The preload force affects the perception threshold of muscle vibration-induced movement illusions

An investigation of proprioception illusion using a stimulator with feedback control

It is possible to create illusions of limb movements using vibrations over the skin. If a muscle is vibrated it can feel as if the limb is moving while it remains still. These illusions have been studied for decades but it is not yet entirely clear how to create them effectively and repeatedly. In this study, three parameters were varied; the frequency of the vibration, the stimulation site and the arm position. A closed loop control of the vibration frequency was used to ensure a fixed frequency over the stimulation time and across the participants. The experiment included twenty-five able-bodied participants (mean age 32±7 years, 9 females). A hanging arm position was introduced with the aim to increase the success rate of illusions compared to other studies. Twenty-four participants felt an illusion across all scenarios. The results highlight that tactile feedback affects the illusion.

The tightening parameters of the vibratory devices modify their disturbing postural effects

The purpose was to specify the impact of two different forces exerted by vibratory devices on the Achilles tendon on postural balance. The postural balance of 13 participants was evaluated on a force platform in two 40 s bipedal stance conditions with closed eyes. Tendon vibrations (80 Hz) were triggered 10 s after the beginning of the postural evaluation and applied during 20 s. Two levels of the force exerted by the vibrators were calibrated using load cells to control the tightening parameters of the vibrators: a strong tightening (ST) condition at 45 N and a light tightening (LT) condition at 5 N. The soleus electromyographic (EMG) activity and the spatio-temporal parameters of displacement of the centre of foot pressure (COP) were analysed. To analyse the effects of the introduction, the adaptation and the end of the stimulation, non-parametric tests were used. The results indicated that the soleus EMG activity increased only in the ST condition. However, during the vibration the anteroposterior COP position was significantly more in a backward position in the LT condition. At the end of the vibration, COP parameters increased more in the LT condition than the ST condition. This study demonstrated that the effects of the vibration depended on the force exerted by the devices on the tendons. The ST increased the vibration effects on EMG activity through greater stimulating effects compared to the LT. However, the ST could also increase the ankle joint stiffness and/or somaesthetic sensory information, which attenuated the COP backward shift.

A Cost-Effective Inertial Measurement System for Tracking Movement and Triggering Kinesthetic Feedback in Lower-Limb Prosthesis Users

Advances in lower-limb prosthetic technologies have facilitated the restoration of ambulation; however, users of such technologies still experience reduced balance control, also due to the absence of proprioceptive feedback. Recent efforts have demonstrated the ability to restore kinesthetic feedback in upper-limb prosthesis applications; however, technical solutions to trigger the required muscle vibration and provide automated feedback have not been explored for lower-limb prostheses. The study's first objective was therefore to develop a feedback system capable of tracking lower-limb movement and automatically triggering a muscle vibrator to induce the kinesthetic illusion. The second objective was to investigate the developed system's ability to provide kinesthetic feedback in a case participant. A low-cost, wireless feedback system, incorporating two inertial measurement units to trigger a muscle vibrator, was developed and tested in an individual with limb loss above the knee. Our system had a maximum communication delay of 50 ms and showed good tracking of Gaussian and sinusoidal movement profiles for velocities below 180 degrees per second (error < 8 degrees), mimicking stepping and walking, respectively. We demonstrated in the case participant that the developed feedback system can successfully elicit the kinesthetic illusion. Our work contributes to the integration of sensory feedback in lower-limb prostheses, to increase their use and functionality.

Altered Proprioceptive Feedback Influences Movement Kinematics in a Lifting Task

Movement control process can be considered to take place on at least two different levels: a high, more cognitive level and a low, sensorimotor level. On a high level processing a motor command is planned accordingly to the desired goal and the sensory afference, mainly proprioception, is used to determine the necessary adjustments in order to minimize any discrepancy between predicted and executed action. On a lower level processing, the proprioceptive feedback later employed in high level regulations, is generated by Ia sensory fibers positioned in muscle main proprioceptors: muscle spindles. By entraining the activity of these spindle fibers through 80Hz vibration of triceps distal tendon, we show the intriguing possibility of inducing kinematics adjustments due to negative feedback corrections, during a lifting task.

Use of imperceptible wrist vibration to modulate sensorimotor cortical activity

Peripheral sensory stimulation has been used as a method to stimulate the sensorimotor cortex, with applications in neurorehabilitation. To improve delivery modality and usability, a new stimulation method has been developed in which imperceptible random-frequency vibration is applied to the wrist concurrently during hand activity. The objective of this study was to investigate effects of this new sensory stimulation on the sensorimotor cortex. Healthy adults were studied. In a transcranial magnetic stimulation (TMS) study, resting motor threshold, short-interval intracortical inhibition, and intracortical facilitation for the abductor pollicis brevis muscle were compared between vibration on vs. off, while subjects were at rest. In an electroencephalogram (EEG) study, alpha and beta power during rest and event-related desynchronization (ERD) for hand grip were compared between vibration on vs. off. Results showed that vibration decreased EEG power and decreased TMS short-interval intracortical inhibition (i.e., disinhibition) compared with no vibration at rest. Grip-related ERD was also greater during vibration, compared to no vibration. In conclusion, subthreshold random-frequency wrist vibration affected the release of intracortical inhibition and both resting and grip-related sensorimotor cortical activity. Such effects may have implications in rehabilitation.