Full length articleGait variability and motor control in patients with knee osteoarthritis as measured by the uncontrolled manifold technique
Introduction
Approximately 23% of people over the age of 60 suffer from knee osteoarthritis (OA) [1]. Knee OA is a degenerative joint disease which causes pain, reduced muscular strength and limited function to the affected joint [2], [3]. Consequently, patients suffering from late- to end-stage OA become limited in their mobility [1].
Koyama and colleagues stated recently that increased stiffness of the joint coupled with reduced muscular strength alters the motor control mechanism, ultimately compromising stability [4]. Stability can be quantified in terms of gait variability [5], [6], [7]. Although having a variable gait is natural, the extent of variability and the patterns observed in variability have been found to differ in those suffering from OA [5], [8], [9]. This could affect their ability to react to perturbations, potentially increasing the risk of falling and limiting the extent and speed of activity the patient feels safe to undertake [7], [10], [11].
Given the complexity of gait, there are limited ways in which researchers and clinicians can objectively assess its overall variability from cycle-to-cycle [7], [12], [13]. Recent studies have used a method known as the uncontrolled manifold (UCM) hypothesis to investigate the relationship between motor control and variability in gait [14], [15], [16]. The populations in the studies by Papi et al.[14] and Black et al. [15] were stroke survivors and pre-adolescents with and without Down syndrome, respectively.
In these studies, the UCM method quantified the combinations of elemental variables (joint degrees of freedom) that successfully stabilised the centre of mass (COM):‘good variability’, and those which compromised the stability of the COM: ‘bad variability’ [14], [15]. Here, stabilisation of the COM refers to the ability of the elemental variables to maintain a consistent mean COM position over numerous trials, despite showing inter-trial variability. Combinations of elemental variables that lead to a deviation of the COM away from its mean position compromise COM stability. Kinematic synergy was found to exist in each population in these studies, meaning that the ‘good variability’ (variance within the UCM) outweighed the ‘bad variability’ (variance orthogonal to the UCM) [14], [15]. However, variability was increased compared to normal, implying that the central nervous system had employed a more variable gait in order to maintain a stable COM during walking. This strategy is believed to reduce COM instability, but to leave the subject more vulnerable to external or internal perturbations as some of the ability to variably respond to these perturbations has already been used [17], [18].
Improving our knowledge of the relationship between gait variability and COM stability in physically compromised populations may enable us to investigate the possibility of using the UCM method as a biomarker for gait stability and risk of falling.
The aim of this study was therefore to use the UCM method to quantitatively evaluate sagittal and frontal plane postural stability during normal walking in an osteoarthritic population.
We hypothesised that our population would display cycle-to-cycle variability when walking while maintaining kinematic synergy.
Section snippets
Study participants
Fifty adults (25 males and 25 females) with bilateral knee osteoarthritis were recruited to this study (Age: 70 ± 9, Mass: 85.4 ± 16.9 kg, Height: 1.65 ± 0.11m, BMI: 31.2 ± 5.1 kg/m2). All patients were scheduled to undergo total knee arthroplasty on the worst-affected knee. The contralateral knee was of similar OA severity in all patients but the pre-operative knee was the more symptomatic. 15 patients had a valgus knee deformity (mean: 6.3 ± 4.9°) and 35 had a varus knee deformity on the pre-operative
Results
Variances within the UCM were greater than the variances orthogonal to the UCM throughout the gait cycle in both planes and limbs (Fig. 2). This suggested that kinematic synergy existed in our patient cohort. In the sagittal plane, a prominent increase in variance was observed at approximately 80% of the gait cycle in the limb of the worst-affected knee (pre-operative side). No such increase was observed in the contralateral limb. Frontal plane variance within the UCM in the pre-operative side
Discussion
The UCM hypothesis states that variability during repetitive movement tasks is necessary and beneficial. It enables us to act appropriately to perturbations or diseases which compromise elements of the kinematic chain of joints or which affect movement [17], [18]. Patients with knee OA have generally been found to have alternative patterns of gait variability when compared to healthy adults, however the way in which the COM is stabilised to complete a walking task remained unclear [5], [7].
Conclusion
A variable gait is employed to stabilise the centre of mass in patients with OA of the knee. Kinematic synergy was confirmed in this population. Weakness of the quadriceps is thought to decrease sagittal plane stability in this patient cohort. This technique may allow specific personalised prescription for prehabilitation and rehabilitation of knee OA patients.
Conflict of interest
None.
Acknowledgement
This study was supported by the University of Strathclyde and Medacta International SA as a PhD studentship. Neither had direct involvement with this study or the writing of this manuscript.
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