Effects of Inspiratory Muscle Training on Respiratory Muscle Strength, Lactate Accumulation and Exercise Tolerance in Amateur Runners: A Randomized Controlled Trial

bjective: This study investigated the dose–response relationship of inspiratory muscle training (IMT) on respiratory muscle strength, lactic acid accumulation and exercise tolerance in amateur runners. Methods: Thirty male amateur runners were randomly assigned to three groups: a high-intensity IMT (HIMT) group, a low-intensity IMT (LIMT) group, and a control group. In addition to their regular training regimen, the high-intensity and low-intensity IMT groups underwent a supervised IMT protocol for a duration of 8 weeks. The primary outcome measures included maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), time to exhaustion (TTE), blood lactate (BLa), rate of perceived exertion (RPE), and rate of perceived breathlessness (RPB). Secondary outcomes encompassed VO2 max, forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio. Results: After 8 weeks of IMT, the MIP of HIMT and LIMT were significantly improved (p< 0.01), and the MEP of both groups also increased (p<

blood lactate (BLa), rate of perceived exertion (RPE), and rate of perceived breathlessness (RPB). Secondary outcomes encompassed VO2 max, forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio. Results: After 8 weeks of IMT, the MIP of HIMT and LIMT were significantly improved (p< 0.01), and the MEP of both groups also increased (p< 0.01). There were no significant changes in FVC and FEV1(p> 0.05), but only FEV1/FVC in HIMT was significantly improved (p< 0.01). Exercise testing showed a significant increase in TTE in both the HIMT and low LIMT groups (p< 0.01). Post-exercise RPE scores were lower in both the HIMT group (p< 0.01) and LIMT group (p< 0.05), and both HIMT and LIMT groups’ post-exercise RPB scores were also reduced in both (p< 0.05). In addition, blood lactate accumulation was significantly lower in both HIMT (p< 0.01) and LIMT (p< 0.05). There were no significant changes in VO2 max (p> 0.05) and HR peak (p> 0.05). Conclusion: IMT for 8 weeks can improve respiratory muscle strength, prolong exercise time, improve blood lactate accumulation, subjective fatigue, and dyspnea during exercise. Among them, high-intensity IMT can better improve exercise tolerance. Keywords:inspiratory muscle training; exercise tolerance; respiratory muscle strength; lactate accumulation; amateur runners 1. Introduction The increasing emphasis on health and athleticism has led to a notable rise in partic- ipation in endurance activities, such as long-distance running and marathon events [1]. As global interest in recreational sports continues to grow, amateur runners face distinct physiological challenges. These challenges stem from their diverse training backgrounds Life2025,15, 705 https://doi.org/10.3390/life15050705

Life2025,15, 705 2 of 13 and relatively lower baseline fitness levels compared to professional athletes [2]. Exercise tolerance is a crucial determinant of an athlete’s stamina, speed, and overall performance in aerobic exercises. As a result, improving exercise tolerance has become a key focus for run- ners [3–5]. However, traditional training paradigms frequently prioritize the development of limb muscles while overlooking the adaptation of respiratory muscles. This oversight is particularly concerning given emerging evidence suggesting that respiratory fatigue may serve as a limiting factor in endurance performance [6,7]. During exercise, a phenomenon known as “inspiratory muscle blood steal” can com- promise the function of respiratory muscles, thereby limiting their capacity to oxygenate active skeletal muscles. This limitation may exacerbate fatigue in skeletal muscles and re- duce exercise endurance [8]. Recent advancements in exercise physiology have emphasized the diaphragm’s dual role as both a respiratory pump and a contributor to postural stability during running, highlighting its potential as a target for performance optimization [9]. Vital to the regulation of blood-gas homeostasis during exercise, the ventilatory pump muscles work in unison to co-ordinate changes in pleural pressure, inspiratory and expiratory flow, lung volume, and aeration. The significance of respiratory muscle fatigue in this context is often underestimated. Respiratory muscle fatigue can significantly limit exercise performance. As physical exertion intensifies, the diaphragm and other respiratory muscles become fatigued, leading to a reduction in their efficiency and an increase in the perceived effort of breathing. This phenomenon is particularly detrimental during endurance ac- tivities, where maintaining a balance between oxygen delivery and muscular demand is critical [10]. Inspiratory muscle training (IMT) has been demonstrated to delay the onset of respiratory muscle fatigue during physical activity. IMT enhances athletic performance by reducing the rate of perceived exertion (RPE) and the rate of perceived breathlessness (RPB), while also modulating metabolic reflexes that regulate blood flow redistribution between respiratory and skeletal muscles [11,12]. Notably, IMT may influence lactate kinetics by improving the efficiency of respiratory muscles, thus delaying the accumulation of blood lactate—a key biomarker associated with anaerobic threshold and fatigue [13]. While existing meta-analyses support the

perceived exertion (RPE) and the rate of perceived breathlessness (RPB), while also modulating metabolic reflexes that regulate blood flow redistribution between respiratory and skeletal muscles [11,12]. Notably, IMT may influence lactate kinetics by improving the efficiency of respiratory muscles, thus delaying the accumulation of blood lactate—a key biomarker associated with anaerobic threshold and fatigue [13]. While existing meta-analyses support the benefits of IMT in elite athletes [14], its translational potential for amateur populations remains insufficiently explored. Recent studies on IMT have shown promising results in enhancing the performance of endurance athletes and physically active individuals. However, the effectiveness of IMT continues to be a subject of ongoing debate within the scientific community. To date, the literature has predominantly focused on the effects of IMT among professional athletes, while data concerning amateur runners are notably limited. This gap is significant, as amateur runners represent the largest demographic in endurance sports yet lack evidence-based guidelines tailored to their phys- iological profiles. Moreover, previous research has seldom incorporated multidimensional assessments of respiratory strength, lactate dynamics, and subjective exertion metrics—an integrative approach that is essential for elucidating the mechanistic pathways underlying IMT’s effects. In response to the identified research gap, this study systematically investigated the effects of IMT on respiratory muscle strength, blood lactate levels, and exercise tolerance in amateur runners. Our research innovatively compares high-intensity versus low-intensity IMT protocols, addressing a significant controversy in training prescription. Assessments were conducted at baseline and after an 8-week IMT regimen. This study elucidates the dose–response relationships between IMT intensity and physiological adaptations, aiming to identify an effective training method to enhance exercise endurance in amateur runners. This study innovatively compares high- vs. low-intensity IMT protocols, addressing a critical gap in training prescription for amateur populations. The findings may have impli- cations for reducing injury risk and promoting long-term adherence to endurance sports.

Life2025,15, 705 3 of 13 2. Materials and Methods 2.1. Participants G*Power 3.1 software was utilized for sample size estimation, and repeated measures analysis of variance (ANOVA) was employed to assess the interaction effects between groups and time. The parameters incorporated into the calculation included an effect size (f) of 0.25, a significance level (α) of 0.05, and a statistical power of 80%. Considering a dropout rate of 10%, we determined that the required total sample size would be 30 participants [15]. Thirty male amateur runners (ages: 18–25 years) were recruited, all possessing at least three years of consistent running training experience. Baseline characteristics—including age, height, weight, BMI, and running experience were homogeneous across groups (Table). Table 1.Descriptive statistics of essential information in each group (Mean±SD). Variable HIMT LIMT Con p Age (yrs) 22.29 ±1.25 22.57±1.51 21.86±1.77 0.684 Height (cm) 175.63 ±4.67 177.86±5.34 178.20±4.97 0.589 Weight (kg) 67.79 ±5.21 67.30±5.04 65.41±4.11 0.630 BMI (kg/m 2 ) 21.9±1.21 21.2 ±1.14 20.5 ±1.05 0.625 Running experience (yrs)5.29±1.11 5.43 ±1.40 5.14 ±1.35 0.918 HR (bpm) 61.80 ±3.74 61.50±4.40 60.70±4.30 0.830 LA (mmol) 1.73 ±0.40 2.07 ±0.63 1.90 ±0.48 0.354 Inclusion criteria: male, aged 18–25 years; at least three years of training experience; no history of cardiovascular, respiratory, or musculoskeletal disorders; participants were asked to be able to follow the study’s intervention and testing protocols. Exclusion crite- ria: subjects with chronic respiratory or cardiovascular disease (such as asthma, chronic obstructive pulmonary disease) or other contraindications; individuals with musculoskele- tal injuries; and individuals who had other training prior to or during the study were deemed ineligible. The study adhered to the principles of the Declaration of Helsinki. Ethical approval was obtained from the Research Ethics Committee of Capital University of Physical Educa- tion and Sport (Approval No. 2022A58), and written informed consent was secured from all participants prior to enrollment. 2.2. Experimental Design This study investigated the effects of inspiratory muscle training (IMT) on amateur runners. The participants were divided into three groups: a high-intensity inspiratory muscle training group (HIMT group, 80% MIP, n = 10), a low-intensity inspiratory mus- cle training group (LIMT group,

(Approval No. 2022A58), and written informed consent was secured from all participants prior to enrollment. 2.2. Experimental Design This study investigated the effects of inspiratory muscle training (IMT) on amateur runners. The participants were divided into three groups: a high-intensity inspiratory muscle training group (HIMT group, 80% MIP, n = 10), a low-intensity inspiratory mus- cle training group (LIMT group, 50% MIP, n = 10), and a control group (CON group, n = 10). Over the course of 8 weeks, individuals in the IMT groups engaged in a super- vised regimen five times per week, utilizing a progressive flow-resistance load breathing trainer. Each training session comprised two sets of 30 maximal inspiratory efforts. In contrast, the control group continued their regular running training without any addi- tional inspiratory muscle training interventions. Randomization was conducted utilizing a computer-generated random number list to ensure an unbiased assignment of participants to groups (www.randomizer.org, accessed on 12 October 2022). Participants were allocated to their respective groups according to the established randomization sequence, and this allocation remained concealed until the completion of baseline assessments. Baseline assessments were conducted at the onset of the study to establish initial values for respiratory muscle strength (MIP/MEP), pulmonary function (spirometry: FVC, FEV1/FVC), and exercise tolerance (time to exhaustion on a cycle ergometer with gas exchange analysis). Post-intervention evaluations mirrored baseline measures and were

Life2025,15, 705 4 of 13 performed by blinded assessors under standardized laboratory conditions (22 ◦ C, 50% humidity) at consistent circadian times (±1 h). Secondary outcomes included blood lactate levels, perceptual exertion assessed through Borg RPE/RPB scales, and aerobic capacity determined via VO2max. Throughout the trial, participants maintained their regular running schedules while adhering to standardized pre-test hydration and dietary protocols verified through 24 h recalls. Additionally, they refrained from engaging in strenuous activities for 48 h prior to testing. 2.3. Respiratory Muscle Strength Test Respiratory muscle strength was quantified via assessments of both inspiratory and expiratory muscle capacities. Inspiratory muscle strength was gauged by recording the MIP, whereas MEP was employed to evaluate the strength of the expiratory muscles [13]. According to the standards of the American Thoracic Society and European Respiratory Society, an RMT device (XeeK BW05, Xiamen, China) was utilized for both pre-intervention and post-intervention assessments. Using this device to measure MIP and MEP has been effective and reliable [16]. For the assessment of inspiratory muscle strength, subjects were required to be in a state of maximal expiration with a nose clip applied to prevent nasal air leakage, followed by a forceful and rapid inhalation. Expiratory muscle strength was determined by having subjects perform a maximal expiration from their normal breathing amplitude [17]. Subjects were instructed to exhale as quickly and forcefully as possible from a state of maximal inhalation. Each subject completed three attempts for both inspiratory and expiratory measurements, ensuring that the variability among trials did not exceed 5%. The highest values recorded were adopted as the definitive MIP and MEP values. All measurements were conducted with subjects in a standing posture. 2.4. Pulmonary Function Test Before and following the intervention, participants underwent a battery of pulmonary function tests utilizing an intelligent respiratory training device (Xeek BW05, Xiamen, China). During the testing protocol, participants were instructed to adopt an erect posture, breathe through a suction mouthpiece, and utilize a nose clip, thereby ensuring the accuracy of the measurement data [18]. The pulmonary function tests were administered in alignment with the protocols delineated by the American

of pulmonary function tests utilizing an intelligent respiratory training device (Xeek BW05, Xiamen, China). During the testing protocol, participants were instructed to adopt an erect posture, breathe through a suction mouthpiece, and utilize a nose clip, thereby ensuring the accuracy of the measurement data [18]. The pulmonary function tests were administered in alignment with the protocols delineated by the American Thoracic Society (1995). These tests included measuring forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio. The variation among the trio of test attempts was constrained to within 5%, ensuring the selection of the most accurate test value to affirm the reliability and consistency of the data. 2.5. Exercise Test Participants were subjected to an incremental exercise test on a cycle ergometer (Ergo- line 100, Erlangen, Germany) during both pre- and post-8-week training intervention. After a 5 min warm-up at a low intensity, the test commenced with an initial workload of 50 W, which was escalated by increments of 10 W every minute. The cadence was maintained at a consistent 60 revolutions per minute until volitional exhaustion [19]. Before the commencement of the test, the demographic and physiological baseline data of the subjects were documented. After these preparations, a respiratory mask was fitted to each subject, and the cycle ergometer seat height was adjusted for optimal er- gonomics. Throughout the exercise test, a CORTEX gas metabolism analyzer (MAX-II) was employed to continuously monitor expired gases and assess maximal oxygen uptake. Upon completion of the incremental exercise test, participants’ perceived exertion and

Life2025,15, 705 5 of 13 breathlessness were evaluated using Borg’s rating of perceived exertion (RPE) and rate of perceived breathlessness (RPB) scales. The RPB scale is used in clinical and exercise physiology research [20,21]. Concurrently, heart rate (HR) was continuously recorded via the Polar V800 (Polar Electro OY; Kempele, Finland). Peak heart rate was recorded immediately after the exercise test. At rest and during peak exercise, blood lactate (BLa) levels were measured immediately using a portable lactate analyzer (EKF Lactate Scout 4, Magdeburg, Germany). Lactate samples were obtained from the earlobe, and the analyzer provides an instantaneous reading of lactate concentration. The predetermined cessation criteria for the exercise test were multifaceted, including (1) achievement of 90% of the age-predicted maximum heart rate (HR max), calculated as 220 minus the subject’s age; (2) a respiratory quotient (RQ) surpassing 1.10; (3) the participant’s inability to maintain the required pedaling cadence, as evidenced by a Borg RPE score exceeding 17; and (4) a discernible plateau in oxygen uptake as depicted by the VO2 curve [22]. 2.6. Inspiratory Muscle Training Subjects are acclimated to the laboratory environment and are briefly introduced to the exercise regimen relevant to the study. They were then introduced to the IMT program. IMT intervention was performed with the flow resistance load breathing training apparatus (Xeek BW05, Xiamen, China) [16,23]. During IMT, participants used disposable filters and nose clips to block nasal breathing, thereby isolating the inspiratory muscles [24]. The IMT regimen involved overcoming a predetermined inspiratory resistance while allowing unimpeded expiration. Inhalation is characterized by rapid and robust initial effort, transitioning to a slower, sustained phase, followed by a protracted exhalation [25]. Participants were assigned to two groups based on the intensity of the IMT program: the HIMT group underwent training at 80% of their MIP, whereas the LIMT group trained at 50% of their MIP. The selection of 50% MIP for the low-intensity IMT (LIMT) group was informed by prior research indicating that this intensity effectively trains respiratory muscles while minimizing the risk of overfatigue in recreational athletes. Additionally, utilizing 50% MIP offers a greater training load [14].

underwent training at 80% of their MIP, whereas the LIMT group trained at 50% of their MIP. The selection of 50% MIP for the low-intensity IMT (LIMT) group was informed by prior research indicating that this intensity effectively trains respiratory muscles while minimizing the risk of overfatigue in recreational athletes. Additionally, utilizing 50% MIP offers a greater training load [14]. Both groups trained for 5 days per week, twice daily, 30 inhalation repetitions per session [26,27]. 2.7. Data and Statistical Analysis The Shapiro–Wilk test was conducted to assess the normality of the distribution of the investigated variables. Results are presented as mean±standard deviation (M±SD). Group differences in baseline characteristics were evaluated using one-way analysis of variance (ANOVA). Independent samples t-tests were employed to compare the general characteristics of the participants. For the analysis of respiratory muscle strength, pul- monary function, and exercise performance between groups at baseline and following an eight-week training period, a 3×2 (group×time) repeated measures ANOVA was utilized, with Bonferroni correction for multiple comparisons. Effect size was calculated to assess the magnitude of the differences between groups. Partial eta squared (η 2) was used as the measure of effect size for the repeated measures analysis of variance (ANOVA). Effect sizes were interpreted according to conventional thresholds: small (η 2= 0.01), medium (η 2= 0.06), and large (η 2= 0.14). Statistical significance was set atp< 0.05. All statis- tical analyses were performed using SPSS software (version 27.0) and GraphPad Prism (version 9.5).

Life2025,15, 705 6 of 13 3. Results All participants completed the eight-week intervention. As shown in the base information (Table p> 0.05). 3.1. Respiratory Muscle Strength For MIP (FigureA), a significant main effect of time was observed (F = 102.64, p< 0.001,η 2partial = 0.792), along with a significant interaction between group and time (F = 36.796,p< 0.001,η 2partial = 0.732). After the 8-week intervention, MIP was significantly improved in both the HIMT and LIMT groups (p< 0.001). Notably, MIP was significantly higher in the HIMT group compared to both the LIMT group (p= 0.016) and the control group (p< 0.01).Life 2025, 15, x FOR PEER REVIEW 6 of 14 dium (η 2 = 0.06), and large (η 2 = 0.14). Statistical significance was set at p < 0.05. All statis- tical analyses were performed using SPSS software (version 27.0) and GraphPad Prism (version 9.5). 3. Results All participants completed the eight-week intervention. As shown in the base infor- mation (Table 1), no significant differences were observed between the groups at baseline (p > 0.05). 3.1. Respiratory Muscle Strength For MIP (Figure 1A), a significant main effect of time was observed (F = 102.64, p < 0.001, η 2 partial = 0.792), along with a significant interaction between group and time (F = 36.796, p < 0.001, η 2 partial = 0.732). After the 8-week intervention, MIP was significantly improved in both the HIMT and LIMT groups (p < 0.001). Notably, MIP was significantly higher in the HIMT group compared to both the LIMT group (p = 0.016) and the control group (p < 0.01). Figure 1. (A) MIP (Maximal inspiratory pressure) and (B) MEP (Maximal expiratory Pressure) pre- and post-intervention (Mean ± SD). * Significant increase within-group post-intervention compared to pre-intervention (p < 0.05), # significant differences between HIMT and LIMT groups versus CON group post-intervention (p < 0.05), Δ significant difference between HIMT and LIMT groups post- intervention (p < 0.05). Regarding MEP (Figure 1B), a significant main effect of time was found (F = 122.932, p < 0.001, η 2 partial = 0.820), and the interaction between

to pre-intervention (p < 0.05), # significant differences between HIMT and LIMT groups versus CON group post-intervention (p < 0.05), Δ significant difference between HIMT and LIMT groups post- intervention (p < 0.05). Regarding MEP (Figure 1B), a significant main effect of time was found (F = 122.932, p < 0.001, η 2 partial = 0.820), and the interaction between group and time was also signifi- cant (F = 41.669, p < 0.001, η 2 partial = 0.755). MEP increased significantly in both the HIMT and LIMT groups after the intervention (p < 0.01). Post-intervention, the HIMT group showed significantly greater MEP values compared to both the LIMT group (p < 0.01) and the control group (p < 0.01), while the difference between the LIMT group and the control group was not statistically significant (p = 0.20) (Table 2). Table 2. Changes in pulmonary function and respiratory muscle strength and post-intervention (Mean ± SD). Variable Time HIMT LIMT Con MIP (cmH 2o) Pre 81.90 ± 9.33 81.97 ± 7.12 85.27 ± 8.08 Post 111.99 ± 16.21 ** ##Δ 95.86 ± 10.69 ** 85.10 ± 7.21 MEP Pre 101.12 ± 9.95 97.64 ± 10.47 98.64 ± 6.75 Figure 1.(A) MIP (Maximal inspiratory pressure) and (B) MEP (Maximal expiratory Pressure) pre- and post-intervention (Mean±SD). # significant differences between HIMT and LIMT groups versus CON group post-intervention (p< 0.05), ∆ significant difference between HIMT and LIMT groups post-intervention (p< 0.05), Compared with before training, **p< 0.01; HIMT or LIMT compared with the control group, ## p< 0.01; HIMT compared with LIMT, ∆∆ p< 0.01. Regarding MEP (FigureB), a significant main effect of time was found (F = 122.932, p< 0.001,η 2partial = 0.820), and the interaction between group and time was also significant (F = 41.669,p< 0.001,η 2partial = 0.755). MEP increased significantly in both the HIMT and LIMT groups after the intervention (p< 0.01). Post-intervention, the HIMT group showed significantly greater MEP values compared to both the LIMT group (p< 0.01) and the control group (p< 0.01), while the difference between the LIMT group and the control group was not statistically significant

This study examines the effects of IMT on respiratory strength and exercise tolerance in amateur runners.