On the expression of joint moments during gait
Introduction
There are four possible reference frames for the expression of the net moment vector in quantitative gait analysis. Three of these possibilities are orthogonal frames: the laboratory (or global) frame (LF); the proximal segment anatomical frame (AF); the distal segment AF. Another possibility is a non-orthogonal frame or joint coordinate system (JCS). When reviewing the previous literature evaluating lower limb joint moments during gait, each reference frame has been implemented in at least one study [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Thus, no consensus currently exists regarding an accepted standard. This makes it difficult to compare joint moment data across studies.
Few studies have investigated the differences in lower limb joint moments during gait when expressed in alternative reference frames [12], [13], [14]. Of those that have, only able-bodied gait has been evaluated and not all possible reference frames have been considered. In contrast, the effect of different inertial parameter sets on lower limb joint moment calculations has been well investigated [15], [16], [17], [18], [19]. This is surprising given that, for relatively slow speed movements such as gait, lower limb joint moments are far more likely to be sensitive to a change in reference frame than they are to a change in the magnitude of an inertial parameter set.
The decision to adopt a given reference frame has typically been based upon preference rather than objective evidence. To our knowledge, Winter et al. [20], [21] provide the only formal attempt to justify a preferred convention. Two theoretical arguments were provided. First, as the dominant velocity of the centre of mass is in the plane of progression and the balance of the trunk in both the sagittal and frontal planes is dictated by gravity, both the centre of mass and trunk are tightly regulated to have trajectories in the plane of progression. Second, even when externally rotated, the lower limb segments do not follow trajectories in a local AF but rather move forward in the plane of progression. Given that under experimental conditions the plane of progression will be aligned with one of the planes of the LF, it was therefore concluded that the net moment vector should be expressed in the LF. Despite this viewpoint, few studies have actually adopted the LF as a preferred convention [2], [7], [9], [10]. Most have used an orthogonal or non-orthogonal local AF [1], [3], [4], [5], [6], [8], [11]. This would suggest that additional clinical factors, such as anatomical interpretation and sensitivity to change post treatment, are also considered by many to be important criteria for making judgments regarding a preferred reference frame.
The aim of the current study was to therefore provide a definitive evaluation of the effect of different reference frames on lower limb joint moments during gait. It was considered likely that such differences would be more prominent for abnormal gait, thus both able-bodied and pathological gait patterns were investigated. An additional aim was to use these results as evidence for identifying a recommended convention for clinical purposes.
Section snippets
Subjects
Ten subjects were voluntarily recruited. Approval was obtained from the Royal Children's Hospital Ethics in Human Research Committee prior to commencement and subjects signed a consent form. The able-bodied adult group comprised of two men and seven women aged 19.8 years (S.D. 2.1) with a mean height of 164.5 cm (S.D. 8.5) and body mass of 60.0 kg (S.D. 1.1). The subject with spastic diplegic cerebral palsy was a 10-year-old boy who was 146.1 cm tall and weighed 39.7 kg.
Instrumentation
Kinematic data were acquired
Joint kinematic profiles
The able-bodied adults displayed typical kinematic patterns in each plane for each joint (Fig. 1). In contrast, the child with cerebral palsy had an increased rearfoot valgus alignment in standing (15.4° compared to 3.5 ± 4.9° for the able-bodied adults) and walked with excessive hip internal rotation (Fig. 1) and foot adduction or toe in (Fig. 2).
Joint moment profiles
Lower limb joint moments for able-bodied adult gait are illustrated in Fig. 3, Fig. 4, Fig. 5. Most joint moments were found to display characteristic
Discussion
Consistent with previous studies [2], [4], [28], most joint moments for able-bodied adult gait were found to display characteristic profiles that were systematic across subjects. Two joint moments, however, were associated with a degree of inter-subject variability: the ankle rotation moment in the foot AF and the ankle invertor–evertor moment in the LF. This is in agreement with findings from a previous study [12].
The inter-subject variability of the ankle rotation moment in the foot AF can be
Acknowledgment
This project was financially supported by a Health Professional Research Training Fellowship from the Australian National Health and Medical Research Council (Grant ID: 237153).
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