Abstract
The purpose of this review is to examine how whole-body vibration can be used as a tool in therapy to help improve common physical weaknesses in balance, bone density, gait, spasticity, and strength experienced by individuals with cerebral palsy. Cerebral palsy is the most common movement disorder in children, and whole-body vibration is quickly becoming a potential therapeutic tool with some advantages compared to traditional therapies for individuals with movement disorders. The advantages of whole-body vibration include less strain and risk of injury, more passive training activity, and reduced time to complete an effective therapeutic session, all of which are appealing for populations with physiological impairments that cause physical weakness, including individuals with cerebral palsy. This review involves a brief overview of cerebral palsy, whole-body vibration’s influence on physical performance measures, its influence on physical performance in individuals with cerebral palsy, and then discusses the future directions of whole-body vibration therapy in the cerebral palsy population.
Introduction
Cerebral Palsy (CP) is a non-progressive neurological condition caused by either a brain injury or brain malformation during fetal development, at birth, or just after birth while the brain is still developing [1]. It is the most common movement disorder in children affecting roughly one in every 500 people and an estimated 17 million people worldwide [1]. The brain areas damaged in individuals with CP can affect muscle tone, gross and fine motor functions, flexibility, balance, reflex control, and postural control [1]. It is common for those with CP to not only have physical disabilities and also comorbidities of intellectual disabilities, such as difficulties with learning, thinking, feeling, behavior, and communication [2], or have problems with one or more senses. Roughly 23–56% of individuals with CP have vision problems, and 28% have epilepsy [3]. These potential disabilities can make adherence to conventional therapies (such as extended physio or physical therapy sessions) difficult.
Many people with CP have an antalgic gait of some kind, and most of these difficulties are due to muscle and tendon tightness. These gait abnormalities can limit a person’s daily activities and their ability to interact socially [4, 5]. In addition, muscle tightness can lead to muscular imbalances, increasing the risk of low bone density [6]. This is because bones need the stresses from normal musculature to grow to a normal shape and size, which is often prevented due to CP [6].
There are multiple types of CP, but some are far more common than others. The two main types are spastic and non-spastic, with roughly 80% of people with CP diagnosed with spastic CP [7]. Non-spastic CP is described as decreased or fluctuating muscle tone with involuntary movement being one of the more common impairments. On the other hand, spastic CP is hyperexcitability of the stretch reflex response [8] and involves hypertonia (increased muscle tone) [9] which causes spastic muscle contractions, unnatural muscular tightness and stiffness, along with muscle fatigue. This can affect gait, movement, postural control, and speech [10, 11]. In addition, individuals with spastic CP have a high prevalence of limb and joint deformity [12–15]. It is theorized that spasticity results from a motor cortex injury that affects the stretch reflex regulation [16]. This commonly causes changes to muscle function, composition, and structure due to secondary neurophysiological adaptations. These adaptations result in unnatural muscle movement and coordination [17], which leads to a gross motor function decline with age [18].
Within spastic and non-spastic CP, the most common subclasses are monoplegia and hemiplegia, which are both unilateral [1]. Other subclasses include paraplegia, diplegia, triplegia, and quadriplegia, which are all bilateral [1]. Monoplegia is where only one limb is affected and is usually the lower limb, whereas hemiplegia involves both the arm and leg on one side of the body (left or right side), and the lower limb usually is less affected compared to the upper limb [1]. Paraplegia involves only the lower body (both legs), whereas diplegia predominantly affects the legs and may or may not slightly affect the arms, which minimally impacts fine motor skills [1]. Triplegia involves three limbs (any combination of the arms, legs, or face) [1]. The usual presentation involves one upper limb and both lower limbs, with the lower limb on the side of the affected upper limb being more affected [1]. For example, if the left arm is affected, the left leg will almost always be more affected than the right leg. Quadriplegia (also known as tetraplegia) affects all four limbs and the trunk [1]. Given all these classifications of CP, the functionality of CP can range considerably. For example, some individuals can be so low functioning that they are wheelchair-bound and entirely dependent on others. In contrast, some are extremely high functioning and are entirely independent with little to no limitations [19, 20].
Due to the wide range of functionality among individuals with CP, several classification methods have been developed. However, the Gross Motor Function Classification System (GMFCS) is the most universally used and is considered to be a reliable and valid classification system for children and adults [19, 20]. The GMFCS is designed to aid clinicians in determining the best treatment with optimal outcomes for the CP population [19, 20]. The GMFCS uses a system composed of five levels that indicate the ability and impairment limitations of those with CP, with lower numbers corresponding to a lower degree of severity [20]. The GMFCS focuses on what can be accomplished instead of the limitations of people [20]. For example, individuals in level I can walk and climb stairs without any limitations [20]. They can also perform gross motor skills such as jumping and running with reduced balance, coordination, and speed [20]. Level II individuals can walk with some limitations over uneven terrain, crowded areas, inclines, or confined spaces and may need some mobility assistance when walking long distances [20]. They can walk up and downstairs with the help of holding onto a railing or with physical assistance [20]. These individuals have difficulty performing gross motor skills like jumping and running [20]. Individuals in level III require a hand-held mobility device to walk, even indoors [20]. Transfers require assistance of some kind, and wheeled mobility of some form is used when traveling long distances [20]. Level IV individuals are transported in a manual wheelchair with physical assistance or in a powered device while out in the community [20]. When properly positioned, they may be able to use a body support walker while at home or school [20]. Individuals in level V are considered the most dependent and have limited or no ability to be self-mobile, even with the use of assistive devices [20]. They are transported in a manual wheelchair by someone else in all settings and may achieve self-mobility with the use of a powered mobility device with proper adaptations for seating and control access [20]. Given the wide range of functionality that the CP population can have, it is imperative to consider that extended conventional physio and physical therapies may be challenging to maintain adherence [21–23]. Therefore, techniques that could reduce time and intensity of therapy should be considered. Next, the role of whole-body vibration (WBV) and how it has been researched as a therapeutic treatment to increase strength and functionality will be discussed.
With significant strength deficits caused by CP [1], improvement in musculature strength is a primary therapeutic goal for these individuals. Van den Berg-Emons [24] demonstrated that due to the secondary neurophysiological adaptations to muscles (hypertonia and hypotonia), bones (reduced bone mineral density), and joints (joint deformities) caused by CP, children and young adults with CP are unable to perform the same type or volume of exercise to improve such physical limitations compared to their able-bodied peers. Unfortunately, these deficits can be exacerbated with deconditioning from a lack of participation in therapy or training routines [25]. This is an issue particularly in adults with CP, who believe that there is a lack of attention and availability of care [26, 27] primarily due to health care coverage limitations [28], and perhaps the belief that therapists feel they have a more noticeable impact working with younger individuals compared to adults [26, 29]. When therapy participation diminishes, these individuals see a decrease in their strength, range of motion (ROM), and flexibility [25, 30]. Due to the limitation of exercise types and volume for individuals with CP, new methods of strength training are often explored for therapeutic purposes, specifically those that can be implemented by a therapist in a clinical setting or at home. One such method that has gained much attention is Whole-Body Vibration (WBV), a generic term for when vibrations of any kind travel through the body, normally when the individuals are standing on a platform [31]. WBV is a neuromuscular training method that has been used by high-level able-bodied athletes to improve strength [32], and in rehabilitation with older adults [33]. The research on the potential of WBV benefits as a therapeutic tool for those with neurological conditions has recently increased [34], but the available literature involving WBV therapy in the CP population is minimal. With the small amount of research involving CP and WBV, evidence has demonstrated that WBV can be an effective therapeutic tool for the CP population, which will be reviewed further in this article. The purpose of this literature review is to examine the potential of WBV as a tool for improving many of the functional deficits in individuals with CP. To understand how WBV can be an effective therapy for individuals with CP, it is important to understand how the body reacts to the mechanisms of WBV.
WBV is an appealing therapeutic tool for some populations because of how varied the intensity and dose can be, depending on the individual’s goals. WBV uses a platform that can be stood or sat on, often with the options of handrails if needed. The platform elicits vibrational displacement, which causes muscular contraction in the individual [32]. WBV can be used as a passive training modality in which the mechanical oscillations of the platform target neuromuscular connections [33, 36]. The frequency (number of complete cycles per second) and amplitude (amount of displacement in mm) can be altered to manipulate a large range of training options [37]. Also, the direction of vibration can vary, such as a side-to-side alternating vertical sinusoidal vibration or the more commonly used vertical synchronous displacement (see Fig. 1, Appendix A) [33, 38]. The side-to-side alternating vibration platforms have larger amplitudes that can increase as individuals place their feet wider and farther from the pivoting fulcrum at the center of the plate. Vertical vibration, on the other hand, offers smaller amplitudes that are evenly dispersed throughout the plate [37]. Though WBV as a therapeutic tool is not well understood [37], it is speculated that during WBV the muscle spindles, Golgi tendon organs, and alpha motor neurons are stimulated by the vibrations, causing muscle contractions which lead to short-term local metabolic effects [39–41]. Potentially, this could be beneficial to individuals with CP as multiple studies have shown that increased muscle strength improves walking ability in children with CP [42, 43]. Further, WBV has been shown to improve strength, power, bone density, spasms, fatigue, pain, gait, balance, stiffness, flexibility, body movement, functional mobility, force, social functioning, and activities of daily life in various populations [41, 44–49], and provides a stimulus that can cause neuromuscular adaptations that are similar to some resistance training protocols [50]. Given that the majority of research on WBV’s impact on physical measures has been conducted on able-bodied populations, some of the benefits observed in physical measures in that population will be briefly discussed in the next section to present justification of why WBV should be considered as a common option for therapeutic treatment for the CP population.
WBV’s popularity has skyrocketed over the last decade, predominantly in gyms and fitness clinics [51], and is gaining traction in rehabilitation and therapy clinics as well [52]. One advantage of WBV is that it takes a much smaller time commitment for a given session than resistance training [51]. For example, one study showed a significantly shorter time for increased muscle activity and exhaustion in a group performing a squat on a vibration platform compared to individuals performing a squat without vibrations with no difference in the paticipant’s effort [46]. Another study showed that after one year, both traditional resistance training and WBV training achieved similar results in improved isometric muscle strength, explosive muscle strength, and muscle mass increase. However, the WBV group averaged around 45 minutes per training session, whereas the resistance training group sessions lasted 1.5 hours per session [53]. Whole-body vibration training has demonstrated benefits in numerous physical performance areas in able-bodied populations, as shown in Table 1 (see Appendix B). In addition, WBV is generally low-impact, with participants often reporting it as a safer training method than traditional exercise, making it a viable tool for conditioning individuals with a low risk of inducing injury [49, 75]. Given previous findings that WBV requires less time and poses a low risk of injury while improving physical functional measures, this makes it a great option as a possible therapy for individuals with CP.
Like the able-bodied population, WBV has shown positive effects for those with CP in both acute [76, 77] and long-term adaptations [32, 76–82]. As in many populations, WBV is an attractive tool for CP individuals due to its passive, low-strain training potential [49] and its vast range of intensity and dosage that can be applied. WBV has been shown to improve numerous secondary limitations (muscle tone, gross and fine motor functions, flexibility, balance, reflex control, and postural control) caused by CP [83]. Indeed, WBV could potentially decrease spasticity when combined with other approaches, and in turn, improve walking speed and motor development [34, 83–86]. One of the ways that researchers suggest WBV may improve spasticity and motor development in individuals with CP is by increasing the muscular neural drive [87]. This increase in drive could allow for the recruitment of previously inactive motor units, similar to the mechanism of strength training [87]. It could produce motor and neural adaptations comparable to strength training in individuals with CP [34, 82]. These adaptations help increase muscle strength and mass through increased efficiency in their function [88]. For example, a systematic review of those with common neurological diseases showed that WBV exposure could immediately increase muscle temperature, oxygen consumption, blood flow, and muscle power [89]. With a lack of muscular strength being common in those with CP, applying WBV therapy as a low-strain and safe method to improve strength can potentially improve many other areas of functionality affected by CP. The purpose of this review, though not systematic, will be to review the literature on WBV as a therapeutic tool for individuals with CP. This review will discuss therapeutic prescriptions of WBV and how it can improve common physical parameters that are hindered by CP, including balance, gait, bone density, spasticity, strength, and power.
Methods
Databases
The following databases were used to select articles pertaining to WBV’s effects on the physical parameters balance, gait, bone density, spasticity, strength, and power in individuals with CP from 1979 to August 2021: PubMed, ScienceDirect, SPORTDiscus, and Google Scholar. Search terms for this review included “cerebral palsy” and “whole-body vibration” combined with the terms “balance”, “gait”, “bone density”, “bone mass”, “flexibility”, “range of motion”, “spasticity”, “strength”, and “power”. In addition, relevant references cited in articles that were examined were also reviewed.
Reviewed studies criteria
The studies reviewed in this article included those specifically looking at individuals with CP receiving a WBV treatment with any physical parameters relating to balance, gait, bone density, spasticity, strength, or power measured as outcome variables. The majority were children with CP, although no criteria for age limit was included. Table 2 (see Appendix C) shows the descriptives of the overall samples in the reviewed studies. This study reviewed both randomized and non-randomized trials. Exclusion criteria included any articles that did not involve individuals with CP, articles that did not look at the aforementioned parameters, and also any articles that did not separate results of the study by participant disability (specifically, in reference to studies that looked at individuals with other disabilities along with CP).
Treatment protocols
In the reviewed studies, treatment protocols were varied. This is not unusual since no standard prescription for WBV in CP populations currently exists. Treatments generally consisted of the following: (1) standing on the vibration platform with bent knees for timed intervals between 1–10 minutes, or (2) holding various squat positions and different knee angles for timed intervals between 1–5 minutes. Stance on the vibration platforms is noted in the tables of this article because standing on a vibration platform with bent knees significantly dampens and reduces the transmission of the vibration to the spine and head compared to standing erect on the platform [90]. Because of this, knee angles and stance were reported in Tables 3–6 (see Appendix D). In addition, all reviewed articles include the information in their methods to facilitate future replication of their study design. The vibration displacements for all reviewed studies were either side-to-side alternating vertical sinusoidal vibration or vertical synchronous displacement if reported. Vibration frequencies ranged from 5–50 Hz, and amplitudes ranged from 1–9 mm.
Whole-Body vibration therapeutic effects on physical parameters in individuals with cerebral palsy
Balance & gait
A lack of balance and an abnormal gait are both main secondary adaptations of CP [1]. Not only do they play a major factor in affecting overall functionality in the CP population, but they can also affect social engagement by making it difficult to integrate into school and society if the individuals are unable to walk or be mobile independently [91]. Improving balance and gait-related function should be important goals for anyone working with this population [81, 93]. Creating sensory stimuli such as vibrations activates the spinal nerves [94] and contributes to improving synaptic expansions and neural communications [95]. This improves the somatosensory system and proprioception, both of which play a major role in balance [96, 97]. Moreover, vibrations sent through the foot during WBV have been shown to promote brain plasticity in areas involved with the somatosensory system by stimulating somatosensory sensations in the heel of the foot [98]. Further, Ritzmann et al. [99] demonstrated that the stretch reflex caused by WBV effectively improves muscle endurance as well as balance control.
Regarding physical measures that have been looked at in the CP population involving WBV, balance and gait are among the more studied areas. In a study that looked at core stability in individuals with CP, both the WBV and core stability program training groups saw a significant improvement in all stability indices (anteroposterior and mediolateral directional sway measured by testing static standing balance using the Biodex Balance System) [100]. However, the WBV group had a significantly greater improvement to both stability indices compared to the core stability program group [100] (see Table 3).
Knowing that WBV can improve stability indices in able-bodied populations, it is important to consider if this can be an effective therapy for individuals with CP. Multiple studies found that WBV treatment for individuals with CP significantly improved many gait and balance metrics such as the 6- and 10-minute walk test [101, 102], walking speed [77], stride length [77], dynamic ankle ROM [77], timed up-and-go test [101], gait sway [101], and limit of stability [101]. Further, many randomized control trials found that just WBV treatment, or WBV in conjunction with conventional therapy outperformed conventional therapies or WBV sham conditions on improving several gait and balance metrics, which included the mediolateral stability index [79, 100], anteroposterior stability index [79, 100], overall stability index [79, 100], sitting ability [103], dynamic ROM [77], timed up-and-go test [78], 6-minute walk test [33, 78], sense of ankle positioning [104], gait speed [80, 104], step length [80, 104], step width [104], cycle time [80], ankle angle during walking [80], standing ability [32], and walking/running/jumping ability [32]. Table 3 displays more study design details and WBV dose for the research reviewed regarding WBV treatment’s impact on gait and balance metrics. Results of the reviewed studies indicate WBV improves many gait and balance parameters in individuals with CP.
Bone density/mass
In addition to strength and balance impairments, the CP population is vulnerable to bone mineral deficiency from a lack of mechanical loading of the skeleton due to a decrease in mobility and weight-bearing capacity [105–110]. Individuals with CP who have lower physical functioning capacity more commonly have low bone density and an increased likelihood of a bone fracture [105–107, 112]. The functional limits and lack of muscle strength are large contributing factors due to a lack of mechanical stimulation and load required to build bone mass and density [113], which makes it imperative that individuals with CP receive therapy to prevent bone mass and density loss. With WBV demonstrating the ability to improve bone density in the able-bodied population [57, 58], it is important to recognize how WBV could improve bone health in individuals with CP.
The research of the impact of WBV on bone mineral density in individuals with CP is sparse but promising. Two studies were found in the literature examining WBV’s impact on bone mineral density specifically in individuals with CP (see Table 4) [102, 114]. Both studies found significant improvements in bone mineral density after WBV treatment in the lumbar spine [102, 114], femur [102, 114], lower limbs [102], and total body [102, 114]. Additionally, one study found that bone mineral content significantly increased in the lumbar spine, lower limbs, and total body after WBV treatment [102]. Moreover, a randomized control trial found WBV in conjunction with physical therapy had significantly greater bone mineral density improvements in the lumbar spine, femur, and total body than physical therapy alone [114]. Though research on WBV’s impact on bone density is limited, the results of the two studies reviewed indicate that WBV does improve bone density in the CP population [102, 114].
Spasticity
Within the CP population, a main contributing factor to gait impairment and physical function is spasticity, specifically due to its involvement with subluxation, contractures, and pain [115, 116]. There are many current treatment options to reduce spasticity, including botulinum toxin injections [117], oral pharmacologic agents, peripheral injectables, physical modalities, intrathecal agents, surgical interventions, and oral magnesium sulfate [25, 118]. However, physical therapy remains the most basic and common treatment [28].
Though WBV is a passive training modality, research shows that this vibration stimulus can modulate muscular weakness and spasticity in CP [32, 113]. WBV exposure causes exponentially more stimulation cycles to the worked muscles compared to walking [119], which may contribute to faster muscle function improvements seen with WBV treatment. For example, nine minutes of WBV at 20 Hz applies over 10,000 stimulatory impulses to the lower extremities [119]. Comparatively, it would require roughly three hours of walking to equate as many impulses [119]. While a child’s brain is still developing, it continues to undergo a maturation and reorganization process [120]. With this in mind, WBV could prevent or improve some of the secondary structural changes caused by CP due to the neural adaptations WBV presents [120]. These neural adaptations are caused by the muscle spindle activating and transmitting through the 1a fibers [78]. This improves the cortical excitability of the muscle being vibrated, which simultaneously reduces the antagonist muscle’s hyperactivity through supraspinal and reciprocal inhibition [78]. This reflex activity reduction has been shown to be associated with improved functional performance like walking ability [121] and postural control [122, 123] in individuals with CP and plays a significant role in reducing muscular spasticity.
It is essential to examine how WBV impacts spasticity since it is directly involved in several other physical deficits in the CP population. Conflicting results were found regarding WBV treatment to improve active and passive ROM in the lower limb joints, where one study did find significant improvement [78] and the other did not [77]. In the handful of randomized control trials conducted involving spasticity parameters and WBV treatment on individuals with CP, WBV significantly improved relaxation index scores, Modified Ashworth Scale scores, and ROM of the knee extensors [32, 78]. Moreover, WBV treatment improved active ROM, passive ROM, relaxation index, and Modified Ashworth Scale scores in individuals with CP greater than sham WBV conditions [78]. Also, knee extensors of the strong and weak leg saw significant improvement in ROM in the WBV treatment groups but not in groups exposed to resistance training and physical therapy conditions [32, 34]. Table 5 displays the results of the randomized control trials of WBV treatment on spasticity parameters in individuals with CP in more detail.
Similar to the research of WBV treatment’s impact on bone parameters in the CP population, the research of its effects on spasticity parameters is sparse. Though little research is available involving WBV and spasticity in individuals with CP, the results of the reviewed studies did demonstrate that it improved spasticity parameters and knee extensor ROM greater than more conventional therapies alone [32, 78].
Strength/power
It has been documented that those with CP have significantly lower force production compared to their able-bodied counterparts. This is important to examine as muscle strength is known to strongly correlate with gait speed in individuals with CP [42, 43], and a muscle’s functional capacity and mass are associated with bone density, which is generally decreased in individuals with CP [113]. Due to various neuromuscular inefficiencies in CP, the functional capabilities of affected muscles are significantly reduced, which in turn causes structural changes in the muscles [10]. For instance, a strong association exists between higher maximum muscle force production and greater muscle cross-sectional area [124, 125]. The reduced cross-sectional area of spastic muscle results in reduced force production compared to the unaffected muscles [126]. For example, a study conducted by Stackhouse, Binder-Macleod, and Lee [126] looked at the force production in the triceps surae and quadriceps femoris in individuals with spastic CP compared to an able-bodied control group during ankle plantar flexion and knee extension. The researchers found that the CP group produced 73% less force in the triceps surae and 56% less force in the quadriceps femoris than the able-bodied control group [126]. Further, another study examined 10-second isometric contraction grip strength in CP and able-bodied individuals [127]. Results indicated that those with CP generated 65% less force production compared to their able-bodied counterparts [127].
Another muscular structural difference between able-bodied and CP muscles is that the resting sarcomere length in those with CP is greater [128–130]. The strength of the neural signal to the muscle as well as the initial resting length of the muscle’s sarcomeres dictates the force production of a muscle [131]. When the sarcomere length is longer than optimal, which is true in spastic CP [128–130], there is a decreased amount of overlap between actin and myosin [132]. This limits the amount of cross bridging that can occur, which greatly reduces force production [132].
In addition to reduced muscle cross-sectional area and increased sarcomere length [128–130], decreased alpha motor neuron recruitment is also involved in the reduced strength of spastic muscles [133]. Reduced alpha motor neuron recruitment in spastic muscles is caused by either a decrease in motor unit discharge rates or incomplete muscle recruitment during max voluntary contractions [134–137]. A study by Burtner, Qualls, and Woollacott [133] showed CP children had increased antagonist coactivation during standing balance and ambulation. This increased antagonist coactivation may contribute to the observable force reduction deficit among those with CP [133]. For example, Ikeda et al. [138] showed that at 60° knee flexion, able-bodied children had a 4.3% hamstring activation during isometric knee extension compared to children with CP who had a 9% hamstring activation.
Other contributions to weakness most likely lie in the morphology of whole muscle and single muscle fibers. For instance, a higher percentage of type I muscle fibers [139, 140], a higher occurrence of muscle fiber atrophy, and an increased amount of connective tissue and intramuscular fat in the most involved muscle groups [140, 141] are among the most common characteristics in spastic muscles that also contribute to strength deficits caused by CP.
The early research on WBV and enhancing muscle strength in CP is positive. Outside gait and balance, it appears to be one of the more researched parameters when examining WBV’s impact on CP functionality. Of the handful of studies that have been conducted on WBV’s impact on strength parameters in individuals with CP, increases in muscle thickness and strength in the lower limbs and increased mass in the trunk and total body were found in these individuals exposed to WBV treatment [32, 142]. Further, randomized control trials found WBV treatment significantly improved muscle thickness in the core and lower limbs and strength in the lower limbs compared to physical therapy and physiotherapy control groups [32, 142]. The limited research has demonstrated that WBV treatment can improve strength measures and muscle mass in individuals with CP (see Table 6). Also, randomized control trials found WBV can improve strength and muscle measures in the lower body more than traditional therapies alone. Overall, the WBV treatment protocols administered in the aforementioned studies did improve strength measures in individuals with CP.
Discussion
In the research reviewed on WBV therapy and its effects on specific physical performance measures (balance/gait, bone density/mass, spasticity, and strength/power) in the CP population, this review found that WBV does improve several physical performance measures which are important to consider when developing a therapy program for individuals with CP. Regarding balance and gait, WBV was documented to enhance the balance and gait of participants with CP, whether the individuals were just static standing or holding a squat or semi-squat position on the WBV platform. Acute interventions of five to nine minutes with varied frequencies (see Table 3) demonstrated improved balance and gait parameters such as the 10-minute walk test, timed up-and-go test, the limit of stability (measured by dynamic balance), walking speed, stride length, dynamic ankle ROM, sway length, and limit of stability [77, 101]. Regarding long-term protocols that lasted between 3–20 weeks, treatment settings varied (see Table 3). All long-term studies showed an improvement in balance and gait parameters including anteroposterior and mediolateral directional sway, sitting ability (Gross Motor Function Measure-88 [GMFM-88] values), overall stability index, anteroposterior stability index, mediolateral stability index, 5-meter walk test, 6-minute walk test, active range of motion of the knee, timed up-and-go, joint-position sense, gait speed, step length, step width, cycle time, ankle ROM, GMFM dimensions (C%, D%, and E%), and chair-stand test [32, 104]. In randomized control trials WBV in conjunction with physical therapy enhanced many gait and balance parameters greater than physical therapy alone [32, 104]. Moreover, WBV treatment on its own demonstrated superior improvements in many gait and balance parameters in randomized control trials compared to core stability and physical therapy interventions [79, 100]. However, one randomized control trial did not find any difference between WBV treatment and physical therapy, though within-subject outcomes showed improvement [80].
Evidence of WBV efficacy on improving gait and balance parameters from the reviewed studies suggests that WBV should be considered in conjunction with traditional therapies or used as a safe additional/alternative therapy for individuals with CP who have difficulty adhering to conventional treatments to improve gait and balance parameters.
As for bone density and bone mass, WBV demonstrated positive results in individuals with CP. Of the two studies reviewed, treatments varied (see Table 4) but yielded significant improvements in bone mineral density [102, 114]. Both study interventions demonstrated that WBV therapy prevented bone loss and improved bone density [102, 114]. Further, Gusso et al., [102] demonstrated an increase in bone mineral content in individuals with CP [102]. Moreover, the randomized control trial reviewed found that WBV in conjunction with physical therapy had significantly greater improvements in bone mineral density in the femur, lumbar spine, and total body compared to physical therapy treatments alone [114]. The research of WBV and bone density in individuals with CP specifically is minimal, but additional studies have found improved bone density after WBV treatment in individuals with CP and muscular dystrophy [82, 143]. However, these studies did not separate their findings between those with CP and muscular dystrophy and were not included in Table 4.
Though limited research is available involving WBV and bone density in CP individuals, the results of the reviewed studies are promising. In fact, in the randomized control trial reviewed, it was found that the bone mineral density in the femur increased by over 50% in six months for the individuals with CP who were exposed to both WBV and physical therapy [114]. This is particularly astonishing given that the control group exposed to physical therapy alone saw no significant increase in bone mineral density in the femur [114]. The limited research and positive results of WBV in individuals with CP suggest WBV treatment should be considered in conjunction with physical therapy to enhance bone mineral density in these individuals, and further investigation be warranted.
Similar to gait, balance, and bone density, spasticity parameters in individuals with CP were improved by WBV treatment. Protocols of the reviewed research articles on WBV’s impact on spasticity in CP varied (see Table 5). However, all lasted three weeks or longer, and all but one were randomized control trials. Conflicting results were noted in two studies regarding active and passive ROM improvements in the lower limbs of individuals with CP (one study found significant improvement [78] and the other did not [77]). On the other hand, improvements in spasticity were reported by demonstrating improvements in the relaxation index (assessed by the Wartenburg Pendulum test), Modified Ashworth Scale, and knee extension measures after WBV treatment for individuals with CP [32, 78]. In terms of longer treatment duration looking at spasticity measures, research by Semler et al. [144] found that those with six months of WBV treatment had decreased spasticity and improved walking ability. However, the study by Semler et al. [144] included a child with CP and children with other disabilities and did not separate their results based on the different conditions when reporting their results and was not included in Table 5.
As with gait, balance, and bone density, the results of the reviewed studies suggest WBV should be implemented with conventional therapies to aid in improving spasticity outcomes. Further research should determine if different protocols, especially the duration of treatment and the frequency and amplitude of vibration, could be more beneficial in improving spasticity in individuals with CP.
In regards to strength measures in individuals with CP, WBV therapy did result in improvement. Treatment duration lasted between eight and 20 weeks, with the frequency of exposure, oscillation speed, and amplitude varying between studies (see Table 6). Increased knee extensor strength in both affected and non-affected legs, improved knee extension peak torque, increased muscle thickness and lean body mass in the lower limbs, core, and total body, improved GMFCS (D% and E%) scores, and improved strength in functional tests [32, 143] were reported after WBV treatment in the reviewed studies. In addition, randomized control trials found WBV in conjunction with physical therapy outperformed physical therapy alone in knee extension strength in the affected leg of the individuals with CP [32]. Moreover, in participants who were only exposed to WBV, greater improvements were reported in quadriceps peak torque and lower body muscle thickness compared to physical therapy and physiotherapy control groups [79, 80].
Data on WBV effectiveness for improving strength parameters from the reviewed studies suggest that WBV should be implemented with traditional therapies or used as an alternative therapy for the CP population who are likely to have attrition from traditional therapies to improve strength. WBV should be further examined as a treatment method used to improve strength and muscle mass in CP, which helps to improve many secondary limitations that tend to stem partially from these strength deficits.
In the reviewed studies, the safety of WBV in the CP population was good [145]. This was expected as previous research found that individuals with CP tolerated WBV well without any adverse effects [145]. One study did report redness of the feet or ankle area after the participant’s first session of WBV, but this is a known and common side effect when beginning WBV, and redness was reported to be transient and occurred only after the first session [32]. In the reviewed studies, most did not report any attrition, but one reported that three children dropped out during the intervention for reasons that did not relate to treatment (one child dropped out because their behavior problems got worse, another because they began an intensive physiotherapy program that caused the child to be too tired to continue with treatment, and the other because they got bored during treatment) [143]. Comparatively, more conventional physical therapy modalities have generally reported low attrition rates [146], but some have noted attrition issues in children with CP [21–23]. Given the decreased time to implement WBV as a therapy as well as its more passive and less-straining treatment style, it has the potential to improve attrition rates in therapy programs for individuals with CP.
Future directions
Most of the current literature of WBV in the CP population focuses on lower body improvement, but other areas of the body should be focused on in future research, specifically the core. Though one study reviewed did examine core hypertrophy [103], strength measures for the core were not taken. Due to physiological impairments that cause spastic muscles to have reduced strength, strengthening the muscles (especially the core) could be one of the most important aspects to focus on during therapy for individuals with CP. Improvement in core stabilization helps increase fine and gross motor function [100] and aids with balance, postural control, gait, stability, and overall muscle strength and functionality [147]. All of these are common areas of weakness in the CP population. The best way to achieve optimal outcomes would be to train both the local and global stability systems to maximize core stability [148]. The local stability system involves deeper core muscles such as the multifidus and transverse abdominis, which are involved in lumbar spine stability [148]. The global stability system includes more superficial, larger muscles such as the rectus abdominis, external obliques, and paraspinals, which are involved more with the movement of the hip and trunk [148]. Both local and global stability is essential in postural control, gait, and motor function, and WBV can potentially enhance core strength in individuals with CP [148]. Future research should examine how WBV therapy could improve core stabilization and core musculature in the CP population.
There was a lack of research that examined electromyography (EMG) in individuals with CP who went through a WBV intervention and only a few studies have looked at EMG with WBV in the able-bodied population. One study examined EMG recordings on the biceps brachii muscle during a maximal dynamic elbow flexion with an applied vibration stimulus [149]. The EMG root-mean-square was almost doubled in value compared to performing the exercise without vibration [149]. Further, two other studies demonstrated that WBV increased leg muscle activity measured by EMG readings [59, 150]. Moreover, a study by Roelants et al. [60], analyzed the effect of WBV on leg muscle activity of the rectus femoris, vastus lateralis, vastus medialis, and gastrocnemius via surface EMG during three standard unloaded isometric exercises: high-squat, low-squat, and one-legged squat. They found that WBV elicited a significantly higher EMG reading for all muscles during all three squat variations than the control [60]. In addition, another study looked at the EMG root-mean-square of the vastus lateralis muscle of participants who stood in a high-squat position on or off a WBV platform [59]. Results indicated a 34% increase in EMG root-mean-square of the vastus lateralis muscle of those performing a high-squat on a WBV platform compared to those not on a WBV platform [59]. Future research should examine EMG readings of individuals with CP performing physical measures and observe if muscle activation improves more with WBV compared to conventional training.
When examining physical measures, cardiovascular capacity was neglected in the CP and WBV research. One of their most common deficits was difficulty with speech and language, mostly related to poor respiratory control [3]. This leads to a much greater risk of respiratory failure [3]. Once in their 40s and 50s, people with CP have to worry about cardiovascular disease much more than their able-bodied counterparts. The incidence of cardiovascular conditions is two to six times higher than in the general population [3]. In the able-bodied population, Milanese et al. [151] demonstrated that WBV increases oxygen consumption. Other studies showed a greater metabolic stimulation and an increase in energy expenditure while static standing, performing dynamic squats, and performing dynamic squats with an additional load during WBV compared to the same conditions without WBV [152]. Similarly, Da Silva et al. [153] found greater metabolic stimulation and energy expenditure when participants performed half-squats on a WBV platform compared to half-squats with no vibration. Moreover, WBV has been shown to facilitate muscle deoxygenation, thus improving muscular oxygen delivery [154] and stimulating lipolysis due to acute lipolytic level elevations [155]. Further, Beijer et al. [156] reported a 20% higher VO2 (whole-body oxygen consumption) and a 27% increase in blood volume in the gastrocnemius muscle after six weeks for healthy male participants who performed squats and calf raises with WBV compared to those who trained without vibration. Given that respiratory-related complications are the leading cause of death among the CP population [3], it is even more vital that future research is warranted in how WBV impacts cardiovascular factors in the CP population.
Regarding vibration patterns, the reviewed research primarily examined sinusoidal vibration platforms. Stochastic resonance WBV (SR-WBV) potentially could be examined as a therapy for individuals with CP. SR-WBV offers only low-frequency vibration exposure, with a maximal degree of complexity and unpredictability compared to the more common WBV platforms, which apply simple sinusoidal vibrations [157]. Stochastic resonance is the reason information flowing through a system can sometimes be maximized by the presence of a certain level of stochastic noise (white noise) [158]. Normally, noise impairs rather than improves signal detection and information transmission. However, stochastic resonance has been well-established as a valid concept in different fields, from electronic circuits to global climate models to human perception [159]. The complex vibrations from SR-WBV cannot be predicted by the human body, which causes the individual’s body systems to be constantly challenged to adapt its muscular and neural reactions appropriately [160–162]. Due to this unpredictable and more challenging setting, SR-WBV causes interactions of different types of neurophysiologic sensors, including the adjustments of efferent and afferent signals that train the sensorimotor system [160]. This could potentially have a greater training effect on the sensorimotor system compared to the more common sinusoidal vibration platforms [163]. Unfortunately, there has been very little research done with this type of vibration compared to the more traditional vertical or side-to-side sinusoidal platforms. In the studies done, most were poorly structured; for example, two studies exposed their control group to SR-WBV (just at a lower frequency) during the intervention, but results of the studies did show promise in their improved gait mechanics and strength [164, 165]. Another study demonstrated that static standing during SR-WBV for three sessions a week for four weeks (three sets for one minute each session) with a frequency of 4–8 Hz and a 3 mm amplitude decreased musculoskeletal pain [166], which could be helpful for the CP population suffering from spasticity. More research is needed before SR-WBV can be applied as a training tool like more common WBV settings (vertical and side to side displacement), which have substantially more research behind them.
Another limitation of the literature involving WBV and CP is that the research is primarily focused on children and adolescents (as shown in Table 2). Most protocols were conducted on children and adolescents with ages that ranged from 4–20 years [32, 142], and just a couple of protocols observed effects in adults (20–51 years of age) [34, 77]. This is likely due to the emphasis on earlier implementation of therapy at a younger age in individuals with CP to achieve optimal results [25, 26]. Although the research on WBV in adults is minimal, it appears they receive similar benefits from these protocols as children and adolescents do. With the limited research provided for WBV effects on adults with CP, the literature would benefit from more studies on how WBV could potentially improve physical parameters for this population.
The current body of WBV research on individuals with CP does have some research that examined WBV compared to traditional resistance training therapy [53]. However, the literature does fall short in determining how much WBV can improve physical functionality when implemented with conventional physical and pharmacological therapy compared to WBV and conventional therapy alone. A review by Ritzmann, Stark, and Krause [167] suggested that vibration therapy in conjunction with conventional physical and pharmacological therapy has the potential to improve physical functionality more than conventional therapy alone and should be investigated further.
Limitations
Several limitations were found in the above studies. One is that the studies did not compare the effectiveness of WBV therapy to common pharmaceutical treatments of oral medication or muscle injection. Future studies should look to compare antispastic pharmaceutical agents to the antispastic effects of WBV. Moreover, other than just a couple of studies [34, 102], most of the existing research protocols did not require participants to stop attending their regular therapeutic activities and just added a WBV program to their participant’s existing daily routines. Future studies should look at only exposure to WBV therapy and compare it to the sole exposure of conventional therapies. Further, the combination of the two therapies should be compared to the sole exposure of WBV therapy and the sole exposure to conventional therapies to better understand the possible benefits and synergistic effects WBV therapy potentially can offer individuals with CP.
Another limitation in the literature for WBV as a treatment for CP is that all of the evaluated studies had a small number of participants, limiting the generalization of the results. Many studies focused on children with CP, and very few articles have looked at adults with CP. Only two studies, to the knowledge of these authors, investigated the effects of WBV on adults with CP [34, 77], but the results seemed promising. Furthermore, many articles only looked at these three types of CP (spastic diplegia, hemiplegia, or quadriplegia) or higher functioning participants (levels I-III on the GMFCS, or between 1 and 2 in the Modified Ashworth Scale). WBV potentially could be just as effective or have stronger effects in participants who are lower functioning according to Gusso et al. [102], who showed that individuals at level III of the GMFCS had greater mobility improvements compared to participants at level II of the GMFCS, warranting further investigation of WBV therapy in the lower functioning CP population.
The minimal reporting of follow-up measurements was another limitation in the literature. Only one study had a follow-up of any kind [78] (only three days after the prescribed intervention was completed), so it is unknown how long these benefits can be retained. In the able-bodied population, research on the sustainability of the physical gains made from WBV therapy is minimal. One study by Prioreschi et al. [168] looked at middle-aged women with rheumatoid arthritis and showed that after a three-month WBV intervention, whole-body bone mineral density, fat mass, and BMI (body mass index) significantly improved compared to baseline. Also, these improvements were found to be sustained at the six-month follow-up. A limitation to this study was that the individual’s activity level, vitamin D and calcium intake, and menopausal status were neither monitored nor controlled for, which may have confounded bone density changes. Nevertheless, the results in this study involving able-bodied women who have rheumatoid arthritis suggest that physical improvements made from WBV therapy in the CP population possibly could be retained for several months after treatment.
Finally, longitudinal studies on consistent exposure to WBV in CP populations are minimal, with only one existing to the knowledge of these authors. One longitudinal study by El-Shamy and Mohamed [114] found improved bone mineral density in the femur, lumbar spine, and total body in children with spastic diplegia after six months of consistent WBV treatment. Regarding the able-bodied population, a few longitudinal studies exist in the literature. One study showed improved maximal leg and trunk flexion strength and decreased pain intensity in the major joints in post-menopausal women after one year of WBV exposure [169]. Moreover, Von Stengel et al. found an improvement in trunk flexion in elderly women after 18 months of WBV exposure when combined with aerobic exercise [170]. Furthermore, two other studies found improvements in bone mineral content and bone mineral density in middle-aged women after six months [58, 171] and improved body composition after nine months of WBV treatment [171]. With the limited evidence but positive benefits reported of longitudinal exposure of WBV therapy in the CP population, and the positive results involving longitudinal exposure of WBV in the able-bodied population, future research should focus on longer interventions of consistent WBV treatment in the CP population.
In summary, future studies with a larger number of participants should look at individuals with CP of all ages and all functionality levels, to determine how WBV therapy alone compares to common pharmaceutical treatments and conventional therapies, how long WBV benefits can be retained, and the impact of consistent WBV treatment over an extended period of time in a longitudinal study.
In conclusion, WBV therapy does improve balance, gait, bone density, spasticity, and strength in individuals with CP and may improve such measures in less training time and at a greater capacity than traditional therapy alone. With the current research of WBV on individuals with CP in its beginnings, many gaps in the literature exist. Future research should examine if individuals with CP can experience any benefits in core strength, increased muscle activation according to EMG readings, or cardiovascular capacity from WBV therapy. Moreover, it should be examined if any greater benefits can be experienced in the CP population using an SR-WBV setting compared to more commonly used vibration settings (vertical and side to side displacement). Finally, it is essential to establish a more standardized protocol to maximize improvements in balance, gait, bone density, spasticity, and strength for individuals with CP. This entails comparing whether any specific stances on the platform, duration and frequency of exposure, and frequency and amplitude settings of the vibration generate superior benefits and should be considered a focus of future research.
