The tuning of robotic lower limb prostheses can be a time consuming and complex process for clinicians and patients with lower limb amputation. Current clinical methods for tuning of robotic lower limb prostheses involve one-on-one sessions between a clinician and the patient, and are done through visual observation and patient-reported feedback. A single patient may require multiple tuning sessions as they acclimate to a device over time. Efforts to reduce this burden through the use of an automated tuning process have the potential to expedite the process for clinicians and patients alike and make the technology more accessible to those who need it most. However, prior to developing an algorithm for automatic prosthesis parameter tuning, an investigation into which gait metrics are most sensitive to changes in gait quality is needed.
A primary challenge in developing an automated prosthesis tuning algorithm is deciding upon an objective and quantifiable definition of what constitutes a ‘good gait’. In typical clinical settings, gait quality is assessed through observation and patient reported feedback due to its simplistic and cost-effective nature(1). Although observational gait analysis can be done quickly, it can be highly subjective (1) and is difficult to translate into a quantitative algorithm. Advanced gait analysis systems can provide objective and quantitative kinematics and kinetics data. Typically, these more involved gait analyses involve comparison to some gold standard, either a set of control data that represents good gait(2), or there is an assumption made that bilateral symmetry represents good gait(3–5). Currently, there is not good agreement in the literature about how to best assess gait quality.
Multiple metrics have been utilized in the evaluation of gait of individuals with lower limb amputation, including observational scores(6), spatiotemporal parameters(7), kinematic and kinetic analyses(8, 9), balance metrics(10, 11), overall gait scores(12, 13), functional clinical outcome measures(9, 14) and patient reported measures(9, 15). In our study, we selected the Prosthetic Observational Gait Score (POGS) (6), the Gait Deviation Index (GDI) (2), Impulse Asymmetry (IA) (8) and truncal Lateral Sway (LS) as our gait metrics under investigation. These metrics were selected as they represent a range of commonly used clinical and biomechanical approaches with the potential for use with wearable sensors capable of providing input into a robotic prosthesis. The POGS is an observational and visual clinical metric which can be used by clinicians to quantify changes in the gait of an individual using a prosthesis or orthosis over time through the systematic analysis of 16 different aspects of an individual’s gait at the anatomical levels of the trunk, hip, knee, ankle and foot; the maximum score is a 32 with lower scores indicating less pathology of gait.(6) Duffy, et al. used POGS to compare differences in gait for individuals with a transfemoral amputation using both a microprocessor knee and a mechanical prosthetic knee joint. (16) While POGS can be performed immediately on-site in a clinical setting, it is preferable to utilize video recordings for improved accuracy.(1)
While observational gait analysis is heavily relied upon in clinical settings, instrumented gait analysis is widely recognized as the preferred method for gait assessment in pathological populations. (1) Accordingly, the GDI is one metric of gait quality which requires the use of an instrumented gait platform and has been reported as an appropriate measure for individuals with lower limb amputation. (12, 13) The GDI utilizes 3-dimensional kinematic data from the pelvis and hip, sagittal plane data from the knee and ankle joints as well as foot progression data in the transverse plane at each 2% increment throughout the gait cycle for a total of 51 points for each gait cycle with scores equal to or greater than 100 indicative of a more normal gait pattern.(2) The GDI has been used to understand levels of disability and impacts of medical interventions in children with cerebral palsy (12) as well as varying suspension types in users of transtibial prostheses, showing significant differences from a healthy control population.(13) GDI has also been shown to be correlated with more simple outcome measures such as step length, self-selected walking speed and the distance traversed during the 6 minute walk test in individuals with amputation.(9)
While kinematics represents one important aspect of evaluation for gait pathology, the use of kinetic information also bolsters the assessment capabilities associated with instrumented gait analysis. Individuals with amputation are noted to spend more time and exert higher loads on their intact side. (5) These temporal and loading asymmetries are important given their associations with higher risks for falls, osteoarthritis and back pain.(5) Because there is a high incidence of low back pain(17, 18) and osteoarthritis (19, 20) seen in prosthesis users, efforts to normalize their gait will make important gains toward improving their overall quality of life. Cutti, et al. showed differences in impulse symmetry between individuals with transfemoral and transtibial amputations; this work further showed that more advanced prosthetic technology improved loading symmetry.(5) The work of Zmitrewicz, et al. also showed a similar trend with Impulse Asymmetry improving with more advanced prosthetic componentry. (8) Specifically, Impulse Asymmetry was utilized in individuals with transtibial amputation to distinguish differences in the response to varying prosthetic feet and improved symmetry was noted during use of an energy storage and return (ESAR) foot compared to a non-ESAR foot.(8)
Trunk sway angular movements have been used as a measure of balance capability in individuals with amputation(21) and in aging populations (22) as well as a marker of disease progression in multiple sclerosis (23) and Parkinson’s disease(24). Balance is a critical metric for clinical populations with functional limitations such as prosthesis users given the high probability for falls and their subsequent detrimental impacts.(10) Trunk position variability has been correlated with step width, which has also been used as a metric of balance during walking(25).
With this motivation, we studied the sensitivity of four representative metrics of gait quality toward various constraints of lower limb joints in an able-bodied population. Our purpose in this study was to define the best gait metric for tuning a robotic lower limb prosthesis with a future goal of combining one (or more) of these metrics with a wearable sensor which may be used to automatically tune a prosthesis. We hypothesized that our selected biomechanically-based metrics, Gait Deviation Index, Lateral Sway and Impulse Asymmetry would outperform the clinical metric, POGS, as these metrics are measured objectively and defined on a continuous scale, compared to the POGS, which is a visual, subjective measure on an ordinal scale.