1Department of Rehabilitation, Vinmec Times City International Hospital, Hanoi, Vietnam
2Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand
3Department of Rehabilitation, Haiduong Medical Technical University, Haiduong, Vietnam
4Niigata University of Health and Welfare, Niigata, Japan
5Department of Rehabilitation Science, Graduate School of Health Sciences, Kobe University, Kobe, Japan
6Department of Ophthalmology, Pham Ngoc Thach University of Medicine, Ho Chi Minh City, Vietnam
Correspondence: Dat Huu Tran Department of Rehabilitation, Vinmec Times City International Hospital, 458 Minh Khai, Hai Ba Trung, Hanoi 100000, Vietnam. Tel: +84-333602710 Fax: +84-439743557 E-mail: tranhuudathmu@gmail.com
• Received: January 13, 2025 • Revised: April 8, 2025 • Accepted: May 12, 2025
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
The effect of inspiratory muscle training (IMT) on cervical spinal cord injury (SCI) remains controversial. This study aimed to assess the efficacy of IMT in enhancing breathing muscle strength, pulmonary function, and quality of life (QoL) among patients with cervical SCI. A search was performed using the PubMed, Cochrane Library, Scopus, Embase, and Web of Science databases through December 2023. This review was conducted according to PRISMA guidelines and the Cochrane Library Handbook. The meta-analysis used mean differences (MDs) or standardized mean differences to pool the results. The Risk of Bias 2 and the GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) were used to assess the methodological quality of the included studies. This systematic review included five randomized controlled trials (202 participants). The results of the meta-analysis showed that IMT significantly improved maximal inspiratory pressure (MIP) with MD 12.13 cmH2O (95% confidence interval [CI] 4.22 to 20.03), maximal expiratory pressure (MEP) with MD 8.98 cmH2O (95% CI 6.96 to 11.00), and vital capacity (VC) with MD 0.25 L (95% CI 0.21 to 0.28). There were no significant improvements in forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), and QoL. The quality of the evidence ranged from very low to moderate, owing to bias and heterogeneity. Our results showed that IMT may improve MIP, MEP, and VC, but not FEV1, FVC, or QoL, in patients with cervical SCI. Further large-scale studies are required to determine this effect’s optimal dosage and duration.
Spinal cord injury (SCI) is defined as an injury to the spinal cord leading to neurological impairment, whether or not there is an associated disruption of the spinal column [1]. According to a report by the World Health Organization, approximately 15 million individuals are living with SCI, accounting for over 4.5 million years of life lived with a disability by 2021 [2]. This condition affect physical and psychological health [3,4]. Cervical SCI is the highest level of SCI and often affects the muscles involved in respiration, such as the diaphragm, abdominal muscles, intercostal muscles, and accessory respiratory muscles. This leads to a significant decline in respiratory function [5-7]. Pulmonary issues continue to be the primary cause of mortality after SCI because pulmonary function and the ability to clear secretions are impaired, particularly in people with higher levels and more complete injuries [7,8], especially in cervical SCI. Individuals with cervical SCI often have more severe impairment of lung function than those with thoracic or lumbar SCI because of the impact on the phrenic nerve.
Respiratory muscle training (RMT) is a promising method to improve the respiratory muscles in patients with SCI. It can be divided into two sub-methods: inspiratory muscle training (IMT) and expiratory muscle training (EMT). IMT, the most common form of RMT, has been suggested to improve inspiratory muscle strength by introducing resistance during inhalation [9]. Numerous studies have demonstrated the advantages of this therapeutic training in patients with SCI, including enhancement of inspiratory muscle strength, alleviation of breathlessness, and improved quality of life (QoL) [10-13]. EMT is another aspect of RMT that involves other muscles, such as the abdominal and internal intercostal muscles, which play vital roles in pulmonary clearance. The EMT significantly prevents phlegm stagnation and pneumonia, particularly in individuals with cervical SCI [14].
Several systematic reviews have assessed the overall impact of IMT on respiratory muscle power, endurance, and QoL in patients with SCI [15,16]. Although IMT has shown benefits in individuals with SCI, the extent to which these benefits apply specifically to patients with cervical SCI remains unclear. Unlike thoracic or lumbar SCI, where diaphragmatic function is largely preserved, cervical SCI often results in diaphragmatic weakness or paralysis, fundamentally altering the respiratory mechanics. This distinction raises critical clinical questions regarding the effectiveness of IMT in this subgroup and whether treatment strategies should be tailored accordingly. By isolating cervical SCI in our study, we aimed to determine whether IMT yields comparable outcomes and whether specific considerations are necessary to optimize respiratory rehabilitation in this population.
METHODS
This systematic review and meta-analysis was conducted in accordance with PRISMA guidelines [17,18] and the Cochrane Handbook for Systematic Review of Intervention [19].
Eligible criteria
Following the PICO framework [19], the review’s inclusion criteria were as follows: (a) Study design: randomized controlled trials (RCTs). (b) Participants: individuals with SCI in the cervical region (from C1 to C8), encompassing both acute and chronic cases. (c) Intervention: the intervention protocol involved IMT and was compared with a control group (placebo, usual care, or other interventions). (d) Studies in English.
We excluded studies that explored the effect of IMT on respiratory conditions unrelated to SCI, such as chronic obstructive pulmonary disease, asthma, and other pulmonary diseases.
Outcome measures
Primary outcome
Maximal inspiratory pressure (MIP) measures inspiratory muscle power. It is a respiratory measurement that assesses the maximum force produced during the vigorous inhalation of a closed airway [20].
Secondary outcomes
(a) Maximal expiratory pressure (MEP) is another way to measure respiratory muscle strength but specifically focuses on the muscles involved in expiration activity. (b) Lung function: vital capacity (VC), forced vital capacity (FVC), and forced expiratory volume in 1 second (FEV1). (c) QoL: the studies utilized the Medical Outcomes Survey (Short Form 12 Health Survey or 36-Item Short Form Survey) [21,22].
Search strategy and study selection
We searched the PubMed, Cochrane Library, Scopus, Embase, and Web of Science databases. The final search was performed on December 20th, 2023. See the Supplementary Material S1 for the entire search strategy. We also reviewed the references of appropriate articles and literature reviews to identify further relevant published and unpublished studies.
Study selection: Two authors (DHT and HTL) reviewed each trial’s titles, abstracts, and full texts using predefined criteria. A third author (HTCP) resolved disagreements between the two reviewers, and the reasons for excluding certain studies were documented. We used the Covidence website (covidence.org) to conduct the screening.
Data collection and quality evaluation
Two authors separately retrieved the data using a standardized Excel form (Microsoft Corp.). Information was collected, including the principal author, year of publication, sample size, training protocol, and outcomes. Two authors independently employed The Cochrane Collaboration’s tool (Risk of Bias 2: RoB 2) [23] to evaluate the methodological quality of the included studies and the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) approach [24] for the quality of evidence [25,26]. We the created a table of characteristics and summary findings based on the GRADE and analysis. Any disagreements were resolved by a third author (ANVL for RoB2 and TTQC for GRADE).
Statistical analysis
Two authors conducted analysis (DHT, HTL), and R programming language with the “meta” package was utilized to perform meta-analysis [27]. The results are presented as mean difference (MD) or standardized mean difference (SMD) with 95% confidence interval (CI). We used the chi-squared test and I2 test to evaluate heterogeneity. A fixed-effects model was used when I2 was <50%. Alternatively, a random effects model was used [28]. Subgroup analyses were conducted to confirm these results potentially. Statistical significance was set at p<0.05.
RESULTS
Study selection
From the electronic database search, 3,471 studies were initially identified. After screening and eliminating duplicates and irrelevant studies, 14 relevant studies were retrieved. After full-text reading, two studies were excluded because they did not utilize IMT [29,30], five studies were not RCTs [31-35], and two studies did not focus on cervical SCI [12,36]. This systematic review included five remaining studies (Fig. 1).
Study features
Table 1 lists the details of the included studies. The five included studies had 202 participants (100 in the treatment group and 102 in the control group), and the level of injury ranged from C4 to C8.
In five studies, the intervention was performed at any stage, including the acute, subacute, and chronic phases. Regarding the severity of the injury, almost all studies included patients with A or B according to the American Spinal Injury Association impairment scale, except for one trial [10], which included a C-type impairment.
Almost all studies mentioned medical instability or cardiopulmonary diseases, chest injuries, or mental illnesses as in their exclusion criteria.
Three trials utilized IMT devices equipped with resistance and spring-loaded mechanisms (POWERbreathe® Plus, Philips Threshold® IMT) [10,13,37], while the other two trials employed instrumentation that lacked spring-loaded features (Respifit S® electronic and DHD inspiratory muscle trainer) [38,39]. Participants were trained with resistance ranging from 60% to 80% of their MIP, except for the study using a DHD inspiratory muscle trainer, in which they started training at the slightest resistance and gradually increased it to reach the target resistance. Four studies involved training twice daily, and one involved training once daily. The follow-up duration ranged from 4 to 8 weeks.
Risk of bias
Evaluating potential bias was challenging because the trials’ detailed reports on their methods were insufficient. Figs. 2 and 3 show an overview of the evaluations of the risk of bias across all the trials incorporated.
Randomization process
All studies considered in the analysis were documented as randomized trials. Adequate allocation concealment was described in only two studies [10,39], whereas the randomization method in the remaining studies was unclear.
Blinding
Only one study demonstrated a clear strategy for blinding participants and assessors [10], while the remaining four studies only focused on blinding participants.
Incomplete outcome data
We identified two studies as having a high risk of incomplete outcome data [37,39] because they failed to provide information regarding the enrolled participants and did not complete the study. The dropout rates in those trials varied from 33% to 36%. Two studies were categorized as low risk due to their low dropout rates (<5%) [10,38].
Selective reporting and measurement methods
All included studies reported predefined outcomes and used appropriate and consistent measurement methods. Thus, we classified all five studies as having a low risk in these domains.
Overall
Only one study [10] showed a low potential for bias; the remaining studies had potential issues or a heightened bias risk.
Impact of IMT
Respiratory muscle strength
In all five studies [10,13,37-39], the impact of IMT on MIP and MEP after the intervention was documented. Respiratory pressures are presented as mean and standard deviation. Compared to control, meta-analysis indicated that IMT improved MIP meaningfully by an average of 12.13 cmH2O (95% CI 4.22 to 20.03, p<0.01; Fig. 4) and enhanced MEP notably by a MD of 8.98 cmH2O (95% CI 6.96 to 11.00, p<0.01; Fig. 5).
Lung function
Three parameters are often presented as lung function, including VC, FVC, and FEV1.
Four studies documented the effects of IMT on VC [10,37-39]. When compared to the control group, IMT showed a meaningful improvement on VC, MD 0.25 L (95% CI 0.21 to 0.28, p<0.01; Fig. 6).
Four studies assessed the effect of IMT on FVC [10,13,37,39]. Meta-analysis revealed that IMT enhanced FVC compared to the control, MD 0.43 L (95% CI -0.32 to 1.18), even though the disparity did not achieve statistical significance (p=0.26; Fig. 7).
Five studies examined the influence of IMT on FEV1 [10,13,37-39]. Meta-analysis indicated an improvement in FEV1, MD 0.27 L (95% CI -0.06 to 0.61) due to IMT, but the disparity did not reach statistical significance compared to the control groups (p=0.11; Fig. 8).
QoL
Two studies [10,38] involving 26 participants presented data on the effect of IMT on QoL. In comparison to control groups (Fig. 9), IMT did not improve QoL, showing a SMD of 0.00 (95% CI -0.14 to 0.15, p=0.90).
Subgroup analysis for the type of threshold devices
Three studies [10,13,37] with 166 participants that utilized a spring-loaded device showed a clinically relevant effect on MIP, with an MD of 8.44 cmH2O (95% CI 1.36 to 15.52; Fig. 10). In contrast, the change of MIP observed in the two remaining studies [38,39] using devices without a spring-loaded feature was insignificant, with an MD of 1.35 cmH2O (95% CI -8.05 to 10.75; Fig. 11).
Quality of evidence
Table 2 shows the evidence quality using the GRADE systems; the outcomes with MIP, MEP, and VC showed effectiveness with moderate-quality evidence. The outcomes of FVC and FEV1 showed that IMT might have an effect with low-quality evidence and no effect with very low evidence in the QoL outcome. Almost all outcomes had problems with the risk of bias. Other reasons that reduced the quality of evidence were inconsistency in FVC, FEV1, and QoL outcomes and imprecision with QoL outcomes because of the small sample size.
DISCUSSION
This meta-analysis focused on the efficacy of IMT for improving inspiratory muscle power, pulmonary function, and QoL in patients with cervical SCI. Our findings showed that IMT may improve respiratory muscle strength (including MIP and MEP) and VC. However, FVC, FEV1, and QoL were unaffected, with little evidence supporting these outcomes.
The meta-analysis with the largest sample size (n=202) focused on respiratory muscle strength. It showed a favorable effect in the intervention group compared to the control group, with MDs of 12.13 cmH2O for MIP and 8.98 cmH2O for MEP. This aligns with findings from numerous other systematic reviews investigating IMT in SCI [14,15,16,40], except for two that showed no effectiveness on MEP [15,16]. This could be because these studies investigated IMT in patients with SCI and those with cervical SCI. IMT directly strengthens inspiratory muscles, especially the diaphragm. As the inspiratory muscles become stronger, they generate higher inspiratory pressure, leading to better lung inflation. This can increase the lung volume (e.g., VC and inspiratory reserve volume), which might result in more effective and forceful expiratory efforts. When the lungs are fully inflated, elastic recoil is more significant during exhalation, contributing to a higher MEP [41]. Although the included trials did not show clinical outcomes such as pneumonia, sense of breathlessness, and peak cough flow, much evidence has demonstrated a correlation between respiratory muscle strength, especially inspiratory muscle power, and these outcomes. Indeed, improving MIP can help reduce the risk of pneumonia [42,43] and improve peak cough flow and secretion clearance in patients with spinal cord injuries [44]. IMT can improve the prognosis of patients with cervical SCI by enhancing respiratory muscle strength.
Regarding the intervention protocol, four studies reported an intensity ranging from 60% to 80% of MIP, categorized as moderate to high intensity, with a frequency of 4 to 5 days per week [10,13,37,38]. Only one study did not specify the exact intensity but reported a frequency of seven days per week [39], and that study showed that IMT did not significantly improve muscle strength. Previous studies have suggested moderate to high-intensity exercise is necessary to achieve the best outcomes [45]. Our meta-analysis showed that a protocol involving 70%–80% MIP with a frequency of 4–5 days per week provides the best results.
VC in individuals with tetraplegia declines notably over time, with a more pronounced decline occurring after 20 years post-injury [46]. In our systematic review, IMT effectively increased VC by a MD of 0.25 L. A systematic review by Berlowitz and Tamplin [40] conducted on 4 of 11 trials, also revealed evidence of a positive effect of RMT on VC in cervical SCI.
This meta-analysis did not show a significant effect of IMT on FEV1 or FVC. Similarly, four previous reviews showed that RMT did not affect FEV1 in SCI patients [14-16,40]. Regarding FVC, to our knowledge, little evidence mentions the effect of IMT on FVC; only one systematic review showed that RMT might affect FVC in SCI [14]. In our review, IMT was performed for a short duration, and the time from the onset of cervical SCI was heterogeneous. Hence, the changes in FVC and FEV1 were unclear, and the recovery phase was much longer than the intervention period [47,48]. To survey the actual effect, future research should be conducted with a longer intervention duration and more specific patients with a similar onset and level of injury. In summary, the results are unclear, and more evidence is needed to clarify the effectiveness of IMT on FEV1 and FVC.
In this meta-analysis, the change in QoL after the intervention was insignificant and remained nearly unchanged. This result is consistent with previous findings [14-16]. To explain this result, it is important to consider that individuals with cervical SCI experience numerous deficits, and their QoL is influenced by various factors, including the time since injury, adaptation to disability, general and mental health, social functioning, emotional issues, and pain [49]. Consequently, individuals with SCI may require multiple interventions to improve their QoL. Furthermore, an intervention duration of four to eight weeks of intervention may not be long enough to induce significant changes in QoL.
By comparing the effectiveness of different IMT devices on MIP and MEP, our results show that spring-loaded devices have clinically relevant effectiveness, whereas devices without a spring-loaded feature do not. This finding aligns with previous studies in patients with SCI [16]. This could be because spring-loaded devices provide consistent resistance and are ideal for inspiratory muscle strength training using structured programs [50]. These results suggest that interventions should involve a spring-loaded device to achieve better outcomes.
Based on these results, IMT effectively improved MIP, MEP, and VC in patients with cervical SCI. This suggests that IMT improves the clinical prognosis of pneumonia and coughing ability [42-44]. While this training method may not improve QoL, it is currently worth performing. Future research should focus on the effectiveness of IMT in other aspects of patients with SCI, such as endurance or daily life activities, or on combining IMT with EMT to evaluate their combined effects in clinical practice. The recommended device is a spring-loaded threshold device.
Limitations and suggestions
Limitations
Existing studies on IMT have limited sample sizes, and the training methods lack specificity, especially intensity and duration. In addition, this study has the following limitations: (i) This review incorporated only five studies, each with a relatively small sample size. Furthermore, the results exhibited considerable heterogeneity owing to significant differences in training duration and devices used. (ii) The study was conducted by searching only five databases, with a limitation in including English trials. (iii) Some studies experienced participant attrition or incomplete data, potentially leading to an elevated risk of bias. (iv) The review was not registered in PROSPERO.
Suggestions
IMT with spring-loaded devices should be used in clinical practice to provide more effective exercise. Second, for research, (i) additional large-scale and rigorous studies are necessary to strengthen this conclusion, define the optimal dosages and duration of the effect, and investigate whether IMT provides any sustained long-term functional benefits in patients with SCI; and (ii) other outcome measures related to ADL, activities, and cardiopulmonary endurance could be investigated.
CONCLUSION
In conclusion, IMT is worth performing in clinical practice based on evidence showing that it improves the strength of respiratory muscles and pulmonary function in patients with SCI by increasing MIP, MEP, and VC. However, there is insufficient evidence regarding the effectiveness of IMT in improving FEV1, FVC, and QoL. The spring-loaded devices substantially impacted the MIP and MEP, whereas the non-spring-loaded devices did not. Additional large-scale studies are necessary to define the optimal dosage and impact duration to investigate whether IMT provides sustained long-term functional benefits in patients with SCI.
CONFLICTS OF INTEREST
No potential conflict of interest relevant to this article was reported.
FUNDING INFORMATION
None.
AUTHOR CONTRIBUTION
Conceptualization: Tran DH. Data curation: Tran DH, Le HT, Chu TTQ, Pham HTC, Le ANV. Formal analysis: Tran DH, Le HT, Le ANV. Methodology: Tran DH, Le HT, Chu TTQ, Pham HTC, Le ANV. Software: Le HT. Project administration: Tran DH. Validation: Chu TTQ. Writing – original draft: Tran DH. Writing – review & editing: Tran DH, Le HT, Chu TTQ, Pham HTC, Le ANV. Approval of final manuscript: all authors.
PRISMA diagram. This figure shows the process of study searching and selection. IMT, inspiratory muscle training; RCT, randomized controlled trial.
Fig. 2.
Bias risk evaluated using the Risk of Bias 2 (RoB 2) tool. This figure displays the results of the summary of the bias risk assessments for each included study, organized into different domains of bias using the RoB 2 tool. Each domain is categorized by colors and patterns representing different levels of bias risk (low risk, some concerns, high risk).
Fig. 3.
Risk of bias overview graph based on each domain. The graph represents each domain as a separate bar, with the length and color indicating the level of bias risk in that domain.
Fig. 4.
Meta-analysis results for maximal inspiratory pressure (MIP). This figure provides a visual summary of the meta-analysis results specifically related to MIP. SD, standard deviation; CI, confidence interval.
Fig. 5.
Meta-analysis results for maximal expiratory pressure (MEP). This figure shows the forest plot of meta-analysis results related to MEP. SD, standard deviation; CI, confidence interval.
Fig. 6.
Meta-analysis results for vital capacity (VC). The forest plot shows the results of the meta-analysis for VC. SD, standard deviation; CI, confidence interval.
Fig. 7.
Meta-analysis results for forced vital capacity (FVC). The forest plot shows the results of the meta-analysis for FVC. SD, standard deviation; CI, confidence interval.
Fig. 8.
Meta-analysis results for forced expiratory volume in one second (FEV1). This figure shows the forest plot of meta-analysis results related to FEV1. SD, standard deviation; CI, confidence interval.
Fig. 9.
Meta-analysis results for quality of life (QoL). This figure represents the forest plot of meta-analysis results for QoL. SMD, standardized mean difference; SE, standard error; CI, confidence interval.
Fig. 10.
Subgroup analysis results of spring-loaded devices on maximal inspiratory pressure (MIP). This figure displays the forest plot of the subgroup meta-analysis results for the impact of training with spring-loaded devices on MIP. SD, standard deviation; CI, confidence interval.
Fig. 11.
Subgroup analysis results of non-spring-loaded devices on maximal inspiratory pressure (MIP). This figure shows the forest plot of the subgroup meta-analysis results for the impact of training with spring-loaded devices on MIP. SD, standard deviation; CI, confidence interval.
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The Effects of Inspiratory Muscle Training in Individuals With Cervical Spinal Cord Injuries: A Systematic Review and Meta-Analysis
Fig. 1. PRISMA diagram. This figure shows the process of study searching and selection. IMT, inspiratory muscle training; RCT, randomized controlled trial.
Fig. 2. Bias risk evaluated using the Risk of Bias 2 (RoB 2) tool. This figure displays the results of the summary of the bias risk assessments for each included study, organized into different domains of bias using the RoB 2 tool. Each domain is categorized by colors and patterns representing different levels of bias risk (low risk, some concerns, high risk).
Fig. 3. Risk of bias overview graph based on each domain. The graph represents each domain as a separate bar, with the length and color indicating the level of bias risk in that domain.
Fig. 4. Meta-analysis results for maximal inspiratory pressure (MIP). This figure provides a visual summary of the meta-analysis results specifically related to MIP. SD, standard deviation; CI, confidence interval.
Fig. 5. Meta-analysis results for maximal expiratory pressure (MEP). This figure shows the forest plot of meta-analysis results related to MEP. SD, standard deviation; CI, confidence interval.
Fig. 6. Meta-analysis results for vital capacity (VC). The forest plot shows the results of the meta-analysis for VC. SD, standard deviation; CI, confidence interval.
Fig. 7. Meta-analysis results for forced vital capacity (FVC). The forest plot shows the results of the meta-analysis for FVC. SD, standard deviation; CI, confidence interval.
Fig. 8. Meta-analysis results for forced expiratory volume in one second (FEV1). This figure shows the forest plot of meta-analysis results related to FEV1. SD, standard deviation; CI, confidence interval.
Fig. 9. Meta-analysis results for quality of life (QoL). This figure represents the forest plot of meta-analysis results for QoL. SMD, standardized mean difference; SE, standard error; CI, confidence interval.
Fig. 10. Subgroup analysis results of spring-loaded devices on maximal inspiratory pressure (MIP). This figure displays the forest plot of the subgroup meta-analysis results for the impact of training with spring-loaded devices on MIP. SD, standard deviation; CI, confidence interval.
Fig. 11. Subgroup analysis results of non-spring-loaded devices on maximal inspiratory pressure (MIP). This figure shows the forest plot of the subgroup meta-analysis results for the impact of training with spring-loaded devices on MIP. SD, standard deviation; CI, confidence interval.
Graphical abstract
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Graphical abstract
The Effects of Inspiratory Muscle Training in Individuals With Cervical Spinal Cord Injuries: A Systematic Review and Meta-Analysis
, Ha Thi Le, MD, MSc3,4
