Journal of NeuroEngineering and Rehabilitation

confounding effect of walking speed on the outcome pa- rameters, which would obscure interpretation of the underlying mechanics. Although strong evidence is lack- ing [ 15 ] , it has been suggested that people tend to walk slower with SACH feet compared to ESAR feet. Such a reduction in walking speed could be a strategy to cope with the indicated constraint on gait stability. While in this study we demonstrate that this specific constraint of SACH feet on step length symmetry and margin of sta- bility exists at equivalent speeds, it should be explored in the future how differences in walking speed influence this constraint. A second limitation is the fact that we included participants that were all currently using the ESAR Variflex foot. The SACH foot was provided to them for the purpose of this experiment. They were allowed a 24-h accommodation period at home to get used to the SACH foot. Moreover, they all had some previous experience with SACH and most of them use a SACH foot in their current bathing/in-house prosthesis. Nevertheless, we cannot rule out if a lack of acclimation on the SACH foot influenced our results. However, the effects found in our study agreed very well with the hy- pothesized differences between feet. This provides some confidence to the fact that hypothesized mechanical constraints indeed exist in prosthetic walking, although with more practice people might find smart strategies to cope with these constraints. Next to the effect of prac- tice other factors might have influence the observed ef- fects. Imperfect socket fit or alignment could affect step length and symmetry. We tried to minimize this effect by allowing participants to use their own socket with both feet, and by having a certified prosthetist optimize alignment of the SACH foot before the experiment. No participant indicated stump problems during the experi- ments. Another limitation of this study is that we only investigated one type of ESAR feet, i.e. Variflex (Össur, Iceland). In general, all ESAR feet do provide increased push power and are expected to allow for the improved control over the backward margin of stability and step length symmetry as found in this study. However, this general effect should be confirmed in other feet. More- over, assessing these parameters could be used as a benchmark test for different prosthetic feet. Finally, re- sults of this study were obtained in a group of relatively active persons with a transtibial amputation as a result of trauma. Generalization of the results to less active persons or different amputation causes or levels should be done with caution. For instance, within the popula- tion of people with transfemoral prosthesis, step length asymmetry is less consistently directed towards a shorter intact step length [ 36 , 40 ] . Given the limitations pre- sented above, generalization of the results to less controlled conditions and their contribution to the expe- rienced benefits of ESAR feet in daily life should be interpreted with care. However, we believe that this study design does reflect a basic mechanical constraint of prosthetic feet and the related effect on step length symmetry and margin of stability. Conclusion In conclusion, this study showed that the energy storing and return (ESAR) prosthetic foot can enhance center of mass propulsion, thereby allowing a symmetric gait pat- tern while preserving the backward margin of stability. These benefits on gait stability and symmetry might pos- sibly contribute to the general preference of people with a transtibial amputation for these dynamic prosthetic feet. Current findings can prove to be helpful in the design, prescription and evaluation of future prosthetic feet. Acknowledgements The authors would like to acknowledge the contribution of Leonardo Cavrini and Fausto Caprara (certified prosthetists/orthotists) for their help during the measurements and Davide Veronesi for the help in recruiting the participants for the measurements. Finally we would like the reviewers of this manuscript for their valuable comments and suggestions. Ethics approval and consent for participation This study was approved by the INAIL research board (Commissione Tecnico Scientifica; Budrio, Italy). No images, other material or personal details, which require individual ’ s consent, are presented in this manuscript. This study was retrospectively registered in the open science framework register ( https://osf.io/ezvt8/ ) . Funding This study was unfunded. The publication cost of this article was funded by the American Orthotic & Prosthetic Association (AOPA). About this supplement This article has been published as part of Journal of NeuroEngineering and Rehabilitation Volume 15 Supplement 1, 2018: Advancements in Prosthetics and Orthotics: Selected articles from the Second World Congress hosted by the American Orthotic & Prosthetic Association (AOPA). The full contents of the supplement are available online at https://jneuroengrehab.biomedcentral.com/ articles/supplements/volume-15-supplement-1 . Authors ’ contributions All authors have read and approved the final version of the manuscript. Consent for publication Not applicable. Competing interests The authors declare that they have no competing interests. Publisher ’ s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author details 1 Department of Human Movement Sciences, Faculty of Behavioral and Movement Sciences, Vrije Universiteit Amsterdam, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands. 2 Department of Research and Development, Heliomare Rehabilitation, Wijk aan Zee, the Netherlands. 3 Department of Health & Technology | Human Kinetic Technology, The Hague University of Applied Sciences, The Hague, The Netherlands. 4 Production Directorate, Applied Research, INAIL Prosthesis Center, Vigorso di Budrio, Bologna, Italy. Houdijk et al. Journal of NeuroEngineering and Rehabilitation 2018, 15 (Suppl 1):76 Page 47 of 72