Introduction

Fibromyalgia syndrome (FMS) is characterised by the presence of widespread and distributed pain in different areas of the body accompanied by the alteration of some functions such as the activity of the neurovegetative system, the sleep quality, or the presence of fatigue [1]. Recently, circulating natural killer (NK) cells in peripheral nerves have been found to be reduced, chronically activated, and redistributed in FMS patients, which directly contributes to the immunopathology associated with FMS [2]. FMS is more prevalent in women than in men and its prevalence in the general population can be as high as 5% [3]. Regarding the aetiology, some studies comment that there may be a presence of hyperexcitability of the central nervous system in patients with FMS, secondarily causing an alteration in pain sensitivity [4, 5]. Regarding the diagnosis of FMS, the American College of Radiology (ACR) established 2016 an update of its criteria and commented that for the diagnosis of FMS, the patient should have at least four of five body regions with pain (widespread pain), where in addition the symptoms must be present for more than 3 months and also show high severity rates of symptoms [6]. Regarding the treatment of FMS, there are some studies that have evaluated the effectiveness of pharmacological [7, 8], psychological [9, 10], educational [11, 12], and also through therapeutic exercise (TE) [13, 14].

The present narrative review aims to evaluate some key studies regarding the effects of different TE modalities on clinical variables of interest in patients with FMS, as well as to discuss some of the possible mechanisms of action of TE in improving pain intensity in patients with FMS.

Aerobic exercise

Aerobic exercise (AE) in patients with FMS is a topic of recognised interest in the scientific field, highlighted by a remarkable increase in research in recent years. We have chosen some relevant studies to comment on. For example, Bidonde et al. [15] evaluated the benefits and drawbacks of AE in adults with FMS. In addition, Bidonde et al. [16] focused on evaluating only aquatic exercise (AqE) in patients with FMS.

In both studies (Bidonde et al. [15, 16]), patients over 18 years of age diagnosed with FMS were selected. The diagnosis could be either ACR 1990 criteria or ACR 2010 criteria [17, 18]. Outcome measures were divided into three types: outcomes representing wellness, outcomes representing FMS symptoms, and outcomes representing physical fitness.

The outcomes that represent wellness were health-related quality of life (HRQoL) or multidimensional function, patient-rated global, clinician-rated global, self-efficacy, and mental health. Moreover, the outcomes that represent FMS symptoms were pain, fatigue, stiffness, sleep quality, tenderness, depressed mood, and anxiety symptoms.

Finally, the outcomes that represent physical fitness were muscle strength, physical function, muscle endurance, muscle power, submaximal cardiorespiratory function, maximal cardiorespiratory function, and muscle/joint flexion.

Of all the outcomes that have been mentioned, we highlight pain, fatigue, HRQoL/multidimensional function, stiffness, physical function, sleep quality, depressed mood, muscle strength, and submaximal cardiorespiratory function.

Bidonde et al. [15] included studies with both unidimensional and composite measures of pain. In addition, if there was more than one type of pain measure, the average pain intensity was prioritised over the others. The most common tools used in the unidimensional assessments were the Visual Analogue Scale (VAS) and the Numerical Pain Rating Scale (NPRS). On the other hand, for joint assessments, pain intensity was assessed in composite form with interference using the 36-Item Short Form Survey (SF-36) or Rand 36 Bodily Pain Scale. Pain intensity was also assessed in conjunction with pain distress using the Multidimensional Pain Inventory or Pain Severity Scale. In Bidonde et al. [16], the VAS scale was the main tool to assess pain. For fatigue, Bidonde et al. [15] and Bidonde et al. [16] accepted both unidimensional and multidimensional assessments of fatigue. The preferred tool for fatigue assessment was the VAS-fatigue. Bidonde et al. [15] assessed HRQoL using as main tools the Fibromyalgia Impact Questionnaire (FIQ), the Short Form Questionnaire (i.e., SF-36 total or SF-12 total) and the EuroQol-5D (EQ-5D). Bidonde et al. [16] assessed multidimensional function, which is directly related to HRQoL. The main tool used for this assessment was the FIQ total. Stiffness was measured using the FIQ Stiffness Subscale in Bidonde et al. [15]. Bidonde et al. [16] only considered the FIQ stiffness VAS as a tool to assess stiffness. As for the assessment of physical function, the following tools were accepted by Bidonde et al. [15]: the FIQ Physical Impairment Scale, the Health Assessment Questionnaire (HAQ), the SF-36 or the Rand 36 Physical Function Scale, the Sickness Impact Profile—Physical Disability Scale and the Multidimensional Pain Inventory Household Chores Scale. Self-reported physical function was mainly assessed by Bidonde et al. [16] using the FIQ physical impairment scale, HAQ, SF-36 physical function, and Sickness Impact Profile. Bidonde et al. [16] included in terms of assessing sleep data mainly from the Pittsburgh Sleep Quality Index (PSQI) and the sleep quality VAS. Bidonde et al. [16] prioritised the data on depressed mood from the Beck Depression Inventory (BDI), Hospital Anxiety and Depression Questionnaire, Centre for Epidemiologic Studies-Depression (CES-D), and FIQ. Muscle strength was assessed using the grip strength in Bidonde et al. [16]. Bidonde et al. [15] and Bidonde et al. [16] collected data on submaximal cardiorespiratory function mainly from the six-minute walk test (6-MWT). The outcome measures highlighted are summarised (Table 1).

It is noteworthy that both Bidonde et al. [15] and Bidonde et al. [16] considered withdrawals and adverse events. In addition, among the total number of articles (29), there was only one article that both reviews repeated [19].

All the interventions included in the above studies had AE as a main feature. AE is understood as a dynamic physical activity performed by large muscle groups which increases heart- and respiratory- rates above resting levels, establishing them for a long period at sub-maximal levels [20]. Bidonde et al. [16] included only exercise programmes performed 50% or more in water, which is called aquatic aerobic exercises (AAE).

Bidonde et al. [15] established exercise intensity using the percentage of maximal heart rate (HRmax) or heart rate reserve (HRR) as a reference. On the other hand, Bidonde et al. [16] classified exercise intensities following the recommendations of the American College of Sports Medicine (ACSM) [21]. Basically, the ACSM established three main parameters as a reference to establish exercise intensity. These parameters are the percentage of HRmax, the percentage of VO2 reserve or HRR, and the Borg Rating of Perceived Exertion Scale (RPE) 6 to 20 scale.

A summary of the interventions is showed in Table 2.

Bidonde et al. [15] concluded with moderate-quality of evidence that AE improved HRQoL and with low-quality of evidence that AE may positively affect pain intensity, physical function, fatigue, and stiffness in patients with FMS. In addition, Bidonde et al. [16] concluded with low to moderate evidence that AqE in patients with FMS could improve fitness, wellness, and symptoms in adults with FMS. Both studies agreed that further research is needed, in particular on the long-term effects.

Body-mind exercise

Meditative movement therapies (MMT) include exercises that combine movement or body positions with a focus on breathing and a clear state of mind aim to achieve deep breathing [22] (Table 3). Langhorst et al. [23] conducted a systematic review assessing the efficacy and safety of MMT in patients with FMS. In this review were included patients of any age diagnosed with FMS by recognised criteria. Moreover, were assessed as outcomes the key domains of the FMS: pain, sleep, fatigue, HRQoL, and depressive mood. In addition, only MMT interventions were added. In conclusion, Langhorst et al. [23] concluded that MMT improved HRQoL and reduced sleep disturbance, depressive mood, and fatigue in FMS.

Resistance exercise

Resistance training (RT) is the most researched exercise modality after AE [24]. Patients with FMS have reduced physical activity and increased sedentary rates. Loss of muscle function is common [25]. RT aims to improve pain, tenderness, fatigue, sleep, depressive mood, and muscle strength in patients with FMS [26]. There is no consensus on the characteristics of the RT intervention. Different protocols have been proposed and evaluated by different systematic reviews. However, RT is a safe and effective intervention when adapted to the individual needs of patients [27].

The aim of the study conducted by Rodríguez-Domínguez et al. [24] was to assess the clinical relevance and effectiveness of RT concerning pain intensity, functionality, and disease severity specifically in women diagnosed with FMS, aged 18 and above. On the other hand, the study conducted by Vilarino et al. [28] aimed to analyze the effects of RT on the mental health of patients with FMS, aged 18 and above.

Pain intensity was evaluated in Rodríguez-Domínguez et al. [24] using VAS and a subscale of the SF-36 (bodily pain). Moreover, in this study functionality was assessed through the Continuous-Scale Physical Functional Performance (CS-PFP), the HAQ, and another subscale of the SF-36 (physical function). Additionally, the FIQ was employed to assess the severity of the disease. Anxiety symptoms were analyzed by Vilarino et al. [28] using the State-Trait Anxiety Inventory (STAI), the Hospital Anxiety and Depression Scale, and the BDI. In addition, depression was assessed through the BDI, the Hospital Anxiety and Depression Scale, and the FIQ.

The interventions in Rodríguez-Domínguez et al. [24] lasted between 8 and 21 weeks, with the most common duration being 12 weeks (four studies). Additionally, most of the interventions involved training sessions twice a week. All workouts included 10 min of warm-up, 50 min of RT, and 10 min of relaxation at the end, which consisted of stretching exercises. During training, patients were instructed to take 1 min of recovery between each set. The RT programs used in the studies included a range of 4–12 exercises. In terms of intensity, several studies began with 40–60% of the one-repetition maximum (1RM). However, others used increased repetitions to increase intensity. In this way, the number of repetitions was inversely proportional to the intensity. While some started in sets of 4 to 5 repetitions and increased to 12 repetitions, others started in sets of more volume (15–20 repetitions) and at the end of treatment increased the intensity to perform sets of 5–10 repetitions. On the other hand, the interventions in Vilarino et al. [28] lasted between 4 and 21 weeks. Additionally, most of the interventions in this review involved training sessions twice a week. All workouts included 5 or 10 min of warm-up, with short walks or light stretches, 30–50 min of RT, and 5–10 min of relaxation at the end, which consisted of stretching exercises. The exercises were performed on machines, with body weight and with free weights, beginning at 40% of 1RM and progressing in some studies to 85%, with one to three sets of 12 repetitions. The summary of the interventions is in Table 4.

Rodríguez-Domínguez et al. [24] concluded that RT was effective in reducing pain intensity and improving functionality in patients with FMS. Whereas Vilarino et al. [28] concluded that RT was effective in improving the mental health of patients with FMS by reducing anxiety and depressed mood.

Mechanisms hypothesized for pain-relieving effects of TE in patients with fibromyalgia

To conclude the narrative review, we have evaluated the article conducted by Neelapala et al. [29] with the aim of extracting some hypotheses about the possible mechanisms involved in the hypoalgesia generated by TE in patients with FMS.

As we have seen above, TE is an approach capable of eliciting changes in clinical variables of interest, and particularly in pain intensity. In fact, TE is one of the management strategies most recommended by treatment guidelines for FMS (e.g., Canadian [30] or European [31] guidelines).

Neelapala et al. [29] comment that TE may lead to improvements in pain intensity through three mechanisms: physical mechanisms, neurophysiological mechanisms, and psychosocial mechanisms.

Conclusions

TE is a clinical tool with great potential for patients with FMS as it can produce hypoalgesia through physical, neurophysiological, and psychosocial mechanisms. All these TE modalities have demonstrated in isolation a remarkable effectiveness in the overall improvement of patients with FMS. However, more research is needed in this field especially on the long-term effects and on the combination of the different training modalities.