Developing an estimate of daily cumulative loading for the knee: examining test-retest reliability

Although the knee adduction moment during gait is a valid and reliable proxy for the dynamic load on the medial compartment of the knee, it represents exposure to loading during one stride only. In contrast, a measure that incorporates both the nature and frequency of loading throughout daily activities might provide additional insight into the effects of cumulative knee loading. The purpose of this study was to introduce a new representation of daily cumulative knee loading and examine its test-retest reliability. Thirty healthy adults participated. Cumulative knee loading was calculated on two testing periods from the mean external knee adduction moment stance phase impulse, measured with a three-dimensional motion capture system over five walking trials, and mean steps/day, measured with a unidimensional accelerometer over one week. Analysis for test-retest reliability included Bland-Altman graphs, intraclass correlation coefficients (ICC 2,1) and standard errors of measurements (SEM). The ICC values for cumulative knee loading, adduction impulse and steps/day ranged from 0.84 to 0.89. Bland-Altman plots suggested daily cumulative knee loading and steps/day measures were less reliable at higher values. The SEM values were 9.67 kNm s, 1.45 Nm s and 1043 steps/day for cumulative knee loading, adduction impulse and steps/day, respectively. Daily cumulative knee loading is reliable and provides a stable measure of the total exposure to knee loading. These findings support further study of cumulative knee loading to determine its potential clinical importance.