Reproducibility and reference values of inclinometric balance and isometric trunk muscle strength measurements in Finnish young adults

The objectives of the study were to evaluate the reproducibility of the maximal isometric trunk muscle strength testing and inclinometric method to measure body sway and to establish reference values for these measurements in young Finnish adults. Reproducibility was assessed with 2 repeated measurements. Reference values are based on the cross-sectional cohort data. Maximal isometric trunk extension, flexion, and rotations were measured in standing posture. Body sway was measured with inclinometric method. The subjects (N = 874) belonged to a subcohort of the Northern Finland Birth Cohort 1986. Intrarater reliability was assessed with 19 volunteers from the birth cohort, and interrater reliability was assessed with 15 young healthy adults. Intrarater reliability intraclass correlation coefficients (ICCs) for body sway measurements ranged from 0.39 to 0.74 and for trunk muscle strength measurements from 0.84 to 0.94. Interrater reliability ICC for body sway ranged from 0.61 to 0.85 and for trunk muscle strength from 0.84 to 0.88. Maximal muscle strength was better in men than in women, but extensor to flexor ratio was higher in women. Women had significantly better values in all body sway measurements than men. Our study showed a remarkable biological variation in isometric trunk muscle strength and inclinometric body sway. Although ICC analysis indicated good reliability, the Bland and Altman analysis revealed quite wide range of measurement error. However, reproducibility of isometric trunk muscle testing is comparable to other measurements of trunk muscle function in use. Reference values collected in our study offer valuable normative data for future studies and can be used as a tool when assessing physical capacity of healthy subjects or patients with different pathological conditions. KEY WORDS lumbar strength, postural balance, reliability of results