If you or someone you know has a lower-limb amputation, you might have noticed volume differences in the residual limb, mainly when a prosthetic limb is often used.
A paper recently published in the Journal of Biomechanics detailed a study that sought to understand muscle volume differences in individuals with lower-limb loss. The researchers hope that the findings can help improve surgical techniques, rehabilitation, and prosthetic devices to enhance prosthetic users’ functional capability.
When an individual loses a major limb, the body changes to compensate for the loss. However, these compensatory strategies lead to muscle hypertrophy (enlargement) and atrophy (decrease in size) caused by over or under using specific muscles.
Since these physical changes are expected, researchers sought to measure the changes in residual limb size in individuals with above- and below-knee limb loss. The study used high-resolution MRI to determine muscle volume changes and predict these changes using the length, height, mass, and width of the femur (bone from hip to knee) and tibia (the larger bone between the knee and ankle).
Twelve individuals with lower-limb loss participated in the study. Their data were compared to six matched control subjects. Among the control subjects, those with unilateral below-knee limb loss showed muscle volume loss in the whole residual limb, while their sound side leg had minor volume changes. These findings show proof for a compensation strategy dominated by the sound side leg.
On the other hand, control subjects with bilateral above-knee limb loss showed significant muscle volume increase in the short adductor muscles. The adductor muscles pull the thighs together to the midline of the body. There is also a considerable volume decrease in the longer adductor muscles, hamstrings, and rectus femoris.
Single-Sided Below-Knee Limb Loss
After running MRI scans on both the control groups and study participants, the researchers confirmed that the total muscle volume in the residual limb and the minor volume changes in the sound side leg show a compensation strategy dominated by the intact leg. But the rectus femoris and lateral hamstrings in the participants’ intact leg were considerably smaller than those in the control subjects.
Furthermore, the researchers found that the participants’ gluteus medius in the residual limb was smaller than in the intact limb. This is most likely a result of compensation movements that decrease outward movements around the residual limb’s socket interface at the knee to avoid pain. Because of these compensatory strategies, there is a near absence of power generation in the residual limb knee at mid-stance. There is also reduced knee power absorption during swing and push-off. This explains why knee extensors in the residual limb are typically atrophied.
Bilateral Above-Knee Limb Loss
In participants with bilateral above-knee limb loss, the researchers discovered a compensation strategy dominated by the gluteus medius (one of the muscles underneath the buttock muscle), short adductor muscles, and hip flexors. Meanwhile, vasti muscles (three muscles located on the inside, middle, and outside of the thigh) are rendered functionally redundant, which explains significant volume reduction in the area.
Findings also show that the biarticular hamstrings in those with bilateral above-knee limb loss are significantly reduced. This is because these muscles are typically not used at the same level before limb loss. In non-amputees, the biarticular hamstrings are essential for maintaining postural stability. It controls and coordinates the hip and knee joints in response to change in upper body movements.
The results show that limb loss changes the function of muscles, which often results in poorly used muscles. The researchers noted a need to investigate the role of these underused muscles in the daily movement of individuals with limb loss.