Respiratory muscle training in people with cervical spinal cord injury – A systematic review

Abstract

Introduction

Spinal cord injury is a physiological disruption often caused by trauma, leading to severe physical and psychological effects, including irreversible impairment and disability. Cervical injuries, particularly between C1 and C8, are the most severe, potentially causing diaphragm paralysis and requiring mechanical ventilation. Reduced respiratory muscle strength not only affects respiratory function but also significantly impacts voice, speech, and communication, which are crucial for quality of life.

Objective

Conduct a systematic review of the literature on respiratory muscle training protocols in individuals with cervical spinal cord injury and evaluate the methodological quality of scientific publications.

Methods

Studies were searched by two independent researchers in the Regional Portal of the Virtual Health Library, EMBASE, SCOPUS and PubMed databases, using the descriptors: “respiratory muscle strength”, “breathing training”, and “cervical spinal cord injury”, with no restriction on the time of publication. Studies containing respiratory muscle strength measurements and respiratory muscle training in people with cervical SCI were included and those that associated other techniques with functional respiratory training, such as electrical stimulation and other complementary techniques were excluded. The studies had the methodological quality (internal and external validity) classified by the PEDro scale (Physiotherapy Evidence Database).

Results

Nine studies were identified and considered valid based on the inclusion criteria. The protocols presented varied parameters. The session time ranged from 15 to 45 minutes, the number of sessions per day ranged from 1 to 2, the number of days per week ranged from 3 to 7, and the number of intervention weeks ranged from 4 to 10. Only three studies presented internal and external validity for respiratory muscle training programs.

Conclusion

This review identified that respiratory muscle training is an effective intervention to improve respiratory function in people with cervical SCI. However, due to the poor methodological quality of the studies, the effect size of the treatment, as well as the ideal dose and intensity, requires further investigation to better determine its overall effectiveness.

Introduction

Spinal cord injury (SCI) is a physiological disruption of the spinal cord (1), often caused by trauma, leading to significant physical and psychological impacts, including irreversible impairment and disability (2). With an annual incidence of 15–40 cases per million people globally, SCI reduces quality of life and can be classified into complete or incomplete lesions (2). Complete lesions involve total loss of sensory and motor innervation, while incomplete lesions retain some sensory or motor function (3). Cervical injuries, particularly those between C1 and C8, are considered severe as they compromise all innervation below the injury level (3). Injuries between C1 and C3 may cause partial or complete diaphragm paralysis, potentially requiring tracheostomy and mechanical ventilation (4). Even injuries below C3 can diminish vital capacity due to paralysis of intercostal and abdominal muscles (4), impairing forced expiration by affecting accessory respiratory muscles.

The weakness of both inspiratory and expiratory muscles in people with SCI has a profound impact on respiratory function (4). Understanding this impact is essential for developing comprehensive rehabilitation strategies that address both aspects of respiratory muscle weakness and improve overall respiratory health in people with SCI.

Reduced respiratory muscle strength impacts not only in the respiratory function, but it also presents a great impact in quality of life of individuals with cervical SCI, for playing an important role in voice, speech and communication. The results of the research (5) indicate that the decrease in respiratory muscle strength (both during inspiration and expiration) and the decrease in maximum phonation time should be addressed as parameters in general rehabilitation and speech therapy (5).

Recognizing the critical role of respiratory muscle rehabilitation in improving the quality of life for individuals with cervical SCI, this systematic literature review aimed to address several key questions: What clinical protocols are described for the rehabilitation and respiratory training of individuals with cervical SCI? What is the internal and external validity of the clinical research studies that developed these protocols? What are the expected outcomes of these protocols? The objective of this research is to conduct a systematic review of the literature on respiratory muscle training protocols for individuals with cervical SCI, evaluate the methodological quality of the scientific publications in terms of internal and external validity, examine the effects of treatment, and describe the respiratory training protocols used.

Method

This systematic review was performed according to the general guidelines of the Cochrane Handbook (6) its design followed the criteria of the PICOS acronym, as shown in the table below and to the updated Preferred Reporting Items for Systematic Reviews and Meta-analyses 2020 statement (PRISMA) (7). The protocol of the review was prior registered in the International Prospective Register of Systematic Reviews (PROSPERO) under the protocol number (CRD42023461810). The literature search was conducted on in the first half of 2023 (Table 1).

Table 1.

PICOS acronym.

Description	Abbreviation	Question components
Population	P	People with cervical spinal cord injury
Intervention	I	Respiratory training
Comparison	C	Intervention types
Outcome	O	Respiratory muscle strength, maximal respiratory pressure
Study type	S	Experimental

Search strategy

Studies were located and selected by searching texts on the topic, with no restriction on the time of publication, in the following databases: Regional Portal of the Virtual Health Library – VHL (http://bvsalud.org/), EMBASE (embase.com), SCOPUS (scopus.com), and PubMed (https://pubmed.ncbi.nlm.nih.gov). The descriptors used in the search were “respiratory muscle strength”, “breathing training”, and “cervical spinal cord injury” and were combined with the logical operator “AND”.

Inclusion and exclusion criteria

Studies were considered valid when they met the following inclusion criteria: (1) Any respiratory muscle strength measurements; (2) Intervention consisting of respiratory muscle training in people with cervical SCI. Studies were excluded if the intervention included were associated with other techniques of functional respiratory muscle training, such as electrical stimulation and other complementary techniques.

Study selection

Firstly, the titles and abstracts were analyzed to verify whether the articles met the inclusion criteria. Two judges analyzed them blindly in the Rayyan platform (https://rayyan.ai), with three possible responses: “yes”, for articles that would be included in the study; “no”, for articles the judge considered outside the inclusion criteria; and “maybe”, when the judge was unsure about including the article in the study. When responses diverged, a third judge made the decision; in cases when there was no consensus between the three judges (considering that there is three possibilities of answers), a fourth judge was invited.

Data extraction

At the end of the studies selection process the full texts were analyzed by one researcher. The following data were extracted from the studies: study type, objective, subjects (number, sex and ages), materials and methods (treatment criteria), results, and conclusion. The selected articles were critically assessed regarding the respiratory exercise protocols for people with cervical SCI. To this end, researchers highlighted whenever possible number of time and number of sessions, number of training days and weeks.

Methodological quality assessment

Physiotherapy Evidence Database (PEDro) scale was used to assess methodological quality of included studies (8). The PEDro scale is valid measure of the methodological quality of experimental studies (9). Its 11-item questionnaire classifies intervention studies in terms of their internal validity (items 2–9) and methodological quality (items 10 and 11). Item 1 addresses the external validity, but it is not used to calculate the PEDro score (8). To each item, the judge answers either “yes” (when the research meets that criterion) or “no” (when it is not met). Articles that score from 6 to 10 points are considered high-quality; from 4 to 5, medium-quality; and from 0 to 3, low-quality (8). This instrument was chosen because the articles found in the search belonged to physical therapy, the field of knowledge for which the scale was developed, considering the said indication and the best application to the selected studies, as most of them were experimental ones.

A judge assessed the articles qualitatively, according to the criteria established by the PEDro scale (8) to classify them.

Data analysis

The data extracted from the studies were described and summarized in Table 2 and the protocols data were expressed in mean, maximum and minimum. The methodological assessment was also described in Table 3.

Table 2.

Description of the studies, highlighting their objectives and conclusions.

Title	Author/ Year	Study type	Objective	Sample number	Injury level and type	Method	Instruments	Results	Conclusion
Effects of Respiratory Muscle Training on Baroreflex Sensitivity, Respiratory Function, and Serum Oxidative Stress in Acute Cervical Spinal Cord Injury	Wang et al., 2021 (15)	Prospective trial	To assess the effects of respiratory muscle training (RMT) in pulmonary and cardiovascular autonomic function and in reactive oxidative species (ROS) production in patients with acute cervical SCI.	44 individuals with acute cervical SCI (20 EG and 24 CG) (men)	Acute cervical injury	EG received high-intensity inspiratory and expiratory training at home, with the individually adjusted load. 40 minutes, 7 days/week, for 10 weeks. CG received a simulated intervention for the same period.	Dofin breathing trainer	RMT significantly improve cardiovascular autonomic function; maximal inspiratory pressure; ROS. Significantly decrease physical and mental components of SF-36 in 6 months follow-up. Did not improve the maximal expiratory pressure and clinical scores.	High-intensity respiratory muscle training at home can improve pulmonary function and resistance and decrease respiratory difficulties in people with post-injury respiratory muscle weakness.
Using an evidence-based protocol to guide rehabilitation and weaning of ventilator-dependent cervical spinal cord injury patients	Gutierrez et al. 2003 (16)	Prospective, case series study.	To develop an evidence-based clinical protocol to improve ventilatory muscle strength and the resistance of people with cervical spinal cord injury who depend on a ventilator.	7 individuals with cervical SCI	Traumatic incomplete cervical injury: 2 individuals with high-level injuries (from the brain to C4) 5 individuals with low-level injuries (C4 to C7)	The protocol consisted of: pretraining optimization (position, suction, aerosolize and hyperinflate); Inspiratory/Expiratory Resistance Training (by inspiratory/expiratory trainer and cuff deflated and red cap on trach tube); On-Vent Endurance Training (by synchronized intermittent mandatory ventilation – SIMV – rate of 1–2; pressure support ventilation – PSV – to maintain ∼400 cc; and progress to off-vent training when patient maintains a tidal volume (Vt) = 400 ml on continuous positive airway pressure (CPAP) 5 PSV 5 for 2 continuous hours per day for 1 week); Off-Vent Endurance Training (by gas injection nebulizer – GIN – wye-piece as tolerated; tracheal gas insufflation (TGI) device as tolerated; red cap to trach tube as tolerated; tracheal decannulation unless contraindicated).The same strategies were used in exhalation training.	Resistex PEP therapy trainer was modified to enable inspiration through one of dour orifices.	Low tetraplegic patients improved by 75% the mean MIP; by 71% the mean MEP; by 59% the mean vital capacity; by 91.6% the mean on-vent endurance time; and by 76.7% the mean off-vent breathing time. Both high and low tetraplegic patients achieved gains in inspiratory and expiratory muscle strength, vital capacity, on-vent endurance, and off-vent breathing times.	The mechanical ventilation weaning rate went from 20% to 100% 24 months after implementing the protocol program, which indicates evident improvement. The findings suggest that low tetraplegic people had better results than high ones and that discontinued mechanical ventilation was the desired clinical outcome.
Inspiratory muscle training is feasible and safe for patients with acute spinal cord injury	McDonald and Stiller, 2019 (17)	Prospective, observational, case series, pilot study.	To investigate the feasibility, safety, and effectiveness of inspiratory muscle training for people with spinal cord injury.	7 individuals with cervical or thoracic SCI.	Acute cervical or thoracic injury	A high-resistance and low-repetition program was established. The training comprised 3–6 series of 6 respirations, beginning at 50% of the MIP, and progressively increasing the training load.	POWERbreathe KH1 respiratory muscle training device	There were 50 sessions in total, in which participants met the criteria to receive training. The training was feasible and safe in all sessions, with stable physiological parameters and no adverse symptoms or events registered before, during, or after the intervention. MIP increased in 4 participants, and the forced vital capacity increased in 3 participants during sessions.	The high-resistance and low-repetition inspiratory muscle training program was feasible and safe in adults with complete cervical or thoracic spinal cord injury whose breathing status was stable.
Impact of respiratory muscle training on blood gases and pulmonary function among patients with cervical spinal cord injury	El-Kader, 2018 (10)	Prospective, case series study.	To investigate the effect of resistance respiratory muscle training on the blood gas test and pulmonary function of people with cervical spinal cord injury.	36 individuals with complete cervical SCI (C5–C8)	Complete cervical injury (C5–C8)	EG received a training load at 20% of MIP and MEP. Six 5-min series, with 3-min rest. 45 minutes/day, 5 days/week, for 6 weeks.	Threshold Respironics Inc., (PEP device) – inspiratory muscle trainer	The mean heart rate, breathing rate, carbon dioxide partial pressure, and pH values decreased, while the forced vital capacity, first-second forced expiratory volume, and oxygen partial pressure significantly increased in EG. Changes in CG, though, were not significant. Significant differences between the groups were also found at the of the study.	Resistance respiratory muscle training improves blood gas and pulmonary function, suggesting this intervention is an effective therapy for people with cervical spinal cord injury.
A comparison of two breathing exercise programs for patients with quadriplegia	Derrickson et al., 1992 (13)	Prospective, case series study.	To assess the effects of two types of respiratory exercises: inspiratory resistive muscle training (IMT) vs abdominal weights (AbWts) training for quadriplegic people.	11 individuals with complete cervical SCI.	Complete cervical injury (C4-5 to C7)	Each patient received their respective training protocol for 7 weeks. The training had two 15-minute sessions per day, 5 days a week. IMT – the amount of resistance was increased when the subject could complete three successive 15-minute sessions. Subjects were instructed to breathe continuously through the equipment during sessions. AbWts – The maximum weight that did not change the inspiratory capacity was placed on the subjects’ abdomens while they performed 10 maximal inspirations, holding each respiration for various seconds. This sequence was repeated three times, totaling 40 respirations per session.	Diemolding Healthcare Division (DHD) breathing trainer – for inspiratory muscle trainer.	There was no significant difference between the treatments. There were significant differences within groups. The IMT group had a greater mean difference from week 1 to week 7 in all pulmonary function tests than the AbWts group – although there were no significant differences in pulmonary function values between groups at the beginning of the study.	There were significant differences within the groups in all 5 pulmonary function tests. Although the data did not support the effectiveness of a training method rather than the other, selected pulmonary function test measures improved. Further studies with larger samples are needed to determine conclusively the benefits of either training protocol.
Effect of 4 weeks of resistive inspiratory muscle training on respiratory functions in patients with tetraplegia during in-patient rehabilitation	Sikka et al., 2021 (11)	Prospective randomized case-control study.	To verify whether a 4-week respiratory muscle training program effectively improves the respiratory functions in tetraplegic people during hospital rehabilitation.	96 individuals with cervical injuries (46 EG and 46 CG)	Complete and incomplete cervical injuries (C4-C7)	Beginning in the first week after the injury. 40 sessions for 4 weeks. 30% of MIP and 30% of MEP; increasing by 10% on alternating days, if tolerated, and limited to 70% of the best weekly MIP or MEP. 2 times/day, 5 days/week. 3 series of 12 inhalations and then 3 series of 12 exhalations. CG: conventional intervention	Threshold Respironics Inc., inspiratory muscle trainer	The mean pre- and post-training values of all result measures revealed highly significant differences within both groups. EG had better results than CG – a highly significant positive effect in all pulmonary function and respiratory force measures.	There were beneficial effects of training on respiratory functions and strength in people with tetraplegia in the first month after the injury.
Respiratory muscle training in individuals with spinal cord injury: effect of training intensity and – volume on improvements in respiratory muscle strength	Raab et al., 2019 (12)	Retrospective cohort study	To investigate the effect of the training intensity and volume on the improvement of respiratory muscle strength in individuals with spinal cord injury.	67 individuals with SCI.	Traumatic or non-traumatic injury, level of motor injury from C4 to T12.	10 individual sessions. EG underwent training for about 6 consecutive weeks, 3–5 sessions/week, and up to 90 repetitions per session. The effort was self-assessed with the Borg Rating of Perceived Exertion (RPE), which ranges from 6 (no effort) to 20 (maximum effort), after each session. Once participants could perform the training session at a lower intensity than in the previous session, resistance was increased.	Threshold Respironics Inc., inspiratory muscle trainer	Altogether, 145 individuals met the inclusion criteria. The data of 78 individuals were excluded for various reasons; hence, 67 sets of data were analyzed. MIP variations were explained by basal MIP and training intensity. The complete model explained 57% (adjusted R2) of lnMIP variance. MIP association with training intensity was independent of the injury type and level. The injury type and level did not have a significant effect on MIP. Controlling baseline MEP variation (ln-transformed; P < 0.0001), there was statistical support that the effect of training intensity on MEP was conditioned to the injury type. While individuals with complete motor injuries increased MEP by 6.8% per 10 units (cmH2O) increased in the training intensity, the corresponding adjusted effect size in the group with incomplete motor injuries was 0.1%. The complete model explained 60% (adjusted R2) of lnMEP variance.	Training intensity was more relevant than volume to successful improvements in respiratory muscle strength muscular in people with spinal cord injury. Thus, the training intensity must be as high as possible. With every increase of 10 training intensity units, MIP improved by 7%. In older people with complete motor injuries, MEP also increased by almost 7% per increase by 10 respiratory muscle training units.
Resistive inspiratory muscle training: Its effectiveness in people with acute complete cervical cord injury	Liaw et al., 2000 (14)	Prospective study	To assess whether resistance inspiratory muscle training (RIMT) can improve pulmonary function in people with complete tetraplegia, 6 months after the trauma.	20 individuals with tetraplegia.	Complete traumatic tetraplegia, from C4 to C7.	Daily training sessions with a 6-resistance-levels incentive spirometer. About 12–16 respirations/minute. 15- to 20-minute sessions, twice/day, 7 days/week, for 6 weeks. When it was easily performed for 3 days, resistance was increased.	Diemolding Healthcare Division (DHD) resistance trainer.	Most pulmonary parameters had statistically significant improvements in both EG and CG, though EG had greater improvements.	Respiratory muscle training can improve ventilatory function, respiratory resistance, and perceived breathing difficulty in people with complete cervical spinal cord injury, within 6 months of the trauma.
Training of the respiratory muscles in individuals with tetraplegia	Uijl et al., 1999 (18)	Case series cohort study.	To assess whether respiratory muscle training in individuals with cervical spinal cord injury improves these muscles’ strength and resistance capacity and these people’ exercise performance.	9 individuals with cervical injuries.	Complete and incomplete cervical injuries (C3–C7)	Individuals underwent simulated training for 6 weeks with no appreciable resistance, followed by actual training for 6 weeks, at 70% of the maximum resistance of inspiratory muscles. Daily 15-minute training sessions, twice/day, for 6 weeks. Imposed 3-to-4-second breathing pattern. The device was adjusted to 2 mm resistance.	INSPIRx incentive spirometer	After the simulated training, the endurance pressure increased from 3.98–4.71 kPa, with a P value of 0.05. Sham training did not influence any other variable. Actual training had no effect on IVC, FIV1, FEV1, or MIP, but increased the respiratory muscle resistance capacity. Oxygen consumption in the maximal effort test improved from 0.87–0.98 l/ min (P = 0.05).	Respiratory muscle training results in a greater resistance capacity of these muscles and simultaneously increased aerobic exercise performance.

Caption: SCI = Spinal cord injury; EG = experimental group; CG = control group; MIP = maximal inspiratory pressure; MEP = maximal expiratory pressure; PEP = Positive expiratory pressure; IVC = Inspiratory vital capacity; FIV1 =  Forced inspiratory volume over 1 s; FEV1 =  Forced expiratory volume over 1 s.

Table 3.

Study scores on the PEDro scale.

	Wang HC; Lin YT; Huang CC; Lin MC; Liaw MY; Lu CH.	Gutierrez CJ; Harrow J; Haines F.	McDonald, T.; Stiller, K.	El-Kader, S.M.A.	Derrickson, J.; Ciesla, N.; Simpson, N.; Imle, P.C.	Sikka, G.; Yadav, J.; Singh, R.; Gupta, K.B.	Raab, A.M.; Krebs, J.; Pfister, M.; Perret, C.; Hopman, M.; Mueller, G.	Liaw, M.-Y.; Lin, M.-C.; Cheng, P.-T.; Wong, M.-K.A.; Tang, F.-T.	Uijl, S.G.; Houtman, S.; Folgering, H.Th.M.; Hopman, M.T.E.
1. Eligibility criteria were specified	yes	yes	yes	yes	yes	yes	yes	yes	yes
2. Subjects were randomly allocated to groups (in a crossover study, subjects were randomly allocated an order in which treatments were received)	no	no	no	no	yes	yes	no	yes	no
3. Allocation was concealed	no	no	no	no	no	no	no	no	no
4. The groups were similar at baseline regarding the most important prognostic indicators	no	no	no	no	sim	yes	no	yes	no
5. There was blinding of all subjects	no	no	no	no	no	no	no	no	no
6. There was blinding of all therapists who administered the therapy	no	no	no	no	no	no	no	no	no
7. There was blinding of all assessors who measured at least one key outcome	no	no	no	no	no	no	no	no	no
8. Measures of at least one key outcome were obtained from more than 85% of the subjects initially allocated to the groups	yes	yes	yes	yes	yes	yes	yes	yes	yes
9. All subjects for whom outcome measures were available received the treatment or control condition as allocated or, where this was not the case, data for at least one key outcome was analyzed by “intention to treat”	yes	yes	yes	yes	yes	yes	yes	yes	yes
10. The results of between-groups statistical comparisons are reported for at least one key outcome	no	no	no	no	yes	yes	no	yes	no
11. The study provides both point measures and measures of variability for at least one key outcome	yes	yes	yes	yes	yes	yes	yes	yes	yes
Total (2–9)	3	3	3	3	6	6	3	6	3

Results

Altogether, 49 scientific articles were identified in the initial search – 18 were excluded due to be duplicated, and 10, for being review studies or not meeting the inclusion criteria. Hence, 21 articles remained to have their titles and abstracts blindly analyzed by judges, from which 11 articles were selected for full-text reading and data analysis. From those 2 were excluded to not meet inclusion criteria, therefore, 9 studies were included in the review (Fig. 1). The studies included are described in Table 2. Overall, these studies demonstrate the effectiveness of respiratory muscle training (RMT) in enhancing respiratory function and quality of life for people with cervical SCI.

Figure 1.

PRISMA flowchart of studies selection.

Studies analysis results regarding internal and external validity and methodological quality, assessed with the PEDro scale questionnaire (8), are described in Table 3. Articles that had the corresponding item were checked with “yes”, whereas those that did not perform it or did not have it described in the text were checked with “no”. Three (33.3%) out of the nine assessed articles were classified as high-quality, and six (66.7%), as low-quality.

The protocols details (time and number of sessions per day, days per week, and the total number of weeks of training) are shown in Table 4. Session time ranged from 15 to 45 minutes, with a mean of 25 minutes; the number of sessions per day ranged from one to two; the number of training days per week ranged from 3 to 7, with a mean of 5.5 sessions per week; and the number of intervention weeks ranged from 4 to 10, with a mean of 6.1 training weeks. However, protocol data were missing in some studies and those were not added to the table.

Table 4.

Time and number of sessions, and number of training days and weeks.

	Minutes/session	Sessions/day	Days/week	No. of weeks
Wang HC; Lin YT; Huang CC; Lin MC; Liaw MY; Lu CH.	40	–	7	10
Gutierrez CJ; Harrow J; Haines F.
1	–	–
McDonald, T.; Stiller, K.	–	–	–	–
El-Kader, S.M.A.	45	–	5	6
Derrickson, J.; Ciesla, N.; Simpson, N.; Imle, P.C.	15	2	5	7
Sikka, G.; Yadav, J.; Singh, R.; Gupta, K.B.	–	2	5	4
Raab, A.M.; Krebs, J.; Pfister, M.; Perret, C.; Hopman, M.; Mueller, G.	–	–	3–5	6
Liaw, M.-Y.; Lin, M.-C.; Cheng, P.-T.; Wong, M.-K.A.; Tang, F.-T.	15–20	2	7	6
Uijl, S.G.; Houtman, S.; Folgering, H.Th.M.; Hopman, M.T.E.	15	2	7	6
Minimum	15	1	3	4
Maximum	45	2	7	10
Mean	25	1.8	5.5	6.1

Discussion

The current review aiming to identify in the literature studies carrying out respiratory muscle training protocols for individuals with cervical SCI and to assess their methodological quality, found a total of nine studies. From those studies, only three were classified as high methodological quality and six as low methodological quality in the PEDro scale classification.

The studies presented indicate that respiratory muscle training is an effective intervention to treat respiratory function in people with cervical SCI. However, the effectiveness appears to vary depending on whether the training focuses on inspiratory or expiratory muscles. While some studies demonstrate significant improvements in inspiratory muscle strength and overall respiratory function, others highlight limited benefits for expiratory muscle strength. This variability suggests that the effectiveness of respiratory muscle training may be influenced by the specific type of training, the protocols used, and the characteristics of the patients. Further research is needed to clarify these factors and determine the most effective training approaches for different aspects of respiratory function in cervical SCI.

Most studies used breathing trainers with various forms of resistance and the Philips Respironics Threshold IMT was the most used breathing trainer (10–12). However, the authors did not mention which criteria they used to define the initial training load, neither which criterion were used to increase the load, the number of repetitions, or the training frequency. Baseline assessment must be considered when selecting the device according to its capacity and defining criteria to begin training and increase loads.

Only one study (13) compared a group that used breathing trainers with another group that used deep breathing with a weight on the abdomen. The group that used breathing trainers had better results than the other one, which leads to the belief that breathing trainers may be effective in rehabilitating people with cervical SCI, but further research is needed to confirm their superiority over other rehabilitation methods to rehabilitate people with cervical SCI.

Thus, the lack of control groups in five studies, (19–23) prevents these studies from determining whether the observed improvements are attributable to the intervention or merely reflect natural variations in the condition. Likewise, the lack of blinding is a bias for data analysis – even though it is difficult to research this population for logistic issues. Therefore, specialized centers that treat this population must contribute to research, giving access to researchers, and helping increase scientific production.

There is a lot of variability in the intervention programs described in the studies included in this review, regarding the dose of exercises, frequency of sessions and duration of the programs. It is therefore not possible to suggest a more appropriate model of intervention as criteria related to the severity and intensity of the sequelae, as well as general condition and dependence on respiratory devices can interfere in the decision of when and how to intervene. In this way, the therapeutic models applied only allow replication in similar clinical conditions, not allowing generalization.

Also, it was demonstrated that the level of methodological quality of most studies was insufficient to produce a high level of scientific evidence, verified with the description of the study types (which were mostly non-controlled prospective case series) and the PEDro scale assessment (8). Therefore, research must have greater methodological rigor to continuously improve the scientific knowledge on respiratory rehabilitation of SCI people and standardize treatments.

The three articles (11, 13, 14) that met the criteria established for this study and scored six in the PEDro scale analysis (8), having met 63.6% of its criteria, seemingly point to a pattern in the number of training sessions per day (twice a day), days per week (5–7 days a week), and the number of weeks (6 weeks). Blinding and the randomized allocation of groups were the main items that compromised the score of the studies.

Considering the techniques proposed by studies of higher methodological quality (in terms of internal and external validity), it was identified that more advanced studies, with greater methodological rigor, are necessary, especially randomized clinical trials. Related areas, such as physiotherapy and speech therapy, have much to contribute to scientific production related to patients with spinal cord injuries, however, they need to advance the methodological rigor of their studies.

Conclusion

This review showed the most common practices and instruments used in respiratory muscle training in people with cervical SCI and verified that respiratory muscle training is an effective intervention to treat respiratory function in this population. However, due to the poor methodological quality of the studies its effectiveness should be further investigated. Studies with greater methodological rigor and evidence-based practices are needed to provide treatment with standardized procedures and specific protocols to this population.

Funding Statement

This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (n° 309108/2019-5) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Conflict of interest

The researchers declare that there is no conflict of interest in this work

Authors’ contributions

1. Study conceptualization and design: FOR and ACCG.

2. Data collection, analysis, and interpretation: FOR, BPL, MMPP, and ACCG.

3. Intellectually important article writing or review: FOR and ACCG.

4. Approval of the final version for publication: FOR and ACCG.

Data availability statement

Data of this research originate from scientific articles published in the databases described in the method and are available in them.