# Prosthesis Embodiment in Lower Extremity Limb Loss: A Narrative Review
## 4 Results
### Agonist-Antagonist Myoneural Interface  
Traditional amputation techniques achieve a stable residuum through either myodesis, the fixation of muscles to bones, or myoplasty, the fixation of muscles to each other [41; 50]. In healthy limbs, the agonist and antagonist muscles function in dynamic opposition: When one muscle contracts, the other stretches. This interplay is vital for proprioception. By leaving the residual muscles in static configurations, traditional amputation techniques disrupt the neuromuscular loops that support proprioception. Mechanoreceptors such as muscle spindles and Golgi tendon organs lose their functionality, severing the feedback to the central nervous system [50]. The agonist-antagonist myoneural interface (AMI) is a surgical construct that was first implemented by the Ewing amputation in [21]. The Ewing amputation technique addresses limitations in traditional amputations by reconstructing the natural agonist-antagonist muscle dynamics. As of 2022, only seven individuals received this procedure [51]. The procedure for BKAs begins with the preparation of the limb, followed by the identification and preservation of key muscle groups [52]. The identified agonist-antagonist pairs are [20] as follows:  
* The peroneus longus and the tibialis posterior, for subtalar eversion and inversion. * The lateral gastrocnemius and the anterior tibialis, for subtalar flexion and dorsiflexion. The distal tendon of each muscle is transected and reattached to its antagonist's proximal tendon, forming a closed-loop system that mimics natural agonist-antagonist dynamics [41; 52]. Peripheral nerves are carefully redirected to the motor branches of the reconstructed muscles through targeted muscle reinnervation to reduce neuroma formation and facilitate sensory and motor signaling [53]. The residual limb is shaped for prosthesis compatibility and then closed in layers. Although clinical applications of AMI have focused on people with BKAs, the technique can be applied to both above-knee amputations (AKAs) and upper-limb amputations [41]. In the context of the system diagram in Figure 1, the reconstructed muscles form the mechanical part of the proprioceptive mechanoneural interface. As described in [41], the neural part works as follows:  
1. The central nervous system contracts the agonist muscle using the usual motor nerve efferent control. 2. The efferent signal, measured limb positions and forces, and other prosthetic inputs are sent to a biomechanical controller that computes an appropriate proprioceptive afferent signal. 3. The nerve innervating the antagonist muscle is stimulated via electrodes to produce an appropriate afferent signal. AMI improves embodiment by restoring proprioception. Starting in the periphery, muscle spindles and Golgi tendon organs are vital receptors in proprioceptive sensation. When these peripheral receptors are activated, the resulting sensory information regarding limb movement and position is processed through the dorsal columns and the spinocerebellar tract of the spinal cord [54]. This information then activates several areas of the brain, including the parietal cortex [19], the frontal cortex, and the insula [55]. A key region is the Brodmann Area 3a (BA3a) in the somatosensory cortex, which integrates signals from mechanoreceptors in muscles, tendons, and joints, allowing the CNS to coordinate motor planning, movement, and force application [20; 56]. In traditional amputations, functional MRI (fMRI) studies reveal significantly lower BA3a activation in amputees with conventional procedures compared to non-amputees while dorsiflexing or plantar flexing their phantom limb. This decreased activity reflects a dependency on compensatory strategies, such as visual or vestibular feedback, which increase cognitive load and reduce movement efficiency [20; 50; 56]. Furthermore, the BA3a activation of AMI amputees was comparable to that of non-amputees [20]. The restoration of proprioception can also be observed functionally.