## 1 Introduction  
Parkinson's disease (PD) is a chronic, highly disabling neurodegenerative disorder characterized by bradykinesia, resting tremor, rigidity and postural instability [1]. Postural instability has been shown to have a major impact on the autonomy, mobility and quality of life of these patients as it often leads to insecure stance and falls [2].  
Balance control is a complex system involving the integration of vestibular, visual and proprioceptive sensory information. Somatosensory information is derived from a variety of sources, most notably the cutaneous mechanoreceptors in the skin and the muscle spindles [3]. It is well known that a decline in skin sensitivity and proprioceptive input naturally occurs with aging. Moreover, kinesthetic sensory deficits may occur due to systemic and neurologic disorders (e.g., diabetes, stroke etc.) [3].  
Previous studies have demonstrated that the integration of sensory information is abnormal in PD patients with strong repercussions on motor function, posture, static and dynamic balance and greater reliance on visual input for postural control [4, 5, 6, 7, 8].  
The effectiveness of current treatments on postural instability and kinesthetic deficits in PD is controversial. [PERSON] et al. [2] observed improvement in postural control after treating rigidity with apomorphine. [PERSON] and colleagues [6] suggest that axial kinesthetic sensitivity worsens with levodopa. Furthermore, important side effects and loss of therapeutic efficacy after years of use of dopaminergic agonists represent a major clinical challenge [9].  
Muscle vibration is a safe, non-invasive rehabilitation technique that holds promise for improving mobility and balance in moderate to severe PD patients. It consists of local vibrations applied on selected muscles or tendons by means of a mechanical device. Stimulation of Ia afferent fibers from muscle spindles and cutaneous receptors strongly increases the exteroceptive and proprioceptive input to the central nervous system (CNS). This has been shown to induce a plastic modulation of the primary sensory motor cortex and spinal reflexes, with long lasting positive effects on muscle tone and motor control [10, 11, 12].  
Despite good results obtained in several other neurological conditions, such as post-stroke spatial heminattention, spastic hypertonia and motor control impairment [11], only a few studies exist on the use of focal vibratory energy for posture and gait disorders in [PERSON]'s disease. Most of the existing experiments in the literature are about whole-body vibration (WBV) which has shown limited efficacy on stability and movement control [13]. In contrast, focal vibration applied over the cervical and soleus muscles [14], and on quadriceps and paraspinal muscles [15], in conjunction with conventional physiotherapy, has shown positive effects on the patient's balance, risk of falling and quality of gait.  
Despite a consistent body of literature on the use of muscle vibration in rehabilitation, there is currently insufficient evidence regarding its application in patients with Parkinson's disease, including muscle targets, duration and frequency of the treatment and technical parameters. Based on these premises, we aimed to evaluate the effects of focal vibration on static and dynamic balance of patients with Parkinson's disease.