**1-. SPECIFIC STRENGTH VS. NON-SPECIFIC AEROBIC**


↓

Cramer et al. [30] VAS at motion ↓

VAS at motion ↓

VAS intensity ↓ > than SI ø

VAS intensity =

Viljanen et al. [31] = VAS

Pain at present = Pain in general = VAS at worst ↓ > than REF

Overall pain =

VAS at present and VAS at worst "exercisers" ↓ > than REF

=VAS ⊕⊝⊝⊝

Overall pain =

Waling et al. [29]

**2.3. LONG ‐TERM EFFECTS**

Viljanen et al. [31]

**3‐. SPECIFIC STRETCH VS. GENERAL STRETCH**

**3.1. ACUTE EFFECTS**: no study **3.2. SHORT‐TERM EFFECTS**

Cunha et al. [27]

↓VAS

ø

⊕⊝⊝⊝

⊕⊕⊕⊝

*J. Clin. Med.* **2021**, *10*, 5946 8 of 24

*J. Clin. Med.* **2021**, *10*, 5946 8 of 25

*J. Clin. Med.* **2021**, *10*, 5946 8 of 25


114

"increased/higher"; ↓ indicates "decreased/lower"; = indicates "no change"; (+) indicates "low risk of bias"; (?) indicates "unclear risk of bias"; (−) indicates "high risk of bias". ø: not evaluated. Abbreviations: NRS, numerical rating scale; NSI, non-specific intervention; REF, reference group; SI, specific intervention; VAS, visual analogical scale; W, week/s. Legend: ↑ indicates "increased/higher"; ↓ indicates "decreased/lower"; = indicates "no change"; (+) indicates "low risk of bias"; (?) indicates "unclear risk of bias"; (−) indicates "high risk of bias". ø: not evaluated.

#### *3.3. Study Characteristics*

A total of 13 articles were reviewed, originating from nine data files (from now on referred to as studies). Although one study generally generated a single article, the results of three studies generated seven articles, whose differentiating aspects can be broadly disaggregated and conveyed as follows: (a) Andersen et al. [32,33], 2008 [32] referred to the short-term effects post-intervention with pain intensity as an outcome, while 2010 [33] referred to a higher sample size and pain regions as an additional outcome. (b) Andersen et al. [26], Nielsen et al. [25] and Søgaard et al. [24] varied their timeframes and outcomes. Andersen et al. [26] assessed the short-term effects post-intervention, similar to their counterparts, but included assessments halfway through the training period and after 10-week follow-up. An analysis was completed, looking at the acute effects after one session. Pain intensity was the outcome. Nielsen et al. [25] analyzed pressure–pain thresholds (PPTs), and Søgaard et al. [24] included repetitive and stressful work tasks as a test to evaluate the training effects on pain intensity. (c) Both Ahlgren et al. [28] and Waling et al. [29] evaluated the short-term effects post-intervention, varying in their assessed pain-related outcomes (pain intensity and PPTs and pain distribution, respectively).

The number of patients in each study varied from 33 to 616. Eight out of 13 articles only included women [22,24–29,31], whereas the other articles included both men and women. A total of 1229 women and 271 men were evaluated, with a mean age varying between 37.6 ± 6.1 years [28,29] and 50.3 ± 14.8 years [22] for the women and between 39.6 ± 9.2 years [23] and 49.0 ± 1.4 years for the men [32]. Most of the patients were office workers [22,24–26,31–33] and assembly line workers [24,26]. Six out of the 13 articles did not specify the patients jobs [21,23,27–30].

Out of 1500 patients, the vast majority (a total of 1269 patients) were diagnosed with non-specific chronic neck-shoulder pain [21–23,27,30–33]. The remaining 231 patients were diagnosed with trapezius myalgia [24–26,28,29]. No study analyzed patients with shoulder pain as a standalone disorder, and all were part of a sample of neck–shoulder pain patients.

Concerning the SE, strengthening exercises using dumbbells were used in six of the articles [24–26,31–33], followed by air machines [28,29], and elastic band [21–23] or isometric exercises using a towel [30]. One study included conventional auto-passive stretching as specific exercise [27].

The NSEs included in the studies were bicycle ergometer training [24–26], nordic walking [22], advice about staying physically active [21,23,32,33], global stretching [27], and body–mind therapies such as yoga [30], relaxation [21,31] or body awareness [21,28,29].

Most outcome measures concerned self-report pain measures. Visual analogue scales [22,24,26–31], 0–9 scales [32,33], and numeric rating scales [21,23] were used to evaluate pain. PPTs were measured in four of the included articles [21,25,29,30]. The other outcomes registered in the different articles were the body pain scale of the 36 item Short Form Health Survey (SF-36) [27,30], neck pain regions (n) [21,33], and pain drawings [21,29].

Frequency of therapies varied from 1 [33] to 5 times/week [30], with 3 times/week the most frequently used [21,23–26,28,29,31–33].

The total duration of the exercise program lasted from 6 weeks [27] to one year [32,33], with a modus of 9 to 10 weeks [22,24–26,28–30]. Follow-up varied between 6 weeks [27] and 9 months [31] after treatment ending.

All studies analyzed the short-term effects of exercise on pain. Four studies analyzed the long-term effects [22,26,27,31], and one study considered the acute effects after one exercise session [26].

Individual study results were clustered based on treatment types and follow-up effects: acute effects after one exercise session and training effects (acute, short-term, and long-term effects), as presented in Tables 3–5**.**




117




**Table 3.** *Cont.*

M, month/s; n, number of subjects; NRS, numerical rating scale; NSI, non-specific intervention; PD, pain drawings; REF, reference group; REP/S, repetition/s; RM, repetition maximum; SI, specific intervention; VAS, visual analogical scale; W, week/s; Y, years.


120


∆SI1 = ∆SI2 = ∆NSI = ∆REF







↑ SI and NSI

 ∆SI = ∆NSI Abbreviations: 0 , minute/s; ", second/s; ↑, increased/higher; ↓, decreased/lower; =, no change; ♀, female subjects; EX, exercise; M, month/s; n, number of subjects; NSI, Non-specific intervention; REF, reference

group; SI, specific intervention; VAS, visual analogical scale; W, week/s; Y, year/s.

#### *3.4. Data Synthesis*

#### 3.4.1. Specific Strength vs. Non-Specific Aerobic Exercises

A total of 8 out of the 13 articles analyzed the effects of specific strength training compared to general aerobic exercises (Table 3).

• Acute effects

One article analyzed the acute effects of a single bout of exercise [26]: non-specific exercise, based on a generic aerobic program, caused an immediate post-exercise pain reduction. Specific strength training showed an immediate post-exercise pain increase during the first half of the training period that flattened near the end of the 10-week training program. Both pain increases and reductions leveled off 2 h after exercise.

• Short-term effects

Seven out of the 13 articles analyzed the short-term effects of physical exercise on pain behavior [21–26,32,33]. Both specific strength training and non-specific physical exercise programs of 10–12 weeks (20–30 min training, 2–3 days/week) resulted in a decrease in general pain [22,23,32,33], in pain during a repetitive task [24], and in the number of pain regions [33] compared to a reference intervention. However, in two articles, specific strength training for 20 min/day, 3 times/week was superior in reducing pain in general [26], pain at worst [26], and pain at rest [24] after 10 weeks of treatment in women with trapezius myalgia.

Two articles reported the effects of exercise on PPTs, reporting no differences between specific and non-specific training [21,25]. While Iversen et al. [21] found no changes after the exercise program, Nielsen et al. [25] reported that pain sensitivity at a pain-free reference muscle was decreased (i.e., higher PPTs) in response to both specific strength training (concentric and eccentric contractions) and non-specific fitness training (bicycle ergometer) after 10 weeks of exercise (20-min training, 3 days/week) in women with trapezius myalgia.

• Long-term effects

Two articles analyzed the effects of specific strength training vs. aerobic exercise 10 weeks after finishing the exercise program, with inconsistent results. Saeterbakken et al. [22] found that both exercise types (specific and non-specific) had a similar effect on pain reduction that lasted during follow-up compared with no effect in a reference group. Nevertheless, the study performed by Andersen et al. [26] reported that specific strength training resulted in significant pain reduction, in contrast to the non-specific aerobic exercise group, which consolidated in the further 10-week follow-up period.

In conclusion, there is low evidence that specific strengthening exercises and nonspecific fitness training produce similar short-term effects regarding pain relief. There is only preliminary evidence that immediate acute response to exercise is more favorable for the non-specific exercise program. There is also very low evidence that the long-term effects are favored by specific strengthening exercises.

#### 3.4.2. Specific Strength vs. Body Mind Exercises

A total of 4 out of the 13 articles analyzed the effects of specific strength training compared to body mind therapies (Table 4) [28–31].

• Short-term effects

All the mentioned articles reported positive short-term effects of exercise programs on pain behavior. There were no differences between specific strength training and NSE (body–mind exercises through body awareness and yoga), as both resulted in a decrease in the intensities of pain at motion [30], pain at present, pain at worst, and pain in general [28,29] after 9 or more weeks of treatment (3 days/week).

However, there were inconsistent results in two of the articles regarding three outcomes: specific strength training during 60 min/day, 3 times/week, reduced pain at worst

after 10 weeks of treatment in women with trapezius myalgia compared to body-awareness exercises [28]. Yoga classes for 90 min/week reduced pain in general and bodily pain items from SF-36, after 9 weeks of practice, in patients with non-specific neck pain, compared to specific strength exercises [30].

Two articles reported the effects of exercise on PPTs [29,30]. Cramer et al. [30] demonstrated better results with non-specific interventions: yoga exercises, practiced with an instructor for 90 min/week for 9 weeks, decreased pressure sensitivity in non-specific neck pain patients compared to strength training for 10 min/day. The study of Waling et al. [29] found no differences between SE and NSE: pressure sensitivity significantly decreased at four myofascial trigger points of the trapezius muscle in both exercise regimens, compared to a reference group.

For pain drawings, no changes were seen in the extent of painful body area in any of the exercise groups (body awareness and specific strength) [29].

• Long-term effects

One article analyzed the effects of a 13-week specific-strength training (3 days/week) compared to relaxation training and a reference group, after 9 months follow-up, concluding that no difference was found in neck pain intensity in questions concerning neck pain disability between the three groups in a sample of 393 female office workers with chronic non-specific neck pain [31].

There is moderate evidence that exercises reduce pain in the short-term compared to a reference group and there seems to be no difference for different types of exercise. For the long-term, there is only preliminary evidence that there is no difference between exercise groups and the reference group.

3.4.3. Specific Stretch vs. General Stretch Exercises

• Short and long-term effects

One article investigated the effects, both short and long-term, of specific versus general stretches on pain reduction (Table 5) [27]. These authors suggest that conventional specific stretching and muscle chain stretching (30 min, 2 times/week), in association with manual therapy (30 min, 2 times/week), were equally effective in reducing the pain of female patients with chronic neck pain, both post-treatment and at six weeks after ending the treatment.

There is preliminary evidence that specific and non-specific stretching exercises are equally beneficial for pain reduction in female patients with chronic neck pain, although more studies are needed.

#### **4. Discussion**

This is the first systematic review specifically examining the effect of SE compared with NSE on pain in the rehabilitation of patients with CNSP.

The aim of this review was to evaluate the effect of SE, involving exercises focused on the neck and/or shoulder region, focused on CNSP patients, looking for pain reduction/increases compared to NSE.

There is considerable evidence of pain reduction after an exercise program, both specific and non-specific, in the short- and long-term [22,24,26,28,29,32,33]. For the short-term effects, 9 out of 13 articles did not favor a particular type of exercise [21–23,27–29,31–33], while 3 articles [24–26] found better effects on pain for specific training, and the other article favored non-specific training [30]. With regard to the long-term effects of exercise on pain, 3 out of 4 articles found that specific [26], or both exercise types independently (specific and non-specific) [22,27], had a lasting effect on pain reduction. The other article found that exercise had no long-term effects on pain [31]. Nevertheless, regarding the acute effect of single bouts of exercise, only one article assessed this aspect [26], reporting an immediate pain reduction after non-specific exercise in contrast to specific resistance exercises. Consequently, more research is needed, specifically about acute and long-term effects.

These results are in line with a Cochrane systematic review evaluating the use of motor control as a specific exercise strategy among a chronic non-specific neck pain population [8]. The study suggested that specific motor control exercises were not superior to more general exercise strategies. Furthermore, the review of Booth et al. [35] did not provide evidence for the superiority of one exercise type in chronic musculoskeletal pain conditions. Therefore, the type of exercise might be less important than the act of doing exercise. Sluka et al. [36] suggest that this lack of specificity of exercise type may be related to the multiple and widespread mechanisms by which exercise works to reduce pain.

Although this aspect is out of the scope of the present review, it is interesting to try to elucidate the mechanisms that could explain our findings. The reason that the research to date has not shown any specific exercise to be superior may be that psychological and/or neurophysiological factors that are common to all exercise approaches have the greatest mediating effects on pain [37]. If changes in pain and disability occur without changes in physical function, then specific modalities of exercise and their dosage seem to be less relevant in chronic musculoskeletal pain [35,38]. It is tempting to speculate that exercise can indeed desensitize the central nervous system. This hypothesis has recently been supported through a review of the current evidence on the central mechanisms underlying exercise-induced pain and analgesia [39].

Exercise is likely to be most effective if tailored to individual patients with spinal pain. As Falla and Hodges [40] stated, current exercise programs for spinal pain treatment often rely on a one-size-fits-all approach and usually fall short of success. These authors provide evidence supporting the hypothesis that the outcome of exercise interventions can be optimized when targeted to the *right* people and adapted to the individual's presentation. In the same line, tailoring exercise to individual patients has been recommended for chronic musculoskeletal pain [17,41], which requires an initial assessment to understand the biological, psychological, and social factors contributing to pain and disability [35]. The dominant pain mechanism must also be considered to optimize exercise prescription. Indeed, a recent systematic review concluded that global (non-specific) exercises are preferred in nociplastic pain conditions, while more SE should be emphasized in non-nociplastic conditions [42]. In the present review, however, all the included studies used standardized exercise programs and no prior assessment was made to determine the patient's profile.

The level of supervision is also an important aspect in promoting treatment adherence and patients' motivation [35]. Supervised exercise programs have been recommended for chronic musculoskeletal pain [17,43]. In the present review, all studies but one [23] included supervised exercise sessions. This could be the reason that the drop-out rate was relatively low in those studies.

Finally, ongoing self-monitoring can be helpful to identify barriers to [14] and facilitators of exercise participation, motivate positive exercise behavior and increase participation [44]. In the present review, only five articles out of 13 did not use diaries to register their adherence to the exercise programs [22,24–26,31], which could also explain the high participation rate.

Thus, supervised exercise, individualized therapy, and self-management techniques may help to promote a successful rehabilitation program [14]; however, the quality of trials assessing these interventions is low [43], and further research is warranted.

#### *Limitations*

First, the main weakness of this review is the risk of bias. Random sequence generation, accounted co-interventions, and concealment of allocation were often not attained. Therefore, a note of caution is due here. Most studies failed to achieve blinding of the patients. Furthermore, the majority of studies relied on self-reported measures, prohibiting blinding of the assessors as well. Although blinding participants and therapists in an exercise trial is difficult to implement and cannot obviate the risk of bias, future studies should endeavor to limit the potential bias with the appropriate blinding of at least the assessors. Keeping the patients and therapists naïve regarding the received treatment should

be attempted, as specific expectations and beliefs could influence outcomes. Assuming naïve patients is only possible in the studies evaluating different exercise modalities. This should be considered in future studies. Second, the number of RCTs included was low. The limited number of studies published in this area also raises the possibility of publication bias. Third, patient activity between post-test and follow-up was not controlled in any study. Finally, only four articles analyzed the follow-up period [22,26,27,31]. In two of these, the follow-up was limited to less than three months, which seems to be insufficient, as CNSP can last for up to several years [26,27]. This aspect limits any comment on the maintenance of the effects of exercise. Ongoing research including acute and follow-up schemes over six months is required to further validate our findings and determine the long-term effects of the intervention.

Furthermore, there was a lack of uniformity in the obtained results regarding the differences in the benefits between specific and non-specific exercises. The term "specific exercise" has been used to describe different types of exercises, such as stabilization [45], strengthening [46], individualized [47], supervised [48], and even what appear to be general exercises [49]. Non-specific exercise protocols usually address general flexibility, strength, and/or endurance training, including all body regions. Such inconsistency, together with an incomplete description of exercise details regarding dosage [8], are a possible reason for the inconsistent results found in different chronic pain populations. Therefore, the working mechanisms and exact definition and dosage of the exercise therapy modalities need to be further elaborated.

#### **5. Conclusions**

This systematic review shows interesting findings for pain relief with regard to training effects using specific and/or non-specific exercise for CNSP. Both specific (neck and/or shoulder exercises) and NSE seem to be effective for short-term pain reduction in patients with CNSP.

Based on the acute effects, there is only preliminary evidence that a bout of non-specific exercise seems to be more tolerable for patients with CNSP, overcoming the exacerbation in the beginning. Regarding the long-term effects, SE seems to be the best option, although the evidence for this is very limited. As the evidence is still rather restricted, this review highlights the need for further RCTs comparing the effects of injury-focused (specific) exercises versus more general (non-specific) exercises, and a need to better understand the definition and dosage of exercise therapy modalities to improve clinical application.

**Author Contributions:** Conceptualization, M.M., L.V. and L.D.; methodology, M.M.; data collection and analysis, L.D., M.A.-R. and M.M.; writing—original draft preparation, L.D. and M.M.; writing review and editing, M.A.-R., L.V., E.L., F.S., M.G.C.A.M.M. and K.D.M.; visualization, F.S. and M.G.C.A.M.M.; supervision, M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** This is a systematic review of published data. We had no access to any individualized patient data. Therefore, no informed patient consent or ethical approval was needed for this study.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.

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