Adding blood flow restriction to isokinetic resistance training provides no additional benefit for strength, but may improve local muscular endurance: a randomized controlled trial

Abstract

Background

This study investigated the effects of adding blood flow restriction (BFR) to high-load isokinetic resistance training on muscle strength and local muscular endurance.

Methods

Forty-two middle- and long-distance runners were randomly assigned to an isokinetic training group with BFR (BFR group; n = 21) or an isokinetic training group without BFR (non-BFR group; n = 21). The training protocol consisted of concentric knee flexion and extension exercises performed at angular velocities of 60°/s (3 sets of 10 maximal repetitions) and 180°/s (3 sets of 30 maximal repetitions), twice weekly for 8 weeks. BFR was applied at 80% of arterial occlusion pressure (AOP) in the BFR group. Muscle strength (peak torque [PT]) and local muscular endurance (fatigue index [FI]) were evaluated before and after the intervention.

Results

Both groups demonstrated significant increases in muscle strength for both knee flexors and extensors at angular velocities of 60°/s and 180°/s. However, no significant between-group differences were observed for strength gains (p > 0.05 for all comparisons). Regarding local muscular endurance, the magnitude of improvement in FI was significantly greater in the BFR group compared to the non-BFR group for both flexors (p = 0.003, η²= 0.102) and extensors (p = 0.005, η²= 0.091).

Conclusions

This study demonstrates that adding BFR to high-load isokinetic resistance training does not enhance muscle strength beyond high-load isokinetic resistance training but significantly improves local muscular endurance, as evidenced by greater reductions in FI in the BFR group. These findings suggest that BFR may be a valuable tool for optimizing endurance-specific adaptations in athletic populations requiring sustained performance. Future research should explore optimal BFR protocols for endurance-focused training across diverse athletic disciplines.

Trial registration

Registration date: 04/11/2024, ClinicalTrials: NCT06678009.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13102-026-01554-7.

Background

Strength training enhances athletic performance by improving muscle strength and endurance, which are key factors for better running economy and overall performance [1–3]. Local muscular endurance, defined as the ability to perform repeated contractions over time [4], positively influences performance across various sports [3, 5]. Incorporating resistance or isokinetic training into running programs improves neuromuscular function, running economy, and endurance, even in well-trained athletes [6–8].

BFR has gained significant attention due to its potential to augment muscular adaptations during resistance training [9, 10]. BFR is an exercise modality in which venous blood flow is completely restricted, and arterial blood flow is partially restricted using a pressurized cuff placed on the proximal portion of the limb [11]. Mechanical tension is considered the primary driving factor behind resistance training adaptations, while the metabolic stress caused by vascular occlusion has primarily been discussed in the context of hypertrophy and may indirectly contribute to long-term strength changes [12, 13]. Strength gains result from both neural factors (e.g., motor unit recruitment and improved skill/coordination) and morphological changes (e.g., hypertrophy and altered muscle structure); molecular processes such as muscle protein synthesis, satellite cell activation, and anabolic signaling play a role in muscle remodeling that supports hypertrophy [14, 15]. Furthermore, hypoxic environment within working muscles, leads to unique physiological responses such as an increase in vascular endothelial growth factor [16] that enhance local muscular endurance.

While the majority of studies have focused on adding BFR to low-load resistance exercises [17], there is less data on its effects when adding to high-load resistance training [18–22]. Previous studies adding BFR to high-load isotonic resistance training reported no additional benefit for muscle strength [19–22] or local muscular endurance [21], likely because such high-force contractions already achieve near-maximal motor unit recruitment [23]. The physiological mechanisms underlying BFR may interact differently with isotonic versus isokinetic training. During high-load isotonic resistance exercise, metabolic stress appears to increase while muscle activation decreases [23]. However, isokinetic resistance exercise differs in that the constant-velocity constraint enables participants to maintain maximal effort throughout each repetition, even under fatigue. This distinct feature may enhance the physiological effects of BFR, offering a novel rationale for investigating its adding to high-load isokinetic resistance training.

The purpose of this study is to examine the effects of adding BFR to high-load isokinetic resistance training on muscle strength and local muscular endurance. Although it has been demonstrated that BFR does not provide additional benefits during high-load isotonic resistance exercises, its effectiveness when added to high-load isokinetic resistance training remains unclear. The primary hypothesis of this study was that adding BFR to high-load isokinetic resistance training would produce greater improvements in muscle strength than high-load isokinetic training alone. The secondary hypothesis was that BFR would enhance local muscular endurance more, due to the metabolic and vascular adaptations associated with restricted blood flow.

Methods

Participants

A priori sample size calculation was performed to ensure adequate statistical power. The power analysis was conducted using G*Power software (version 3.1) [24], assuming an effect size of d = 0.8, based on Rivera et al. (d = 1.28) [9], the primary outcome variable was the isokinetic knee extension peak torque, a significance level of α = 0.05. The analysis indicated that a sample size of 42 participants (21 per group) was required to achieve 95% confidence and 80% power. This calculation was based on independent samples t-tests to compare the primary outcomes between groups. Fifty-three healthy middle- and long-distance runners were recruited via local running clubs. There were 27 participants in the BFR group and 26 participants in the non-BFR group. Eleven participants dropped out before completion because of low adherence. Athletes who were unable to complete 2 sessions per week or a total of 16 sessions were dropped from the study. Therefore, data from 42 participants were considered in the analysis (Table 1). A total of 84 extremities from 42 participants were included in the study. Both extremities underwent the same training protocol unilaterally. Inclusion criteria were as follows: absence of heart disease or musculoskeletal conditions affecting the lower extremities; no use of dietary supplements during the study or within two months prior; and running more than 20 km per week. Participants were excluded if they had a history of thromboembolism, cardiovascular or peripheral vascular disease, uncontrolled hypertension, diabetes, smoking or tobacco use, or a body mass index ≥ 30 kg/m². Participants had not participated in any kind of regular resistance training within the previous 6 months before the experimental period. Throughout the intervention period, participants were instructed not to run during the 24-hour period prior to isokinetic training, nor during the 48-hour period prior to the initial and final tests. To ensure the applicability of the results across sexes, both male and female athletes were included. The study group consisted of 57% female and 43% male athletes; the non-BFR group consisted of 38% female and 62% male athletes. Participants were divided into subgroups based on sex, age, and weekly running distance to ensure homogeneity. Randomization was then performed within these subgroups using a computer-generated random sequence.

Table 1.

Descriptive characteristics of the participants

Variable	BFR Group (mean ± SD)	non-BFR group (mean ± SD)
Age (year)	26.43 ± 7.49	26.57 ± 6.44
Body height (cm)	168.05 ± 9.6	171.86 ± 10.3
Body weight (kg)	61.22 ± 10.76	68.89 ± 12.16
Body mass index (kg/m²)	21.67 ± 2.13	22.95 ± 2.35
Sports age (years)	4.30 ± 3.13	3.62 ± 2.99
Weekly sports time (hours)	8.43 ± 5.35	9.07 ± 5.13
Weekly running distance (km)	28.57 ± 12.3	27.57 ± 8.45

SD Standard Deviation, BFR Blood Flow Restriction

Study design

Forty-two middle- and long-distance runners were randomly assigned to either an isokinetic training group with BFR (BFR Group; n = 21) or an isokinetic training group without BFR (non-BFR group; n = 21). Lower-body AOP of the BFR group was measured before the isokinetic knee flexor and extensor muscle strength and endurance evaluation. Isokinetic knee flexor and extensor muscle strength and endurance were evaluated with the isokinetic dynamometer. The training protocol consisted of concentric knee flexion and extension exercises, twice weekly for 8 weeks. Both groups performed the same isokinetic training protocol. BFR was applied while the athletes doing isokinetic training protocol, but not to the non-BFR group. Isokinetic knee flexor and extensor muscle strength and endurance were re-evaluated at the end of the intervention (48 h after the last training session). This study adhered to the Consolidated Standards of Reporting Trials (CONSORT) 2010 guidelines for reporting randomized controlled trials [25]. The completed CONSORT checklist has been submitted as supplementary material. The study protocol was approved by the Health Sciences University Gülhane Training and Research Hospital Clinical Ethics Committee (approval date: 11.05.2022, number: 2022/61) and registered on ClinicalTrials.gov (registration date: 04/11/2024, NCT06678009).

Procedures

Determination of arterial occlusion pressure

AOP was assessed before the start of the isokinetic knee flexor and extensor muscle strength and endurance evaluation to determine the pressure used in the BFR group. In the same position in which the isokinetic training protocol was to be performed, a vascular Doppler probe (ES100 V3; Hadeco, Inc.; Japan) was placed over the tibialis posterior artery and an auscultatory pulse was obtained. A cuff (C3 Model; KAATSU Global, Inc.; USA; 5 × 40–66 cm) was placed on the upper thigh and inflated to the lowest point at which the auscultatory pulse could no longer be detected. This value was defined as the AOP. AOP was determined independently for each leg using a manual vascular Doppler device. During the isokinetic training protocol, 80% of the AOP of each leg was used [18]. This conservative approach ensured that BFR was consistently applied across both legs, minimizing variability in the intervention.

Evaluation of isokinetic knee flexor and extensor muscle strength and endurance

Isokinetic knee flexor and extensor muscle strength and endurance evaluations were conducted with an isokinetic dynamometer (System 3, Biodex Medical Systems, Inc., Shirley, New York). The isokinetic testing protocol was performed before and after the 8-week training intervention to assess changes in muscle strength (PT) and local muscular endurance (FI). Before the isokinetic muscle strength and endurance evaluations, the athletes performed a 10-minute warm-up exercise on the bicycle ergometer at a pedaling speed (revolutions per minute, rpm) of 60–80. Then, participants were positioned on the isokinetic dynamometer seat, with safety belts fastened on the trunk, pelvis, and thigh to minimize extra body movements that could affect the peak torque values. The lateral epicondyle of the femur was used as a marker to align the knee rotation axis and the instrument rotation axis. Before each angular velocity, three repetitions of knee flexion and extension at the same angular velocity were performed to familiarize the athlete with the movement at this angular velocity. Both extremities were assessed unilaterally; in half of the athletes, the dominant limb was evaluated first, and the non-dominant limb was evaluated 3 min later, and vice versa in the other half. The order of the leg tests was consistent in the baseline and final tests. One minute rest was given between 60°/s and 180°/s angular velocities and 3 min rest was given when switching from one leg to the other.

Participants were evaluated for bilateral knee flexion and extension movements at angular velocities of 60°/s and 180°/s. Participants were encouraged to maximal performance with verbal commands. Each athlete performed 5 repetitions of knee flexion and extension at 60°/s angular velocity and 30 repetitions at 180°/s angular velocity, and PT and FI were determined [26]. PT was the peak value measured during the 5 collected trials followed by the 30 collected trials and was measured in Newton meters. The endurance of the knee flexor and extensor muscles was determined by the FI. FI was calculated as the percentage decrease from PT. Athletes performed 30 maximal extensions and flexions at an angular velocity of 180°/sec. The FI was calculated as the percentage decrease in PT over 30 repetitions using the following formula: FI (%) = 1 - {[(mean PT from first 5 repetitions – mean PT from last 5 repetitions)/mean PT from first 5 repetitions] × 100}, based on previous studies [27, 28]. Higher values indicate lower fatigue resistance.

Isokinetic training protocol

The isokinetic training protocol consisted of concentric knee flexion and extension exercises performed at two different angular velocities, representing low-velocity (60°/s) and high-velocity (180°/s) conditions. The protocol included three sets of 10 maximal repetitions at 60°/s and three sets of 30 maximal repetitions at 180°/s. Verbal encouragement was provided to ensure maximal effort during training. Rest periods were standardized as follows: 1 min between sets at the same angular velocity; 3 min when switching between angular velocities (60°/s to 180°/s); 3 min when switching from one leg to the other. This protocol was designed based on prior research demonstrating that isokinetic training at multiple angular velocities effectively improves muscle strength and endurance [26]. The inclusion of both low-velocity (60°/s) and high-velocity (180°/s) conditions allowed for a comprehensive evaluation of the effects of BFR on different aspects of muscle performance.

Based on the study of Curran et al. the training protocol was carried out twice a week for eight weeks [18]. This distribution allowed for adequate recovery between sessions while maintaining consistent training stimuli. To ensure equalized volume and load control, all participants performed the same number of repetitions and sets at identical angular velocities (60°/s and 180°/s). Participants were also reminded before each session that they should not perform lower limb resistance exercises outside of this study to keep load control similar between participants. Before commencing the training program, the athletes engaged in a 10-minute warm-up session on a bicycle ergometer, maintaining a pedaling speed (revolutions per minute, RPM) between 60 and 80. For the BFR group, isokinetic training in conjunction with BFR was conducted with the use of lower extremity cuffs (C3 Model; KAATSU Global, Inc.; USA; 5 × 40–66 cm), with an applied continuous pressure of 80% of the AOP [18], as determined by a manual vascular Doppler device. The cuff was inflated 10 s before the start of the isokinetic training protocol and remained inflated both between sets and during rest from 60°/s to 180°/s. The cuff was deflated at the end of the exercise for that leg. After a 3 min rest, the other leg was started. Again, in the other leg, the cuff remained inflated during 3 min of rest as the leg went from 60°/s to 180°/s. The maximum occlusion time for each extremity was set as 10 min [29]. The non-BFR group underwent the same exercise protocol without BFR. Participants completed the training protocol without complications.

Statistical analyses

The study was designed with a total of 42 athletes, to provide 95% confidence level and 80% test power to determine the effect size of d = 0.8 for independent samples t-test. Prior to between-group comparisons, limbs were evaluated as separate observations. Analyses were performed for 42 extremity outcomes in 21 participants. Normal distribution of isokinetic strength and local muscular endurance data was assessed using the Shapiro-Wilks test. Data were presented as mean ± SD for normally distributed data and as median ± IQR for non-normally distributed data. Isokinetic strength and local muscular endurance data were each assessed using a 2 × 2 (group × time) repeated-measures ANOVA with group allocation (BFR vs. non-BFR group) as the between-subjects independent factor, and time (baseline and final) as the within-subjects dependent factor. Pairwise comparisons were performed using the Bonferroni test. The effect size (partial eta squared effect sizes (η²) was examined to investigate the size of the difference between the variables (classified as 0.01 = small, 0.06 = medium, 0.14 = large). All statistical analyses were performed using IBM SPSS Statistics version 25 (Armonk, NY, USA), and an alpha of p ≤ 0.05 was considered statistically significant for all comparisons.

Results

The descriptive characteristics of the participants are presented in Table 1.

Both groups showed improvement within themselves, and muscle strength increased after the training protocol (Table 2; Fig. 1); but there was not a significant difference between the two groups (Table 3).

Table 2.

Baseline and final test measurements of knee flexor and extensor muscle strength at angular velocities of 60°/sec and 180°/sec within groups

Variables	Group	Test	Mean ± SD or Median ± IQR	Mean Difference (95% CI Lower-Upper)
Flexion PT 60°/sec	BFR	Baselineb	73.6 ± 34.8	12.65 (8.55–16.4)
		Finala	95.3 ± 31.6
	non-BFR	Baselineb	93.0 ± 42.2	6.90 (3.7–11.3)
		Finala	104.0 ± 23.4
Extension PT 60°/sec	BFR	Baselineb	158.0 ± 78.9	18.82 (12.05–25.6)
		Finalb	182.0 ± 107
	non-BFR	Baselineb	202.0 ± 78.9	16.41 (10.9–21.9)
		Finalb	209.0 ± 90.1
Flexion PT 180°/sec	BFR	Baselineb	56.4 ± 31.0	12.55 (8.85–15.4)
		Finalb	67.3 ± 31.9
	non-BFR	Baselineb	67.7 ± 30.4	11.60 (8.4–13.9)
		Finalb	77.2 ± 31.2
Extension PT 180°/sec	BFR	Baselineb	99.6 ± 60.0	20.24 (15.85–24.6)
		Finalb	123.0 ± 71.4
	non-BFR	Baselineb	127.0 ± 68.1	17.35 (13.75–20.6)
		Finalb	146.0 ± 61.1

SD Standard Deviation, IQR Interquartile Range, CI Confidence Interval, PT Peak Torque, BFR Blood Flow Restriction

^aMean ± SD for normally distributed data; ^bMedian ± IQR for non-normally distributed data

Fig. 1.

Knee flexor and extensor muscle strength measurements of BFR and non-BFR groups. PT, peak torque; BFR, blood flow restriction

Table 3.

The effects of isokinetic muscle strength variables with the corresponding p value and partial Eta squared effect size

Variables	ANOVA		p-value	Effect size (η²)
Flexion PT at 60°/sec	Group		0.080	0.037
	Time		0.000	0.237
	Group × Time		0.299	0.013
		BFR	0.000	0.194
		non-BFR	0.036	0.133
Extension PT at 60°/sec	Group		0.125	0.028
	Time		0.000	0.448
	Group × Time		0.578	0.004
		BFR	0.000	0.308
		non-BFR	0.000	0.271
Flexion PT at 180°/sec	Group		0.060	0.043
	Time		0.000	0.348
	Group × Time		0.704	0.002
		BFR	0.000	0.234
		non-BFR	0.000	0.256
Extension PT at 180°/sec	Group		0.114	0.030
	Time		0.000	0.618
	Group × Time		0.267	0.015
		BFR	0.000	0.542
		non-BFR	0.000	0.371

PT Peak Torque, BFR Blood Flow Restriction

For knee flexion at 60°/sec PT, there was no significant main effect of group (p = 0.080, η²= 0.037) or group × time interaction (p = 0.299, η²= 0.013). However, a significant main effect of time was found (p = 0.000, η²= 0.237), indicating overall improvement in muscle strength across groups (Table 3).

Similarly, for knee extension at 60°/sec PT, no significant group effect (p = 0.125, η²= 0.028) or group × time interaction effect (p = 0.578, η²= 0.004) was found, while a significant main effect of time (p = 0.000, η²= 0.448) persisted (Table 3).

For knee flexion at 180°/sec PT, the findings followed a similar pattern, with no significant group effect (p = 0.060, η²=0.043) or group × time interaction effect (p = 0.704, η²= 0.002), but a significant main effect of time (p = 0.000, η²= 0.348) (Table 3).

Similarly, for knee extension at 180°/sec PT, no significant group effect (p = 0.114, η²= 0.030) or group × time interaction effect (p = 0.267, η²= 0.015) was found, while a significant main effect of time (p = 0.000, η²=0.618) persisted (Table 3).

FI significantly decreased in both flexor and extensor muscle groups in favor of the BFR group, indicating improved local muscular endurance following the exercise with BFR (Tables 4 and 5; Fig. 2).

Table 4.

Baseline and final test knee flexor and extensor muscular endurance measurements of the BFR and non-BFR groups

Variables	Group	Test	Mean ± SD or Median ± IQR	Mean Difference (95% CI Lower-Upper)
Flexion FI (%)	BFR	Baselinea	53.4 ± 11.17	−9.40 ((−12.78)-(−6.03))
		Finalb	44.03 ± 10.3
	non-BFR	Baselinea	56.7 ± 10.91	−2.91 ((−6.29)-(0.45))
		Finalb	53.7 ± 12.13
Extension FI (%)	BFR	Baselinea	44.4 ± 7.77	−3.38 ((−5.48)-(−1.29))
		Finala	41.0 ± 5.17
	non-BFR	Baselinea	47.1 ± 7.91	−0.41 ((−2.51)-(1.67))
		Finala	46.7 ± 9.27

SD Standard Deviation, IQR Interquartile Range, CI Confidence Interval, FI Fatigue Index, BFR Blood Flow Restriction

^aMean ± SD for normally distributed data; ^bMedian ± IQR for non-normally distributed data

Table 5.

The effects of isokinetic local muscular endurance variables with the corresponding p value and partial Eta squared effect size

Variables	ANOVA		p-value	Effect size (η²)
Flexion FI (%)	Group		0.003	0.102
	Time		0.000	0.244
	Group x Time		0.008	0.082
		BFR	0.000	0.273
		non-BFR	0.089	0.035
Extension FI (%)	Group		0.005	0.091
	Time		0.012	0.074
	Group x Time		0.050	0.046
		BFR	0.002	0.112
		non-BFR	0.692	0.002

FI Fatigue Index, BFR Blood Flow Restriction

Fig. 2.

Knee flexor and extensor local muscular endurance measurements of the BFR and non-BFR groups. FI, fatigue index; BFR, blood flow restriction

Results for the FI of the knee flexor muscles revealed significant main effects of group (p = 0.003, η²= 0.102) and time (p = 0.000, η²= 0.244), as well as a significant group × time interaction (p = 0.008, η²= 0.082) (Table 5).

Similarly, for the FI of the knee extensor muscles, there were significant main effects of group (p = 0.005, η²= 0.091) and time (p = 0.012, η²= 0.074). A significant group × time interaction was also found (Table 5).

Discussion

Our study’s results suggest that adding BFR to high-load isokinetic resistance exercise does not provide any additional benefit in terms of increasing muscle strength but may increase local muscular endurance. The secondary hypothesis was supported as the BFR group showed a significantly greater reduction in FI for both knee flexors and extensors compared to the non-BFR group.

While there are numerous studies investigating the effects of adding BFR to low-load resistance training, there is less evidence regarding its use in high-load resistance training [17–22, 30]. Previous studies using high-load isotonic exercise have not found additional improvements in strength or local muscular endurance [16–20]. When investigating studies that combined isokinetic exercise and BFR, it is seen that low loads are mostly used [31–33], similar to isotonic exercise, and the evidence investigating the effects of high-load isokinetic resistance training on muscle strength is quite limited [18, 30]. No studies investigating its effect on local muscular endurance were found in the literature.

Looking at studies with BFR added to high-load isokinetic resistance exercise, Curran et al. found similar results to our study, no significant difference was observed in peak torque measurement of the quadriceps muscle between isokinetic exercise groups with and without BFR after training [18]. Similar to our study, they used 80% AOP and performed an exercise program twice a week for 8 weeks. In another study by Santos et al., maximal muscle strength did not increase more with the addition of BFR to high-load isokinetic resistance training [30]. The increase in muscle strength induced by resistance exercise is thought to result from 2 main mechanisms: (1) increased mechanical stress and/or (2) metabolic stress [34]. The study by Biazon et al. [22] showed that although metabolic stress was highest during high-load isotonic resistance exercise with BFR, muscle strength gains were similar to training protocols with lower levels of metabolic stress (high-load resistance exercise only) or mechanical stress (BFR with low-load exercise). The authors suggested that their results may support that muscle protein synthesis reaches its peak with high-load exercise and that adding metabolic stress to the muscle may not lead to further improvements. Similar to the study by Biazon et al., the studies by Hammert et al. [21], Teixeira et al. [19], and Laurentino et al. [20] also found that BFR did not provide an additional strength benefit to high-load isotonic exercise. BFR does not elicit greater muscle activation when combined with high-load isotonic resistance exercise [23, 35]. The maximum motor unit activation required to develop muscle strength seems to be achieved by high-load resistance training without BFR. Our findings are consistent with the literature, and the reason for the lack of further gains in strength may be that muscle activation and muscle protein synthesis have already reached their maximum levels during high-load resistance exercise performed without BFR. Although isokinetic resistance exercise is quite different from isotonic resistance exercise in that it allows participants to maintain maximum effort throughout each repetition even under fatigue due to the constant angular speed constraint, when applied at high loads and combined with BFR, it does not provide additional benefits in muscle strength similar to isotonic exercise.

We could not find any studies in the literature investigating the effect of adding BFR to high-load isokinetic resistance training on local muscular endurance. Existing studies using isokinetic resistance training evaluated short-term and acute effects of low-load isokinetic resistance training combined with BFR [31, 36] on local muscular endurance. Keller et al. calculated the decrease in maximal isometric torque in measurements made before and after a 2-week isokinetic training program between two groups with and without BFR and found that the addition of BFR made no difference between the two groups on fatigue [31]. This study is quite different from our study. In their study, they performed a 2-week low-load (30% PT) concentric forearm flexion and extension isokinetic resistance exercise program. A 2-week exercise program does not seem to be sufficient for local muscular endurance development. In addition, the low-load exercise may not have created the metabolic stress necessary to develop muscular endurance. Lauver et al. evaluated the acute effect of a single session of low-load (30% PT) eccentric isokinetic exercise and found that the decrease in maximal isometric torque was similar between the groups with and without BFR [36]. In our study, we did not evaluate the short-term effect of BFR, but rather the long-term effect of adding BFR on the FI, and found that adding BFR to high-load isokinetic training caused a significant decrease in the FI after 8 weeks of training. The observed improvements in local muscular endurance, particularly in the BFR group, can be explained by several physiological mechanisms. BFR creates a hypoxic environment within the working muscles, accumulating metabolites such as lactate, hydrogen ions, and inorganic phosphate [23]. This metabolic stress is known to stimulate the production of growth factors like vascular endothelial growth factor (VEGF) and insulin-like growth factor-1 (IGF-1), which promote angiogenesis, mitochondrial biogenesis, and muscle fiber recruitment [16, 37]. Resistance training combined with BFR may increase capillarization in type I muscle fibers more than in type 2 muscle fibers [38]. Type I fibers are fatigue-resistant and contribute significantly to endurance performance. These adaptations could contribute to the observed improvements in local muscular endurance and may explain the greater reduction in FI observed in the BFR group compared to the non-BFR group. However, Hammert et al. investigated the effect of applying BFR with high-load isotonic resistance training on local muscular endurance and found that BFR did not provide additional benefit to the improvement of local muscular endurance [21]. This may be due to the isokinetic resistance exercise, which differs from isotonic resistance exercise in that it allows one to maintain maximum effort throughout each repetition, even under fatigue. This distinguishing feature can enhance the physiological effects of BFR, and adding BFR to high-load isokinetic resistance training can improve local muscular endurance. Our study is the first to demonstrate that adding BFR to high-load isokinetic resistance training leads to an improvement in the FI, a parameter of local muscular endurance. Further studies are needed to investigate the effect of adding BFR to high-load isokinetic resistance exercise on local muscular endurance.

Limitations

While outcomes were assessed before and after the intervention, intermediate assessments (e.g., at 4 weeks) were not performed. This was a deliberate design choice to minimize potential confounding factors associated with repeated testing (e.g., learning/familiarization effects, fatigue, and other measurement reactivity) and to preserve the interpretability of the overall pre-post training effect. However, this approach does not allow characterization of the time course of adaptations or whether responses plateaued during the intervention period. Future studies incorporating carefully controlled multi-time-point assessments could better describe the progression of adaptations and help identify optimal training durations while accounting for influences related to testing. Another limitation of the present study is that randomization was performed before pre-testing of PT and FI, which may have introduced a minor risk of bias. Future studies should consider randomization after pre-testing to further minimize this potential source of bias. Additionally, one limitation of this study is the lack of a time-matched, non-exercise control group. This limits our ability to rule out the potential effects of repeated testing, natural variation, or measurement familiarity [39, 40]. Also, menstrual cycle phase and contraceptive use among female participants were not controlled or recorded. Given that hormonal fluctuations may influence muscle performance and fatigue, future studies should consider controlling for these variables.

Practical applications

This study has important practical implications for athletes, coaches, and clinicians involved in designing resistance training programs aimed at improving local muscular endurance. The findings suggest that adding BFR to high-load isokinetic training can enhance endurance-specific adaptations without compromising strength gains. This approach may be particularly beneficial for middle and long-distance runners, who require sustained muscle performance over extended periods. Additionally, the use of individualized AOP measurements and isokinetic dynamometry provides a framework for implementing BFR safely and effectively in athletic populations. Coaches and practitioners can use these findings to tailor training protocols to specific performance goals, optimizing the balance between strength and endurance development.

Conclusions

This study demonstrates that adding BFR to high-load isokinetic resistance training does not enhance muscle strength but significantly improves local muscular endurance, as evidenced by a greater reduction in FI in the BFR group. These findings highlight the potential of BFR as a tool for optimizing endurance-specific adaptations in athletic populations, particularly runners requiring sustained performance. Practically, this approach can be integrated into high-load isokinetic resistance training programs to enhance local muscular endurance without increasing training volume or load. Future research should focus on investigating the effect of adding BFR to high-load resistance training on local muscular endurance.

Authors’ contributions

H.G., T.K., and B.A. planned and conceptualized the study. G.B. and A.Ö. assisted in the recruitment of the patient population and final review. E.A. performed the statistical analyses.

Funding

This study was supported by the Scientific and Technological Research Council of Türkiye (TUBITAK) under Grant Number 321S398. The authors thank TUBITAK for their support.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

The study was approved by the Health Sciences University Gülhane Training and Research Hospital Clinical Ethics Committee (approve date 11.05.2022 and number: 2022/61) and performed according to Helsinki declaration criteria; signed informed consent forms were collected from all participants.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.