# Impact of Visual Kinesthetic Illusions on Reciprocal Inhibition and Motor Function
## 1 Introduction  
Reciprocal inhibition (RI) is crucial in facilitating smooth joint movement. Research has indicated that inhibitory interneurons in the spinal cord contribute to RI [1, 2]. Peripheral inputs and the activity of supraspinal regions, including the primary motor cortex (M1) and supplementary motor cortex, modulate the interneurons, contributing to RI [3, 4, 5, 6, 7, 8]. Previous studies have indicated that RI declines in individuals with upper motor neuron disorders or older adults [9, 10]. A reduction in RI results in joint movement issues due to the co-contraction of opposing muscles, negatively impacting walking and balance capabilities [11].  
Repetitive passive movement (RPM) is a technique used to enhance RI. This intervention involves executing repetitive, passive motions on the targeted joints. Our research team has identified optimal RPM parameters for the ankle joint: a higher angular velocity (\(40^{\circ}/s<160^{\circ}/s\)) and a broader range of motion (\(20^{\circ}<40^{\circ}\)) [12]. Our research demonstrates that focusing attention on the RPM intervention site prolongs the after-effect on RI compared to not directing attention there [13]. Also, focusing attention on the RPM intervention site during RPM reduces the activity of intracortical inhibitory circuits, including short latency afferent inhibition (SAI) [14], and increases motor evoked potential (MEP) [15], which indicates corticospinal tract excitability. However, while it is necessary to bolster RI, RPM may not deliver the intended intervention results for patients who face limitations relating to its use, such as spasticity and restricted range of motion caused by upper motorneuron disorders, spinal cord injuries, and aging. In particular, in clinical settings, patients suffering from upper motor neuron disorders and spinal cord injuries and older individuals requiring RI enhancement due to diminished RI function may experience limited joint mobility stemming from excessive spasticity or joint contractures. Consequently, they might find it challenging to receive RPM with effective intervention parameters, leading to a lack of anticipated benefits.  
Sensory input manipulation can induce the experience of body illusion [16]. One specific type of illusion, known as visual kinesthetic illusion, occurs when an individual at rest perceives bodily movement or feels the urge to move a body part while observing a video of that part in motion [17; 18]. The body illusion enhances motor cortex excitability by decreasing SAI [19; 20; 21]. Moreover, the visual kinesthetic illusion of ankle joint movement reportedly increases corticospinal tract excitability [22]. We consider that visual kinesthetic illusion enhances M1 excitability and promotes the activation of RI's inhibitory interneurons by descending the input, thereby improving RI performance.  
We herein aimed to explore how visual kinesthetic illusion impacts RI and ankle joint motor function. We hypothesized that visual kinesthetic illusions boost the activation of inhibitory interneurons of RI via descending input, which results from heightened M1 excitability, thereby enhancing RI. Enhancement of RI is also expected to diminish co-contraction between antagonistic muscles and improve ankle joint motor function.