Residual muscle activity associated with phantom limb movements after transhumeral amputation without surgical reinnervation: a preliminary ultrasound study.

Transhumeral amputations often result in significant functional limitations. Multiarticulated myoelectric prostheses for individuals with transhumeral amputations are often unreliable because of limitations in signal quality and challenges with accurately detecting motion intent from the available musculature in the residual limb, leading to high prosthesis rejection rates. Surgical approaches like targeted muscle reinnervation have been proposed to mitigate these challenges by biologically amplifying motor signals but are not suitable for all individuals. There is intriguing evidence that distinct muscle activation patterns associated with voluntary mobilization of the phantom limb may be produced in the residual limb after transhumeral amputation even without surgical intervention. However, this phenomenon has not been sufficiently investigated. This study explores the potential of ultrasound imaging to visualize muscle activity associated with phantom hand movements in an individual with transhumeral amputation who has not undergone targeted muscle reinnervation. We demonstrate that ultrasound imaging can capture these patterns, with each tested phantom movement corresponding to unique residual limb muscle activity. The activation patterns for each motion showed greater consistency across repetitions (correlation range: 0.41-0.86) compared to their average similarity with other motions (correlation range 0.06-0.32). These findings provide preliminary imaging evidence suggesting functional innervation of the residual upper arm muscles with digit-level specificity after transhumeral amputation. We recommend further research into modalities that can sense phantom limb mobilization to improve prosthesis control after transhumeral amputation.