Effects of inspiratory muscle training on pulmonary function, trunk stability, and balance in stroke patients: a stratified randomized controlled trial

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J Exerc Rehabil Vol. 21, No. 3, 131-139, June, 2025

Publication date (electronic) : 2025 June 25

doi : https://doi.org/10.12965/jer.2550256.128

Dae-Hee Lee, Hye-Joo Jeon

Department of Physical Therapy, U1 University, Yeongdong, Korea

^*Corresponding author: Hye-Joo Jeon, Department of Physical Therapy, U1 University, 310 Daehak-ro, Yeongdong-eup, Yeongdong 29131, Korea, Email: bluestar@u1.ac.kr

Received 2025 May 5; Revised 2025 May 17; Accepted 2025 May 26.

Abstract

INTRODUCTION

Stroke is one of the primary causes of illness and long-term disability globally (Markus, 2023). Stroke is one of the leading causes of death and disability worldwide, with approximately 12 million new cases each year and over 100 million stroke survivors globally (Johnson et al., 2019). It affects not only sensory, motor, cognitive, and language functions but also significantly impairs respiratory function (Ghoreyshi et al., 2022). Respiratory dysfunction in stroke patients can result in reduced oxygen supply, weakened cough, difficulty clearing airway secretions, and decreased cardiopulmonary endurance, all of which can adversely affect physical health and quality of life (de Almeida et al., 2011).

Individuals who are bedridden or rely on a wheelchair often experience trunk muscle weakness and imbalance (Verheyden et al., 2014). Hemiplegia commonly leads to asymmetrical posture and increased trunk flexion, both of which can limit inspiratory capacity (Lima et al., 2014). Impaired trunk extension and postural asymmetry reduce the efficiency of the respiratory muscles and restrict chest wall movement, leading to diminished inspiratory and expiratory function (Britto et al., 2011). Studies have shown that stroke patients have significantly lower maximal inspiratory pressure (MIP, 17–57 cmH₂O) and maximal expiratory pressure (MEP, 25–68 cmH₂O) compared to healthy individuals (Menezes et al., 2016).

Respiratory muscle training is a structured approach designed to strengthen the inspiratory or expiratory muscles by increasing MIP and MEP (Illi et al., 2012). Among these methods, inspiratory muscle training (IMT) has been shown to improve respiratory function, reduce dyspnea, enhance postural stability and balance, and increase quality of life in individuals with stroke (Xiao et al., 2012). IMT is grounded in key principles of skeletal muscle conditioning—overload, specificity, and reversibility—and promotes increased diaphragmatic strength and thickness through progressive resistance (Souza et al., 2014; Zhang et al., 2020). The diaphragm plays a dual role in respiration and trunk stability through its co-activation with abdominal and spinal muscles, particularly during postural adjustments (Develi et al., 2021; Nason et al., 2012). This functional linkage supports the use of IMT to influence trunk control and balance in individuals recovering from stroke.

Stroke often results in respiratory dysfunction due to weakened inspiratory muscles, reduced lung volumes, and impaired neuromuscular control, increasing the risk of complications such as pneumonia (Sutbeyaz et al., 2010). IMT plays a vital role in rehabilitation by strengthening respiratory muscles, thereby improving pulmonary function, enhancing cough efficiency, and promoting oxygenation (Britto et al., 2011). IMT also improves exercise tolerance and functional recovery, making it a key component of poststroke rehabilitation.

Recent evidence from systematic reviews and meta-analyses has further substantiated the role of IMT in stroke rehabilitation. Zhang et al. (2024) conducted a meta-analysis demonstrating that respiratory muscle training significantly enhances MIP, MEP, forced vital capacity (FVC), and overall functional capacity in patients with early-stage stroke. In addition, Fabero-Garrido et al. (2022) reported that IMT contributes to measurable improvements in diaphragmatic thickness, inspiratory muscle strength, and exercise tolerance, highlighting its efficacy in promoting structural and functional recovery of the respiratory system in stroke survivors. Likewise, Li et al. (2024) found that threshold respiratory muscle training led to significant gains in both inspiratory and expiratory muscle strength, as well as improvements in pulmonary function parameters (e.g., FVC etc.) and exercise endurance, further supporting its integration into poststroke rehabilitation protocols.

Although a number of studies have investigated the effects of IMT on respiratory muscle strength and exercise capacity in individuals with stroke (Tovar-Alcaraz et al., 2021; Vaz et al., 2021), relatively few have explored its impact on trunk control and balance—both of which are critical for functional independence and fall prevention. Moreover, many existing studies are limited by small sample sizes or lack of methodological rigor. For instance, Aydoğan Arslan et al. (2022) reported notable improvements in respiratory function, trunk control, and balance following IMT; however, comprehensive, well-designed trials remain scarce.

This study aims to address these gaps by investigating the effects of IMT on respiratory function, trunk control, and balance in patients with subacute stroke using a stratified randomized controlled design. In particular, the study focuses on how targeted respiratory intervention may influence neuromuscular coordination essential for postural control. We hypothesized that a structured IMT protocol would elicit significant enhancements in respiratory performance, trunk stability, and balance, thereby contributing to a more comprehensive functional recovery in individual’s poststroke.

METERIALS AND METHODS

Study design

This prospective, single-blind, stratified randomized controlled trial was conducted at Suwon Centum Hospital in South Korea between December 2024 and March 2025, with the study design illustrated in Fig. 1. The study protocol was approved by the Institutional Review Board of U1 University (IRB No. U1IRB2024-12).

Fig. 1

Flow chart of study participants.

Participants

Thirty stroke patients who were hospitalized during the study period and voluntarily provided informed consent after receiving a full explanation of the study objectives were enrolled. The required sample size was determined using the G*Power software (ver. 3.1, Franz Faul, University of Kiel), based on the effect sizes reported in prior research (Faul et al., 2007). A paired sample t-test was used, with an alpha level of 0.05, a power of 0.95, and an effect size of 1.61, indicating a minimum of 10 participants per group (20 in total). To accommodate possible dropouts, 30 participants were recruited. The inclusion criteria were (a) a clinical diagnosis of stroke, (b) age between 30 and 70 years with either patient or legal guardian consent, (c) the ability to walk independently for at least 10 m (use of assistive devices allowed), (d) no history of cardiovascular or respiratory disease, and (e) a score of 24 or higher on the Korean version of the Mini-Mental State Examination. Exclusion criteria were (a) not meeting the inclusion criteria and (b) voluntary withdrawal from the study for personal reasons. Participant characteristics are shown in Table 1.

Table 1

General characteristics of both groups

RESULTS

Inspiratory function

In the study group, MIP significantly increased from 36.10± 4.29 cmH₂O at baseline to 50.77±4.81 cmH₂O after the intervention (P<0.001). In contrast, the control group showed no significant change, with MIP increasing slightly from 36.22±3.16 cmH₂O to 36.60±2.98 cmH₂O (P>0.05). The maximal inspiratory flow rate (MIFR) in the study group also increased significantly, from 1.54±0.50 L/sec to 3.22±0.65 L/sec (P<0.001). The control group showed no significant change, with MIFR increasing slightly from 1.69±0.47 L/sec to 1.72±0.49 L/sec (P>0.05). Maximal inspiratory capacity (MIC) in the study group significantly increased from 1.28±0.39 L to 1.71±0.33 L (P<0.001), whereas the control group showed no significant change, with MIC increasing slightly from 1.25±0.35 L to 1.30±0.35 L (P>0.05). Between-group comparisons revealed significant differences in MIP (P<0.001) and MIFR (P<0.05), while the difference in MIC was not statistically significant (P>0.05), as shown in Table 2.

Table 2

Comparison of inspiratory function within and between groups

Expiratory function

In the study group, PEF significantly increased from 277.40± 51.75 L/min to 383.30±59.94 L/min (P<0.001). The control group showed no significant change, with PEF rising from 266.80± 43.69 L/min to 289.40±58.90 L/min (P>0.05). FEV₁ in the study group significantly increased from 1.60±0.35 L to 2.27± 0.48 L (P<0.001). In contrast, the control group showed no significant change, with FEV₁ changing slightly from 1.67±0.31 L to 1.68±0.32 L (P>0.05). Between-group comparisons showed significant differences in PEF (P<0.001) and FEV₁ (P<0.05), as presented in Table 3.

Table 3

Comparison of expiratory function within and between groups

Trunk impairment scale

TIS scores significantly improved in both groups. In the study group, scores increased from 11.40±3.59 to 14.73±2.98 (P<0.001), and in the control group, from 11.53±2.45 to 12.80±2.23 (P< 0.001). However, the between-group difference was not statistically significant (P>0.05), as shown in Table 4.

Table 4

Comparison of balance ability within and between groups

Group, time, and interaction effects on functional outcomes following IMT

A mixed-design analysis of variance revealed statistically significant main effects of time for all outcome variables, indicating that both groups experienced overall improvements following the intervention. Specifically, significant time effects were observed for MIP, MIFR, MIC, PEF, FEV₁, TIS, TUG, and BBS, with all P-values less than 0.001 and large effect sizes (partial η² ranging from 0.469 to 0.822).

Significant main effects of group were also found for MIP (P< 0.001, η²=0.575), MIFR (P<0.001, η²=0.384), and PEF (P= 0.006, η²=0.238), indicating that the experimental group performed better than the control group regardless of time. The group effect for FEV₁ approached significance (P=0.057), while no significant group effects were found for MIC, TIS, TUG, or BBS.

Importantly, significant interaction effects between group and time were observed for all variables. This indicates that the magnitude of improvement over time differed significantly between the experimental and control groups. Notably, the experimental group demonstrated greater gains in respiratory function (MIP, MIFR, MIC, PEF, FEV₁), trunk control (TIS), mobility (TUG), and balance (BBS) compared to the control group, with all interaction effects reaching statistical significance (P<0.001) and effect sizes ranging from moderate to large (partial η²=0.359 to 0.680), as shown in Table 5.

Table 5

Results of mixed-design analysis of variance for within- and between-group effects

DISCUSSION

The aim of this study was to examine the effects of IMT on respiratory function, trunk control, and balance in stroke patients. IMT plays a vital role in stroke rehabilitation, focusing primarily on maintaining and restoring pulmonary function (Chen et al., 2016). In this study, the study group showed significant improvements in MIP, MIFR, MIC, PEF, and FEV₁ after the intervention (P<0.001). In contrast, the control group did not demonstrate statistically significant changes (P>0.05). Between-group comparisons also revealed significant differences in MIP, MIFR, MIC, PEF, and FEV₁ (P<0.05), indicating that the intervention was effective in enhancing respiratory function.

The improvement in respiratory function was linked to increased respiratory muscle strength, especially in the study group, where MIP, MIC, and MIFR showed significant increases (P<0.001). This suggests that the intervention effectively strengthened the inspiratory muscles. Similar results have been observed in previous studies (Sutbeyaz et al., 2010), reinforcing the notion that IMT positively affects lung capacity and respiratory muscle strength. Additionally, the significant rise in MIFR indicates improved airway resistance and airflow dynamics, suggesting that the intervention may have enhanced respiratory efficiency. In a 9-week study on stroke patients, Britto et al. (2011) found that participants who underwent IMT showed significant improvements in MIP and endurance (Kim et al., 2014). Likewise, Chen et al. (2016) conducted a study on 21 stroke patients and reported a substantial increase in MIP in the group that received IMT compared to the control group.

The significant improvements in PEF and FEV₁ in the study group (P<0.001, P<0.05) indicate enhanced expiratory effort and lung function. These results suggest that IMT effectively boosts lung capacity and airflow velocity. The increase in FEV₁ also points to improved lung elasticity and airway patency. Since the control group did not show similar changes, it is likely that the intervention directly contributed to the improvement in pulmonary function. A study by Sutbeyaz et al. (2010) found that an IMT program led to improvements in both FEV₁ and PEF. Previous research has shown that IMT enhances FEV₁, suggesting that IMT may reduce the risk of stroke recurrence (Hodges and Gandevia, 2000). An increase in PEF reflects stronger respiratory muscles, a lower risk of respiratory infections, and improved expiratory function (Myint et al., 2005). In stroke patients, PEF is approximately one-third lower than in healthy elderly individuals (Hodges et al., 2001a), and a reduced FEV₁ has been linked to a higher risk of stroke and its recurrence (Hodges et al., 2001b). Both groups demonstrated significant improvements in TIS, TUG, and BBS, reflecting gains in balance and functional mobility. However, no significant differences were found between the groups (P>0.05). This suggests that both groups likely participated in a comparable level of physical activity or that the balance and mobility assessment tools used in this study might not have been sensitive enough to detect subtle effects of the intervention. Nevertheless, the study group showed a stronger trend of improvement, indicating that a longer intervention period might produce more noticeable differences between the groups.

IMT may also aid in trunk control and balance by strengthening the diaphragm, which is essential for core stability. The diaphragm’s contraction increases intra-abdominal pressure, working in conjunction with the pelvic floor and abdominal muscles to improve postural control (Ferraro et al., 2019). These findings are consistent with research by Aydoğan Arslan et al. (2022), which emphasized IMT’s positive effects on breathing, trunk control, and balance in stroke patients. Moreover, growing scientific evidence supports the role of IMT in enhancing balance. IMT targets the diaphragm and related core muscles, which are crucial for maintaining postural stability. For instance, Ferraro et al. (2019) conducted a randomized, double-blind, placebo-controlled study showing that 8 weeks of unsupervised, home-based IMT significantly improved balance in healthy older adults, as assessed by the Mini-Balance Evaluation Systems Test (Xiao et al., 2012). These improvements were attributed to enhanced inspiratory muscle strength and function, which likely contributed to better postural control. More recent studies also affirm IMT’s comprehensive benefits. For example, Lee and Kim (2018) reported that respiratory training significantly improved trunk control, pulmonary function, and trunk muscle activity in chronic stroke patients. Similarly, Jung and Bang (2017) demonstrated improvements in walking ability and inspiratory muscle strength following IMT in subacute stroke patients. Oh et al. (2016) found that IMT positively affected balance and abdominal muscle thickness. These studies support the growing consensus that IMT is not only beneficial for respiratory recovery but also for functional mobility and core stability. The findings of this study demonstrated that IMT produced significant improvements in respiratory function, trunk control, mobility, and balance in stroke patients. While both the experimental and control groups showed improvements over time, the magnitude of change was significantly greater in the experimental group across all measured outcomes. These results support the efficacy of IMT as an effective adjunct to conventional rehabilitation in enhancing functional recovery following stroke. The large effect sizes observed, particularly in inspiratory function and trunk stability, highlight the clinical relevance of incorporating IMT into stroke rehabilitation protocols.

This study has several limitations. First, the relatively small sample size may limit the statistical power and restrict the generalizability of the findings. Second, the absence of long-term follow-up hinders the ability to determine the sustained effects of the intervention over time. Third, participants in the control group may have engaged in unsupervised or incidental physical activity, potentially affecting the outcomes and contributing to the lack of statistically significant differences in certain measures such as balance and mobility. Fourth, all participants were recruited from a single institution, which may result in a relatively homogeneous sample and limit the external validity of the results. To address these limitations, future research should include a larger and more diverse sample population, employ multicenter trials to enhance generalizability, and incorporate long-term follow-up assessments to evaluate the persistence of treatment effects. Moreover, using objective and standardized tools to monitor physical activity and functional performance-especially in the control group-would help clarify the true effects of the intervention.

In conclusion, this intervention effectively improved trunk control, balance, and both inspiratory and expiratory functions. These findings suggest that the intervention is a valuable exercise rehabilitation approach for individuals with impaired respiratory function.

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