## 1 Introduction  
Multiple sclerosis (MS) is an inflammatory autoimmune demyelinating disease of the central nervous system [1] affecting 2.8 million people worldwide [2]. Inflammation is the main cause of brain and spinal cord damage, which causes the destruction of myelin and axons [3]. The etiology of MS is largely unknown; however, it is assumed to be triggered by environmental factors in individuals with complex genetic risk profiles [2, 3]. Nevertheless, it is well known that the disruption of the electrochemical coupling of neurons, due to nerve damage, leads to dysfunction of the sensory and motor systems, appreciable through several symptoms [2]. The most common symptoms of physical disability may include fatigue susceptibility, motor weakness, sensory loss and/or ataxia [1]. This multitude of neurological symptoms affects basic motor tasks (i.e., gait and postural balance), thus impairing the quality of life of people with MS (PwMS) [4]. Motor impairments are the main causes of disability in PwMS; overall, 85% of patients report mobility problems [5; 6], and more than 50% of them require assistive devices within 15 years of disease onset [1]. PwMS motor impairment is typically associated with a reduced muscle strength [7] due to the following disease-related morphological and nervous factors: the cross-sectional area (type I, II, and IIa fibers are smaller); the tendency of the leg muscles to a higher fat percentage and a lower lean mass [7]; altered neural motor drive [8], causing a deficit of motor unit recruitments [9]; and factors related to postural control [10]. Gait alteration is perceived by PwMS as the most invalidating impairment across the spectrum of physical deficits [11]. The current literature reports that PwMS exhibit slower walking, take shorter steps, spend a greater percentage of the gait cycle in 'double support', and exhibit high joint kinematic alteration in the knee, and ankle joints [5; 6; 10; 11; 12; 13]. The kinematic parameters appear to be predictive of the level of gait impairment in PwMS [10] and could be explained by altered patterns of leg muscle activation during walking [5; 13]. The functional meaning of the EMG abnormalities, of the muscles involved in ankle control, is unclear but highlights impaired motor control in PwMS [5]. Postural control impairment in PwMS is relevant and well-known; however, there are not conclusive results that enable us to clarify the neuropathological mechanisms underlying balance disorders. Considering the damage variability of the central nervous system in PwMS, it is hypothesized that the impairment of postural control has different causes that vary from one person to another [14]. It has been suggested that the postural balance deficit is due to a slowing conduction of proprioceptive afferent along demyelinated tracts of the spinal cord, resulting in an altered central integration input (visual, vestibular, and somatosensory) [15]. Other studies identify the cause as focal or diffuse damage to the cerebellum or cerebellar peduncles and associative regions [16]. In the literature, there is clear evidence of the effectiveness of exercise to counteract the motor impairments in PwMS. Different types of exercise (i.e., moderate resistance or endurance training, aerobic activity, hydro kinesis) have been proposed to reduce the motor symptomatology of MS disease [17; 18; 19; 20; 21]. Following these kinesiological interventions, it is possible to appreciate neuroprotective adaptations (i.e.,: increase in the thickness of some area of the cortex) [22], probably linked to an increase in the release of neurotrophic factors [20; 23; 24; 25] and inhibition of demyelination [26]. In the past two decades, whole-body vibration (WBV) has proven to be an alternative exercise intervention in neurological disease [27; 28] and it has been suggested as a valid noninvasive form of exercise [29]. WBV generates a neuromuscular response that appears to mediate motor unit recruitment [30], involving not only reflex mechanisms but also cortical processes [31].