Variability of knee moment arms in the frontal and sagittal planes during normal gait

A new measure, the length of the moment arm at the knee joint in the frontal and sagittal planes during normal gait, was assessed, and the variability tested. Simultaneously the external moments about the hip, knee and ankle were analysed. A gait analysis system with a force platform and two video cameras was used. Two recordings in each plane with a few minutes' interval were collected in 10 healthy normal subjects walking at free cadence in their own shoes. For one subject, two recordings in each plane were obtained for 5 consecutive days. The mean moment arm length in the frontal plane at mid-stance, when trials were done minutes apart, was 48 mm, with a coefficient of variation of 7.9%. The mean moment arm length for the peak knee moment was 63 mm with a coefficient of variation of 7.9%. For the moment arm lengths at the peak moments in the sagittal plane, the variability was very high. The variability of the trials done minutes apart was in most cases less than of those done days apart. In the frontal plane, the smallest variability in trials done minutes apart was in the external hip peak and mid-stance moments and the external knee peak and mid-stance moments. In the sagittal plane, the smallest variability was in the external peak dorsiflexing moments and mid-stance dorsiflexing moments about the ankle. All ankle moments in the frontal plane and the peak moments about the hip and knee in the sagittal plane showed a high variability. Thus, care should be taken when these moments are evaluated with similar methods.