In this study, we evaluated the influence of different prosthetic ankle power settings on users&#; metabolic cost, using a commercially available powered prosthesis (BiOM). We hypothesized that 1) to minimize their energy cost, users would require a higher power setting than the power setting chosen by the prosthetist, which approximated the work of the biological ankle, and 2) the highest power setting (100%) would not be the optimal power setting to minimize metabolic cost.
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In support of our first hypothesis, we found that the best tested power setting was higher than the prosthetist-chosen power setting for all subjects. Moreover, we found that on average, subjects walking with their best tested power setting significantly reduced their cost of transport (COT) by 0.04&#;±&#;0.02 compared to walking with the prosthetist-chosen power setting. This change is larger than the within-day minimum detectable change (MDC) for COT reported by Davidson et al.26, which is 0.022. Therefore, we consider this difference a meaningful change and not likely due to measurement variation. On an individual basis, 6 of the 9 subjects analyzed had differences in COT greater than the MDC; the exceptions to this were Subject 1 (0.167), Subject 4 (0.012), and Subject 8 (0.005). The cost of transport calculated here is similar to values reported in other studies of people with transtibial amputation walking with the BiOM at similar walking speeds. In this study, the average COT for the prosthetist-chosen setting was 0.39&#;±&#;0.04, while the average was 0.40&#;±&#;0.05 in Gardinier et al.14 and 0.36 in Herr et al.10.
Since the prosthetist-chosen power setting was chosen to approximate the work done by a biological ankle during walking, our results suggest that individuals with transtibial amputation may require ankle work in excess of biological norms in order to reduce their metabolic effort. The biological net ankle work for non-amputee subjects walking at 1.25&#;m/s is approximately 0.1&#;J/kg10,27. On average, the best tested power setting in our study corresponded to net ankle work of 0.24&#;±&#;0.07&#;J/kg, which is roughly double that of non-amputee subjects. As aforementioned, this finding is supported in studies conducted with able-bodied individuals wearing both exoskeletons17,18 and an ankle prosthesis emulator19. In contrast, a recent study by Quesada et al.21 did not show an effect of prosthesis work on the metabolic work rate of amputee subjects. Differences between our study and the study by Quesada et al. could have arisen from a variety of factors, including the functional level of the subject population and/or different prosthesis controllers. The subject cohort analyzed in our study comprised six K4 and three K3 individuals, while the cohort tested by Quesada et al. included six K3 and no K4 individuals. Gardinier et al.14 found that individuals with K4 functional level were significantly more likely to receive an metabolic benefit from the BiOM than those with a K3 level, which may partially explain the different outcomes of these studies. Additionally, the various prosthesis emulator work conditions tested in by Quesada et al. were generated by modifying the torque-angle relationship, and as such, the prosthesis work remained mostly constant between steps. In contrast, the BiOM device is controlled using a reflexive controller, which utilizes a neuromuscular model to command a torque at each step, and varies depending on how the user loads the prosthesis10,28,29. The reflexive controller and powered plantarflexion on the BiOM device are hypothesized to help users maintain balance, especially on variable terrain29,30. Similarly, Kim et al.31 demonstrated that a stabilizing controller which modulates ankle push-off work each step reduced metabolic cost and step width variability in able bodied-users walking with an ankle prosthesis emulator. The reflexive nature of the BiOM&#;s controller may have played an important role in the observed reduction of metabolic cost in the amputee subjects in our study, perhaps by reducing balance-related compensation efforts or allowing users to explore different assistance strategies by changing how they load the device step-to-step. Further studies will be required to determine the exact role that the reflex controller plays in reducing energy cost in prosthesis users.
The results of this study did not fully support our hypothesis that we would see an increase in COT at the highest power setting, indicative of too much ankle power. In fact, no significant group differences in COT were found between the 50%, 75%, and 100% conditions (corresponding to mean net ankle work of 0.19&#;±&#;0.08&#;J/kg, 0.22&#;±&#;0.05&#;J/kg, and 0.24&#;±&#;0.07&#;J/kg respectively). When examining the data on a subject-specific basis, we saw that 5 of the 9 subjects analyzed had the best tested power setting at 75%. Yet, of these subjects, only Subjects 5 and 6 exhibited the anticipated trend of increased COT at 100% (both S5 and S6 increased their COT by 0.049 from 75% to 100%; the minimum detectable change (MDC) is 0.022). The other three subjects (S3, S4, S8) had changes in COT smaller than the MDC between 75% and 100%. The remaining three participants had their best tested settings located at the maximum (100%), which did not support our hypothesis. It is possible that these findings are also tied to the reflexive controller of the BiOM. Many subjects exhibited a plateau or a decrease in net ankle work past the 50% power setting, which also corresponded to little change in COT past this power setting. Accordingly, it appears that some subjects down-regulated the amount of work they received from the device at the higher power settings, most likely by not loading the device with their full body weight and exploiting the reflexive nature of the controller. As our experimental setup did not include force plates, we were unable to experimentally confirm this in the current study.
When we examined the relationship between net ankle work and COT (with all subjects pooled), we found a moderate linear correlation (r&#;=&#;&#;0.55). Yet, the best fit linear model only resulted in R2&#;=&#;0.30, which highlights the variability between subjects. On an individual basis, we found much stronger linear relationships between net ankle work and COT (r&#;=&#;&#;0.82&#;±&#;0.15; R2&#;=&#;0.69&#;±&#;0.22). Accordingly, most of the subjects tested (6 of 8, excluding Subject 8) had best tested conditions that corresponded to their maximum ankle work (see Fig. 5). For Subject 3, the difference in COT between the best tested condition (75%) and the condition with the maximum net ankle work (50%) was well below the minimum detectable change for COT (<0.01)26; this difference was much larger in Subject 6 (0.04). We could not complete this analysis for Subject 8, because we were unable to collect BiOM ankle work data during the 100% condition. Therefore, the power setting that maximized the ankle work delivered by the device was energetically optimal (or very close to energetically optimal) for nearly all participants in this study. Given this trend, however, it is also interesting to note that the net ankle work that corresponded to the best tested condition was quite variable between subjects, ranging from approximately 1.5 times (min: 0.14&#;J/kg) to 3.5 times (max: 0.36&#;J/kg) the work of a typical biological ankle.
The reflexive controller and corresponding variability in net ankle work that users produced between 0% and 25% power conditions could also explain why we saw an increase in COT for some subjects between these conditions. Although there were no significant group differences between these conditions, we observed an increase in COT between the 0% and 25% power conditions greater than the MDC for Subjects 1, 2, and 5 (mean difference: 0.04&#;±&#;0.02, n&#;=&#;3, see Fig. 2). Qualitatively, we noticed that these same subjects were those who elicited only a modest increase in net ankle work between 0% and 25% power conditions (mean difference: 0.01&#;±&#;0.01&#;J/kg, n&#;=&#;3, see Fig. 4). Similarly, those five subjects (S3, S4, S6, S8, S10) who elicited substantially more ankle work at 25% than at 0% (mean difference: 0.07&#;±&#;0.02&#;J/kg, n&#;=&#;5) exhibited a decrease or very small increase in COT between these conditions (mean difference: 0.00&#;±&#;0.01). The final subject (S9), exhibited very little change in ankle work or COT between the 0% and 25% power conditions. These results seem to classify our subject cohort into two groups when a small amount of power was provided: those who took advantage of the power, and those who appeared to modify their behavior to avoid receiving power from the device and correspondingly increased their COT. If the work the device performed at this power setting was disruptive to the natural walking dynamics or balance of the subject, it is possible that they were &#;fighting&#; the device by increasing muscular co-contraction or adopting atypical compensatory gait mechanisms, which may have driven up metabolic cost for those participants32. Given our small sample size and the high level of breath-by-breath variability in the metabolic measurements, further analysis of additional biomechanical quantities (e.g., electromyography, inverse kinematics, spatiotemporal parameters) will be conducted to investigate these mechanisms in detail and to determine quantitative relationships, if any.
This study presents quantitative evidence regarding how users respond differently to various ankle power settings and exploit the BiOM&#;s reflexive controller in order to reduce their metabolic cost. There are several hypothetical reasons that users might adapt their gait to reduce the plantarflexion power they receive from the device at various power settings. Users could be actively off-loading the device at higher power settings because they feel uncomfortable or unstable, and resultant compensatory gait strategies or &#;fighting&#; the device could lead to higher energy consumption. However, it is also possible that users are subconsciously or passively adapting their interaction with the device in order to optimize a physiological objective function, such as minimal metabolic cost33, minimal impact forces34, or maximal stability35, among others. Further detailed analyses of additional biomechanical measures (e.g., electromyography, ground reaction forces) are required to elucidate the underlying causes for these observations. The results of this study provide some insight into additional elements of prosthetic control that may be necessary to reduce metabolic cost, beyond only the magnitude of ankle power delivered. The complex interactions between prosthetic control, metabolic cost, muscle activity, joint kinetics, stability, and patient satisfaction remain an important topic for continued future research in order to inform powered prosthetic ankle prescription and improve patient outcomes.
Our study is not without its limitations. First, there was a limited sample of participants, and due to the walking stamina necessary to complete the experiment, we only tested active, healthy individuals. People with higher levels of ambulatory function (K3-K4) are capable of walking with variable cadence and performing advanced ambulation tasks, and may be able to better adapt their gait in order to take advantage of power from the device. Additional studies with a modified protocol will be necessary to determine how these results extend to individuals with lower levels of ambulatory function. Second, it is possible that the five-minute acclimation time was insufficient for some participants to adjust to each power setting. Compared to other studies in which the users had hours10, multiple sessions21, or even weeks13 to acclimatize to a powered device and its conditions, the users in our study received less time to familiarize themselves with the power settings. Although the results from our study suggest that some participants were able to adapt to the power delivered from the device in this short amount of time, it is possible that some subjects may have required more time and/or specific training to fully adapt to each setting. Third, to prevent physical fatigue and respect time constraints, we tested participants in increments of 25%, which is a very coarse sampling of the parameter space, and may have limited our ability to identify the true energetically optimal power setting for all users. Due to device limitations, we could not test users past 100% power (0.24&#;±&#;0.07&#;J/kg), so it is also possible that a more energetically favorable setting exists outside our tested range. As we do not know the exact physiological relationship between power setting and COT, we cannot currently extrapolate these results outside the tested range. Fourth, our experimental setup did not include an instrumented force treadmill so we were not able to experimentally validate the agreement between the BiOM&#;s step-by-step calculations of net ankle work and average peak ankle power and those values obtained through inverse dynamics. Future studies with the BiOM prosthesis that include an instrumented treadmill will further improve the generalizability of this study&#;s results. Finally, it is important to point out that our study was limited to evaluating the best power setting while participants walked on a level treadmill at a constant speed. It is likely that the optimal power setting would change when users walked at different speeds, at an incline or decline, or over variable terrain, so this study can not make universal claims about the optimal power setting for all tasks.
In conclusion, to minimize their metabolic energy consumption, subjects in this study required a higher power setting than the setting chosen by the prosthetist to approximate biological ankle kinetics. On average, the power setting setting which minimized energy cost corresponded to approximately double the net ankle work of the biological ankle. Furthermore, subjects walking at their best tested power setting exhibited a meaningful decrease in cost of transport compared to walking with their prosthetist-chosen power setting, which suggests that individuals may benefit metabolically from prescribed ankle power that exceeds biological norms. However, the varied responses between subjects also point to the need for subject-specific parameter tuning. As one solution, recent work has demonstrated the feasibility of automatically tuning assistive device parameters to minimize metabolic cost (i.e., body-in-the-loop optimization)36,37,38,39, and continued research in this area has the potential to impact clinical device prescription. Finally, subjects&#; net ankle work was highly variable at different power settings, likely due to the reflexive controller of the BiOM and the user&#;s adaptation to the device. As such, future work should focus on quantifying the mutual adaptation of the human user and the device to inform the design of optimal powered prosthesis controllers for individuals with transtibial amputation.
10 Questions Doctors Should Ask About Prosthetics
Questions you should ask your chosen central fabrication provider.
Doctors only want the best for their patients. Thus, when choosing a prosthetist, they naturally want to find a reliable partner who can deliver the best prosthetic device for their patients. At the end of the day, prosthetics aim to improve your patients&#; quality of life, and this is only possible if you work with some skilled and experienced.
So, to help you, check these 10 key questions about prosthetics you should ask to find a central fabrication partner for your patients.
How long have you been in the industry?
While not directly related to fabricating prosthetics, this is still one of the most important questions about prosthetics to ask. It gives you a good idea of the company&#;s experience in the field, which is critical to ensure that they know the best practices.
Do you have proper accreditation?
Another way to verify expertise is to ask them if they have industry accreditations. When it comes to orthotic prosthetic work, you should look out for ABC accreditation since this is the highest standard given to professionals in central fabrication.
Are your technicians certified?
Contact us to discuss your requirements of china prosthetic ankle joint. Our experienced sales team can help you identify the options that best suit your needs.
Get to know more about the people who will be designing and building the prosthetics. Like the previous point, your goal is to find ABC-certified technicians with proper knowledge and qualifications for central fabrication.
What type of devices can you fabricate?
This question gives you a good idea of the range of prosthetics that a company can make. Naturally, it would be best to work with someone who can fabricate all types of devices to cater to your patients&#; needs. Generally, these include upper extremity, above knee, and below knee prosthetics.
How long does it take to get the prosthetics?
Fast turnaround times is an important consideration since you want to give your patients their prosthetics as early as possible. Thus, you should ask this early on to set expectations on how long it will take to get the prosthetics you order.
What do you use to make the prosthetics?
Of course, you want your patients to always get high-quality and durable prosthetics. So make sure you ask about the components and materials a company uses to see if these are industry standard.
How much do the prosthetics cost?
While it might be hard to get exact rates due to differences in each prosthetic device, you should still be able to get an estimate. This gives you a good idea of whether their services are within your budget.
How do you ensure that prosthetic devices fit properly?
Prosthetic limbs are never a one-size-fits-all since they need to be made based on a user&#;s needs and measurements. Given this, it helps to learn about what processes or tools a company uses to get the optimal fit. The last thing you want is to have problematic devices for your patients down the line, so asking this question early can help you decide better.
Do your prosthetics look like real limbs?
The prosthetic design can vary from one maker to another. However, some patients may have concerns about how their prosthetics will look, so you can try finding a company with technologies to create realistic-looking devices. For example, some have spray skins that can replicate a patient&#;s skin tone accurately.
What happens if there are problems with the prosthetics?
You never know when problems may arise with prosthetics, so it helps to have some form of assurance that you can repair or return them. As much as possible, find a company that offers warranty for their devices.
Contact Us Today for Your Prosthetic Needs
Grace Prosthetic Fabrication is a leading provider of central fabrication services with over 30 years of experience in the industry. With our advanced technologies, high-quality craftsmanship, and certified technicians, you can trust us to take over the fabrication work while you work with your patients&#; physical therapist and healthcare team.
Whatever your prosthetic needs may be, we are ready to help you. Contact us today at 1-800-940- to get answers to your questions about our prosthetics and orthotics.
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