For decades, whole-body vibration (WBV) has widely been applied in different areas of rehabilitative medicine, geriatrics, and as a training method for elite athletes (1). More recently, the respective research has increasingly been focused on the application and potential benefits of WBV as a therapy in neuro-rehabilitation, such as in adolescent patient groups with spastic cerebral palsy (CP) (2). With the advantage of great neuroplasticity in children, training regimes commonly achieve higher efficiency compared to adolescent patient groups (3, 4). To date, scientific evidence about possible beneficial effects of WBV on spasticity in CP has only reported the outcome, such as functional parameters of movement control. This is, for example, assessed by the clinical diagnostic tool “(Modified) Ashworth Scale” (5, 6). Apart from the fact that this scale has been characterized with significant limitations of reliability (7–9) and validity for children with CP (10), objective neurophysiological diagnostics, which provide a deeper understanding of WBV and its underlying mechanisms, are lacking.

Depending on the respective origin, severity and position of the lesion, spastic CP as a neurological motor disorder leads to several motor impairments (11, 12). Spasticity, as one of the most prominent spinal phenomena, is defined as a motor disorder with velocity-dependent elevated tonic stretch reflexes (SRs) and exaggerated tendon jerks (13). Those include hyperexcitability of short-latency (14, 15), but reduced long-latency reflexes (16). Coexisting with spasticity, also muscle weakness (17, 18) as well as diminished selective and voluntary supraspinal motor control (19) are expressed in developmental disorders of functional movement: patients suffer from gross motor problems (12), such as impaired mobility (20) and postural control (19, 21), as well as disturbances in gait (22–24). On a neuromuscular basis, reduced electromyographic activity (22), a strong co-activation of antagonistic muscles during movement (15, 22, 23, 25), diminished reciprocal inhibition (17, 26), and changes in perception (12) have been associated with those functional impairments and should be considered regarding therapy of movement disorders.

The benefits of WBV include its uniqueness as a passive training modality during which neuromuscular structures are modulated by mechanical oscillations of the support-surface (1, 27, 28). Thus, its application in neuro-rehabilitation has emerged as a particularly valuable tool. The positive effects of WBV exercise for CP involve acute (29, 30) and long-term training-induced adaptations (31–36). Those are well documented on a functional level: while acute modulations comprise enhancements in gait-related parameters (30, 37) and joint mobility, mutually accompanied with decreased spasticity (29), long-term adaptations also include improvements of strength (2, 32, 33), increased ankle excursions (34), gross motor functioning (2, 37), and balance (32). Nevertheless, those results predominantly focus on the effects of WBV on a functional level, but evidence about the underlying mechanisms is still deficiently researched for patients with neurological disorders. Based on evidence that WBV has an impact on the central motor control in a healthy population, such as a decrease of spinal excitability and Ia afferent reflex activation after WBV (38–41), it could have an impact on deficiently controlled neuromuscular structures in subjects with CP. Nonetheless, a transfer of investigations in a neuro-rehabilitation setting with objective assessment tools is still lacking.

For this purpose, the aim of the current study was to identify neurophysiological changes after WBV in children with spastic CP. In particular, it should be examined whether the phenomenon of reduced spinal excitability after WBV might lead to a reduction of spasticity-related parameters in neuromuscular control. Based on the deficient voluntary movement control in subjects with CP, our approach included three protocols with electrophysiological and functional assessments that are of relevance during everyday movements: besides (1) controlling SR activity, changes were examined including (2) voluntary neuromuscular activation and (3) active joint mobility in the lower limb.

We hypothesized that WBV reduces SR responses, and thus reduces exaggerated sensory input via Ia afferent pathways. We further expected that this release in the spasticity-related condition would produce a better movement control reflected by a reduction in co-contraction and enhanced joint mobility during voluntary activation.

Grand means and SDs are displayed in Table 1.

Protocol 1: Mechanically evoked SOL SR were significantly reduced after acute exposure to WBV (−12 ± 16%, p < 0.01), changes observed for GM SR remained not significant (−5 ± 30%, p = 0.20, Figure 3). Latencies increased in SOL SR and GM SR by +1% (p = 0.01) corresponding to 0.5–1.0 ms, respectively. Muscular pre-activation prior to stretch and kinematic starting position (goniometry) in the ankle and knee joint remained unchanged with respect to different points in time (cf. Table 2, A). Reliability was assured with good to excellent Cronbach’s α at levels of 0.88–0.96 for muscular pre-activation and 0.91–0.99 for initial joint angles, respectively.

Protocol 2: Significant pre- to post-effects were observed after WBV: iEMGs during VA were significantly elevated for all lower limb muscles (p = 0.03). No interaction effects of time x muscle were observed (p = 0.78). While VA/SR-ratio in SOL was distinctly increased (19 ± 41%, p = 0.04), no changes could be observed for GM (−8 ± 71%, p = 0.32) (Figure 4). Cronbach’s α yielded excellent results of 0.90–0.99. SEM values are listed in Table 2 (B).

Protocol 3: No significant changes could be observed in angular excursion of the ankle joint (−1 ± 42%, p = 0.46). In the knee joint, however, active angular excursion was increased during flexion and extension (+15 ± 20%, p < 0.01), accompanied by significantly greater co-activation ratios over time reflected by values >1 for all recorded agonist–antagonist muscle pairs (p = 0.01). No interaction effects were revealed for the variables time × muscle group (p = 0.86). Cronbach’s α was excellent for kinematic (0.94–0.96) and acceptable for neuromuscular measures (0.73–0.96). Test–retest reliability was questionable for co-activation during plantarflexion (SOL/TA) and unacceptable for knee extension (RF/BF and VM/BF, cf. Table 2, B). SEM values are listed in Table 2 (B).

The current study demonstrated acute modulations after a 1-min bout of WBV including (1) decreased SOL SR activation, (2) elevated maximal voluntary muscle activation in lower limb muscles, and (3) increased aROM in the knee joint only, accompanied by improved intermuscular coordination in subjects with CP. The results point toward modulations opposed to the characteristics of pathological movement disorders and indicate improved voluntary movement control.

Voluntary and Involuntary Movement Control: Changes of the muscle and its neuronal control are causal for impairments in CP children (17, 57), including spasticity-associated hyperexcitability of spinal reflexes (58, 59). In the current study, neurophysiological consequences following WBV were investigated in regard to spinal excitability: subjects displayed a reduction of reflex activity by −5 to −12%, with minimally prolonged latencies of 1% in the m. triceps surae—the muscle that is closest to the vibration platform, and thus most affected by the WBV stimulus (44). These results are in line with those observed in healthy subjects (38–40) and demonstrate in general that modulations in SR sensitivity to WBV may be comparable to subjects suffering from CP. Vibration is known to affect the receptor organs and reduce the sensitivity of primary or secondary muscle spindle endings (60–63). Furthermore, the integration of muscle spindle input is changed as well, which is illustrated by changes of Ia afferent transmission. Spasticity is, among others, associated with a decrease of inhibition of reflex activity (64), and thus by lowering the Ia afferent sensory input, spinal excitability might become comparable to that existing in healthy subjects (40). A reduction of SR sensitivity is of functional relevance during simple movement, because the muscle does not overreact to small stretch loads, leading to better “access” for voluntary muscle activation. This is in accordance with results demonstrating corticospinal facilitation (65) concomitant with spinal inhibition (66) after local vibration applied in healthy subjects. Therefore, facilitated input from supraspinal and brain-descending (corticospinal) structures could be associated with enhanced movement control (67, 68).

In spasticity, reciprocal inhibition is pathologically affected (64, 69–71). This can be reflected in pathological co-contractions of antagonistic muscles leading to augmented joint stiffness (20, 22) as well as muscle weakness leading to deficient movement patterns in children with CP (17, 72). In the current study, however, neuromuscular control during voluntary movement execution emphasizes improved neuromuscular coordination following WBV: the results clearly demonstrate an enhanced muscular activation of lower limb muscles (values up to +37%) as well as improved coordination between agonists and antagonists during maximal excursion of the ankle and knee joint (ratios up to +117%). Especially, modulations in the m. triceps surae are of great relevance, as this muscle is predominantly affected in children with CP (69). It could point toward a possible involvement of reciprocal inhibition as being facilitated after WBV, which would be in line with evidence concerning modulations during local vibration (73), involving a neuronal inhibitory modulation of the non-vibrated muscle (74).

Functional Relevance and Application: From a functional point of view, current neuromuscular modulations clearly point toward improved voluntary motor control which is assumed to be guided by supraspinal structures after WBV. For instance, a reduction of reflex activity has previously been associated with greater functional performance such as postural control (75–77) or concerning walking ability in subjects with CP (78). In accordance with the neuromuscular benefits, greater motor control is also displayed by a wider range of motion in the knee joint, which is in line with previous investigations (29).On the basis of this interrelation between spasticity, joint angular velocity and the ability for gross motor functioning performance (79), increased joint mobility contributes to a better functional performance in subjects suffering from CP. Another aspect deals with the reflex latencies, which are decreased in subjects suffering from spasticity (80). WBV-induced minimally prolonged reflex latencies indicating a reduced transmission velocity over afferent and efferent nerve fibers, which may counteract the pathological symptoms in CP patients (80). With an emphasis on leg muscles which are of great relevance for everyday life activities due to their involvement in locomotion and posture control (2, 36, 37), these modulations provide a perspective for advantageous WBV training regimes applied in the field of neuro-rehabilitation.

Limitations: The main difficulty of the current investigation was to balance standardization and moral support to the young participants while securing ethical tenability during the assessments. As a result, effects were controlled by test–rest reliability, but not with a separate control group. For instance, in Protocol 3, neuromuscular activation failed reliability regarding knee joint excursion, and thus results should be considered with caution. The demonstrated effects after WBV emphasize the beneficial application of vibration. However, without any comparison with other treatments, it cannot be concluded whether those effects are solely specific to vibration. In addition, even though variables (type, amplitude, and frequency) of the vibration protocol were chosen carefully, based on current evidence, immediate results following WBV cannot be generalized and information about the temporal maintenance of the current modulations needs to be addressed in future research. Therefore, the evaluation of vibratory effects on spasticity during the described protocols can solely be discussed on the basis of evidence described in the literature.

The current study is in accordance with the latest revision of the Declaration of Helsinki, which is approved by the ethics committee of the University Hospital of Cologne. All subjects and parents gave written informed consent to the diagnostic procedure in accordance with the latest revision of the Declaration of Helsinki, which is approved by the ethics committee of the University Hospital of Cologne. Children with spastic cerebral palsy were involved in the current study. They were medically examined by the attending physicians regarding inclusion criteria of the study and both, children and parents, were informed about the procedure. Children and parents gave their consent to take part but could drop out at any time during the measurements without stating any reason.

All the authors AK, ES, AG, ID, AF-M, KF, and RR made substantial contributions to the conception or design of the work, the acquisition, analysis, and interpretation of data for the work. Further, they contributed drafting the work and revising it critically, they helped with the final approval of the version to be published, and made the agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.