Low-Intensity Blood Flow-Restricted Multi-Joint Exercise Improves Muscle Function in Patients With Patellofemoral Pain Syndrome: A Randomized Trial

Abstract

Background

Patellofemoral pain syndrome (PFPS) limits physical activity and quality of life, especially during weight-bearing tasks. Although high-load resistance exercises are recommended for rehabilitation, they may worsen symptoms in pain-sensitive individuals. Low-intensity blood flow restriction (BFR) training has emerged as a potential alternative. However, its effects on functional performance and mechanical properties remain unclear.

Material/Methods

In this assessor-blinded, randomized controlled trial, 41 individuals with PFPS were randomly assigned to either the experimental group (EG, n=20) or the control group (CG, n=19). The EG performed multi-joint resistance exercises combined with BFR, while the CG performed the same program without BFR. Both groups completed the same multi-joint resistance exercise program twice weekly for 6 weeks. Outcome measures included pressure pain threshold (PPT), muscle mechanical properties such as tone and stiffness in the vastus medialis and vastus lateralis, isometric knee extensor strength, and balance ability. Balance was evaluated using the Y-Balance Test and the stair-descending task.

Results

The EG showed significantly greater improvements in knee strength, PPT, and balance (P<0.05). Notably, significant increases in muscle tone were observed in the vastus medialis and lateralis muscles, as well as muscle stiffness in the vastus medialis and semitendinosus muscles.

Conclusions

Low-intensity BFR multi-joint resistance exercise may be an effective intervention for improving physical function, pain, and mechanical properties in patients with PFPS.

Introduction

Patellofemoral pain syndrome (PFPS) is a common musculoskeletal disorder characterized by pain in and around the anterior aspect of the knee. PFPS has a high prevalence rate of approximately 10–28% in women and physically active individuals [1–3]. Pain typically worsens during squatting, climbing or descending stairs, prolonged sitting, or kneeling. Various contributing factors to PFPS include quadriceps weakness, impaired hip muscle function, lower-limb malalignment, and neuromuscular control deficits [4,5]. PFPS consequently restricts sports, daily life, and occupational activities, leading to reduced quality of life and contributing to socioeconomic burdens through increased healthcare costs and decreased productivity [6,7].

Conservative treatments for PFPS typically include strengthening the quadriceps and hip muscles, stretching, and taping, which improves patellar alignment and relieves pain [7–11]. However, these exercises generally require moderate intensity – over 60% of the patient’s 1-repetition maximum or a rating of ≥13 on the Borg perceived exertion scale – to induce muscle hypertrophy and neuromuscular adaptation [12]. This recommendation is consistent with the American College of Sports Medicine (ACSM) and clinical practice guidelines for PFPS [6,13,14]. However, such intensity may be difficult to tolerate for patients with PFPS with high pain sensitivity or joint load intolerance, potentially leading to discomfort and reduced adherence [15,16].

Consequently, interest in blood flow restriction (BFR) training, a low-load intervention that aims to improve strength while minimizing joint stress, has been increasing. BFR restricts venous return by using a pneumatic cuff during low-load exercise, which stimulates metabolic stress and mechanical tension, ultimately leading to muscle hypertrophy and strength gain [17–21]. This method is a promising alternative for patients with PFPS and has shown positive results in prior studies on acute postoperative ACL reconstruction and PFPS [22–24]. Recent randomized controlled trials further reported that intermittent BFR combined with low-load resistance training produced pain and functional improvements in PFPS patients comparable to those achieved with high-load resistance exercise, suggesting that BFR may serve as a practical alternative for patients unable to tolerate traditional high loads [20].

Multi-joint exercises involving hip abductors and external rotators are more effective than isolated quadriceps exercises for alleviating PFPS symptoms [25–27]. In particular, gluteus medius and maximus weakness contribute to impaired lower-limb alignment and trunk stability [25,26], which can increase patellofemoral joint compression forces and prolong symptoms [28,29]. Therefore, rather than focusing solely on isolated joint exercises, BFR interventions must adopt a multi-joint approach that enhances lower-limb coordination.

Furthermore, recent studies have shifted their attention from merely increasing strength to understanding how mechanical muscle properties, such as tone and stiffness, affect functional recovery and pain modulation. These properties are closely related to joint stability, shock absorption, and postural control and are particularly relevant to the pathophysiology of PFPS, which often involves malalignment and repetitive mechanical irritation [30,31]. However, most existing PFPS exercise interventions have primarily focused on quantitative metrics such as muscle strength and hypertrophy, and studies investigating mechanical muscle properties are limited [32,33]. Moreover, previous BFR research has largely concentrated on quadricep-targeted protocols, with limited exploration of multi-joint exercises or integrated analysis of mechanical characteristics, particularly in PFPS populations.

Therefore, this study aimed to evaluate the effects of a multi-joint BFR training protocol involving both the hip and knee joints on strength, balance, PPT, and mechanical muscle properties in patients with PFPS.

Material and Methods

Study Design and Setting

This single-blind, randomized, controlled pre–post trial examined the effects of BFR-based multi-joint exercise in patients with PFPS. Before participation, all participants were provided with a full explanation of the study purpose, procedures, expected benefits, and potential risks, and written informed consent was obtained from each participant.

This study was conducted in accordance with the ethical principles of the Declaration of Helsinki and was approved by the Institutional Review Board (IRB) of Sahmyook University (approval no. SYU 2024-07-039-002). The study was prospectively registered with the Clinical Research Information Service (CRIS), a primary registry of the World Health Organization International Clinical Trials Registry Platform (registration ID: KCT0009780), on September 10, 2024. The first participant was enrolled on September 27, 2024, following the registration.

All adverse events were monitored throughout the intervention period. Any reported adverse events were documented by the study physical therapist, who assessed their severity and relatedness to the intervention. Serious adverse events were promptly reported to the IRB.

Study Participants

A total of 44 participants aged 18–40 years residing in Seoul, South Korea, who exhibited PFPS symptoms were recruited. Participants were eligible if they presented with non-traumatic anterior or peripatellar knee pain lasting for more than 4 weeks with a visual analog scale score of ≥4; experienced pain during at least 2 daily activities, such as stair climbing, squatting, kneeling, or standing after prolonged sitting; and showed at least 1 positive result on physical examination, including the patellar compression test. These diagnostic criteria were based on a comprehensive clinical evaluation that integrated symptom-provoked pain and physical examination findings in accordance with the clinical guidelines of Barton and Crossley [34] and Willy et al [6].

The exclusion criteria were any of the following: history of knee surgery or orthopedic treatment within the past year, traumatic knee injury, arthritis or structural deformities, central nervous system disorders, or other orthopedic or neurological conditions that could interfere with the intervention [35]. A board-certified orthopedic specialist conducted all inclusion and exclusion evaluations. Only participants who were clinically diagnosed with PFPS were included in the study.

The sample size was calculated using G*Power software (version 3.1.9.7; Heinrich Heine University, Düsseldorf, Germany) based on the effect size (Cohen’s d=0.87) [36] which examined the effect of BFR exercise on the peak torque of the quadriceps and hamstring muscles. With an alpha level (α) of 0.05 and statistical power (1-β) of 0.80, a minimum sample size of 36 participants was required. Accounting for an estimated dropout rate of 20%, 44 participants were recruited. Three participants were excluded based on the inclusion/exclusion criteria, and 2 dropped out during the intervention period. Thus, 39 participants were included in the final analysis, with 20 in the experimental group (EG) and 19 in the control group (CG). The EG performed multi-joint exercises with BFR, while the CG performed the same program without BFR (Figure 1).

Figure 1.

Flow chart of patient recruitment during the trial.

Procedures

The resistance band color was selected based on a pretest in which each participant rated their perceived exertion for each band using the Borg Rating of Perceived Exertion scale (6–20 points). The initial exercise intensity was set using a color corresponding to a Borg Rating of Perceived Exertion (RPE) level of 6, defined as very very light, and a level of 8, defined as very light. This resistance level was consistently maintained throughout the 6-week intervention to control potential confounding variables that could otherwise affect the study outcomes.

Participants were randomly assigned to either the EG or the CG. Randomization was conducted by an independent research assistant using a computer-generated algorithm, and simple randomization without restrictions such as blocking or stratification was applied. Allocation concealment was maintained so that participants were not aware of their group assignments in advance.

The research team comprised 4 personnel, including a primary therapist, a primary assessor, an assistant therapist, and an assistant assessor. All participants received the intervention from the same primary therapist. In the EG, the application and adjustment of the BFR cuff required the therapist to be aware of group allocation. In contrast, the primary assessor was blinded to group assignment, was not involved in the intervention, and performed all outcome evaluations independently. In addition, all participants were blinded to their group allocation to minimize the risk of bias during the study.

Both the assistant therapist and assessor completed protocol training in advance to ensure methodological consistency and were prepared to deliver or evaluate the intervention with the same level of proficiency as the primary personnel. They were designated as substitutes in cases of scheduling conflicts or absence of the primary personnel, thereby maintaining continuity and consistency in both the treatment and assessment processes. In addition, to minimize potential performance bias, both the primary and assistant therapists followed the same standardized protocol across groups, providing consistent verbal instructions and maintaining neutral feedback to avoid unnecessary motivational influence.

Intervention

The participants in the EG wore a 10-cm-wide BFR cuff (Hyper Recovery, XMSJ, USA) on the proximal one-third of the thigh. The cuff pressure was set to 80% of the individual’s systolic blood pressure to apply BFR during exercise. The cuff remained inflated between sets but was released during rest periods between exercises. The CG performed the same exercises without restriction of blood flow.

Each exercise consisted of 4 sets: the first set was performed for 30 repetitions, and the second through fourth sets were performed for 15 repetitions. The remaining intervals were standardized to 30 s between sets and 2 min between exercises.

All interventions were directly supervised by a physical therapist with more than 5 years of clinical experience. The therapist continuously monitored and corrected compensatory movements such as pelvic lifting and trunk rotation during the exercise. During the first session, demonstrations and feedback were provided for all the exercises to ensure proper execution. The detailed intervention protocols are presented in Table 1 and Figures 2–4.

Table 1.

Structure and application of low-intensity multi-joint exercise protocol.

	Details	Reps/sets	Rest time
Knee extension	The participant sat on a treatment table with the hip and knee flexed at 90°. An elastic resistance band was attached to the ankle, and the participant slowly extended the knee while the pelvis remained in contact with the table. After full extension, the leg was slowly returned to the starting position	A total of 4 sets First set, ×30 Second to fourth sets, ×15	Between sets, 30 s. Between exercises, 2 min.
Hip abduction	In the side-lying position, the bottom leg was slightly flexed, and the top leg remained straight. With an elastic band wrapped around the top leg, the participant lifted the leg approximately 30° vertically while maintaining 5° of external rotation. Care was taken to avoid trunk rotation throughout the movement	A total of 4 sets First set, ×30 Second to fourth sets, ×15	Between sets, 30 s. Between exercises, 2 min.
Hip external rotation	From a side-lying position with both the hip and knee joints flexed to approximately 45°, the participant externally rotated the upper knee while keeping the feet together. Special attention was paid to prevent pelvic rotation during the movement	A total of 4 sets First set, ×30 Second to fourth sets, ×15	Between sets, 30 s. Between exercises, 2 min.
Squat	The participant began in a standing position with feet shoulder-width apart. An elastic band was positioned around the distal thighs. The participant slowly squatted by flexing the knees to approximately 30°, pushing the hips backward as if sitting down, and then returning to a standing position. Care was taken to ensure that the knees did not move past the toes	A total of 4 sets First set, ×30 Second to fourth sets, ×15	Between sets, 30 s. Between exercises, 2 min.

Figure 2.

Structure and application of low-intensity multi-joint exercise protocol. (A) BFR cuff. (B) Knee extension. (C) Hip abduction. (D) Hip external rotation. (E) Squat. BFR, blood flow restriction

Figure 3.

Measurement devices used in this study. (A) Isometric dynamometer. (B) PPT. (C) MyotonPRO. PPT, pressure pain threshold.

Figure 4.

The measurement sites for the MyotonPRO and PPT assessments of the lower limb muscles were based on consistent anatomical landmarks. (A) Rectus femoris. (B) Vastus medialis. (C) Vastus lateralis. (D) Semitendinosus. (E) Biceps femoris.

Outcome Measures

All outcome measures were evaluated at baseline and after completion of the 6-week intervention. Subsequently, the specific procedures for each assessment are described below.

Outcome Measures: Muscle Strength

The isometric strength levels of the quadriceps and hamstrings were evaluated using an isometric dynamometer (K-Pull, KINVENT, Montpellier, France), and measurements were collected using dedicated software (K-force Pro, KINVENT). For quadricep strength testing, the participants sat with their hip and knee joints flexed at 90° and their arms crossed in front of the chest. A strap was secured around the lateral malleolus of the affected limb, and the participants were instructed to exert maximal isometric contraction with the knee at 45° extension [37,38]. Hamstring strength testing was conducted after a 2-minute rest, with the participant in a prone position and the arms placed beside the shoulders. A strap was similarly placed around the lateral malleolus, and maximal isometric contraction was performed with the knee at 45° flexion [37,39]. Each muscle was tested thrice with 5-second maximal contractions, and 10-second rests were provided between trials. The average of 3 trials was used for the analysis [38], with results measured in units of kilogram-force (kgf). The inter-rater reliability for this protocol was ICC=0.976, and the intra-rater reliability was ICC=0.958 [40].

Outcome Measures: Balance

Balance was assessed using the Y-Balance Test (YBT) and the stair-descending task, both of which require lower-limb strength and postural stability [41]. For the YBT, the participants stood on the affected leg and reached in anterior, posteromedial, and posterolateral directions. Trials were considered invalid if balance was not maintained. Each direction was tested thrice after 4 practice attempts and a 2-minute rest [42]. The composite score was calculated by summing the maximum reach distances in the 3 directions, dividing by 3 times the limb length, and multiplying by 100 [43]. The inter-rater reliability for each direction was anterior ICC=0.79, posteromedial ICC=0.79, and posterolateral ICC=0.83 [41].

The stair-descending task is a functional test designed to comprehensively evaluate dynamic alignment and neuromuscular control [44]. The test was used to assess the effectiveness of the intervention in improving alignment and motor control. The participants stood with the affected leg on the edge of a step, their hands on their hips, and their contralateral leg raised off the ground. They were instructed to bend the stance knee, lower their body, gently tap the floor with the contralateral heel, and then return to the starting position. This sequence was repeated 5 times [45].

Video recordings were taken from the front and scored using a 6-point scale (0–5) based on the number and type of errors observed, which was developed by Rabin and Kozol [44]. The 1-point errors included hands lifted from the hips, trunk leaning, pelvic misalignment, tibial tuberosity medial to the second toe, and contralateral leg contact or shaking. A 2-point error was assigned if the tibial tuberosity crossed the medial border of the foot. The total score ranged from 0 to 5, with lower scores indicating better alignment and control. The inter-rater reliability of this test was κ=0.74, indicating substantial agreement [44].

Outcome Measures: Pressure Pain Threshold

Pressure pain threshold (PPT) was measured using a digital algometer (FPX25, Wagner Instruments, Riverside, USA, 2024). Measurements were performed with the participant in a supine position for the rectus femoris, vastus medialis, and vastus lateralis muscles and in a prone position for the semitendinosus and biceps femoris muscles.

Anatomical landmarks for PPT assessment were based on protocols described by Bravo-Sánchez et al [46] and Ramazanoğlu et al [32]. The measurement site for the rectus femoris was defined as the midpoint between the anterior superior iliac spine and the inferior border of the patella. The vastus medialis was measured 4 cm superior and 3 cm medial to the inferior border of the patella, with the probe applied at an angle of approximately 55° to the horizontal plane following the muscle fiber orientation. For the vastus lateralis, the site was at the midpoint between the medial border of the patella and the greater trochanter. The semitendinosus was assessed at the midpoint between the ischial tuberosity and medial epicondyle of the femur, whereas the biceps femoris was evaluated at the midpoint between the ischial tuberosity and lateral epicondyle of the femur.

The pressure was applied at a constant rate of 10 N per second. The participants were instructed to press a handheld button at the exact moment when the sensation shifted from pressure to pain, and the pressure readings were recorded. Each site was measured twice using the same protocol with a 1-minute rest interval between trials. The mean values of the 2 trials were used in the final analysis. Higher PPT values indicated lower pressure sensitivity, whereas lower values reflected higher sensitivity to pressure-induced pain. The intra-rater reliability of this assessment was ICC=0.90–0.95, and the inter-rater reliability was ICC=0.70 [47].

Outcome Measures: Mechanical Properties

The mechanical properties of the muscles were assessed using the MyotonPRO device (Myoton AS, Estonia), which measures muscle tone and stiffness. This non-invasive handheld myotonometer quantitatively evaluates the viscoelastic characteristics of soft tissues [48,49]. The device applied a brief mechanical impulse of 0.18 N to the skin surface over the target muscle and recorded the resulting damped oscillations of the tissue.

Muscle tone reflects the intrinsic tension and elasticity of the muscle and is measured by the oscillation speed after mechanical stimulation. Muscle stiffness quantifies the resistance of soft tissue to an external force, thereby indicating tissue hardness [50].

Measurements were conducted using the multi-scan mode (5 consecutive impulses at 1-second intervals). Each site was measured in 3 sets, and the average value was used for the final analysis [51]. All measurements were performed with the participants in a fully relaxed position, and the probe was placed perpendicular to the skin surface over the muscle.

The validity of this device has been supported by comparison with shear wave elastography, showing a moderate to strong correlation (r=0.42–0.67) [9]. Its intra-rater reliability has been reported as ICC=0.79–0.93, and inter-rater reliability as ICC=0.74–0.99 [52] (Figure 3).

Results

General Participant Characteristics

A total of 39 participants were included in the final analysis, 20 and 19 in the EG and CG, respectively. No adverse events were reported during the study period. The EG included 11 men (55.0%) and 9 women (45.0%), whereas the CG included 12 men (63.2%) and 7 women (36.8%).

Measurements were performed based on the limb to which the BFR cuff was applied. In the EG, the left limb was tested in 12 participants (60.0%) and the right limb in 8 participants (40.0%). In the CG, the left limb was tested in 9 participants (47.4%) and the right limb in 10 participants (52.6%).

The mean patient height was 168 cm in both groups. The mean age was 28.05 years in the EG and 27.63 years in the CG. The mean body weight was 66.65 kg and 66.74 kg in the EG and CG, respectively. The mean body mass index was 23.29 kg/m² and 23.34 kg/m² in the EG and CG, respectively (Table 2). Statistical analysis revealed no significant differences in any of the general characteristics between the 2 groups, confirming baseline homogeneity (P>0.05).

Table 2.

General characteristics of the participants.

	EG (n=20)	CG (n=19)	χ2/t(P)
Sex (Male/Female)	11/9	12/7	0.268 (0.605)
Measurement site (left/right)	12/8	9/10	0.626 (0.429)
Height (cm)	168±6.20	168±6.93	0.079 (0.938)
Age (years)	28.05±2.81	27.63±1.86	0.544 (0.590)
Weight (kg)	66.65±12.37	66.74±10.72	−0.131 (0.896)
Body mass index (kg/m2)	23.29±3.37	23.34±2.81	−0.054 (0.957)

EG – experimental group; CG – control group.

Comparison of Pre–Post Changes in Muscle Strength Within and Between Groups

Knee Flexor Strength

In the EG, knee flexor strength significantly increased from 11.20±3.07 kgf to 16.43±4.08 kgf (P<0.05). In the CG, knee flexor strength also significantly increased from 13.02±2.67 kgf to 14.99±2.60 kgf (P<0.05). The EG showed a significantly greater improvement than the CG (P<0.05).

Knee Extensor Strength

In the EG, knee extensor strength improved significantly from 18.12±4.62 kgf to 25.54±3.50 kgf (P<0.05). In the CG, it increased significantly from 19.44±3.50 kgf to 21.69±3.81 kgf (P<0.05). Between-group comparison of the pre–post changes also revealed a significantly greater increase in the EG than in the CG (P<0.05) (Table 3).

Table 3.

Comparison of pre–post changes in muscle strength within and between groups.

Variable (kgf)	Group	Pre	Post	Change	t(P)	t(P)b
Knee flexor	EG	11.20±3.07a	16.43±4.08	5.22±2.20	10.574 (0.001*)	4.768 (0.001*)
	CG	13.02±2.67	14.99±2.60	1.96±2.04	4.199 (0.001*)
Knee extensor	EG	18.12±4.62	25.54±4.11	7.41±1.47	22.554 (0.001*)	13.441 (0.001*)
	CG	19.44±3.50	21.69±3.81	2.25±0.81	12.065 (0.001*)

^aMean±standard deviation;

^bP-value for the difference in pre–post change between groups;

^*P<0.05.

kgf – kilogram-force; EG – experimental group; CG – control group.

Comparison of Pre–Post Changes in Balance Within and Between Groups

In the EG, the YBT score significantly increased, from 86.43±10.24% to 98.86±11.49% (P<0.05). In the CG, it also significantly increased, from 91.49±9.67% to 96.29±8.10% (P<0.05). A comparison of the change in scores between the groups revealed a significantly greater increase in the EG than in the CG (P<0.05).

In the EG, the score on the stair-descending task significantly decreased, from 4.30±0.80 to 1.75±0.85 (P<0.05). In the CG, the score also significantly decreased, from 3.53±0.61 to 1.89±0.80 (P<0.05). The between-group comparison of pre–post change demonstrated a significantly greater reduction in the EG than in the CG (P<0.05) (Table 4).

Table 4.

Comparison of pre–post changes in balance within and between groups.

Variable (kgf)	Group	Pre	Post	Change	t/z(P)	t/z(P)b
Y-balance test (%)	EG	86.43±10.24a	98.86±11.49	12.42±6.00	9.253 (0.001*)	4.348 (0.001*)
	CG	91.49±9.67	96.29±8.10	4.79±4.86	4.297 (0.001*)
Stair-descending task	EG	4.30±0.80a	1.75±0.85	−2.55±0.99	−3.880 (0.001*)c	−2.931 (0.003*)d
	CG	3.53±0.61	1.89±0.80	−1.63±0.83	−3.789 (0.001*)c

^aMean±standard deviation;

^bP-value for the difference in pre–post change between groups;

^cWilcoxon signed-rank test;

^dMann-Whitney U test;

^*P<0.05. EG – experimental group; CG – control group.

Comparison of Pre–Post Changes in PPT Within and Between Groups

Rectus Femoris

In the EG, the PPT of the rectus femoris significantly increased, from 31.48±15.16 N/cm² to 44.68±14.16 N/cm² (P<0.05). In the CG, it also significantly increased, from 31.81±13.07 N/cm² to 37.13±12.87 N/cm² (P<0.05). Between-group comparison of the pre–post changes showed a significantly greater increase in the EG than in the CG (P<0.05).

Vastus Medialis

In the EG, the PPT of the vastus medialis significantly increased, from 33.84±17.01 N/cm² to 45.04±16.70 N/cm² (P<0.05), whereas that of the CG also significantly increased, from 31.23±12.88 N/cm² to 36.01±12.02 N/cm² (P<0.05). The EG demonstrated significantly greater improvement than the CG (P<0.05).

Vastus Lateralis

The PPT of the vastus lateralis significantly improved in the EG, from 30.93±12.43 N/cm² to 40.89±13.66 N/cm² (P<0.05) and in the CG, from 25.80±8.26 N/cm² to 31.87±8.96 N/cm² (P<0.05). The EG showed a significantly greater increase in the PPT compared with the CG (P<0.05).

Semitendinosus

In the EG, the PPT of the semitendinosus significantly increased, from 33.03±12.09 N/cm² to 47.89±11.51 N/cm² (P<0.05), and that of the CG also significantly increased, from 38.15±14.07 N/cm² to 45.40±13.19 N/cm² (P<0.05). The EG exhibited a significantly greater change compared with the CG (P<0.05).

Biceps Femoris

The PPT of the biceps femoris significantly increased, from 36.45±14.50 N/cm² to 48.39±12.47 N/cm² in the EG (P<0.05) and from 37.00±9.55 N/cm² to 42.20±9.01 N/cm² in the CG (P<0.05). Between-group comparisons indicated a significantly greater increase in the EG than in the CG (P<0.05) (Table 5).

Table 5.

Comparison of pre–post changes in pressure pain threshold within and between groups.

Variable (N/cm2)	Group	Pre	Post	Change	t(P)	t(P)b
Rectus femoris	EG	31.48±15.16a	44.68±14.16	13.19±6.11	9.646 (0.001*)	3.944 (0.001*)
	CG	31.81±13.07	37.13±12.87	5.31±6.35	3.646 (0.002*)
Vastus medialis	EG	33.84±17.01	45.04±16.70	11.20±6.77	7.398 (0.001*)	3.058 (0.004*)
	CG	31.23±12.88	36.01±12.02	4.78±6.31	3.302 (0.004*)
Vastus lateralis	EG	30.93±12.43	40.89±13.66	9.96±4.61	9.663 (0.001*)	2.837 (0.007*)
	CG	25.80± 8.26	31.87± 8.96	6.06±3.93	6.722 (0.001*)
Semitendinosus	EG	33.03±12.09	47.89±11.51	14.85±6.64	9.995 (0.001*)	3.584 (0.001*)
	CG	38.15±14.07	45.40±13.19	7.25±6.59	4.793 (0.001*)
Biceps femoris	EG	36.45±14.50	48.39±12.47	11.94±4.44	12.029 (0.001*)	2.612 (0.013*)
	CG	37.00±9.55	42.20±9.01	5.19±2.24	10.071 (0.001*)

^aMean±standard deviation;

^bP-value for the difference in pre–post change between groups;

^*P<0.05.

EG – experimental group; CG – control group.

Comparison of Pre–Post Changes in Mechanical Properties Within and Between Groups

Rectus Femoris

In the EG, muscle tone significantly increased, from 12.16±1.20 Hz to 12.85±1.47 Hz (P<0.05), whereas that of the CG showed a non-significant increase, from 12.55±1.12 Hz to 13.13±1.86 Hz (P>0.05). The between-group comparison of the changes in muscle tone was not significant (P>0.05). Muscle stiffness significantly increased in the EG, from 203.63±36.55 N/m to 235.24±27.98 N/m, and in the CG, from 199.89±61.56 N/m to 226.06±34.58 N/m (P<0.05). However, no significant difference was observed in the changes in scores between the groups (P>0.05).

Vastus Medialis

Muscle tone significantly increased in the EG, from 11.79±1.23 Hz to 13.26±1.32 Hz, and in the CG, from 11.71±1.26 Hz to 12.57±1.35 Hz (P<0.05). A between-group comparison revealed a significantly greater increase in the EG than in the CG (P<0.05). Muscle stiffness also significantly increased, from 188.59±50.97 N/m to 215.93±49.05 N/m, in the EG, and from 186.90±45.54 N/m to 198.34±45.32 N/m in the CG (P<0.05). Between-group analysis revealed a significant difference favoring the EG (P<0.05).

Vastus Lateralis

In the EG, muscle tone significantly increased, from 13.44±1.47 Hz to 15.81±2.78 Hz, and from 14.16±1.52 Hz to 15.36±2.37 Hz in the CG (P<0.05). The between-group comparison showed a significantly greater improvement in the EG than in the CG (P<0.05). Muscle stiffness significantly increased, from 269.30±47.32 N/m to 307.79±72.76 N/m, in the EG, and from 273.42±34.91 N/m to 288.29±53.47 N/m in the CG (P<0.05). However, the change in scores did not differ significantly between the groups (P>0.05).

Semitendinosus

Muscle tone significantly increased in the EG, from 12.75±1.29 Hz to 14.25±1.54 Hz, and in the CG, from 13.06±1.49 Hz to 14.32±2.05 Hz (P<0.05). However, no significant difference in the change in scores was observed between the groups (P>0.05). Muscle stiffness significantly increased in the EG, from 218.08±36.77 N/m to 239.16±36.03 N/m and from 224.26±45.53 N/m to 235.47±47.80 N/m in the CG (P<0.05). Between-group comparisons showed a significantly greater increase in the EG than in the CG (P<0.05).

Biceps Femoris

In the EG, muscle tone significantly increased, from 13.04±1.18 Hz to 14.12±1.35 Hz, and in the CG, from 12.89±1.29 Hz to 13.87±1.45 Hz (P<0.05). However, the between-group comparison of the pre–post changes was not significant (P>0.05) (Table 6).

Table 6.

Comparison of pre–post changes in muscle tone within and between groups.

Variable [61]	Group	Pre	Post	Change	t(P)	t(P)b
Rectus femoris	EG	12.16±1.20a	12.85±1.47	0.68±0.85	3.592 (0.002*)	0.316 (0.754)
	CG	12.55±1.12	13.13±1.86	0.58±1.24	2.030 (0.057)
Vastus medialis	EG	11.79±1.23	13.26±1.32	1.47±0.18	36.489 (0.001*)	7.608 (0.000*)
	CG	11.71±1.26	12.57±1.35	0.86±0.30	12.232 (0.001*)
Vastus lateralis	EG	13.44±1.47	15.81±2.78	2.37±1.67	6.317 (0.001*)	2.426 (0.020*)
	CG	14.16±1.52	15.36±2.37	1.19±1.31	3.985 (0.001*)
Semitendinosus	EG	12.75±1.29	14.25±1.54	1.49±0.59	11.344 (0.001*)	1.015 (0.317)
	CG	13.06±1.49	14.32±2.05	1.26±0.84	6.499 (0.001*)
Biceps femoris	EG	13.04±1.18a	14.12±1.35a	1.08±0.56	8.602 (0.001*)	0.586 (0.561)
	CG	12.89±1.29	13.87±1.45	0.97±0.53	7.925 (0.001*)

^aMean±standard deviation;

^bP-value for the difference in pre–post change between groups;

^*P<0.05.

EG – experimental group; CG – control group.

Muscle stiffness significantly increased, from 227.96±42.49 N/m to 243.39±40.47 N/m in the EG and from 220.46±42.21 N/m to 229.77±53.17 N/m in the CG (P<0.05); however, the between-group difference was not significant (P>0.05) (Table 7).

Table 7.

Comparison of pre–post changes in muscle stiffness within and between groups.

Variable (N/m)	Group	Pre	Post	Change	t(P)	t(P)b
Rectus femoris	EG	203.63±36.55a	235.24±27.98	31.60±18.14	7.790 (0.001*)	0.448 (0.656)
	CG	199.89±61.56	226.06±34.58	26.17±50.94	2.239 (0.038*)
Vastus medialis	EG	188.59±50.97	215.93±49.05	25.82±9.83	5.015 (0.001*)	5.384 (0.001*)
	CG	186.90±45.54	198.34±45.32	11.80±5.81	5.302 (0.001*)
Vastus lateralis	EG	269.30±47.32a	307.79±72.76a	38.49±48.85	3.524 (0.002*)	1.822 (0.077)
	CG	273.42±34.91	288.29±53.47	14.86±29.11	2.226 (0.039*)
Semitendinosus	EG	218.08±36.77	239.16±36.03	21.08±10.38	9.079 (0.001*)	3.471 (0.001*)
	CG	224.26±45.53	235.47± 47.80	11.20±6.94	7.036 (0.001*)
Biceps femoris	EG	227.96±42.49	243.39±40.47	15.43±17.23	4.005 (0.001*)	1.245 (0.221)
	CG	220.46±42.21	229.77±53.17	9.31±3.00	3.096 (0.006*)

^aMean±standard deviation;

^bP-value for the difference in pre–post change between groups;

^*P<0.05.

EG – experimental group; CG – control group.

Discussion

This study evaluated the effects of low-intensity BFR multi-joint resistance training on muscle strength, balance, pain, and mechanical properties in individuals with PFPS. The pre–post changes in key outcome variables were compared between an EG receiving low-intensity BFR multi-joint training and a CG performing non-BFR low-intensity training.

Both groups demonstrated significant improvements in muscle strength after the intervention (P<0.05), with the EG showing significantly greater increases compared with the CG (P<0.05). These findings suggest that BFR-induced metabolic stress and hypoxic conditions may contribute to increased muscle protein synthesis through mTORC1 pathway activation [53] and may elicit enhanced neuromuscular excitability and physiological responses similar to those observed with high-intensity training [20,54]. These results are consistent with those of previous studies [12,23,55] and are further supported by mechanistic evidence highlighting mTORC1-mediated hypertrophic signaling as a key driver of muscle adaptation under BFR conditions, as well as by PFPS and knee rehabilitation trials [20,22,23] that emphasize the clinical relevance of integrating BFR into multi-joint protocols reflecting functional daily and sport activities.

Both groups showed significant improvements in the YBT and stair-descending task after the intervention (P<0.05), with the EG demonstrating greater changes than the CG (P<0.05). Balance is closely associated with strength as well as proprioception and neuromuscular control [56,57], and BFR intervention had a beneficial effect on these factors. The stair-descending task is a valuable clinical tool for assessing dynamic alignment and functional stability [58]. These findings align with prior evidence that hip-inclusive, multi-joint programs improve PFPS-related balance outcomes [25,27]. Moreover, Werasirirat et al [59] observed enhanced YBT performance after BFR training in chronic ankle instability, supporting the broader role of BFR in neuromuscular control.

The PPT significantly increased in all measured muscles in both the EG and CG after the intervention (P<0.05), with significantly greater increases observed in the EG than in the CG (P<0.05). These findings suggest that BFR training may reduce pain sensitivity, potentially through exercise-induced hypoalgesia [60], modulation of central sensitization, and the activation of descending pain inhibitory pathways via beta-endorphin release [61]. This is consistent with Kong et al [20], who reported comparable pain relief effects between low-load BFR and high-load resistance training in PFPS patients.

Muscle tone significantly increased in most muscles in both groups (P<0.05), with the EG showing greater improvements in the vastus medialis and lateralis than the CG (P<0.05). These muscles play a key role in patellar stability, and the observed improvements indicate enhanced neuromuscular function [62,63]. These findings suggest that BFR-assisted multi-joint training may accelerate restoration of stabilizing muscles around the knee, thereby enhancing patellofemoral joint stability and supporting functional recovery.

Muscle stiffness also increased significantly in both groups (P<0.05), with the EG demonstrating significantly greater changes in the vastus medialis and semitendinosus muscles than the CG (P<0.05). These adaptations reflect improvements in muscle contractility and viscoelastic properties, consistent with prior reports using myotonometric assessment [32,33]. Such mechanical improvements may play a role in pain modulation and long-term joint protection.

In this study, for the mechanical properties, muscle tone showed significant between-group improvements in the vastus medialis and vastus lateralis, while stiffness demonstrated significant between-group differences in the vastus medialis and semitendinosus. However, for the rectus femoris and biceps femoris, no significant between-group differences were observed in either variable. These findings suggest that low-intensity multi-joint BFR exercise does not exert uniform effects on all muscles, but rather induces differential adaptive responses depending on the mechanical demands and physiological characteristics of each muscle.

In summary, low-intensity BFR multi-joint resistance exercise is an effective intervention for improving muscle strength, balance, pain sensitivity, and mechanical properties in patients with PFPS. This approach may serve as a viable clinical alternative for patients unable to perform high-intensity exercises, thereby offering practical applications in rehabilitation settings.

However, this study had several limitations. Long-term follow-up was not conducted to assess the sustainability of the intervention effects, and the absence of electromyographic analysis limited the quantification of neuromuscular mechanisms. Furthermore, the exclusion of hip muscle assessments restricted the evaluation of knee joint interactions. This study was conducted in a young adult population aged 18–40 years, and therefore the findings should be interpreted with caution when generalizing to other groups such as older adults or professional athletes. Future studies should investigate whether the observed improvements are maintained over time and whether symptom recurrence occurs, and more comprehensive approaches are required to address these limitations.

Conclusions

The BFR EG showed greater improvements than the CG in all outcome measures of muscle strength, balance, and PPT (P<0.05), and significant between-group differences were observed in muscle tone of the vastus medialis and lateralis and in muscle stiffness of the vastus medialis and semitendinosus (P<0.05). These findings emphasize that BFR-based multi-joint training is an especially effective rehabilitation strategy for patients who cannot tolerate high-intensity loading, while also expanding its clinical relevance and supporting potential applications across diverse patient populations. Future studies should explore the long-term effects and underlying mechanisms of BFR-based multi-joint training.