Research & Presentations
Finding the Right Balance Both prosthetic knee and ankle-foot technologies influence standing balance outcomes By David Moser, PhD, BEng, BSc, and Mike McGrath, PhD
Background: Standing The act of standing still and maintaining balance may seem quite simple. However, despite this apparent simplicity, it is quite literally a complex balancing act and a marvelous feat of biomechanical control. If you reach forward to pass an object or simply point to something, your body’s center of mass (COM) will move, and whether you realize it or not, you’ll make subtle motor adjustments that affect balance to allow for this change. Next time you are standing in a crowd, observe those around you and you will realize that most people don’t stand still for very long. You will notice that they are continuously moving, swaying slightly, and shifting their weight around. For an individual with limb loss, this is much more challenging and often hazardous, as the body COM is decentralized due to the missing limb mass. Compromised sensory feedback and a loss of active motor control at the knee and ankle, both of which are used to recover from unbalanced situations, add to the difficulty and risks when standing, particularly on uneven surfaces. A key problem that arises is a lack of adaptation within the prosthetic limb to adjust for variations in changing ground inclines, which affects limb loading. The consequence is often pain and discomfort at the socket interface, which can result in an unloading compensation strategy to alleviate the discomfort. A vicious circle is created because the
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resulting uneven inter-limb loading then further compromises balance stability, safety, and clinical outcomes.
Why Alignment and Combined Component Selection Matter When considering normal biological control of multiple joints during standing, it is interesting to observe that we rarely stand with the hip, knee, and ankle joint at the full extent of motion. When standing on level ground, only the knee is close to a fully extended position; both the hip and ankle joints
will be positioned well within the available range of motion. When static, the ground-reaction force is projected from the center of pressure (COP) and passes ahead of the ankle joint (creating an external flexion moment), while at the same time passing slightly ahead of the knee joint, and through or slightly behind the hip joint. Upright stability is created with very little muscular effort in the form of a closed isokinetic chain by the interplay of passive tension created within the limb. Therefore, when considering the combined effect of multiple
Figure 1
Illustration showing the effect changes to ankle joint AP alignment can have on limb stability. The red line represents the ground-reaction vector. Large changes to AP ankle position can cause excessive flexion and extension tendencies, which require physical compensation.
