Journal of NeuroEngineering and Rehabilitation

Background Lower-limb amputees tend to walk asymmetrically when looking at gait temporal and loading parameters, with more time spent and load exerted on the intact limb [ 1 – 9 ] . Temporal asymmetry is typically measured based on step or stance duration; loading asymmetry based the magni- tude of the first peak of the vertical ground reaction force (GRF), and the impulse of GRF [ 2 , 3 , 6 , 10 ] . Temporal and loading asymmetries were associated to several comorbidities [ 5 ] : increased falls [ 11 ] , osteoarth- ritis of the sound limb [ 10 , 12 – 15 ] , osteoporosis of the contralateral limb [ 15 , 16 ] , back pain [ 17 – 20 ] . In addition, walking in public with noticeable asymmetries attracts the general attention [ 21 ] , which can be very uncomfortable for some prosthesis users. With this background, it is not surprising that a common, almost unquestioned [ 22 ] , goal for rehabilitation is to regain a symmetric walking [ 9 , 23 ] . However, the literature does not clearly indicate that striving for perfect symmetry is really and always the best option. Already in 1998, Winter & Sienko [ 1 ] stated that “ human system with major structural asymmetries in the neuromuscular skeletal system cannot be optimal when gait is symmetrical. Rather, a new non-symmetrical opti- mal is probably being sought by the amputee within the constraints of his residual system and the mechanics of his prosthesis ” . Later in 2005, Schmid and co-workers [ 3 ] compared the center of pressure trajectories under the sound and prosthetic foot of transfemoral amputees and concluded that the longer stance on the sound side can be ascribed to the greater ability of the sound leg to advance the step and maintain balance until the prosthetic limb can sustain the body weight. Hof et al. [ 4 ] corroborated this explanation in the theoretical framework of the “ extrapolated center of mass ” [ 24 ] , concluding that stance time asymmetry is a “ sensible adaptation ” of experienced transfemoral amputees to improve stability during walking, to overcome the missing lateral ankle strategy of prosthetic feet. More recently, Adamczyk & Kuo [ 8 ] , with a theoret- ical and experimental approach involving transtibial ampu- tees, concluded that “ some asymmetry may be unavoidable in cases of unilateral limb loss ” due to the reduced ankle plantar flexion of the ankle, with direct consequences on stance duration, greater collision work at the sound side, greater work overall, and increased peak force at loading response [ 25 – 27 ] . Imposing symmetry can actually be detrimental, as also observed by [ 27 , 28 ] . The evidences from the literature, therefore, indicate that optimal symmetry ratios might exist, to obtain a com- promise among stability, forward progression, preservation of body structures and perception of a “ normal and sym- metric biped locomotion ” [ 21 ] . Unfortunately, at present not only optimal symmetry ratios are unknown, but we also don ’ t know typical symmetry ratios or which measures of symmetry are essential and which are redundant. In our opinion, 3 methodological and 3 clinical questions should be answered to clarify these open issues. The methodological questions are: – Q1: do all amputees show the typical M-shaped pattern of the GRF [ 29 ] , with presence and appropriate timing of its two peaks? In case of a negative answer, the measure of loading symmetry based on the first peak of GRF will be restrict to patients presenting the M-shaped pattern; – Q2: can we limit the study of temporal symmetry to stance, leaving out step symmetry? We will give a positive answer if stance and step symmetries are very strongly correlated for all amputees, with a coefficient of determination R 2 > 0.64 [ 30 ] ; – Q3: can we further limit the study of gait symmetry to just stance symmetry, leaving out loading symmetry, whose measure requires more cumbersome and expensive equipment? We will give a positive answer if stance symmetry is very strongly correlated ( R 2 > 0.64) with the symmetry of the first peak and impulse of GRF. The clinical questions are: – Q4: does gait symmetry depend on the level of amputation? In case of a positive answer, typical ranges of symmetry should be established, which can be used to understand how far a new patient is from well adapted prosthesis users in terms of percentiles; – Q5: do advanced prosthetic components improve temporal and loading symmetry? In particular, do C-leg users have better results than mechanical knee users of the same mobility level? – Q6: is it always true that amputees overload the sound side both in terms of first peak and impulse of GRF, thus contributing to the development of osteoarthritis? Unfortunately, at present it is difficult to answer to these questions based on the available literature, because there are no studies that considered, at the same time 1) both temporal and loading asymmetries, 2) both trans- femoral and transtibial amputees treated at the same prosthetic & rehabilitation center, 3) mechanical and electronic knees, 4) energy-storage-and-return feet instead of the SACH (Solid-Ankle Cushion-Heel) foot. Moreover, the number of patients included is typically limited to 8, both for studies on transtibial and transfemoral amputees. Finally, no studies addressed the correlation between temporal and loading parameters. The aim of this study was to overcome these limita- tions and answer to questions Q1-Q6 on three groups of Cutti et al. Journal of NeuroEngineering and Rehabilitation 2018, 15 (Suppl 1):61 Page 30 of 72