Tendinopathy is a disease entity which can cause significant pain and functional limitations for individuals and collectively places a tremendous burden on society through high healthcare costs (Hopkins et al., 2016; Dean et al., 2017). In chronic tendinopathy, tendons experience morphological changes and can present with increased tendon thickness, fibril disorganization, and neovascularization caused by repetitive tendon microtrauma (Magnusson and Kjaer, 2019; Millar et al., 2021). Tendinopathy prevalence has been shown to be higher in athletes due to frequent jumping, landing, running and change of direction movements (Zwerver et al., 2011). Collectively, tendinopathies can account for up to 30% of all musculoskeletal conditions requiring medical attention, with up to 22% of elite athletes having patellar tendinopathy at least once during their sporting careers (Lian et al., 2005; Skjong et al., 2012; Canosa-Carro et al., 2022). Complete and partial tendon ruptures are also common in both athletes and the general population with the Achilles tendon having the highest prevalence of ruptures (Nyyssonen et al., 2008). Like tendinopathy, tendon ruptures can also cause significant pain, disability and functional limitations and are associated with significant societal and healthcare costs, whether treated surgically or conservatively, with there being a lack of consensus on optimal treatment methods (Holm et al., 2015).

Resistance training has long been considered the treatment of choice in the rehabilitation of chronic tendinopathies, with both eccentric and heavy slow resistance training (HSRT) demonstrating positive clinical effects, for both improving symptoms and tendon structure (Kongsgaard et al., 2010; Beyer et al., 2015). Progressive resistance training is also considered an essential element of rehabilitation following tendon rupture to counteract muscle atrophy and stimulate tendon repair, whether treated conservatively or surgically (Christensen et al., 2020). The application of progressive tendon loads during rehabilitation is essential to not compromise tendon healing, with the precise dosage parameters of resistance training loading a critical consideration (Bohm et al., 2015). Prolonged time under tension with traditional heavy loads during the early phase of tendon rehabilitation could be counterproductive and compromise tendon healing (Loenneke et al., 2012a; Couppe et al., 2015). Blood flow restriction training (BFRT) is a method of resistance training which utilizes pneumatic cuffs or straps around a limb to partially restrict arterial blood flow, while simultaneously occluding venous outflow until the cessation of cuff pressure (Lorenz et al., 2021). BFRT also known as occlusion, hypoxic or Kaatsu training has become increasingly popular over the last decade as a method for enhancing strength gains in healthy populations such as athletes and more recently as a rehabilitation tool in those with musculoskeletal pathologies (Hughes et al., 2017; Barber-Westin and Noyes, 2019; Nitzsche et al., 2021). For example, BFRT has been found to be an efficacious method for increasing strength gains and muscle hypertrophy in rehabilitation following surgery for anterior cruciate ligament (ACL) rupture (Hughes et al., 2018; Caetano et al., 2021). The physiological benefits associated with BFRT, include beneficial adaptations to the cardiovascular, endocrine, and musculoskeletal systems with psychosocial benefits also reported such as mood and performance improvement (Karabulut et al., 2013, 2021; Neto et al., 2016; Silva et al., 2018; Bowman et al., 2019; da Silva et al., 2019; Okita et al., 2019; Freitas et al., 2021a; Miller et al., 2021).

Whilst traditional eccentric or HSRT for tendinopathy utilizes heavy training loads of up to 70% of 1 repetition maximum (1-RM), low-load BFRT (LL-BFRT) typically uses lower training intensities, and loads in the range of 20–40% of 1RM, which may be more tolerable for patients not able to tolerate high muscle-tendon training loads, while still preventing muscle atrophy and promoting hypertrophy (Centner et al., 2019a; Krzysztofik et al., 2019; Shiromaru et al., 2019; Kataoka et al., 2022). Interventional studies have found superior or similar clinical outcomes with LL-BFRT compared to conventional high-load resistance training (HL-RT) in knee rehabilitation for ACL reconstruction, patellofemoral pain, and knee osteoarthritis (Ohta et al., 2003; Bryk et al., 2016; Giles et al., 2017; Ferraz et al., 2018; Korakakis et al., 2018a; Ferlito et al., 2020; Grantham et al., 2021). BFRT has been shown to cause exercise-induced hypoalgesia through endogenous opioid and endocannabinoid mechanisms, so could therefore be a useful pain management tool in early musculoskeletal rehabilitation, particularly in the presence of an acute pain response (Korakakis et al., 2018b; Hughes and Patterson, 2019, 2020; Hughes et al., 2021). Recent evidence suggests that LL-BFRT may be a superior method for augmenting muscular adaptations in early musculoskeletal rehabilitation, which has been found to be comparably effective for inducing muscular hypertrophy and only minimally inferior for increasing muscular strength compared to HL-RT (Manini and Clark, 2009; Abe et al., 2012; Loenneke et al., 2012b; Yasuda et al., 2012; Martin-Hernandez et al., 2013; Lixandrao et al., 2018; Hughes et al., 2019a). The mechanisms of action of BFRT in muscular adaptation are thought to be related to increased inflammation and metabolic stress which can increase blood supply to muscles potentially stimulating muscle growth (Loenneke et al., 2012c; Pearson and Hussain, 2015; Rossi et al., 2018; Freitas et al., 2021a). Other speculated physiological mechanisms explaining muscle hypertrophy adaptations in response to BFRT includes activation of chemoreceptors, muscle swelling, and increased protein synthesis (Freitas et al., 2021a). Due to a paucity of research, it is unclear what effects BFRT may have on tendons, but the induced ischemic muscular milieu may facilitate morphological and mechanical tendon properties through enhanced collagen metabolism and tendon remodeling (Klein et al., 2001; Boesen et al., 2013). Despite these potential beneficial physiological mechanisms of BFRT on tendon healing, the method of training has received a dearth of attention in tendon rehabilitation, despite the clinical benefits found for other musculoskeletal conditions and the knowledge of resistance training being the most evidence-based treatment available for tendinopathies. Therefore, the objective of this scoping review is to evaluate current research on the use of BFRT for treating tendon injuries. The scoping review will be guided by addressing the following review questions on specific aspects of BFRT interventions within tendon rehabilitation: 1. What outcomes have been reported for BFRT in healthy tendons and rehabilitation for tendon injuries and which outcome measures have been used? 2. What BFRT intervention and cuff parameters have been used in published studies? 3. What physiological mechanisms explaining effects of BFRT on tendons and tendon injuries have been investigated in published studies?

Due to the exploratory nature of the research questions a scoping review was conducted as they are recommended for mapping key concepts, evidence gaps and types of evidence within a particular field and can help guide future research and the possibility of conducting systematic reviews on the topic (Tricco et al., 2018). The scoping review is reported in accordance with the Preferred Reporting Items for Systematic reviews and Meta-analysis extension for Scoping reviews known as the PRISMA-ScR (Tricco et al., 2018). This scoping review aimed to evaluate current BFRT interventions in healthy tendons and the rehabilitation of tendon injuries for the first time in the literature. The results will allow dissemination of the parameters of research BFRT interventions to clinical practitioners through peer-reviewed journal publication, allowing increased likelihood of implementation in clinical practice. The review will also outline future research and exercise reporting needs within BFRT interventions in tendon rehabilitation.

The main findings of this scoping review were that despite the dearth of studies available on the effects of BFRT on tendons, studies do indicate that BFRT can produce beneficial effects on tendons. Preliminary evidence from case series and case reports indicates that BFRT may be helpful for improving clinical outcomes such as pain in function in rehabilitation of tendinopathy and tendon ruptures, however no RCTs have been conducted in these populations. The evidence for beneficial changes in healthy tendons is more robust due to several RCTs on the topic, showing beneficial physiological effects on tendon morphology and mechanical properties, including increases in tendon stiffness, with reductions in tendon thickness, vascularity, signal intensity (echogenicity) and skin temperature. Although it is unclear if these beneficial effects found in healthy tendons would also occur with pathological tendons, the preliminary evidence suggesting clinical improvement with BFRT in tendon pathology, is suggestive of potential comparable physiological benefits in tendon pathology. There is a clear need for further interventional studies of BFRT in tendinopathy and tendon rupture rehabilitation, with high quality large scale RCTs required to reach definitive conclusions and recommendations for BFRT in tendon pathology. However, there is a clear scientific rationale for the potential of clinical improvements in tendon pathology with BFRT as evidenced by the beneficial effects seen in healthy tendons, and the improvement of clinical outcomes with BFRT in other musculoskeletal disorders (Ohta et al., 2003; Bryk et al., 2016; Giles et al., 2017; Ferraz et al., 2018; Korakakis et al., 2018a; Ferlito et al., 2020; Grantham et al., 2021). Given the increased research interest and clinical use of BFRT in musculoskeletal rehabilitation for non-tendon pathologies, the dearth of available studies applying BFRT to tendon pathologies could be considered somewhat surprising. This is particularly relevant considering resistance training is considered the gold-standard first-line treatment intervention for tendinopathies, particularly Achilles and patellar tendinopathy, due to a plethora of evidence showing the clinical efficacy of resistance training such as eccentric and heavy slow resistance training (Burton and McCormack, 2021). Perhaps the belief that resistance training in tendinopathy must include high training loads has been a limiting factor in the application of LL-BFRT and could explain why it is an underutilized tool in tendon rehabilitation.

The evidence from RCTs comparing LL-BFRT with HL-RT, suggests comparable outcomes for improving muscle and tendon properties (Centner et al., 2019b, 2021), with these changes possibly serving as the mechanisms to explain the clinical benefit seen with BFRT in the case reports in tendinopathy and tendon rupture rehabilitation. The first RCT investigating the effects of LL-BFRT compared to HL-RT in patellar tendinopathy has been registered in Denmark, by the authors who conducted the positive case series included in this review (Skovlund et al., 2020). This trial will be the first step in determining if a shift is required in the tendinopathy rehabilitation field, from the belief that HL-RT is a prerequisite for improving outcomes in tendinopathy, to a possible future where both HL-RT and LL-BFRT are both viable rehabilitation methods, giving clinicians and patients more options and choice during rehabilitation. This may be particularly relevant for non-athletic patients who are unaccustomed to training with heavy loads, sedentary elderly patients, or those who may have contraindications to heavy training and those with an acute painful or reactive tendinopathy or recent tendon rupture, who would be unable to tolerate the loads required with HL-RT. In the rehabilitation of ACL ruptures, LL-BFRT has been found to be a beneficial training method for increasing muscular adaptations in those who have difficultly performing HL-RT (Palmieri-Smith and Lepley, 2015). Furthermore, LL-BFRT has been shown to attenuate pain, increase strength and improve function in rehabilitation for hospital inpatients (Ladlow et al., 2018), ACL rupture (Patterson et al., 2019), patellofemoral pain (Constantinou et al., 2022), rheumatoid arthritis (Rodrigues et al., 2020), ankle fractures (Larsen et al., 2021), and knee osteoarthritis (Ferraz et al., 2018), suggesting pain improvement may be possible with lower training loads in tendon injuries without requiring all patients to undertake HL-RT.

Included studies used low training intensities, with most programming training based on a percentage of an individual's 1-RM, typically 30%, which is congruent with loads between 20 and 40% of 1RM which are typically recommended in the BFRT literature (Kilgas et al., 2019). It is well-established that LL-BFRT requires a higher volume of repetitions to derive physiological adaptations (Kraemer and Ratamess, 2004), with the 30-15-15-15 program of 75 repetitions per set, completed with four sets typically recommended (Patterson et al., 2019). Whilst seven studies implemented this regime, the number of sets across studies ranged from 3 to 6, with repetitions ranging from 5 to 30, with one study using muscular failure instead of predefined repetitions. It is unclear if training to volitional muscular failure with BFRT is required to derive adaptations, with previous BFRT studies suggesting it may be unnecessary (Patterson et al., 2019). Previous studies have shown that muscular failure is not required for muscle hypertrophy, with overall training load volume considered more relevant for augmenting hypertrophy (Schoenfeld et al., 2017, 2019; Lasevicius et al., 2018, 2019). Details on rest periods and whether cuff pressure was maintained or deflated between sets and exercises varied across studies. However, previous research has shown that rest with an inflated or deflated cuff are viable options (Yasuda et al., 2013), although longer rest periods may reduce metabolic stress and therefore limit potential adaptations compared to short rest periods (Loenneke et al., 2010a,b; Patterson et al., 2019). Despite large variances in the BFRT arterial occlusion pressure of included studies which ranged from 30 to 80%, recommendations for occlusion pressure in the literature do range from 40 to 80% (Loenneke et al., 2011; Patterson et al., 2019), suggesting pressure should be individualized based on measures of arterial pressure and comfort levels (Jessee et al., 2016; Mattocks et al., 2017).

This review has several limitations, particularly the small number of studies included, with only five studies on tendon pathology, all being case series or case reports, highlighting the need for future high-quality studies with larger sample sizes, as there are no RCTs on BFRT in tendon pathology currently available. Future studies should also investigate the effects on specific subgroups known to be at increased risk for tendon injuries such as athletes. There was considerable heterogeneity of the BFRT parameters implemented in studies, with standardized methods and reporting of interventions required in future BFRT studies in tendon rehabilitation to enhance clinical translation of the research interventions. The longest follow-up times of included BFRT interventions were 14 weeks, with much longer follow up times required to assess the long-term adaptations and outcomes of BFRT on healthy and pathological tendons. Methods for monitoring and recording adherence to BFRT should also be emphasized in future studies as several included studies did not report the adherence level to BFRT, which may vary due to perceptual responses and comfort which may affect reported clinical outcomes (Loenneke et al., 2011; Martin-Hernandez et al., 2017; Freitas et al., 2021b; Suga et al., 2021).

The superiority of LL-BFRT over standard LL-RT for muscular adaptations have been previously highlighted (Takarada et al., 2002; Madarame et al., 2008; Abe et al., 2010a,b; Yasuda et al., 2010; Centner et al., 2019c; Lambert et al., 2021), with findings from this review suggesting the same may be true for tendon adaptations. However, it remains unclear whether LL-BFRT or standard HL-RT is a superior method for inducing muscular adaptations, with some studies finding equal benefit for muscle strength gains (Lixandrao et al., 2015; Vechin et al., 2015; Curran et al., 2020; Gronfeldt et al., 2020; Hill et al., 2020) and others suggesting HL-RT is a superior method (Hughes et al., 2019b). Some studies included in this review suggest that the tendon adaptations in the healthy Achilles and patellar tendon following LL-BFRT are comparable to those evoked by HL-RT, which is an encouraging finding for the field of tendon rehabilitation (Centner et al., 2019b, 2021). However, these comparable beneficial tendon adaptations found in the high-quality RCTs on healthy tendons need to be investigated in high-quality RCTs in tendon pathology before conclusions can be drawn and recommendations made. Such findings, if found to be comparable and translate in tendon pathology may require a paradigm shift in the tendinopathy rehabilitation field in relation to the prescription of resistance training interventions, particularly for select populations not able to tolerate the standard and proven HL-RT interventions (Loenneke et al., 2013).

Despite a dearth of studies to date on the effects of BFRT on healthy tendons and in tendon pathologies such as tendinopathy, preliminary evidence for beneficial effects of BFRT on tendons and clinical outcomes is encouraging. As BFRT is a relatively novel method, particularly its application in musculoskeletal rehabilitation, definitive conclusions, and recommendations on BFRT in tendon rehabilitation cannot be made at present, which should be addressed in future research, due to the potential therapeutic benefits highlighted in this review. The addition of LL-BFRT as a viable rehabilitation method in tendinopathy rehabilitation would be complimentary to currently utilized HL-RT interventions and provide more rehabilitation options for patients unable to tolerate HL-RT during tendon rehabilitation.

IB conceptualized the work, developed the methods, search strategy, and framework for the review. IB and AM contributed to the development of the research questions and the study design. Both authors developed the first and subsequent drafts of the manuscript and reviewed and approved the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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