The proprioception illusion can simulate limb movement in persons with limb difference

The proprioception illusion occurs when cutaneous vibrations are applied to a muscle tendon which triggers nerves to signal to the brain that the muscle is lengthening, and therefore the limb is moving. This phenomenon has been used in a wide range of studies. One potential use of the illusion is for sensory feedback for limb prostheses, but this has not been investigated using non-invasive methods. This is the first reported non-invasive test of the illusion with limb different participants. The responses of sixteen persons with upper arm differences (eight congenital and eight acquired) were measured over a range of frequencies and locations. Eighty seven percent of participants (n = 14) confirmed feeling illusionary movements, only two participants did not. Participants felt extension (n = 7) and flexion (n = 7) of the elbow, humeral abduction (n = 10) and adduction (n = 6) and rotation of their upper arm (n = 9). Statistical analyses of 5-point likert scores revealed that arms in the hanging position had significantly more vivid (mean ± SD: 2.47 ± 1.44 vs. 2.13 ± 1.35) and longer duration (2.52 ± 1.52 vs. 2.19 ± 1.47) illusions with a greater perceived range of movement (1.91 ±1.09 vs. 1.78 ± 1.09) compared to when the arm was supported. There were no significant differences in illusionary movements between stimulation frequencies or sites.