Prosthetic Categories

The Differences in Hip Muscle Strength and Balance Among Above-Knee Amputees

    After amputation and before acquiring a new prosthetic limb, people with limb loss are often classified into five functional levels, also known as K-levels, ranging from the lowest, K0, to the highest, K4. Medicare uses this system to determine financial coverage. However, experts saw a need to refine further how classification is done.

     The differences in hip muscle strength and balance among above-knee amputees can help refine K-level classification.

    To determine a patient’s K-level, doctors and orthopedic technicians subjectively evaluate an individual’s ability to walk with a prosthesis. Only the range of motion of joints is assessed objectively. Healthcare professionals recognized this lack of objectivity.

    A clinical tool called the Amputee Mobility Predictor was developed to address the subjectivity of the existing K-level classification. It consists of 21 ambulation and balance tasks to be performed by patients with and without prostheses. While performance in functional and walking tests is typically associated with lower limb strength, several studies have shown that patients with above-knee amputation have significantly reduced strength in the residual limb compared to the sound side leg.

    Experts recognized the importance of the muscles surrounding the hip to stabilize the pelvis while standing and walking. They know that having weak hip abductors leads to the body compensating by shifting the trunk to the prosthetic side. This leads to poor balance and inefficient posture. Despite hip strength’s important role in finding the right prosthetic limb, the K-level classification doesn’t have standardized hip strength tests.   

    So, researchers sought to investigate the differences in hip muscle strength of the residual limb and differences in static balance parameters among patients with above-knee amputation assigned to different K-levels.

    The study  

    A total of 22 participants were considered for the study. Four participants were assigned to K1 or K2, six to K3, and 12 to K4. All had a single above-knee amputation and a post-amputation time of at least one year. The researchers excluded candidates if the amputation was caused by diabetes or if they had acute pain, open wounds, or edema in the residual limb.

    Using a custom-made diagnostic device, the researchers assessed the participants’ maximum isometric hip strength of the residual limb in hip flexion, abduction, extension, and adduction. Meanwhile, their static balance was examined in the bipedal stance on a force plate in eyes-open and eyes-closed conditions. 


    The researchers found visible strength differences between the K3 and K4 groups and between the K1/2 and K4 groups. Only a tiny difference was detected between the groups for hip abduction and adduction, hip extension, and hip flexion. Although the disparity was not statistically significant, the researchers noted that the values still imply a distinction between K-level groups.

    When determining balance capabilities, the researchers observed more significant body sway in the K1/2 group than in the K4 group, especially when the participants had their eyes closed.  

    The bottom line  

    The researchers concluded that the results show that muscle strength tests of the residual limb and static balance tests may serve as additional measures to improve K-level assignment for patients with lower-limb amputation.

    However, they emphasized that this was only an initial study, and they recommend studying these factors again with a larger number of participants.