Journal of NeuroEngineering and Rehabilitation

Background Energy storing and return prosthetic (ESAR) feet have been available for decades. These prosthetic feet include carbon fiber components, or other spring-like material, that allow storing of mechanical energy during stance and releasing this energy during push-off [ 1 ] . This prop- erty has long been claimed to reduce the metabolic en- ergy required for walking and hence improve walking economy. However limited scientific evidence has been found to corroborate this hypothesis [ 2 – 7 ] . Biomechan- ical studies have demonstrated enhanced mechanical en- ergy storage in early stance and a considerable increase in positive power during push-off while using ESAR feet compared to conventional rigid feet [ 8 – 11 ] . In addition, studies have demonstrated that the increased external mechanical work during prosthetic walking seems to de- pend on a reduced push-off power [ 12 ] and that this is mitigated when walking with ESAR feet [ 9 , 13 ] . Never- theless these effects on mechanical energy transfers during walking, do not clearly translate into positive ef- fects on metabolic energy expenditure and gait economy [ 14 , 15 ] . It has been suggested that positive effects of in- creased mechanical ankle push-off power, are negated by an increased muscle activation required for body support or to control power transfer across residual joints in the prosthetic leg [ 16 – 18 ] . Despite the apparent absence of increased walking economy, ESAR feet remain the feet of preference for most people using lower limb prostheses [ 19 , 20 ]. This gives rise to the consideration that other functional ben- efits, beyond economy, should exist. It has previously been shown that ESAR feet could reduce mechanical load, and therefore potentially prevent overload injuries in prosthetic or intact leg [ 21 ] . Alternatively, recent in- sights in the gait pattern of people with a lower limb amputation suggest that the enhanced ankle push-off power with an ESAR foot might enhance gait stability and improve gait symmetry [ 22 ] . A stable gait requires that the body ’ s center of mass is controlled relative to the continuously changing base of support, i.e. the stance foot. In the fore-aft direction, this entails that the body ’ s center of mass needs to pass the leading foot during each stance phase, otherwise an interrupted forward progression or backward fall will occur [ 23 ] . The likelihood for the center of mass to success- fully pass the leading foot can be assessed using the ‘ margin of stability concept ’ postulated by Hof et al. [ 24 , 25 ] . Based on the inverted pendulum characteristics of human gait the position of the center of mass over time can be predicted based on its initial position, its velocity and the natural fre- quency of the inverted pendulum. Using these parameters, the so-called extrapolated center of mass can be calculated (Fig. 1 ) . To maintain forward progression and make a subsequent step, the extrapolated center of mass needs to project anterior to the posterior border of the leading foot at the instant of toe-off of the trailing leg. The distance be- tween the extrapolated center of mass and posterior border of the foot indicates the backward margin of stability. The smaller the backward margin of stability, the bigger the chance that the center of mass will not pass the foot in the presence of perturbations during single leg stance. Recently, we have [ 22 ] demonstrated the effect of a re- duced ankle push-off power on regulating the backward margin of stability during the intact step in people with a lower limb prosthesis. It was shown that due to a re- duced ankle push-off power the center of mass velocity at toe-off of the prosthetic leg is lower compared to the contralateral step. This results in a reduction in the for- ward projection of the extrapolated center of mass and hence a potentially reduced backward margin of stability. To preserve sufficient backward margin of stability, people walking with a lower limb prosthesis appear to reduce intact leg step length, even though this inevitably leads to step length asymmetry (Fig. 1 ) . From this mech- anism, it can be derived that a prosthetic foot and ankle that increases push-off power might be beneficial as it would allow the user to improve gait symmetry without reducing the backward margin of stability. In this study, we investigated the potential effect of en- ergy storing and return feet on gait symmetry and back- ward margins of stability in a group of people with a transtibial amputation. We compared level ground walking using an ESAR foot and a SACH foot and hypothesized that the higher push-off power of the ESAR foot compared to the SACH foot will increase center of mass velocity at toe-off, increase intact step length and step length sym- metry without reducing the backward margins of stability. Methods Data used for this study was previously collected and published to assess differences in external work during walking with ESAR and SACH feet [ 9 ] . The specific de- tails on data collection and analysis relevant for the current study are outlined below. Participants Fifteen male participants with a transtibial prosthesis (age 55.8 ± 11.1 yr., weight 86.0 ± 12.6 kg, height 1.74 ± 0.04 m) were included in this study. All participants underwent am- putation due to trauma, were classified at K3 level, and were free from other musculoskeletal, neurological or cardiovas- cular co-morbidities. All participants had walked with an ESAR prosthetic foot for at least two years before inclusion in the study. They were all informed on the study aim and procedure and provided written informed consent. The study was approved by the INAIL research board (Commissione Tecnico Scientifica, Budrio, Italy), and per- formed in accordance with the declaration of Helsinki. Houdijk et al. Journal of NeuroEngineering and Rehabilitation 2018, 15 (Suppl 1):76 Page 42 of 72