The single-leg sit-to-stand test is valid and reliable for assessing lower limb performance and asymmetry in international cross-country skiers

Objective: To evaluate the validity and test–retest reliability of single-leg sit-to-stand (SLSTS) performance metrics derived from a mobile application in elite cross-country skiing (XCS) athletes. Design: Cross-sectional, repeated-measures. Setting: Laboratory, field. Participants: 22 international-level XCS athletes (13 females, 9 males; median age: 18 yrs). Main outcome measures: SLSTS rising time, velocity, power, and inter-limb asymmetry were collected across two sessions separated by three days. Isokinetic knee extension peak torque and countermovement jump (CMJ) height were assessed to establish convergent and construct validity, respectively. Reliability was determined using intraclass correlation coefficients (ICC2,1) and coefficient of variation (CV). Validity was assessed using Pearson correlations. Results: SLSTS metrics demonstrated good inter-day reliability (ICC2,1 = 0.81–0.89; CV = 6.1–9.9 %) for both limbs, with no differences in inter-limb asymmetry between sessions. These metrics showed moderate convergent validity (|r| = 0.34–0.54) with isokinetic peak torque, and moderate–strong construct validity (|r| = 0.46–0.75) with CMJ height. Notably, the strength of the associations with CMJ height was comparable to those observed between isokinetic torque and CMJ height (Steiger's Z-test, p > 0.05). Conclusion: The app-based SLSTS is a reliable and valid alternative for assessing lower-limb performance and asymmetry in elite XCS athletes when laboratory-based assessments are not feasible.