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IINTRODUCTION Research shows that transfemoral amputees are a high risk group for low back pain/injury occurring from repetitive overuse caused by inherent inequality in muscular use (Devan et al., 2014). Powered prosthetic components have been anecdotally believed to have the potential to aid in minimizing the risk of occurrence of such overuse low back injury in transfemoral population. The impact of powered prosthetic components in the context of minimizing the risk of low back pain/injury in transfemoral population has just begun to be explored (Jayaraman et al., 2018). Through this case series, we compare the performance between microprocessor controlled knee (MPK) and a powered Knee-Ankle prosthesis (PKA) (Lawson et al. 2014) in transfemoral population (K3 and K4). We investigate the efficacy of powered knee-ankle prosthesis for clinically relevant aspects like energy efficiency, low back muscle activation profiles and range of variable cadence in comparison to MPK. METHOD The Northwestern University Institutional Review Board approved this study and informed consent was obtained from subjects prior to participation. K3 or high activity level amputees using MPK’s were recruited. In this case series, performance metrics analyzed from 4 subjects (Genium (n=2), Rheo (n=1) and Cleg (n=1)) will be presented. Participants were randomized to start with either their clinical MPK or prototype PKA. Participants were evaluated by a prosthetist for appropriate component fitting with the study device. No changes were made for the predicate MPK device. The PKA was custom tuned for each participant to optimize gait kinematics similar to healthy ambulation, comparable to gait optimization done for clinical MPK fittings. Following optimization, participant underwent up to 12 training sessions to ensure acclimation. Safe and independent performance of the benchmark activities over three consecutive training sessions were used as a threshold to indicate the successful completion of training, with both devices. The testing protocol included a myriad of tests to compare the performance of each device on metabolic, gait kinematics, and low back muscle activation between the devices. The tests included, a modified graded treadmill test, walking over a force plate with motion markers for high resolution biomechanics and a dual task protocol. Cross over to the second device occurred following a minimum two-month wash out period. RESULTS Walking with a PKA improved energy efficiency (reduced weight normalized O2 cost ml/kg//min), reduced exertion (Borg RPE), promoted relatively greater symmetry in low back muscle (Lumbar L3 erector spinae (Figure 1)) activation and allowed participants to achieve a wider range of variable cadence than with their MPK (C-Leg, Genium, Rheo). [ABSTRACT FROM AUTHOR]