Individuals with transfemoral amputations (TFAs) face both physical and psychological challenges to community ambulation tasks (Penn-Barwell, 2011). A common clinical solution for restoring mobility is the use of prosthetic devices, with microprocessor-controlled prosthetic knees (MPKs) considered the current standard of care (Stevens and Wurdeman, 2019). Previous studies have highlighted the benefits of MPKs, including the restoration of the ability to walk on different terrains (Highsmith et al., 2010), improvement in joint kinetics symmetries (Kaufman et al., 2012), and an overall enhancement in quality of life (Samuelsson et al., 2012). The research community has also investigated powered prostheses over the past decades utilizing active power transmission to assist users in walking on varying terrains (Azocar et al., 2018, Lawson et al., 2014, Tran et al., 2022). For individuals with transtibial amputation, the active assistance allows powered prosthetic devices to generate significantly greater peak ankle power in level walking which results in decreases of metabolic cost of transport (COT) by 14 % compared to passive prostheses (Au et al., 2009, Ferris et al., 2012). It is shown that powered prostheses can replicate the biomechanics of the knee and ankle of the sound side joint kinematics, kinetics, and power with respect to gait phase during walking (Ingraham et al., 2016, Lawson et al., 2013, Quintero et al., 2018, Tran et al., 2022). While research has extensively focused on the functional performance of prostheses themselves, their impact on the biomechanics and health of the intact joints has been largely neglected.

Due to the loss of knee and ankle joints, individuals with TFA load the sound side leg more heavily than the residual leg (Harandi et al., 2020, Zhang et al., 2019). They often use compensatory strategies such as forward trunk motion and increased lateral pelvis tilt towards the prosthetic limb (Harandi et al., 2020, Henao et al., 2020, Koehler-McNicholas et al., 2016, Persine et al., 2022, Rougier and Bergeau, 2009). This shifts load bearing to the sound side leg (Nolan et al., 2003, Schaarschmidt et al., 2012). Overexertion of joints of the amputated limb that compensate for passive prostheses has been linked to joint diseases like knee osteoarthritis and lower back pain (Kulkarni et al., 1998, Norvell et al., 2005, Struyf et al., 2009). The risk of osteoarthritis could be reduced by an equal loading across the prosthetic and intact limbs during ambulation such as stairs (Gailey et al., 2008). In addition, walking on ramps and stairs is essential for individuals with TFA as their activities of daily living, contributing to their independence, overall well-being, and quality of life. Thus, understanding how prostheses affect the biomechanics of both biological and prosthetic joints and during challenging community ambulation tasks such as stairs and ramps is significant.

Computing a joint’s mechanical work as either the integral of its moment/torque with respect to its angular displacement or as the time integral of its instantaneous power provides a comprehensive measure of joint loading. This approach accounts for both the magnitude and duration of forces exerted on the joint, offering greater insight compared to analyses based solely on joint kinematics, moment, or power. Our previous study showed a 57 % reduction in sound side knee joint positive work and a 26 % reduction in sound side hip positive work per gait cycle during stair ascent with a powered prosthesis compared to step-to-step gait when using passive prostheses. In stair descent, a 28 % reduction in negative work at the sound side ankle was observed compared to passive prostheses (Camargo et al., 2023). In studies of individuals with transtibial limb loss powered prosthetic ankles did not significantly reduce the work required by the sound side hip in ramp tasks (Jeffers and Grabowski, 2017, Montgomery and Grabowski, 2018), despite achieving similar peak ankle power (Elery et al., 2020, Ferris et al., 2012, Takahashi et al., 2015). Current literature has separately examined the biomechanics of powered and passive prostheses for individual lower limb joints. However, no study has comprehensively compared the biomechanics and joint work of powered and passive knee-and-ankle prostheses during ramp and stair walking. Biological work—calculated as the sum of the joint work from the sound side lower limb and the prosthetic side intact hip joint—can help scientists and clinicians understand joint degradation and its long-term effects on prosthesis users. It serves as a valuable outcome measure in this context. As prostheses with powered (i.e., robotic) joints continue to advance, a comprehensive understanding of how a powered or passive prosthesis affects all lower limb joint biomechanics of an individual with TFA is critical.

The objectives of this research were to 1) deliver a detailed biomechanical comparison between a powered and passive knee-and-ankle prosthesis system, 2) quantify the effect of powered and passive knee-and-ankle prostheses on biological work. We hypothesized that, while both systems provide energy restoration at the ankle and energy dissipation at the knee, the powered prosthesis would 1) reduce positive biological work during ramp and stair ascent due to active knee extension and 2) reduce negative biological work during descent due to controlled energy dissipation.