Effects of exercise combined with whole body vibration in patients with patellofemoral pain syndrome: a randomised-controlled clinical trial

Knee pain is a common condition with a high prevalence in the general population, with patellofemoral pain (PFP) being one of the most common forms of knee pain [1]. Patellofemoral pain is described as a diffuse non-traumatic pain in the anterior area of the knee, which is aggravated when performing patellofemoral joint loading activities such as squats, running, going up and down stairs or sitting for a long time [2]. Its annual prevalence is approximately 23% in adults, increasing to 29% in adolescents; and it is more frequent in women [3].

There is no specific cause of pain in these patients and the aetiology of PFP is considered multifactorial. For this reason, the diagnosis is complex and essentially based on the clinical history, always ruling out lesions of adjacent structures such as ligaments or menisci [2]. This syndrome is considered to be a ‘black hole’ in orthopaedic medicine because no single explanation clarifies the patellofemoral problem and no single therapeutic intervention can reduce all patellofemoral dysfunction [4].

Pain around or behind the patella, weakened knee extensor muscles and associated symptoms usually reduce knee functionality and entail a limitation in daily and occupational activities, negatively impacting a patient’s quality of life and social aspects [5].

Exercise therapy is an evidenced-based non-pharmacological intervention often prescribed for this condition [6]. There is strong evidence that supports its use to improve pain and function in both the short and long term [7, 8]. Particularly, strength training is considered an essential part of the exercise programmes when conservative treatment is applied [9, 10]. Traditionally, quadriceps strengthening has been used for functional re-education in these patients [11]. However, currently, it is strongly recommended to combine quadriceps and proximal hip exercises in patients who suffer PFP in order to obtain better results in terms of pain reduction and improved function [6, 12].

Whole body vibration (WBV) training has demonstrated improvements in muscle strength, power, balance, flexibility, proprioception or human gait in healthy adults, the elderly [12‐15]. In this type of training, vibration is transmitted from a platform on which an individual remains standing while holding on to the device with the upper limb, or even passively being the patient seated in a chair in front of the platform and the feet (barefoot) on the platform [16]. The amplitude of the vibration generated by these devices ranges from 0.7 to 14 mm, with a specific oscillation frequency that can range from 0.5 to 80 Hz [17]. Different mechanisms have been described by which vibration can contribute to flexibility improvement, especially as an acute effect, such as reduction of pain sensation, increase in blood flow or reduction of muscle stiffness [18].

In clinical settings, there is growing evidence about the positive effects of WBV training on reducing pain intensity and increasing lower limb functionality in patients with several musculoskeletal painful conditions [19, 20]. Zafar et al. [21] concluded that WBV training reduces pain and improves function in individuals with knee OA. The inclusion of the WBV in rehabilitation programs was suggested by Wang et al. [22] and Simao et al. [23] considering its benefits for improving functionality in patients with knee osteoarthritis. More recently, Wang et al. [24] supports this effect on pain reduction in patients with non-specific chronic low back pain.

To our knowledge, only one study using WBV training has been developed in patients with PFP [25]. The improvements in quadriceps and hip strength induced by the WBV make it a potential therapeutic resource for treating patients with this syndrome. The dosage of the exercise is one of the most diffuse aspects of this type of training, and it is necessary to establish optimal load parameters to maximise the benefits and reduce potential injury risks [22, 23, 26]. As far as we know, no reports exist about short-term training periods (four weeks), including knee, hip and core exercises in patients with PFP, and considering in the exercise dosage the contraction time during a global position (30 s), and not the number of repetitions of a movement. Therefore, the aim of this study was to determine the effects of adding WBV to an exercise protocol on pain and disability in patients with anterior knee pain. We hypothesised, a priori, that an exercise protocol performed on the vibratory platform for a 4-week period, with a three days per week frequency, is more effective than exercise alone for reducing pain levels and improving knee function in patients with PFP.

A sample of 55 subjects, between 19 and 67 years of age, were selected for the study. A participant flow-diagram is shown in Fig. 2. After the enrolment phase, 5 subjects were excluded due to different reasons. The final sample included 50 individuals, 24 men and 26 women (mean age ± SD, 50 ± 12.0 years).

The right lower limb side was affected for 54% of the participants (n = 27) and the left for 46% (n = 23). The participants’ level of education was: 40% primary studies (n = 20), 30% secondary (n = 15) and 30% university degree (n = 15). With respect to pharmacology treatment, 28 subjects used to take nonsteroidal anti-inflammatory drugs and analgesics before the study and 22 did not take any medication. Around 58% of the sample had an occupation in the service sector, 20% in the construction field and 22% in industry.

Table 2 shows the mean values and the standard deviation of the main variables at the baseline for both groups. The gender of the participants in each group is also presented. There were no differences between groups at the beginning of the study in any of the studied variables. Although the knee extension variable was recorded by the goniometric measurement, all the knees in the study presented a value of 0 degrees (full extension).

Table 3 includes the differences between the baseline and post-treatment scores of the studied variables for each group and between groups. Statistical significance was found in favour of the experimental group in the between-groups comparison and in the interaction of the experimental group before and after treatment in terms of pain perception: VAS F _{1, 48} = 82.4; p = 0.000; η² = 0.63 and DN4: F_{1, 48} = 35.7; p = 0.000; η² = 0.43; as in the lower limb functionality and disability outcomes, LEFS: F _{1, 48} = 49.1; p = 0.000; η² = 0.51 and KUJALA: F _{1, 48} = 30.5; p = 0.000; η² = 0.39; and, finally, in the knee flexion range of motion: F _{1, 48} = 52.7; p = 0.000; η² = 0.53.

There were no adverse events reported by the participants in either group.

The aim of this investigation was to determine the effects of adding WBV to an exercise protocol to improve pain and disability in patients with anterior knee pain. Present findings indicate that performing knee, hip and core exercises on the vibrating platform achieves better immediate results in terms of pain relief and functional knee self-reported outcomes in comparison with exercises on their own (without a vibratory stimulus). The average change on self-reported instruments obtained by the subjects in the experimental group (VAS, LEFS and Kujala scores) reached the respective thresholds of minimal clinically important difference. Therefore, the combination of exercise and vibration may be considered to have a high short-term value in clinical practice for pain relief and improve lower limb functionality.

Pain intensity reduction after WBV training has been reported in previous studies but the mechanism for this effect remains unclear [19]. Several theories have been proposed to explain the analgesic effect of global vibration, such as the gate control or the inhibitory effect of vibration-induced non-noxious stimulus over spinothalamic tract neurons [19, 21, 23]. In addition, the stimulation generated by the vibration on the musculature through the tonic vibration reflex can generate a positive contribution to this previously mentioned analgesic effect [42]. Most likely, this marked reduction in pain intensity may be the mechanism by which patients in the experimental group showed a clinically and statistically significant improvement in self-reported knee and lower limb functionality at the end of the intervention. Avelar et al. reported that WBV may be a strategy to use to improve the functionality and self-perception of knee osteoarthritis in older adults [43]. Simao et al. also described an improvement in the self-perception of pain in elders with knee osteoarthritis after 12 weeks (3 times per week) of adding vibration to a squat exercise [23]. In our study, the intervention period is shorter but includes several exercises that can lead to the reported benefits in self-reported knee pain and functionality.

Respect to the pain quality in PFP syndrome, Jensen et al. hypothesised that the observed sensory aberrations in these patients may cause neuropathic-like knee pain [44]. Using the DN-4 questionnaire, we obtained results that could point to neuropathic components in the subjects of our sample. An alteration in the repetition or processing of A-beta inputs could generate a reduction of the pain inhibitory capacity [45]. A possible explanation for the pain reduction after WBV training may be that the increase in sensory inputs from the mechanoreceptors of the skin, joint and muscle responses caused by vibration could also favour a reduction in pain level caused by physical activity in subjects with PFP or even improve the quadriceps arthrogenic muscle inhibition [46]. In any case, better understanding of the pain mechanisms that underlying the PFP syndrome becomes essential for administering adequate treatment [44].

We suggest that the present findings about the WBV training on pain level reduction effect in short-term could help patients with PFP to overcome the negative emotional impact of this injury, which include confusion and low expectations for improvement as well as low perceived self-efficacy [3]. It is known that psychosocial impact of a musculoskeletal disease correlates with pain level and reduced physical function [47], and achieving a clinically relevant reduction in pain relief could help reduce this psychosocial impact.

In most studies, exercise programs that reported positive effects on pain intensity and muscle strength for patients with PFP were 8 weeks in duration [19]. Therefore, 4 weeks seems to be most likely a short time to show neuromuscular adaptations in these patients. In our study, we did not evaluate the effects of training on neuromuscular parameters and, therefore, we cannot confirm effects at this level. However, Abassi et al. reported beneficial effects of WBV training on the muscle strength electromyographic parameters in patients with knee osteoarthritis in a 4-week period [48]. This indicates that short-term improvements in muscle strength parameters are possible and could potentially lead to functional improvements of the self-reported scales employed. Additionally, the tonic vibration reflex, induced by the mechanical platform vibrations, can potentially increase the recruitment of the motor units and the activity of the proprioceptive system, resulting in a clinical improvement [10, 41].

In our study, a statistically significant improvement in the knee flexion range of motion after completing the protocol was observed. Osawa & Oguma, in their meta-analysis, showed that vibration interventions had significant effects on flexibility [18]. These observed changes in the knee flexion range of motion could be mediated by the knee pain level reduction. Other potential mechanism could be involved such a potential increase in blood flow after vibration or a reduction of muscle stiffness [41, 43]. In any case, we do not test directly any of these mechanisms. Nevertheless, how long the effects can be maintained after the sessions remains unclear and chronic effects of the WBV training on the knee ROM should be studied in subjects with PFP.

In this study, the exercise programme was carried out by decentralising the focus of attention on the patellofemoral joint and trying to improve the strength and function of the full lower limb and core [3, 10]. Previous investigations have shown that adding hip muscle strengthening exercises (hip abductors, external rotators and abdominal core muscles) produces a faster improvement in anterior knee pain in comparison to a standard knee rehab program [49]. Fukuda et al. reported that adding hip musculature strengthening to a knee strengthening and stretching programme in sedentary women with PFP was more effective than knee exercises alone in improving long-term function and pain outcomes. For this reason, we included additional exercises to the traditional squat training [50].

The results of this study should be considered with caution due to some methodological limitations. First, the main outcomes were obtained using self-reported measures of pain and knee functionality without any other neuromuscular objective parameters. A longer follow-up period is most likely needed in order to objectively register neuromuscular adaptations. Second, only acute effects on clinical outcomes were assessed during the post-intervention period and, therefore, we do not know how long the registered benefits can be maintained. Studies with longer follow-up periods are necessary. Third, we did not stratify our sample into subgroups of patients with homogeneous clinical statuses as has been suggested [5]. Due to the complex nature and aetiology of this painful syndrome, it is necessary to group patients with similar clinical characteristics together in order to know the effects of clinical interventions more specifically. For example, in this work, although not statistically different there are some differences in the subject’s characteristics such age or weight (i.e. intervention group subjects are a little younger than controls) which could have influenced the results. Finally, potential adaptations on the trained musculature have not been evidenced. To the best of our knowledge, previous results of WBV training have been reported in women with PFP [51]. We have reported results for a male sample, which should be contrasted in future studies with a greater sample and a longer follow-up period.

In conclusion, the realisation of a training protocol of hip, knee and core exercises on a vibratory platform produces positive effects on the pain level and functional capacity of patients with PFP and is more effective than exercise alone in improving pain and function in patients with PFP in the short term.