observations from routine community physical activity programs remain limited. This exploratory secondary observational analysis compared changes in postural stability between two archival cohorts of University of the Third Age participants assessed over different observation periods. The dataset comprised 48 paired assessment records representing 43 unique participants. Five individuals appeared in both historical datasets; therefore, the primary independent comparison included 24 participants in the longer-observation cohort and 19 in the shorter-observation cohort. The respective intervals between baseline and follow-up assessments were 408–568 and 21–76 days. The cohorts corresponded to programs offering 77 and eight scheduled physical activity sessions, respectively, although individual attendance was not documented. Postural stability was assessed using the HUMAC Balance System under two 30-s firm-surface conditions with eyes open and eyes closed. Ten outcomes were evaluated using Wilcoxon signed-rank tests for within-cohort changes and Mann–Whitney U tests for between-cohort differences in change scores, with Holm adjustment for multiple comparisons. No outcome changed significantly in the longer-observation cohort after adjustment. In the shorter-observation cohort, the eyes-closed sway index and eyes-closed stability score (ECSS) changed in a less favorable direction (Holm-adjusted p = 0.043 and p = 0.048, respectively). Between-cohort differences in change were significant for both outcomes (Holm-adjusted p = 0.019 for each; rank-biserial correlations −0.553 and 0.557, respectively). Sensitivity analyses using alternative strategies to exclude overlapping records retained the statistical significance of these findings. The results demonstrate differences in selected eyes-closed postural stability measurements between archival cohorts; however, scheduled activity exposure and follow-up duration were inseparable. Consequently, the findings do not establish an independent effect of exercise duration or preservation of postural control attributable to physical activity. Prospective studies with comparable observation intervals and verified individual exercise exposure are required to determine their clinical significance.