Along with pain and stiffness, patients with KOA experienced decreased muscle strength, impaired neuromuscular function and limited physical function [(30)]. As a type of strength exercise, WBV training has been proposed for the treatment of KOA. However, several systematic reviews about the effect of WBV for KOA were explored, thereby leading to dispersed results [(20, 21, 31, 32)]. These studies failed to indicate the superiority of WBV compared with a control group or a similar strength training group. Thus, this study attempted to determine the effects of 8-week WBV training on pain, physical function and neuromuscular function in individuals with KOA.
In this study, the participants were randomly assigned to WBV, ST, and HE group and performed corresponding intervention for 8 weeks. The results showed that the 8-week WBV training effectively increased isokinetic strength of knee extensors compared with ST or HE, while these three interventions didn’t promote the knee passive motion sense. As for the self-reported pain intensity and physical function, WBV, ST, and HE led to a similar effect in these clinical outcomes, although the pain and the time of TUG test improved significantly after 8 weeks of all three interventions. Generally speaking, the finding of this study demonstrated that compared with similar training and health education, whole body vibration intervention could significantly promote the increment in knee extensors strength and have a similar effect on knee pain relief and physical function improvement.
4.1 Muscle strength
The main finding of this study is the significant improvement in muscle strength (park torque and peak work) in the WBV group compared with similar strength training. As reported in previous studies, the improvements in muscle strength and power might be the results of several neural factors [(33)] and biochemical factors [(34)]. Several factors were speculated to be involved in the possible mechanism underlying the effect of WBV training on strength gains, such as increased recruitment, synchronisation, muscular coordination and proprioception [(30)]. However, the results of proprioception in our study showed that the WBV training did not promote the improvement of proprioception. Considering the lack of electromyography data, whether the recruitment, synchronisation and coordination of the muscles around knee joint were enhanced after 8 weeks of WBV training cannot be confirmed.
There are several reasons that can explain the increased effect of training on muscle strength. Firstly, during vibration training, the length of the muscle-tendon complex in skeletal muscle changed, and vibration elicited the “tonic vibration reflex,” which is one kind of muscle response produced by the activation of muscle spindles, mediation of Ⅰa afferents and activation of muscle fibres [(23)]. Furthermore, during the ST, the force depended on the mass and the gravity acceleration. However, for the participants in the WBV group, the acceleration was changed by platform’s vibration, which adjusted the resistance during training sessions. Another possibility might be that the WBV training stimulated growth hormone secretion, which was beneficial to the gain of muscle strength.
Currently, there is no consensus on the effect of WBV training on muscle strength in patients with KOA [(18, 23, 35, 36)]. Several researchers have affirmed the advantage of WBV training on muscle strength in KOA, which was consistent with our result [(24, 25, 36)]. Due to the absence of guidelines for optimal protocol, previous studies employed different parameters, such as frequency, amplitude, duration, posture of WBV training, and even the vibration device. As mentioned previously, mechanical vibrations provoke a reflexive muscle contraction, which is referred to as tonic vibration reflex [(37)]. The magnitude of provoked tonic vibration reflex is related to vibration frequency, vibration displacement, initial position applied in WBV training, vibration type and the training protocol [(38)]. The difference between these parameters employed might be responsible for the inconsistent conclusions.
4.2 Proprioception
It has been generally accepted that impaired knee proprioception played an important role in the onset and progression of KOA as a local factor [(4, 39)]. Compared with age-matched healthy controls, the patients with KOA showed significant impairment in position sense or motion sense [(40, 41)]. WBV exposure is a neuromuscular training in the management of several neurological disorders and musculoskeletal disease. It was speculated that WBV training could help improve muscle strength and proprioception and neuromuscular responses [(42)]. However, the result of our study showed that WBV training did not improve proprioception in participants with KOA. Currently, limited studies have been designed to investigate the effect of WBV training on proprioception in KOA. Trans et al. [(24)] compared the effects of two different vibration trainings on KOA and found that the TDPM was improved in WBV in the balance-vibration group but not in the conventional stable-vibration group. The stable WBV device (vertical platform) was applied to this study, and the difference in the device might have contributed to the ineffectiveness in proprioception. Segal collected the vibration perception threshold of the lower extremities [(22)], and similar to our experiment, positive results were not found.
4.3 Pain
Compared with the baseline, the knee pain decreased significantly in the HE and WBV groups, and knee relief tendency was found in the ST group, although no significant intergroup effect was found. The results of the pain state are consistent with those in previous studies. For example, in Tsuji’s study [(17)], the participants with knee pain underwent WBV training 3 times a week for 8 weeks. However, no significant effect of WBV training on pain was found in comparison with the control group, which performed a similar exercise without the vibration stimulus. Similar to our results, Wang [(19)], Trans [(24)] and Bokaeian [(18)] found that the WBV training did not help relieve the pain of KOA patients more effectively. Furthermore, a systematic review and meta-analysis showed that vibration training does not have an additional effective effect on knee pain [(31)].
4.4 Physical function
The TUG and 6MWD tests were used to determine the ability to perform daily activities, as commonly reported in related studies [(17, 30)]. Compared with other groups, the physical function did not improve more effectively in the WBV group, whereas the TUG and 6MWD results were enhanced after 8 weeks of WBV training and ST. During the physical function test, several factors would the test outcomes, such as the function of balance, mobility, muscle strength, etc. Although the WBV training obviously promoted the increasement of knee muscle, other multiple factors might contribute to undifferentiated results of physical function. Besides, similar to our results, several studies failed to verify the superior effect of WBV on physical function [(17, 22, 43)]. However, after WBV training with longer or more frequent interventions, the physical function of patients improved significantly compared with those that underwent a similar training, as reported by Wang et al. [(19)], Simao et al. [(43)] and Osugi et al. [(25)]. Therefore, we speculate that prolonged WBV training might promote functional improvement.
This study had several limitations. Firstly, we did not address the factors that might affect the effect of WBV, such as the vibration frequency, displacement and type. Secondly, all the participants were measured at a single centre, and this might have skewed the clinical outcomes. In addition, the participants with mild and moderate KOA were included in this study. Considering the eligibility criteria, the results have limited generalisability and are difficult to apply to patients with severe KOA. In this study, the pain intensity, physical function and neuromuscular function were addressed to determine the efficacy of WBV training. A full exploration of the effects of WBV training on KOA patients was not possible due to the lack of analysis of disease-related biochemical indicators and neuromuscular response.