Effects of High-Intensity Interval Training Versus High-Intensity Circuit Training on Airflow Limitation in Older Adults with Pre-COPD: A Pilot RCT
Article information
Abstract
PURPOSE
Pre-COPD, characterized by mild airflow limitation, represents a critical window for preventing progression to chronic obstructive pulmonary disease (COPD). This pilot randomized controlled trial examined the effects of high-intensity interval training (HIIT) versus high-intensity circuit training (HICT) on pulmonary function in older adults with pre-COPD.
METHODS
Twenty older men aged 60-75 years with pre-COPD, defined as normal or near-normal FEV1 with symptoms, risk factors, and mild airflow limitation (FEV1/FVC 0.70-0.95), were screened. Fourteen eligible participants were randomized to HIIT (n=7) or HICT (n=7) for a 9-week supervised intervention. Due to three HIIT dropouts, on-treatment analysis included 4 HIIT and 7 HICT participants. Primary outcomes were pulmonary function, with secondary measures of cardiopulmonary and functional fitness.
RESULTS
Despite the limited sample size, significant time×group interactions were observed for FEV1/FVC ratio (p=.004, η²p =0.62) and FEV1 % predicted (p=.018, η²p =0.37). The HIIT group improved FEV1/FVC ratio (0.82±0.08 to 0.91±0.06, +11.0%) and FEV1 (91.4±15.2% to 98.7±14.8%, +8.0%), while the HICT group showed unexpected decrements (FEV1/FVC: 0.88±0.06 to 0.84±0.07, -4.5%; FEV1: 97.3±14.6% to 89.2±15.3%, -8.3%). Functional fitness improvements were observed exclusively in the HIIT group, with significant gains in lower extremity strength and aerobic endurance.
CONCLUSIONS
This preliminary investigation indicates that HIIT may be associated with favorable changes in pulmonary function in older adults with pre-COPD, although these findings should be interpreted with caution. Given the limited sample size, the results primarily provide exploratory effect size estimates and feasibility data for future studies. The observed differences between HIIT and HICT underscore the need for further research on exercise prescription in this population.
INTRODUCTION
Chronic obstructive pulmonary disease (COPD) is one of the leading causes of death worldwide. Because it is characterized by largely irreversible airflow limitation, early prevention is of paramount importance. Age-related decline in pulmonary function is an inevitable physiological process. Even in healthy adults, forced expiratory volume in one second (FEV1) decreases by approximately 25-30 mL per year after age of 25, resulting in an approximately 30-40% reduction in lung function by the age of 80 [1]. This physiological decline may be further accelerated by environmental risk factors, such as cigarette smoking, exposure to air pollution, occupational exposure to harmful substances, and recurrent respiratory infections, eventually leading to chronic airflow obstruction [2].
Recent studies have increasingly focused on individuals with early respiratory impairment who do not yet meet the diagnostic criteria for COPD. In particular, older adults with mild airflow limitation (FEV₁/ FVC 0.70-0.95) do not meet the diagnostic criterion for COPD (FEV₁/ FVC <0.70) but are considered to be at high risk of developing COPD [3]. Accordingly, these individuals have recently been described as having pre-COPD or borderline airflow limitation in the respiratory literature [4]. A large cohort study reported that individuals with mild airflow limitation had a three- to four-fold higher risk of developing COPD within five years than those with normal lung function [5]. Moreover, these individuals already exhibit features similar to those observed in patients with COPD, including increased systemic inflammation, reduced exercise capacity, and limitations in activities of daily living, suggesting that they may be more likely to progress to COPD without early intervention [5].
Therefore, mild airflow limitation is considered a critical window for intervention to prevent the development of COPD, highlighting the need for effective preventive strategies. To date, pharmacological therapies have primarily been developed for patients with established COPD and clinically significant symptoms, whereas no standardized pharmacological interventions have been established for individuals with mild airflow limitation. Consequently, non-pharmacological approaches, particularly exercise interventions, have emerged as promising alternative strategies. Previous studies have shown that regular exercise can attenuate the decline in lung function, improve cardiorespiratory fitness and respiratory muscle function, and reduce systemic inflammation [6-8]. However, evidence remains limited regarding which type of exercise is most appropriate for older adults with mild airflow limitation.
An appropriate exercise intervention for this population should (1) promote cardiorespiratory adaptations while minimizing excessive ventilatory burden, (2) improve both respiratory muscle function and peripheral muscle function, and (3) be feasible for implementation in both community and clinical settings. Considering these requirements, high-intensity interval training (HIIT) and high-intensity circuit training (HICT) have emerged as two promising exercise strategies for older adults with mild airflow limitation [9,10].
HIIT consists of repeated bouts of short, high-intensity exercise interspersed with recovery periods, allowing cardiorespiratory adaptations to be achieved within a relatively short time [11]. Its intermittent nature may also improve aerobic capacity while limiting the excessive ventilatory demand associated with lactate accumulation, making it a potentially suitable exercise strategy for individuals at risk for respiratory disease [12-15]. In contrast, HICT combines aerobic and resistance exercises in a circuit format, offering both time efficiency and broader improvements in physical fitness [16]. The American College of Sports Medicine (ACSM) and the American Heart Association (AHA) recommend combining aerobic and resistance exercise for older adults [17]. In line with these recommendations, HICT is considered a practical approach for improving functional fitness, muscular strength, and the ability to perform activities of daily living simultaneously [18-21].
Previous studies suggest that HIIT is particularly effective for improving cardiorespiratory fitness, which may contribute to better pulmonary health [11], whereas HICT may offer greater benefits for muscular strength, balance, and functional performance [21]. However, no study has directly compared these two exercise modalities (HIIT vs. HICT) in older adults with mild airflow limitation. Therefore, it remains unclear which approach is more effective for preserving pulmonary function and preventing the progression to COPD.
Therefore, this pilot randomized study aimed to compare the effects of a 9-week HIIT and HICT program on pulmonary function and physical performance in older adults with mild airflow limitation. As a pilot study, it was designed to provide preliminary estimates of effect sizes and feasibility data to inform the design of a future large-scale randomized trial. The findings of this study may contribute to the development of optimal exercise strategies for the early prevention of COPD.
METHODS
1. Study design and participants
This study was designed as a randomized, assessor-blinded, parallel-group pilot study to compare the effects of HIIT and HICT on pulmonary function and physical performance in older adults with mild airflow limitation. The study protocol was approved by the Institutional Review Board of Pusan National University Hospital (IRB No. 1809-020-071) and was conducted in accordance with the ethical principles of the Declaration of Helsinki. Written informed consent was obtained from all participants before enrollment. The study was conducted and reported in accordance with the CONSORT 2010 extension for pilot and feasibility trials [22].
Participants were recruited between September 2018 and January 2019 from the outpatient pulmonary clinic at Pusan National University Hospital and the local community. Recruitment was conducted through the outpatient pulmonary clinic, hospital bulletin board advertisements, and local senior community centers.
The inclusion criteria were as follows: (1) men aged 60-75 years; (2) mild airflow limitation on pulmonary function testing (FEV₁/FVC ratio, 0.70-0.95); (3) predicted FEV₁ ≥80%; (4) resting oxygen saturation ≥90%; (5) no acute respiratory exacerbation within the previous 3 months before enrollment; (6) no participation in a structured exercise program (moderate-intensity exercise fewer than twice per week); and (7) ability to walk independently for at least 100 m without an assistive device.
The exclusion criteria were as follows: (1) hospitalization or systemic corticosteroid treatment for an acute respiratory exacerbation within the previous 3 months; (2) unstable cardiovascular disease, including unstable angina, uncontrolled arrhythmia, or New York Heart Association (NYHA) class III-IV heart failure; (3) musculoskeletal disorders limiting exercise participation; (4) uncontrolled metabolic disease (HbA1c >9.0%); (5) cognitive impairment (Korean version of the Mini-Mental State Examination [K-MMSE] score <24); and (6) active treatment for malignant disease.
A total of 20 individuals were screened for eligibility. Of these, six were excluded (three did not meet the inclusion criteria, two declined to participate, and one was excluded for other reasons), leaving 14 participants for randomization. Participants were randomly assigned in a 1:1 ratio to either the HIIT group (n=7) or the HICT group (n=7). Randomization was performed by an independent researcher using a computer-generated random sequence with a block size of four. The allocation sequence was concealed in sequentially numbered, opaque, sealed envelopes and was not revealed until participant enrollment had been completed, thereby ensuring allocation concealment. The researchers who delivered the exercise interventions were not involved in the randomization process and were unaware of group allocation before participant assignment [23].
No serious exercise-related adverse events occurred during the study, and all exercise sessions were completed without medical complications. During the 9-week intervention, three participants in the HIIT group withdrew from the study because of dyspnea during the early phase of the intervention (n=2) or personal reasons (n=1). The per-protocol analysis included four participants in the HIIT group and seven participants in the HICT group (Fig. 1).
CONSORT flow diagram of participant recruitment, retention, and analysis.
2. Exercise program
Both groups participated in a supervised exercise program three times per week (Monday, Wednesday, and Friday) for 9 weeks, with each session lasting 40 minutes. All exercise sessions were supervised by an exercise specialist with a master’s degree in exercise physiology and more than 5 years of clinical experience in exercise training. The exercise sessions were conducted in the exercise therapy room of the Department of Rehabilitation Medicine at Pusan National University Hospital between 9:00 AM and 11:00 AM.
The HIIT program was performed on a treadmill (h/p/cosmos mercury 4.0; h/p/cosmos sports & medical GmbH, Germany). Each session consisted of a 5-minute warm-up, 20 minutes of interval exercise, and a 5-minute cool-down. Exercise intensity was prescribed using the heart rate reserve (HRR) calculated according to the Karvonen formula [24]. High-intensity intervals were performed at 85-95% of HRR, whereas active recovery intervals were performed at 40-45% of HRR. The interval protocol consisted of three sets of 4-minute high-intensity exercise followed by 2-minute active recovery. To allow for progressive adaptation to training, the target intensity for the high-intensity intervals was gradually increased from 65-75% HRR during weeks 1-2, to 75-85% HRR during weeks 3-4, and finally to 85-95% HRR during weeks 5-9.
The HICT program was based on a circuit consisting of six exercises: jogging in place, modified push-ups, chair squats, abdominal crunches, step-ups using a 15-cm step box, and arm circles. Participants performed each exercise for 30 seconds, followed by a 15-second transition period, and completed the full circuit three times. A 2-minute rest period was provided between circuits. The target exercise intensity was 65-95% of HRR, and the speed and range of motion of each exercise were adjusted according to each participant’s fitness level.
Because the HIIT and HICT programs differed substantially in their exercise characteristics, it was not feasible to match the absolute exercise workload based on exercise duration, number of repetitions, or external work. Therefore, an equal-workload design was not adopted. Instead, physiological exercise intensity was standardized using heart rate reserve (65-95% HRR) to achieve comparable physiological demands between the two interventions.
3. Outcome assessments
All assessments were performed by the same assessor 1 week before the intervention (baseline) and within 3-5 days after completion of the 9-week intervention.
1) Pulmonary function testing
Pulmonary function was assessed using a diagnostic spirometer (PONY FX; COSMED, Rome, Italy) in accordance with the 2019 American Thoracic Society/European Respiratory Society standardization guidelines [25]. Before testing, volume calibration was performed using a 3-L calibration syringe. All tests were conducted by the same experienced examiner.
Participants performed forced expiratory maneuvers in a seated position while wearing a nose clip. Each participant completed a minimum of three and a maximum of eight attempts. Among the acceptable maneuvers, only tests that met the reproducibility criteria were retained. Reproducibility was defined as a difference of no more than 150 mL between the largest and second-largest values for both FEV₁ and FVC.
The spirometer and its software underwent daily calibration and quality checks before testing to maintain consistent measurement accuracy. The outcomes included FEV₁, forced vital capacity (FVC), the FEV₁/FVC ratio, and peak expiratory flow (PEF). Percent-predicted values were calculated using Korean reference equations derived from the Korea National Health and Nutrition Examination Survey (KNHANES) [25]. To minimize measurement bias, the examiner was blinded to participants’ group allocation (HIIT or HICT).
2) Cardiopulmonary exercise testing
Cardiopulmonary exercise testing (CPET) was performed using a treadmill and a breath-by-breath gas analysis system (Quark CPET; COSMED, Rome, Italy) with the Modified Bruce Ramp Protocol. Considering the characteristics of older adults with mild airflow limitation, a symptom-limited maximal exercise test was performed. Before each test, the system was calibrated according to the manufacturer’s instructions. A 12-lead electrocardiogram was continuously monitored throughout the test. Blood pressure was measured every 2 minutes, and oxygen saturation was monitored continuously. The primary outcome measures included peak oxygen uptake (VO₂peak), peak minute ventilation (VEpeak), respiratory exchange ratio (RER), peak heart rate, and exercise duration. Exercise testing was terminated according to the ACSM guidelines.
3) Functional fitness assessment
Functional fitness was assessed using the Senior Fitness Test (SFT) battery developed by Rikli and Jones [26].
(1) Muscular strength
Upper-body strength was assessed using the 30-second arm curl test. Participants sat in a chair with both feet flat on the floor and held a 2-kg dumbbell in one hand. The number of complete elbow flexion and extension repetitions performed in 30 seconds was recorded using a stopwatch (CASIO, Japan). Lower-body strength was assessed using the 30-second chair stand test. Participants sat with both feet flat on the floor and their arms crossed over the chest. The number of times they rose to a full standing position and returned to the seated position within 30 seconds was recorded.
(2) Aerobic endurance
Aerobic endurance was assessed using the 2-minute step test. The target stepping height was set at the midpoint between the patella and the iliac crest and marked with tape. Participants marched in place for 2 minutes, and the number of times the knee reached the marked height was recorded.
(3) Flexibility
Upper-body flexibility was assessed using the back scratch test. Participants reached one hand over the shoulder and the other up the middle of the back, attempting to bring the middle fingers together. The distance between the tips of the middle fingers was measured in centimeters. Overlap was recorded as a positive value, whereas a gap was recorded as a negative value. Lower-body flexibility was assessed using the chair sit-and-reach test. Participants sat near the front edge of a chair, with one leg bent and the other extended. With the extended leg straight, they reached forward toward the toes. The distance between the fingertips and the tip of the toes was measured in centimeters. Reaching beyond the toes was recorded as a positive value, whereas failing to reach the toes was recorded as a negative value.
(4) Dynamic balance and agility
Dynamic balance and agility were assessed using the 8-foot up-and-go test. Participants stood up from a chair, walked around a cone positioned 2.44 m away, returned to the chair, and sat down. The time required to complete the test was recorded.
4) Six-minute walk test
The 6-minute walk test (6MWT) was performed in accordance with the American Thoracic Society guidelines on a 30-m indoor corridor [27]. Before testing, resting blood pressure, heart rate, and oxygen saturation were measured, and standardized instructions were provided to all participants. Standardized encouragement and the remaining test time were provided at 1-minute intervals, and participants were allowed to rest if necessary. The primary outcome was the total walking distance (m). The lowest oxygen saturation during the test and the Borg dyspnea score (0-10) immediately after the test were also recorded. The test was terminated immediately if participants experienced chest pain, severe dyspnea, dizziness, or other symptoms requiring test discontinuation.
4. Statistical analysis
As this was a pilot study, the sample size was determined primarily by feasibility rather than by an a priori power calculation. All statistical analyses were performed using SPSS Statistics version 26.0 (IBM Corp., Armonk, NY, USA). Continuous variables are presented as mean±standard deviation, and categorical variables as frequency and percentage. The normality of continuous variables was assessed using the Shapiro-Wilk test.
Baseline characteristics were compared between the two groups using an independent-samples t-test or the Mann-Whitney U test, as appropriate. The primary analysis of intervention effects was performed using a two-way repeated-measures analysis of variance, with time (pre- and post-intervention) as the within-subject factor and group (HIIT and HICT) as the between-subject factor. The primary effect of interest was the time-by-group interaction.
When a significant interaction was observed, paired-samples t-tests were used for within-group comparisons and independent-samples ttests for between-group comparisons. Effect sizes were reported as Cohen’s d and partial eta squared (η²p). Cohen’s d values were interpreted as small (0.2), medium (0.5), or large (0.8), according to Cohen’s criteria. Given the exploratory nature of this pilot study, 95% confidence intervals were also reported where applicable. Statistical significance was set at p<.05.
RESULTS
1. Baseline characteristics
The baseline characteristics of the participants are presented in Table 1. A total of 11 participants were included in the final analysis (HIIT, n=4; HICT, n=7). The mean age was 68.5±4.1 years in the HIIT group and 67.0±5.2 years in the HICT group, with no significant difference between groups (p=.587). Anthropometric characteristics, including height, body weight, and body mass index (BMI), were comparable between the two groups (all p>.05).
Baseline characteristics of study participants
Pulmonary function testing showed that the baseline FEV₁/FVC ratio was significantly lower in the HIIT group than in the HICT group (0.82 ±0.08 vs. 0.88±0.06, p=.009), despite randomization. This baseline imbalance likely reflects the limitations of randomization in a small pilot sample. Predicted FEV₁ did not differ significantly between the groups (91.4 ±15.2% vs. 97.3±14.6%, p=.208).
Cardiopulmonary exercise testing showed no significant difference in baseline VO₂peak between the HIIT and HICT groups (19.2±4.1 vs. 22.3 ±4.8 mL·kg⁻¹·min⁻¹, p=.284). These values corresponded to approximately 70-80% of the age-predicted values, indicating mildly reduced exercise capacity.
In the functional fitness assessment, the HICT group performed significantly better on the 30-second chair stand test than the HIIT group (26.7±7.9 vs. 15.0±3.1 repetitions, p=.015), indicating greater baseline lower-body strength. No significant difference was observed in the 6-minute walk distance (498.5±110.6 vs. 576.8±77.6 m, p=.189).
2. Changes in pulmonary function
Changes in pulmonary function are presented in Table 2. A significant time-by-group interaction was observed for the primary outcome, the FEV₁/FVC ratio (F(1, 9)=14.47, p=.004, partial η²p =0.62). In the HIIT group, the FEV₁/FVC ratio increased from 0.82±0.08 to 0.91±0.06, representing an absolute increase of 0.09±0.04 and an 11.0% improvement (p=.006). In contrast, the HICT group showed a decrease from 0.88±0.06 to 0.84±0.07, corresponding to an absolute reduction of 0.04±0.03 and a 4.5% decline (p=.022).
Changes in primary and secondary outcomes after 9-week intervention
A time-by-group interaction was also observed for percent-predicted FEV₁ (F(1, 9)=5.38, p=.018, partial η²p =0.37). Percent-predicted FEV₁ increased by 11.8 percentage points in the HIIT group, from 88.7±17.1% to 100.5±16.5%, whereas it decreased by 15.3 percentage points in the HICT group, from 100.8±17.1% to 85.5±18.8%.
3. Changes in cardiorespiratory fitness
Neither VO₂peak nor VEpeak changed significantly in either group. For the primary indicator of cardiorespiratory fitness, VO₂peak increased from 19.20±4.08 to 21.29±2.90 mL·kg⁻¹·min⁻¹ in the HIIT group and from 22.36±4.85 to 26.52±6.18 mL·kg⁻¹·min⁻¹ in the HICT group; however, neither increase reached statistical significance (Table 2).
4. Changes in functional fitness
Changes in functional fitness are presented in Table 2. A significant time-by-group interaction was observed for the 30-second chair stand test (F(1, 9)=5.73, p=.040, η²p =0.39). In the HIIT group, the number of repetitions increased from 15.0±3.1 to 19.5±2.6, representing an improvement of 4.5±1.9 repetitions (30.0%; p=.016). In contrast, the HICT group showed a decrease from 26.7±7.9 to 21.0±6.2 repetitions (−5.7±7.1 repetitions), although this change was not statistically significant (p=.077).
In the 2-minute step test, the HIIT group improved from 118.7±9.7 to 190.0±28.0 steps, an increase of 71.3±23.2 steps (60.1%; p=.008). The HICT group also increased from 133.0±30.6 to 171.7±53.6 steps (+38.7±47.9 steps), although the change did not reach statistical significance (p=.074).
In the 8-foot up-and-go test, the HIIT group improved significantly, with completion time decreasing from 8.6±1.0 to 6.6±1.0 seconds (−2.0±0.7 seconds, 23.3%; p=.016). The HICT group also showed a shorter completion time, decreasing from 7.1±1.3 to 6.5±1.0 seconds (−0.6±1.2 seconds), but the change was not statistically significant (p=.221).
5. Changes in the six-minute walk test
No significant changes were observed in the 6-minute walk test in either group. The 6-minute walk distance increased from 498.5±110.6 m to 510.9±50.5 m in the HIIT group and from 576.8±77.6 m to 593.8±81.8 m in the HICT group. However, neither group achieved the minimal clinically important difference of 30 m (Table 2).
DISCUSSION
This pilot study compared the effects of HIIT and HICT in older adults with mild airflow limitation and provides preliminary evidence to inform the design of future definitive trials. The findings suggest that HIIT is a feasible exercise intervention for individuals with pre-COPD and may induce favorable improvements in pulmonary function and selected aspects of functional fitness. However, given the small sample size and the differential dropout rates between the two groups, the findings should be interpreted as exploratory. Therefore, larger adequately powered randomized controlled trials are warranted to confirm the reproducibility and clinical significance of these findings. Despite the limited sample size (HIIT, n=4; HICT, n=7), the magnitude and consistency of the observed effects provide a rationale for future investigation.
In the present study, the HIIT group demonstrated an increase in the FEV₁/FVC ratio, suggesting that short-term HIIT may improve indices of airflow obstruction in older adults with pre-COPD. The mean FEV₁/FVC ratio increased from 0.82 to 0.91, representing an approximately 11% relative improvement. The FEV₁/FVC ratio is widely recognized as a key indicator of airflow obstruction [28]. However, given the very small sample size and the baseline imbalance between the groups, these findings should be interpreted with caution. Notably, three of the four participants in the HIIT group achieved an FEV₁/FVC ratio greater than 0.90 following the intervention. Although this observation suggests that HIIT may promote favorable changes in pulmonary function, the potential influence of regression to the mean and random variation cannot be excluded. Therefore, these preliminary findings warrant confirmation in larger, adequately powered randomized controlled trials.
The improvement in FEV₁/FVC observed in the present study may have implications for COPD prevention. Previous large-scale cohort studies have reported that a lower FEV₁/FVC ratio is associated with an increased risk of developing COPD [29]. However, given the small sample size and pilot design of the present study, such epidemiological associations cannot be directly inferred from our findings. Therefore, the present findings should be interpreted as preliminary evidence that short-term exercise intervention may induce favorable changes in pulmonary function, rather than as evidence that HIIT reduces the long-term risk of COPD.
Compared with previous exercise intervention studies in patients with established COPD, the changes in FEV₁/FVC observed in the HIIT group in the present study showed a somewhat different pattern. Numerous pulmonary rehabilitation studies have reported that moderate-intensity aerobic exercise or physical activity promotion programs improve exercise capacity and health-related quality of life but do not produce significant changes in pulmonary function indices such as FEV₁ or FEV₁/FVC [29,30]. In contrast, an increase in FEV₁/FVC was observed following HIIT in the present study. This finding is partly consistent with recent experimental evidence suggesting that high-intensity interval exercise may enhance the oxidative capacity of the respiratory muscles and improve autonomic regulation [10,31,32]. These findings raise the possibility that a greater physiological training stimulus may be required to elicit measurable improvements in pulmonary function during the pre-COPD stage. In contrast, once structural airway obstruction has become established, short-term exercise interventions may be insufficient to induce meaningful changes in airway mechanics or respiratory muscle function.
Several physiological mechanisms may underlie the improvements in pulmonary function observed following HIIT, particularly adaptations in respiratory muscle function and airway inflammation. Repeated high-intensity exercise provides a substantial training stimulus to the respiratory muscles, which may enhance their oxidative capacity and endurance [33], thereby improving ventilatory efficiency. In addition, HIIT has been reported to improve the airway microenvironment by increasing anti-inflammatory cytokines and reducing pro-inflammatory mediators [34]. Alterations in autonomic regulation and improvements in pulmonary vascular function have also been proposed as potential contributing mechanisms [35,36]. However, because inflammatory biomarkers, oxidative stress, respiratory muscle function, and autonomic nervous system activity were not directly assessed in the present study, these mechanistic explanations remain speculative. Further studies are therefore needed to determine whether the short-term functional improvements observed in this study translate into long-term structural adaptations.
In contrast, no significant improvement in pulmonary function was observed in the HICT group, and some pulmonary function indices showed a trend toward decline. These findings are generally consistent with previous studies. Several studies evaluating combined aerobic and muscular endurance training in patients with COPD or mild airflow limitation have reported improvements in functional fitness but little or no consistent improvement in pulmonary function indices such as FEV₁ and FEV₁/FVC. Likewise, the systematic review by O’Shea et al. [37] concluded that resistance- and endurance-based training improved peripheral muscle function but provided limited evidence for improvements in pulmonary function.
Taken together with previous findings, the HICT protocol used in the present study may not have provided a sufficiently sustained ventilatory stimulus to induce measurable improvements in pulmonary function. This may be related to the characteristics and exercise dose of the HICT program. Because each exercise bout was relatively short, the ventilatory stimulus may not have been maintained continuously throughout the session. In addition, the resistance exercise component may have increased intrathoracic pressure through the Valsalva maneuver, while the frequent transitions between exercises may have disrupted the establishment of a stable breathing pattern [38].
Although the two exercise programs were matched for relative exercise intensity, they were not matched for absolute workload or total energy expenditure because of their inherently different exercise modalities. Therefore, the decline in pulmonary function observed in the HICT group may have been influenced not only by differences in breathing patterns and the Valsalva maneuver but also by a relatively lower ventilatory stimulus, shorter exposure to high-intensity exercise, or lower total energy expenditure compared with HIIT. However, because ventilatory responses, energy expenditure, and breathing patterns were not directly measured, these explanations remain hypothetical and should be interpreted with caution. Future studies should quantify these physiological variables to better distinguish the differential effects of different exercise modalities.
In addition to improvements in pulmonary function, the HIIT group showed significant improvements in several measures of physical performance. Performance improved by 30% in the 30-second chair stand test, 60% in the 2-minute step test, and 23% in the 8-foot up-and-go test, suggesting that HIIT may enhance overall physical function. These improvements may reflect the combined effects of HIIT on lower-body strength and aerobic endurance. The improvement in lower-body strength may be attributable to the repeated high-intensity treadmill stimulus imposed on the quadriceps and gluteal muscles [39]. In addition, HIIT may enhance muscle function by promoting metabolic and hormonal adaptations and recruiting fast-twitch (type II) muscle fibers [40,41]. The improvement in the 2-minute step test also suggests enhanced aerobic endurance, potentially through adaptations such as improved mitochondrial function and greater peripheral oxygen utilization [42]. However, because these physiological mechanisms were not directly assessed, these explanations remain speculative.
This pilot study provides preliminary information to guide the design of future definitive trials. Although the large effect size observed for the change in FEV₁/FVC (η²p =0.62) should be interpreted cautiously because of the small sample size, our findings suggest that at least 20-30 participants per group may be required in future studies. No serious adverse events occurred during the intervention, indicating that supervised high-intensity exercise may be feasible for older adults with mild airflow limitation. However, the relatively high dropout rate in the HIIT group (43%, 3/7) suggests that a more gradual and individualized training progression may improve adherence. The absence of significant pulmonary function improvements in the HICT group also warrants further investigation, particularly regarding the influence of resistance exercise components and baseline pulmonary function. Finally, by focusing on individuals at high risk of COPD rather than patients with established disease, this study provides preliminary support for a preventive exercise-based approach in the pre-COPD stage.
This study has several limitations. First, the extremely small sample size and high dropout rate substantially limited the statistical power of the study. Only 11 participants were included in the final analysis (HIIT, n=4; HICT, n=7), and the particularly small number of participants in the HIIT group increased the risk of selection bias, type I error, and overestimation of effect sizes. In addition, because multiple outcome variables were analyzed simultaneously, the possibility of false-positive findings cannot be excluded. Therefore, the present findings should be considered exploratory and require confirmation in larger, adequately powered randomized controlled trials. Second, despite randomization, baseline imbalance between the groups complicates the interpretation of the findings. In particular, the significantly lower baseline FEV₁/FVC ratio in the HIIT group raises the possibility that part of the observed improvement may reflect regression to the mean rather than the intervention alone. Furthermore, because a non-exercise control group was not included, it is difficult to distinguish the specific effects of exercise from nonspecific influences such as the passage of time or learning effects. These design limitations restrict causal interpretation of the findings, particularly for physiological outcomes such as pulmonary function, which exhibit natural biological variability. Third, although the 9-week intervention was sufficient to observe short-term physiological adaptations, it was not long enough to determine whether these changes translate into structural adaptations or long-term clinical benefits. In addition, several potentially important outcomes, including inflammatory biomarkers, oxidative stress, and health-related quality of life, were not assessed. Finally, because of the inherent differences between HIIT and HICT, the two interventions were matched for relative exercise intensity (based on heart rate reserve) but not for absolute workload or total energy expenditure. Future studies should incorporate quantitative measures of exercise dose, such as MET-min, VO₂-based workload, or total energy expenditure, to enable more rigorous comparisons between different exercise modalities.
Despite these limitations, they are inherent to the nature of a pilot study, whose primary purpose is to generate hypotheses and inform the design of future definitive trials rather than to provide confirmatory evidence. In this context, the present findings provide valuable information for improving the design of subsequent studies. Future trials should include adequately powered sample sizes and a non-exercise control group to allow more robust comparisons. Longer intervention periods (at least 12 weeks, and preferably 6 months or longer) with follow-up assessments are needed to determine the durability of the observed effects. In addition, comprehensive evaluation of inflammatory biomarkers, oxidative stress, respiratory muscle function, health-related quality of life, and mechanistic variables such as ventilatory patterns and respiratory muscle activation would help clarify the mechanisms underlying exercise-induced adaptations. From a clinical perspective, exercise programs should be individualized according to baseline pulmonary function, physical fitness, and symptom severity. A gradual progression during the first 3-4 weeks, together with breathing instruction, is recommended, and close supervision during the initial training period may help ensure exercise safety.
CONCLUSION
This pilot study provides preliminary evidence that a 9-week HIIT program may induce more favorable changes in pulmonary function, particularly the FEV₁/FVC ratio, than HICT in older adults with mild airflow limitation. Although the observed improvements may be partly related to physiological adaptations involving airway mechanics, respiratory muscle function, or ventilation-perfusion relationships, these mechanisms remain speculative and require further investigation. The direction of these findings is consistent with previous studies reporting beneficial effects of HIIT on cardiorespiratory adaptation and exercise capacity in individuals with mild COPD or early airflow limitation.
Given the small sample size, high dropout rate, baseline imbalance, and exploratory nature of this pilot study, the findings should be interpreted with caution and should not be generalized. Nevertheless, this study is among the first to explore the feasibility of exercise intervention during the pre-COPD stage rather than after the onset of established COPD.
These findings suggest that HIIT may represent a promising intervention for older adults at risk of COPD. Larger randomized controlled trials with longer follow-up and comprehensive mechanistic assessments are needed to confirm these preliminary findings and to establish evidence-based exercise strategies for individuals with mild airflow limitation.
Notes
CONFLICT OF INTEREST
The authors received no financial support from any organization for the conduct of this study and declare that they have no competing interests that could have influenced the work.
AUTHOR CONTRIBUTIONS
Conceptualization: SJ Lee, KU Kim, MJ Shin, JJ Park; Data curation: SJ Lee, KU Kim, MJ Shin, TS Park; Formal analysis: EA Jo, SJ Lee; Methodology: EA Jo, SJ Lee, KU Kim, MJ Shin, TS Park, JJ Park; Project administration: KU Kim, MJ Shin, JJ Park; Visualization: EA Jo; Writing - original draft: EA Jo, SJ Lee; Writing - review & editing: EA Jo, KU Kim, MJ Shin, TS Park, JJ Park.