Plastic changes induced by muscle focal vibration: A possible mechanism for long-term motor improvements

Abstract

Repetitive focal vibrations can induce positive and persistent after-effects. There is still no satisfactory interpretation of the underlying mechanisms. A rationale, which can provide consistency among different results, is highly desirable to guide both the use of the application and future research. To date, interpretive models are formulated to justify the results, depending on the specific protocol adopted. Indeed, protocol parameters, such as stimulus intensity and frequency, intervention time and administration period, are variable among different studies. However, in this article, we have identified features of the protocols that may allow us to suggest a possible common mechanism underlying the effectiveness of focal vibration under different physiologic and pathologic conditions. Since repetitive focal muscle vibration induces powerful and prolonged activation of muscle proprioceptors, we hypothesize that this intense activation generates adaptive synaptic changes along sensory and motor circuits. This may lead to long-term synaptic potentiation in the central network, inducing an enhancement of the learning capability. The plastic event could increase proprioceptive discriminative ability and accuracy of the spatial reference frame and, consequently, improve motor planning and execution for different motor functions and in the presence of different motor dysfunctions. The proposed mechanism may explain the surprising and sometimes particularly rapid improvements in motor execution in healthy and diseased individuals, regardless of specific physical training. This hypothetic mechanism may require experimental evidence and could lead to extend and adapt the application of the “learning without training” paradigms to other functional and recovery needs.

Effectiveness of Focal Muscle Vibration in the Recovery of Neuromotor Hypofunction: A Systematic Review

“…As pointed out in other reviews [ 6 , 10 ], positive and persistent after-effects were obtained by applying small sinusoidal muscle stretches (<1 mm, preferably between 0.2–0.5 mm) at 100–150 Hz (most commonly at 100 Hz). The low amplitude is recommended to avoid triggering the tonic vibratory reflex that causes muscle fatigue during treatment [ 40 , 41 ].…”

“…Persistent and powerful proprioceptive improvements could favour compensatory conditions or trigger virtuous circles, capable of enhancing and ensuring the persistence of positive sequelae. Furthermore, as proposed in the model described above [ 10 ], improvements in spatial coordinates could allow for the refinement of developing motor strategies or the rapid development of new motor programmes.…”

“…RFV can generate synaptic potentiation in proprioceptive circuits and can shift the activity of neural circuits to a different level to allow the system to be more responsive and adaptive. The potentiation and increased resolution of proprioceptive signals induced by proprioceptive activation [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ] may lead to LTP-like plastic changes in cortical and subcortical circuits, promoting more effective motor planning and more accurate spatial coordination [ 10 ]. A study on a volleyball team [ 35 ] offered indirect evidence for this hypothesis.…”

“…RFV is demonstrated to be a powerful stimulus of the proprioceptive system and, when administered with proper characteristics at the local level, induces a neural response able to modify the motor drive [ 6 , 10 ]. On this basis, the RFV have the same target as proprioceptive training in rehabilitation.…”

“…The literature shows that sensory modalities can be improved by applying repeated sequences of a specific vibratory stimulation (repeated focal muscle vibration, RFV), which has been reported to selectively activate the neuromuscular spindle [ 10 ]. This prolonged artificial activation of the proprioceptive system can induce persistent after-effects in proprioceptive circuits and increase the sensitivity and discrimination of the sensory system, as occurs in the tactile, visual and auditory systems after repetitive activation [ 11 ].…”