Effect of the BFR-7211 on lower limb muscle strength in older adults: A randomized controlled trial

Abstract

Objective

To evaluate the efficacy of a proprioceptive neuromuscular facilitation (PNF) stretching program integrated into a massage chair (BFR-7211) on lower limb strength, balance, and functional mobility in older adults, as a potential intervention for preventing sarcopenia and enhancing musculoskeletal health.

Design

A randomized controlled trial with a 4-week follow-up (12 visits) and 1:1 allocation ratio, conducted without blinding due to the nature of the interventions.

Setting and participants

Sixty-two community-dwelling older adults, 65 years and older, from a senior welfare center were randomized into an experimental group (massage chair with PNF stretching) or a control group (standard trunk exercise program) three times per week for a 4-week intervention period.

Methods

The experimental group received mechanical massage therapy utilizing the PNF stretching mode (XM module, level 2) of the massage chair for 20 min, three times weekly over four weeks. The control group performed a standard trunk exercise program consisting of trunk curl-ups, diagonal trunk curls, and single-legged extensions for 30 min per session. The primary outcome was measured by the Short Physical Performance Battery (SPPB). Secondary outcomes included maximal hamstring strength, body composition, depressive symptoms (GDS-K), fear of falling (KFES), and quality of life (K-CASP-16). Adverse events were monitored throughout the study. Outcomes were measured at baseline, 6th visit, and the final visit (visit 12). Within-group changes were assessed using paired t-tests, while between-group differences were analyzed with Welch’s t-tests and a prespecified ANCOVA adjusting for baseline SPPB, age, and sex. Adjusted mean differences with 95% CIs and p-values are reported, and a sensitivity analysis using a linear mixed model accounted for repeated measures at visits 6 and 12 with baseline SPPB as a covariate.

Results

The experimental group showed significant improvements in SPPB total score (from 9.42 ± 2.38 to 11.58 ± 0.67, p < 0.001), chair-stand, balance, and gait speed, while the control group showed minimal or negative changes. Hamstring muscle strength increased significantly in the experimental group for both legs, whereas the control group experienced a decline. The experimental group also demonstrated a slight increase in soft lean mass, which is also statistically significant. The experimental group also demonstrated a significant reduction in fear of falling (KFES: −3.00, p = 0.045) and improved quality of life (K-CASP-16: +2.32, p = 0.032), with no significant change in depressive symptoms or body composition. No serious adverse events were reported.

Conclusion

A four-week massage chair-based PNF stretching program significantly improved lower limb strength, physical function, and perceived health in older adults. This intervention offers a safe, accessible, and effective strategy for enhancing musculoskeletal health and reducing fall risk in older adults.

1. Introduction

As global populations rapidly age, enhancing the fundamental physical fitness of older adults has emerged as a critical social and public health priority. Historically, humanity has long aspired toward longevity, and advancements in modern medicine and health technologies have significantly extended life expectancy [1]. However, increased longevity alone does not necessarily guarantee a healthy life; instead, it may contribute to elevated risks of chronic disease, sarcopenia, and physical disabilities [2].

An analysis of health-adjusted life expectancy among Organisation for Economic Cooperation and Development (OECD) countries has revealed notable discrepancies between overall life expectancy and health expectancy, with a reported increase in the prevalence of physical disabilities, particularly during later stages of life [3]. The World Health Organization (WHO) has proposed various guidelines that individuals and societies should adopt to promote healthy aging, prominently highlighting the importance of regular physical activity [4]. Furthermore, sustained physical activity contributes to maintaining or enhancing musculoskeletal function, thereby reducing the risk of falls and effectively preventing or managing chronic diseases [5,6].

In particular, lower limb strength is regarded as a critical indicator of physical health among older adults, as its decline is closely associated with impaired gait, reduced balance, and limitations in physical activities [7]. Additionally, diminished lower limb strength heightens the risk of falls, potentially resulting in decreased quality of life [8]. According to research by van Alphen HJ et al., a reduction in thigh circumference demonstrated a significant correlation with increased dementia risk, suggesting that sarcopenia may not only impair physical functioning but also negatively influence cognitive abilities [9]. Therefore, maintaining and enhancing lower limb strength is essential for preserving health in older age, underscoring the importance of developing suitable exercise interventions and supportive environments specifically tailored for older adults [10,11].

Recently, among various physical activities, proprioceptive neuromuscular facilitation (PNF) stretching has garnered attention as a practical exercise modality for improving essential physical fitness in older adults [12]. PNF stretching is a technique for improving flexibility that combines passive stretching and isometric contractions to enhance static flexibility [13]. Older adults often encounter significant barriers to sustained physical activity, including the absence of exercise partners, lack of safe exercise environments, chronic pain, and previous injury experiences. PNF stretching is considered an effective approach to mitigate these barriers while providing substantial fitness benefits [14]. According to the study by Ferber et al., PNF stretching contributes significantly to improved lower limb strength, balance, and flexibility, effectively reducing the risk of falls among older adults [12]. In particular, the PNF contract-relax stretching technique has been reported to enhance muscle flexibility and joint range of motion, alleviating muscle stiffness and positively impacting balance [15]. Additionally, evidence from various sports disciplines supports the efficacy of PNF stretching in promoting flexibility among elite athletes [16].

Improving musculoskeletal health through physical activity does not apply equally to all older adults. Specifically, some elderly individuals may find engaging in regular physical activity challenging, making it necessary to explore alternative methods to maintain musculoskeletal function. Electric massage chairs have recently emerged as promising healthcare tools alongside traditional exercise programs. Recent studies have highlighted the positive effects of massage chairs on musculoskeletal health. One study compared the effectiveness of massage chairs and traditional physical therapy in alleviating lower back pain. While the traditional physical therapy group showed superior functional improvement, as measured by the Functional Rating Index, massage chairs offered significant advantages in terms of cost-effectiveness and patient satisfaction, as assessed by the McGill Pain Questionnaire.

A study conducted among office workers found that electric massage chairs effectively relieved neck and shoulder pain, enhancing musculoskeletal health. In another investigation involving 15 patients with lower back pain, it was observed that using heated massage chairs more than three times reduced muscle tension in both the upper and lower back, increased muscular elasticity, and significant pain relief. Furthermore, a comparative clinical study that assessed massage chair therapy alongside conventional physical therapy for patients with lower back pain revealed that the outcomes were similar for both methods in terms of pain relief and functional improvement. These findings suggest that massage chairs can serve a purpose beyond mere relaxation, showing considerable potential as therapeutic and preventive tools for musculoskeletal conditions.

This study aims to evaluate the effects of proprioceptive neuromuscular facilitation (PNF) stretching program integrated into a massage chair (BFR-7211) on lower limb strength in older adults as an alternative approach for preventing sarcopenia and enhancing lower extremity muscle function in an aging society. Specifically, the present investigation will assess the impact of massage chair-based PNF stretching on lower limb strength and balance, and perform a comparative analysis with traditional stretching techniques. Furthermore, this study seeks to determine whether using a massage chair represents a practical and efficient method for improving musculoskeletal function among older adults. Ultimately, the findings of this study will provide foundational data for proposing effective strategies to maintain and enhance physical function in older adult communities and explore the future potential of massage chairs as therapeutic and rehabilitative medical devices.

2. Methods

2.1. Study design

The present study was designed as a randomized controlled trial with a 4-week follow-up, utilizing a 1:1 allocation ratio between the experimental and control groups to assess the efficacy of an electric massage chair (Model BFR-7211, Bodyfriend Co., Ltd.) [17]. A blinded trial was not feasible due to the nature of the interventions. This study adhered to the principles outlined in the Declaration of Helsinki and employed the relevant standard items for clinical trials by the CONSORT statement for transparent reporting [18]. Additionally, the study received approval from the institutional review board (IRB) at Hwasung Medi-Science University (HSMUIRB-2024-01) and was registered with the Clinical Research Information Service (CRIS) of South Korea (KCT0009678, registered on August 5, 2024).

2.2. Participants

The participants in this study were recruited from the Hwaseong City Seobu Senior Welfare Center. Initially, an assessment for eligibility was conducted after signing an informed consent form. Inclusion in this study required participants to be 65 or older, to have no disabilities that hinder the ability to perform daily activities, and to be able to communicate effectively. The exclusion criteria were as follows: (1) individuals diagnosed with dementia, cognitive impairment, epilepsy, or similar conditions; (2) those who have difficulty walking due to degenerative spinal disease or joint disease; (3) individuals with lower extremity disabilities resulting from brain disorders, spinal cord diseases, or neuromuscular diseases; (4) participants currently taking anticoagulants for thrombosis or embolism or those with conditions that predispose to bleeding; (5) individuals with a history of mental illnesses such as schizophrenia, severe depression, or anxiety disorders; (6) participants suffering from serious neurological or medical conditions, such as cardiovascular disease; (7) those with severe hearing or vision impairments; (8) participants receiving treatment for severe osteoporosis or malignant diseases; (9) individuals with severe skin diseases or those using implantable medical devices, such as pacemakers; (10) participants being treated for acute intervertebral disc herniation, acute spinal sprains or fractures, or traumatic spinal injuries; (11) any other cases deemed inappropriate for participation by the research team; (12) individuals who have participated in another clinical study within the last three months.

Participants meeting the inclusion criteria were randomly assigned to either the experimental or control groups using a block randomization method. Withdrawal from the study was permitted at any time. Out of the 73 participants screened, seven did not meet the eligibility criteria. Consequently, 66 participants were randomly assigned to the experimental group (n = 33) and the control group (n = 33). During the trial, two participants from the experimental group dropped out—one at visit seven due to medication for rheumatoid arthritis and another at visit nine due to an ankle sprain from jogging. Additionally, two participants from the control group dropped out due to hospitalization related to pneumonia. Therefore, 62 participants completed the trial (Fig. 1).

Fig. 1.

Consort flow diagram of the study.

2.3. Intervention

The experimental group received mechanical massage therapy using the PNF stretching mode (XM module, level 2) of the massage chair, administered for approximately 20 min [17]. The PNF stretching protocol consisted of six phases targeting the hip and knee. Participants began by performing downward presses and upward lifts with the knee extended, each held for 10 seconds across three sets; this was followed by hip flexion and extension movements, also held for 10 seconds in three sets. The session ended with a prolonged downward press and hip flexion; each held for 15 seconds in a single set.

The control group followed a standard trunk exercise program that included trunk curl-ups, diagonal trunk curls, and single-legged extensions, performing these exercises for about 30 min [[19], [20], [21], [22]], three times a week, over the course of a month. Each exercise consisted of 3 sets of 10 repetitions, with a 30-second rest between repetitions and 60 seconds of rest between sets. If any participants were unable to attend a session on the scheduled date due to unavoidable circumstances, the reason was recorded, and the visit was rescheduled within ± 3 days of the original date. The intervention was conducted three times a week for four weeks, totaling 12 visits. All participants were assessed for primary and secondary outcomes on the 1st, 6th, and 12th visits, with a follow-up call conducted within 7 days of the completion of the final visit.

2.4. Outcomes

2.4.1. Primary outcome

The primary outcome was evaluated using the Short Physical Performance Battery (SPPB), a validated and objective tool for assessing lower extremity physical function and predicting adverse health outcomes such as falls, disability, and mortality [[23], [24], [25]]. Participants completed three tasks: (1) balance tests (side-by-side, semi-tandem, and full-tandem stances held for up to 10 seconds each); (2) gait speed (measured by a 4-meter walk at a usual pace); and (3) repeated chair stands (five unassisted rises from a seated position). Each component received a score from 0 to 4, resulting in a total score range of 0 to 12, where higher scores indicate better function. To measure the SPPB test results, a toolkit consisting of sensors and software (Dyphi, Daejeon, Korea) was used, as validated and reported in previous studies [26,27].

2.4.2. Secondary outcomes

Hamstring isometric strength was measured using the ActivForce Digital Dynamometer (ActivForce LLC, San Diego, CA, USA), a validated portable device known for its high intra- and inter-tester reliability (ICC > 0.85) in lower extremity assessments [28,29]. Participants lay prone on an examination table with their hips at 0 ° and knees flexed to 90 °. The dynamometer was secured against the posterior ankle using an immobilization belt. Each participant was instructed to perform a maximal voluntary isometric contraction of the hamstring muscle group by pressing their ankle against the resistance of the dynamometer for approximately 3–5 seconds. This procedure was repeated for a total of three trials per leg, with sufficient rest between trials to prevent fatigue. The peak force value (in kilograms) achieved across the three trials was identified as the highest force and was used for further analysis. Body mass index (BMI), body fat mass (BFM), lean body mass (LBM), skeletal muscle mass (SMM), and soft lean mass (SLM) were assessed using the InBody770 (InBody Co., Ltd., Seoul, South Korea), a multi-frequency bioelectrical impedance analysis (MF-BIA) device [30]. During measurement, participants stood barefoot on the foot electrodes and gripped the hand electrodes with palms and thumbs aligned. This 60-second test provided whole-body and segmental body composition metrics, including BMI (kg/m²), BFM (kg), LBM (kg), SMM (kg), and SLM (kg). Perceived health assessments were conducted using the Geriatric Depression Scale-Korean Version (GDS-K), the Korean Falls Efficacy Scale (KFES), and the Korean version of the Quality of Life in Old Age (K-CASP-16). The 15-item GDS-K, validated for screening major depression in older adults in Korea (Cronbach’s α = 0.88), was administered [31]. Scores of eight and higher indicate clinically significant depressive symptoms. The 16-item KFES evaluates the fear of falling during daily activities, such as bathing and shopping, on a 1–4 Likert scale [32]. Total scores range from 16 to 64, with higher scores indicating greater fear. The K-CASP-16 scale assesses quality of life across four domains: control, autonomy, pleasure, and self-realization [33]. Scores range from 0 to 48, with higher values indicating better well-being. Lastly, all adverse events were recorded twice: immediately after the intervention and again two days later.

2.5. Sample size

The sample size was determined a priori using G*Power 3.1.9.7 software. Based on previous RCTs investigating PNF stretching interventions in older adults, a moderate-to-large effect size (Cohen’s d = 0.65) was anticipated for lower limb muscle strength improvements [34,35]. For an independent t-test with a two-tailed α of 0.05, power (1-β) of 0.80, and an allocation ratio of 1:1, the calculation yielded a minimum requirement of 52 participants (26 per group). To account for a 15% attrition rate, which is common for a geriatric intervention study [36], a total of 73 participants were recruited for this study. Final analyses included 62 participants (31 per group), exceeding the minimum required sample size while maintaining adequate statistical power (post-hoc power = 0.83). This approach aligns with methodological recommendations for geriatric trials and ensures robustness against potential confounding variables such as inter-individual variability in age-related muscle responsiveness [36,37].

2.6. Statistical analysis

Descriptive statistics were calculated and presented as means with standard deviations. Within-group change from baseline (visit 1) to visit 12 was performed with paired-samples t-tests. Between-group differences were evaluated on individual change scores (visit 12 minus baseline) using Welch’s two-sample t-tests. An intention-to-treat primary analysis was performed using prespecified analysis of covariance (ANCOVA), with the post-intervention SPPB total score as the dependent variable. The treatment group was treated as a fixed factor, and baseline SPPB scores were included as a covariate, along with age and sex, which were specified a priori. Adjusted mean differences (AMD) are reported along with 95% confidence intervals (CI) and two-sided p-values. For sensitivity analysis, a linear mixed model (LMM) was applied to data collected at both visit 6 and 12. This model included fixed effects for the treatment group, visit timing, and the interaction between these factors, as well as a random intercept for each participant. Baseline SPPB scores were used as a covariate to model changes from baseline. All statistical analyses were performed using R Studio (version R4.4.2).

3. Results

3.1. Characteristics of the participants

A total of 62 participants successfully completed the trial, which was evenly randomized into two groups: the control group (n = 31) and the experimental group (n = 31). Baseline characteristics indicated differences in age and sex distribution; the experimental group was older, with an average age of 75.74 ± 4.29 years compared to 71.03 ± 4.18 years in the control group. Additionally, the intervention group had a higher percentage of males, comprising 52.84% compared to 29.03% in the control group. Body composition was similar between the groups; however, the experimental group showed a slight increase in height, weight, BFM, LBM, SMM, and SLM. There was a notable difference in perceived health status: 80.65% of participants in the control group reported their health as good or very good, while only 29.03% in the experimental group rated their health positively, with 58.06% reporting their health as moderate and 12.90% rating it as bad. Baseline measures of functional performance and strength were also lower in the experimental group, with the mean SPPB total score at baseline being 9.42 ± 2.38 for the experimental group compared to 11.16 ± 9.42 for the control group. Furthermore, hamstring strength in both legs was significantly lower in the experimental group. Detailed baseline characteristics for each group are outlined in Table 1.

Table 1.

Baseline characteristics of the study participants.

Characteristic	Control (n = 31)	experimental (n = 31)	Total (n = 62)
Age in years, mean (SD)	71.03 (4.18)	75.74 (4.29)	73.39 (4.82)
Sex, n (%)
Male	9 (29.03)	17 (52.84)	26 (41.94)
Female	22 (70.97)	14 (45.16)	36 (58.06)
Height, cm	158.61 (6.31)	161.06 (6.99)	159.8 (6.72)
Weight, kg	63.30 (8.31)	64.56 (10.30)	63.93 (9.29)
BMI, mean (SD)	25.14 (2.85)	24.85 (3.38)	24.99 (3.10)
Health status, n (%)
Very good	4 (12.90)	3 (9.68)	7 (11.29)
Good	25 (80.65)	6 (19.35)	31 (50.00)
Moderate	2 (6.45)	18 (58.06)	20 (32.26)
Bad	0 (0)	4 (12.90)	4 (6.45)
Very bad	–	–	–
Measurements, mean (SD)
BFM	16.17 (4.27)	16.37 (5.20)	16.27 (4.72)
LBM	47.13 (4.59)	48.18 (5.55)	47.65 (5.08)
SMM	23.74 (2.51)	24.73 (2.78)	24.24 (2.70)
SLM	41.12 (4.35)	42.84 (4.82)	41.98 (4.67)
Hamstring strength, mean (SD)
Right, kg	4.33 (1.24)	3.01 (0.93)	3.67 (1.27)
Left, kg	4.16 (1.19)	3.12 (0.98)	3.64 (1.20)
SPPB, mean (SD)
Walk (0−4)	3.68 (0.83)	3.45 (0.85)	3.56 (0.84)
Sit to stand (0−4)	3.87 (0.87)	2.94 (1.15)	3.40 (0.98)
Balance (0−4)	3.61 (0.76)	3.03 (1.14)	3.32 (1.00)
Total score (0−12)	11.16 (1.66)	9.42 (2.38)	10.29 (2.22)

3.2. Effects of the intervention on primary outcome

In the primary outcome of functional strength, measured by SPPB, the experimental group showed significant improvement over the 4-week period, which included 12 visits, while the control group exhibited minimal changes (Table 2). The SPPB total score for the experimental group increased from 9.42 ± 2.38 at baseline to 11.58 ± 0.67 by the 12th visit (p < 0.001), representing a mean improvement of +2.16 ± 1.86 points, whereas the control group’s score remained essentially unchanged. A between-group comparison confirmed that the experimental group significantly increased the SPPB total score (p < 0.001). An examination of the SPPB sub-components revealed similar trends, with the experimental group improving in the chair-stand (sit-to-stand) component from 2.94 ± 1.15 to 3.94 ± 0.25 (p < 0.001) and in the balance component from 3.03 ± 1.14 to 3.77 ± 0.50 (p < 0.001); in contrast, the control group showed a slight decline in the chair-stand score, decreasing from 3.87 ± 0.87 to 3.74 ± 0.68 (p = 0.043), with no significant change observed in balance. Moreover, the experimental group demonstrated improvement in gait speed (walking), with scores increasing from 3.45 ± 0.85 to 3.87 ± 0.34 (p = 0.010), while the control group had stable gait scores, moving only slightly from 3.68 ± 0.83 to 3.84 ± 0.52 (p = 0.362). Differences in change scores between the groups were statistically significant for the chair-stand and balance sub-components (p < 0.001 for both), indicating that the experimental group achieved significantly greater functional gains. However, the difference in the walking sub-score was insignificant (p = 0.266).

Table 2.

Changes in the outcomes during the 12-Visit (4 weeks) intervention period.

	Within-Group Differences						Between-Group Differences
	Control (n = 31)			Experimental (n = 31)			Control (n = 31)	Experimental (n = 31)
	Baseline	12th Visit	p-value	Baseline	12th Visit	p-value	Difference (95% CI)	Difference (95% CI)	Between groups (95% CI)	p-value
SPPB, mean (SD)
Walk (0−4)	3.68 (0.83)	3.84 (0.52)	0.362	3.45 (0.85)	3.87 (0.34)	0.010	0.16 (−0.19 to 0.52)	0.42 (0.11 to 0.73)	0.26 (−0.20 to 0.72)	0.269
Sit to stand (0−4)	3.87 (0.43)	3.74 (0.68)	0.043	2.94 (1.15)	3.94 (0.25)	<.001	−0.13 (−0.25 to −0.00)	1.00 (0.60–1.40)	1.13 (0.71–1.55)	<.001
Balance (0−4)	3.61 (0.76)	3.48 (0.68)	0.354	3.03 (1.14)	3.77 (0.50)	<.001	−0.13 (−0.41 to 0.15)	0.74 (0.39–1.10)	0.87 (0.43–1.31)	<.001
Total score (0−12)	11.16 (1.68)	11.06 (1.26)	0.703	9.42 (2.38)	11.58 (0.67)	<.001	−0.10 (−0.61 to 0.42)	2.16 (1.48–2.84)	2.26 (1.42–3.10)	<.001
Hamstring strength, mean (SD)
Right, kg	4.33 (1.24)	4.00 (0.96)	0.001	3.01 (0.93)	3.97 (0.83)	<.001	−0.33 (−0.50 to −0.15)	0.95 (0.82–1.09)	1.28 (1.06–1.50)	<.001
Left, kg	4.16 (1.19)	3.72 (0.75)	0.007	3.12 (0.98)	3.85 (1.03)	<.001	−0.44 (−0.76 to −0.13)	0.73 (0.62 to 0.84)	1.17 (0.84–1.50)	<.001
Measurements, mean (SD)
BFM	16.17 (4.27)	16.19 (4.42)	0.854	16.37 (5.20)	16.45 (5.27)	0.481	0.02 (−0.19 to 0.23)	0.08 (−0.14 to 0.30)	0.06 (−0.24 to 0.35)	0.699
LBM	47.13 (4.59)	47.11 (4.80)	0.868	48.18 (5.55)	48.23 (5.54)	0.486	−0.01 (−0.17 to 0.14)	0.05 (−0.09 to 0.19)	0.06 (−0.14 to 0.27)	0.548
SMM	23.42 (2.13)	23.50 (2.20)	0.118	23.46 (2.22)	23.64 (2.12)	0.069	0.08 (−0.02 to 0.18)	0.18 (−0.02 to 0.38)	0.10 (−0.12 to 0.32)	0.355
SLM	40.79 (3.71)	40.94 (3.71)	0.006	42.22 (4.45)	42.56 (4.41)	<.001	0.14 (0.05 to 0.24)	0.34 (0.24 to 0.44)	0.20 (0.06 to 0.33)	0.005
GDS, mean (SD)	6.06 (4.97)	5.74 (4.56)	0.693	12.97 (8.84)	6.32 (5.26)	<.001	−0.32 (−1.96 to 1.32)	−1.39 (−3.13 to 0.35)	−1.07 (−3.43 to 1.29)	<.001
KFES, mean (SD)	17.87 (2.99)	18.65 (4.25)	0.21	21.55 (7.74)	20.32 (6.11)	0.318	−0.68 (−2.11 to 0.75)	−3.00 (−4.81 to −1.19)	−2.32 (−4.59 to −0.05)	0.146
K-CASP-16, mean (SD)	28.84 (4.96)	31.97 (4.49)	<.001	35.81 (2.63)	45.32 (3.87)	<.001	1.45 (0.64 to 2.26)	2.32 (0.72 to 3.92)	0.87 (−0.87 to 2.61)	<.001

After adjusting for baseline SPPB scores, age, and sex, the intervention group showed a significantly higher post-intervention SPPB score compared to the control group, with an AMD of 0.97 points (95% CI: 0.41–1.54; p = 0.001) (Supplementary Table S1). The results from the sensitivity LMM analysis revealed a significant group-by-visit interaction (p < 0.001). At the 12-visit mark, the difference in SPPB change from baseline favored the experimental group by 1.95 points (95% CI: 0.97–2.93; p < 0.001), which is consistent with the findings from the ANCOVA analysis.

The gender-specific ANCOVA, adjusted for baseline SPPB scores, demonstrated a significantly greater improvement in SPPB total scores among participants in the experimental group compared to those in the control group for both males and females (main effect of group: p = 0.005). Among female participants in the experimental group, the adjusted mean change in SPPB total score was 2.36 ± 1.86 (95% CI: 1.38–3.33), whereas male participants in the same group showed an adjusted mean change of 2.00 ± 1.90 (95% CI: 1.09–2.91). In the control group, the adjusted mean change was –0.18 ± 1.50 (95% CI: −0.81–0.45) for females and 0.11 ± 1.17 (95% CI: –0.65–0.87) for males. Although a numerically greater improvement was observed in females, the interaction between group and gender did not reach statistical significance (p = 0.949), indicating that the intervention effect did not differ significantly between male and female participants after adjusting for baseline SPPB.

3.3. Effects of the intervention on secondary outcomes

Several secondary outcome measures were assessed in the study. The experimental group demonstrated substantial improvements in lower-limb muscle strength compared to the control group, with maximal hamstring strength in the right leg increasing from 3.01 ± 0.93 kg to 3.97 ± 0.83 kg (p < 0.001), while strength in the control group decreased from 4.33 ± 1.24 kg to 4.00 ± 0.96 kg during the same period (p < 0.001 for the within-group decline). Similarly, left-leg hamstring strength increased from 3.12 kg ± 0.98 kg to 3.85 kg ± 1.03 kg in the experimental group (p < 0.001), while it fell from 4.16 ± 1.19 kg to 3.72 ± 0.75 kg in the control group (p = 0.007). The gains in muscle strength in the experimental group were greater than the changes observed in the control group for both legs.

Regarding psychosocial and functional health measures, the intervention did not significantly affect depressive symptoms but improved fear of falling. GDS scores showed no significant change within either group; the experimental group changing from 7.71 ± 2.62 to 6.32 ± 5.26 (p = 0.113), and control group changing from 6.06 ± 4.97 to 5.74 ± 4.56 (p = 0.693) with no significant between-group difference in GDS changes (p = 0.368). The KFES score improved markedly in the experimental group, decreasing from 21.55 ± 7.74 at baseline to 18.55 ± 4.90 post-intervention (p = 0.002), indicating reduced fear of falling, whereas the control group's KFES score changed only from 18.81 ± 2.57 to 18.13 ± 4.15 (p = 0.338), with a statistically significant difference between groups (p = 0.045). Finally, both groups showed an increase in quality of life as measured by the K-CASP-16, with the experimental group's score rising from 31.94 ± 4.76 to 34.26 ± 3.74 (p = 0.006) and the control group's score increasing from 28.84 ± 4.96 to 30.29 ± 4.88 (p < 0.001); however, there was no significant difference between the groups in terms of improvement (p = 0.320).

In addition, changes in muscle mass were also observed. In the experimental group, SMM increased from 23.46 ± 2.22 kg to 23.64 ± 2.12 kg (p = 0.069). In contrast, the control group showed only a minimal change, with SMM shifting from 23.42 ± 2.13 kg to 23.50 ± 2.20 kg (p = 0.118). The difference in change between the two groups was not statistically significant (p = 0.355). Regarding SLM, the experimental group improved from 42.22 ± 4.45 kg to 42.56 ± 4.41 kg (p < 0.001). The control group also experienced a smaller increase, from 40.79 ± 3.71 kg to 40.94 ± 3.71 kg (p = 0.006). Once again, the difference in changes between the groups favored the experimental group (p = 0.005). Moreover, neither group experienced significant changes in body composition, as BFM remained essentially constant from baseline to post-intervention, and LBM showed minimal change with no significant differences between the groups.

3.4. Adverse effects

No serious adverse events were observed in either group during the trial. A few participants in the experimental group reported minor and temporary side effects, such as limb discomfort or mild bruising at the cuff site. However, these issues were resolved independently and did not result in dropouts. These minor side effects were not significantly different between the experimental and control groups.

4. Discussion

The present study demonstrates that a massage chair exercise program integrating PNF stretching can significantly enhance older adults' lower-extremity functional performance and muscle strength. Participants experienced substantial improvements in both leg strength and mobility following the intervention. To the best of our knowledge, this is the first study to use an automated massage chair designed to deliver PNF stretching to address age-related functional decline. The positive results validated the effectiveness of this innovative exercise approach and set it apart from previous interventions. Unlike traditional PNF routines, which typically require a trained therapist or partner to apply resistance and assist with stretches, this chair allows individuals to perform PNF movements independently and safely. This unique design has the potential to lower barriers to exercise for populations with limited access to supervised training, thereby broadening the availability of effective interventions for sarcopenia.

This study evaluated changes in physical function among older adults using the SPPB. The experimental group demonstrated improvements across all components of the SPPB, resulting in a statistically significant increase of 2.16 points (p < 0.001) in total scores, which exceeds the minimal clinically important difference and surpasses the improvements commonly seen with other interventions. For example, structured exercise and multimodal programs in similar populations typically resulted in gains of 0.5–1.5 points over 8–12 weeks [38]. Notably, the sit-to-stand test revealed that the experimental group improved by 1.00 point (p = 0.001), while the control group experienced a decline of 0.13 points (p = 0.043), indicating a significant difference between the two groups. These findings align with previous research; Silva et al. reported that PNF training significantly increased lower limb muscle strength in older women, with an average improvement of 31% [39]. Additionally, Kumari et al. demonstrated that pelvic PNF significantly enhanced balance and strength compared to lower-limb strengthening exercises alone, particularly in the Functional Timed Up and Go Test (FTSTS) and the Performance-Oriented Mobility Assessment (POMA) [40]. Overall, these results reinforce the effectiveness of PNF-based interventions for improving physical function in older adults, as evidenced by the substantial improvements observed in the experimental group.

In the balance assessment, the experimental group demonstrated a statistically significant improvement of 0.74 points (p < 0.001). This suggests that the PNF stretching-integrated massage chair may enhance not only lower limb strength but also balance and body stability. Supporting this finding, Sefton et al. reported significant improvements in balance among older adults following a 6-week therapeutic massage intervention, which resulted in enhanced stability and reductions in both displacement area and velocity [41]. Similarly, Soni and Parikh showed that PNF stretching improved neuromuscular control and trunk stability, strengthening balance abilities [42].

A gender-specific analysis showed numerically greater improvements in SPPB scores among female participants compared to males in the experimental group. However, after adjusting for baseline differences, the interaction effect between gender and intervention was not statistically significant (p = 0.949), indicating that the intervention’s effectiveness did not differ meaningfully between male and female participants. This suggests that the PNF stretching-integrated massage chair can be considered broadly effective regardless of gender. While prior studies have emphasized the importance of tailored interventions due to the higher prevalence of sarcopenia among older women [43,44], the present findings do not support a gender-specific effect. Future studies with larger, gender-balanced samples may be necessary to determine if more subtle differences in response can be detected with increased statistical power.

A comprehensive analysis of lower limb muscle strength outcomes revealed that the experimental group using the PNF stretching-integrated massage chair showed more significant improvements than the control group. The experimental group increased hamstring strength by approximately 32% (right leg: +0.95 ± 0.36 kg; left leg: +0.73 ± 0.30 kg). Comparable resistance training studies in older adults report absolute gains of 0.5–1.5 kg in knee flexor/extensor strength over 8–12 weeks [45]. Thus, the gains observed in this study are both clinically meaningful and favorable compared to prior interventions. Further, improvements of 10–20% in lower limb muscle strength are typically considered meaningful in the context of aging and fall prevention [46].

Although exercise programs for older adults often result in only modest increases in muscle mass, typically ranging from 0.2 to 1.0 kg over several months, these small gains are clinically significant, considering the natural decline in muscle mass that occurs with aging [47]. In this study, both SMM and SLM showed slight increases in the experimental group, indicating a potential protective effect against sarcopenia. While these changes may seem small, regular, customized exercise programs and supportive devices are crucial for preventing sarcopenia and improving functional mobility [48].

In assessing the efficacy of fall prevention using the KFES, the experimental group demonstrated a statistically significant decrease of 3.00 points (p = 0.045). According to Delbaere et al., a reduction of 2–3 points on the KFES is considered clinically meaningful, as it indicates improved confidence and a decreased fear of falling, which is associated with a lower risk of falls [49]. In comparison, the control group experienced a non-significant reduction of 0.68 points (p = 0.338). These findings suggest that using a massage chair effectively reduces the fear of falling in older adults. Hawkins et al. reported that an increased fear of falling is associated with a higher risk of falling, primarily due to decreased physical function and gait instability [50]. Similarly, Elmaghfuroh et al. emphasized that reduced walking stability and balance in older populations are closely linked to lower-limb muscle weakness and poor coordination, significantly increasing the risk of falls [51]. That said, massage chair intervention is also beneficial for strengthening muscles, relieving pain, and potentially lowering fall risk among older adults.

In the present study, the quality of life was assessed using the K-CASP-16, revealing a significant improvement in the experimental group with an increase of 2.32 points (p = 0.032), while the control group showed a non-significant upward trend with an increase of 1.45 points (p = 0.195). These findings suggest a strong link between enhanced physical function and improved quality of life in older adults. According to Andari et al., personalized and regular physical exercise that improves mobility and balance, especially when combined with health education and routine health assessments, significantly enhances physical, psychological, and social well-being, thus improving overall quality of life [52]. Similarly, another study noted that customized physical rehabilitation programs incorporating aerobics, strength training, flexibility, and balance exercises can boost vitality and overall quality of life among elderly populations [53]. Moreover, Vancea et al. demonstrated that intensive rehabilitation significantly increased functional independence, with Functional Independence Measure (FIM) scores improving by an average of 1.436 times, underscoring the positive impact of comprehensive physical rehabilitation on the quality of life for older adults [54]. The findings of this study further highlight the potential of the PNF-integrated massage chair to effectively improve physical functionality and enhance the quality of life among elderly individuals.

In addition, the depression scores decreased by 1.39 points (p = 0.305) in the experimental group and by 0.32 points (p = 0.693) in the control group. Both groups exhibited reductions in depressive symptoms, but the experimental group showed a greater tendency toward improvement. A systematic review by Blake et al. confirmed that even modest improvements in depressive symptoms, specifically 1–2 points on the GDS or similar measures, are clinically significant for older adults [55]. These findings suggest that PNF stretching may positively impact mental well-being and enhance physical function. According to Ha et al., patients with post-mastectomy lymphedema who engaged in a pain-free range PNF exercise program experienced significant reductions in depressive and anxiety symptoms starting from the fourth week of the intervention [56]. Also, Nagare et al. reported that low-intensity stretching alleviated depressive symptoms in middle-aged women [57], indicating that PNF-based interventions may promote mental health by improving both musculoskeletal function and physical recovery.

Despite its promising results, this study has several limitations. First, the sample size was modest. A larger sample would provide more robust data. Second, the intervention duration was relatively short. While improvements were observed after the intervention, it remains unclear whether these gains are sustainable or if a longer training period would lead to additional benefits. Future studies should include an extended duration to assess the durability of functional improvements and to investigate whether continuous use of the PNF massage chair can prevent the progression from pre-sarcopenia to overt sarcopenia. Third, there may be bias in the participant composition regarding age and gender, suggesting that future research should involve a more diverse range of ages and include both genders. Lastly, differences in baseline functioning may have led to greater responsiveness in the experimental group. This phenomenon is well-documented in several studies involving older adults, which show that those with poorer baseline functions tend to experience the most significant improvements after participating in physical activity or rehabilitation programs [58,59]. Future studies should consider adjusting for covariates or using stratified randomization to account for baseline frailty, thereby ensuring more balanced comparisons between groups. Nonetheless, this study is significant as it explores the potential of integrating PNF techniques into massage devices as a novel therapeutic approach, demonstrating preliminary effectiveness in improving lower limb muscle strength and physical flexibility.

With all that mentioned, PNF stretching utilizes a unique mechanism that differs from traditional static or dynamic stretching, promoting neuromuscular control and enhancing muscle activation through repeated muscle relaxation and contraction cycles [60]. This physiological approach allows PNF stretching programs to improve lower limb muscle strength and, balance, and functional mobility [61,62]. Additionally, PNF stretching activates neuromuscular sensory receptors such as muscle spindles and Golgi tendon organs, helping to regulate muscle tone, increase muscle flexibility, and enhance coordination and proprioception [63,64]. These features suggest that massage chairs equipped with PNF functionality may go beyond merely relieving fatigue and could serve as therapeutic devices for neuromuscular enhancement and muscle strengthening. Hence, this study represents the first systematic evaluation of the clinical effectiveness of a PNF stretching-integrated massage chair, highlighting its potential as an effective intervention strategy for improving physical function in older adults.

5. Conclusion

This randomized controlled trial demonstrates that a 4-week (12 visits) PNF stretching program delivered through a massage chair significantly improves lower limb strength, balance, and functional mobility in older adults living in the community. The experimental group showed meaningful enhancements in the SPPB, hamstring strength, and a reduced fear of falling, performing better than the control group. These findings emphasize the potential of mechanized PNF stretching as a safe, accessible, and time-efficient intervention to combat sarcopenia-related functional decline and fall risk in aging populations. Integrating PNF principles into a massage chair addresses key barriers to exercise adherence among older adults, such as lack of supervision, physical limitations, and safety concerns. Future research should investigate the long-term sustainability of these benefits and explore broader applications in clinical settings. Ultimately, this study points to a promising avenue in geriatric rehabilitation and the promotion of musculoskeletal health, where technology-assisted interventions may increasingly play a vital role in preventing sarcopenia and functional decline in aging societies.

Statement of ethics

The ethical approval was granted by the local IRB at the Hwasung Medi-Science University (HSMUIRB-2024-01). Consent to Participate Statement. The informed written consent was obtained from all respondents to participate in the study. The research was conducted ethically in accordance with the World Medical Association Declaration of Helsinki.

Declaration of Generative AI and AI-assisted technologies in the writing process

I have not used any AI-assisted technologies at all.

Funding

This study was funded by Bodyfriend Co., Ltd. (BF-CRW-24-03).

Data availability statement

Data will be made available on request to the corresponding author.

Declaration of competing interest

The authors have no conflict of interest to declare.

Appendix A. Supplementary data

The following is Supplementary data to this article: