
==== Front
Int J Sports Phys Ther
Int J Sports Phys Ther
2159
International Journal of Sports Physical Therapy
2159-2896
NASMI Website: International Journal of Sports Physical Therapy

35949372
36437
10.26603/001c.36437
Scoping Review
Inertial Flywheel Resistance Training in Tendinopathy Rehabilitation: A Scoping Review
https://orcid.org/0000-0002-6224-4401
Burton Ian 1
McCormack Aisling 2
1 Musculoskeletal physiotherapy NHS Grampian https://ror.org/00ma0mg56
2 Aberdeen City Council https://ror.org/04cfb2973
Corresponding author: Aisling McCormack BSc Aberdeen City Council, Aberdeen, U.K. ianburton_10@hotmail.co.uk
1 8 2022
2022
17 5 775786
19 1 2022
9 4 2022
© The Author(s)
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (4.0) which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

Background

Inertial Flywheel Resistance Training (IFRT) has recently emerged as a beneficial rehabilitation option for some musculoskeletal disorders. Although the use of resistance training as treatment for tendinopathy has become widespread, it is unclear if IFRT has efficacy as a treatment option for tendinopathies.

Objective

To identify current evidence on IFRT in the treatment of tendinopathy, evaluating intervention parameters and outcomes.

Methods

This scoping review was reported in accordance with the PRISMA Extension for Scoping Reviews (PRISMA-ScR). Databases searched included MEDLINE, CINAHL, AMED, EMBase, SPORTDiscus, Cochrane library (Controlled trials, Systematic reviews), and five trial registries. Two independent reviewers screened studies at title, abstract and full text. Following screening, data were extracted and charted, and presented as figures and tables alongside a narrative synthesis. Any study design conducted on adults, investigating the effects of IFRT for tendinopathy were included. Data were extracted on intervention parameters and outcomes of IFRT interventions.

Results

Four studies on patellar tendinopathy were included. A variety of outcomes were assessed, including pain, function, strength, power, and tendon morphological and mechanical properties, particularly changes in tendon thickness. IFRT intervention parameters were largely homogenously prescribed, with slight variances.

Conclusion

Despite a paucity of studies to date on the effects of IFRT for treating tendinopathy, preliminary evidence for beneficial effects of IFRT on clinical outcomes in patellar tendinopathy is encouraging. As IFRT is a relatively new and unexplored method in tendinopathy rehabilitation, definitive conclusions, and recommendations cannot be made at present, which should be addressed in future research, due to the potential therapeutic benefits highlighted in this review.

Flywheel training
Tendinopathy
Resistance training
Exercise
Physiotherapy
Tendon
No sources of funding were used to assist in the preparation of this article.
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pmcINTRODUCTION

The use of flywheel devices in resistance training can be dated back over one hundred years, with an increased interest since 1994 due to the development of flywheel devices to counteract muscle strength loss and atrophy for astronauts undertaking space flight.1 Inertial flywheel resistance training (IFRT) has been found to be a beneficial alternative or compliment to traditional resistance training methods in both healthy athletic populations and in rehabilitation for some musculoskeletal injuries.2 The beneficial effects of flywheel training in athletes for increasing muscle strength and hypertrophy have been well documented, but only recently has evidence emerged regarding its beneficial applications in the rehabilitation setting.3 IFRT incorporates the use of inertial resistance via a rotating flywheel device that requires increased muscle contraction speeds compared to standard resistance training, which with repeated training over time can lead to increases in muscular power and strength.4–6 During maximum effort concentric movements, rotation of the flywheel generates kinetic energy, which when braked during the return eccentric movement causes increased eccentric resistance to working muscles and tissues.7 Due to the isoinertial type of resistance involved, greater eccentric muscle activation can occur compared to traditional resistance training using free weights.8 A plethora of beneficial effects have been demonstrated on muscle tissue with flywheel training, including increased muscle power, strength, length, muscle activation, hypertrophy, and an increase in fast twitch muscle fibers.9 Improvements in athletic performance indicators such as change of direction ability, post activation potentiation, maximal running speed and vertical jump height has also been demonstrated in athletes.10–13 Despite eccentric overload of muscles occurring during flywheel training, muscle adaptations have been found to occur without increased pain or risk for musculoskeletal injury during training.14 Eccentric biased training such as flywheel training activates the stretch-shortening cycle more efficiently than traditional isotonic resistance training, which can increase muscle adaptation through the addition of sarcomeres known as sarcomerogenesis, which may lead to greater adaptations if applied during injury rehabilitation.15 The use of IFRT has been found to not only improve athletic performance but it may also decrease the risk of musculoskeletal injury during competition in athletes.16,17 Due to its beneficial effects on muscle tissue, IFRT can also be used in isolation or in combination with traditional rehabilitation methods, which may lead to enhanced outcomes, particularly for acute muscle injuries and degenerative muscle conditions such as sarcopenia.18,19 Muscle weakness and atrophy can also be reduced during post-surgical early rehabilitation for joint injuries, such as after anterior cruciate ligament reconstruction, when muscle atrophy is a common problem.20 Studies have shown that IFRT can be used in all stages of musculoskeletal injury rehabilitation to increase muscle strength and functional performance, from post-surgical to return to sport phases.21–23

Tendinopathy represents a spectrum of tendon pathology, associated with changes to the structural tendon collagen matrix, presence of various inflammatory cells and clinical symptoms of pain and impaired performance, with potential for progression to a chronic degenerative condition.24 Tendinopathy is common in athletes with high training demands and is associated with repetitive tendon microtrauma, with Achilles and patellar tendinopathy having the highest prevalence in athletes.25 Traditional eccentric training has been the gold standard first-line management approach for tendinopathies for the last two decades, due to its documented beneficial clinical effects for both upper and lower limb tendinopathies.26,27 Despite the largest body of evidence existing for eccentric training, heavy slow resistance training (HSRT) involving both concentric and eccentric actions has been shown to have comparable positive clinical outcomes.28,29 Due to these findings, some experts have suggested there is no scientific rationale for eliminating concentric actions in tendinopathy rehabilitation, with IFRT a potentially efficacious rehabilitation method due to having both a concentric and eccentric overload component and therefore requiring less overall training exposure, which may improve adherence.30–32 Despite this, there are a lack of recommendations for the use of IFRT and it is not routinely used in clinical rehabilitation for tendinopathies. The effects of flywheel training on healthy and pathological tendons are not well documented, with some evidence indicating improvements in tendon stiffness.33 Due to a paucity of research, it is unclear what effects IFRT may have on tendons, but the induced muscular adaptations also suggest mechanical tendon properties, collagen metabolism and tendon remodelling may be enhanced due to the eccentric overload effect.34,35 The application of progressive tendon loads during rehabilitation is essential to not compromise tendon healing, with the precise dosing parameters of resistance training loading a critical consideration.36 Prolonged time under tension with traditional heavy loads and increased training frequency during the early phase of tendon rehabilitation could be counterproductive and compromise tendon healing.37 Despite the potential applicability and possible beneficial physiological mechanisms of IFRT 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.38,39 Therefore, the objective of this scoping review is to evaluate current research on the use of IFRT for treating tendinopathies. The scoping review will be guided by addressing the following review questions on specific aspects of IFRT interventions within tendinopathy rehabilitation: 1. What outcomes have been reported for IFRT in rehabilitation for tendinopathy and which outcome measures have been used? 2. What IFRT intervention and device parameters have been used in published studies? 3. What physiological mechanisms explaining effects of IFRT for tendinopathy have been investigated in published studies?

METHODS

Due to the exploratory nature of the research questions of this review, 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.40 The scoping review is reported in accordance with the Preferred Reporting Items for Systematic reviews and Meta-analysis extension for Scoping reviews (PRISMA-ScR).40

Eligibility criteria

The inclusion criteria for the scoping review were guided by a modified PICO (PCoCo), which is recommended for scoping reviews.40 The review considered studies with adult participants aged eighteen years or older with a diagnosis of any tendinopathy for any time duration. All lower and upper limb tendinopathies were considered for inclusion, such as gluteal, hamstring, patellar, Achilles, rotator cuff and lateral elbow. Any tendon condition characterized by common tendinopathy symptoms, in the absence of a full thickness tendon rupture were considered for inclusion. A clinician’s diagnosis based on verifiable clinical features including pain location, response to palpation or tendon loading with specific tendinopathy tests were accepted for inclusion. Studies including participants with other concurrent injuries or medical conditions were excluded. The concept of interest was IFRT for the treatment of any tendinopathy, including any type or format. Therefore, any muscle contraction type performed with IFRT, including eccentric, concentric, isotonic, isometric, plyometric, general strength training, or combinations of these exercise types were considered. The IFRT intervention could be used as a first or second-line intervention and may be delivered in isolation or combined with other treatments. The context considered for inclusion included any setting in which IFRT was delivered by health or exercise professionals, in a supervised or unsupervised manner, using any methods for training progression and compliance. This scoping review considered both experimental and quasi-experimental study designs for inclusion, including randomized controlled trials, non-randomized controlled trials, prospective and retrospective cohort studies, case series and case reports. Unpublished studies, systematic or narrative reviews or reports were not considered for inclusion.

Search strategy

The search was carried out using a uniform search strategy across all databases (Appendix 1) and it included key words from two main concepts: IFRT (‘flywheel’, ‘inertial’, ‘isoinertial’, ‘eccentric overload’), and Tendon (‘tendon’, ‘tendinopathy’, ‘tendon injury’). The Boolean operators “Or” and “And” were used to link the key words from each concept and to link the concepts themselves, respectively. A 3-step search strategy was implemented in this scoping review. It incorporated the following: 1) a limited search of MEDLINE and CINAHL using initial keywords as, followed by analysis of the text words in the title/abstract and those used to describe articles to develop a full search strategy; 2) The full search strategy was adapted to each database and applied to MEDLINE, CINAHL, AMED, EMBase, SPORTDiscus, and the Cochrane library (Controlled trials, Systematic reviews). The following trial registries were also searched: ClinicalTrials.gov, ISRCTN, The Research Registry, EU-CTR (European Union Clinical Trials Registry), ANZCTR (Australia and New Zealand Clinical Trials Registry). Databases were searched from inception to January 18th, 2022 (Search performed on January 18th, 2022). The search for relevant grey literature included Open Grey, MedNar, Cochrane central register of controlled trials (CENTRAL), EThOS, CORE, and Google Scholar. 3) For each article located in steps 1 and 2, a search of cited and citing articles using Scopus and hand-searching where necessary, was conducted. Studies published in a language other than English were only considered if a translation was available as translation services are not available to the authors.

Study selection

Following the search, all identified citations were collated and uploaded into RefWorks and duplicates removed. Titles and abstracts were screened by two independent reviewers and assessed against the review inclusion criteria. Potentially relevant studies were retrieved in full, with their details imported into Covidence software (Veritas Health Innovation, Melbourne, Australia). Two independent reviewers assessed the full text of selected articles in detail against the review inclusion criteria. Any disagreements that arose at any stage of the study selection process were resolved through discussion or by input from a third reviewer. The results of the search are reported in accordance with the PRISMA-ScR. In accordance with guidance on conducting scoping reviews, critical appraisal was not conducted.40

Data extraction

Data were extracted from sources included in the scoping review by one reviewer, with independent data extraction by a second reviewer for at least 10% of studies. The data extracted included specific details regarding the population, study methods and key findings relevant to the review questions. The data extracted included dimensions such as study type, purpose, population & sample size, methods, details of the IFRT intervention, specific exercises, outcome measures used and clinical outcomes. Details of the IFRT interventions included type, dosage, device and intervention parameters, and methods used to progress the training stimulus. Data were also extracted on any physiological mechanisms such as effects on tendon morphological and mechanical properties, which have been investigated to explain the effects of IFRT in tendinopathy. The extracted data are presented in Table 1 with an accompanying narrative synthesis.

92719 Table 1. Characteristics, intervention parameters and outcomes of included studies

Author, Study design, population	Intervention, exercises, duration	Training parameters	Device parameters	Outcome measures	Outcomes, results	
Ruffino et al. 2021,41 RCT, n=42, patellar tendinopathy	1. Heavy slow resistance training (squat, hack squat, leg press). 2. Inertial Flywheel training (squat, leg press, knee extension). 12 weeks	Sets: 4, Reps: 12(10 max effort), Freq: 1 x WK, Prog: NR, Int: 8-RM, reps 1–2 were used for increasing the inertial resistance, and reps 3–12 were executed
with maximal effort. Rest: 2-3 MIN between sets. Session time: 20 MIN	Three custom inertial flywheel machines: 2-legged squat, leg press and knee extension (Ivolution, Sunchales, Argentina). Each coupled concentric and eccentric actions were completed with a repetition cycle of about 3 seconds. Inertia loads were: 2.5 kg flywheel (moment inertia 0.05 kg/m2) from week 1-6 and 4 kg flywheel (moment inertia 0.10 kg/m2) from week 6-12.	Pain & function (VISA-P), Patient Specific Functional Scale (PSFS), health status (EuroQol-5D), patient impression of change on pain and function, adherence, adverse events, pain provocation test for the patellar tendon (0-10), physical tests (strength & power), patellar tendon thickness and doppler signal on ultrasound.	Both groups improved clinical outcomes, with no significant difference between groups in clinical outcomes, physical tests (strength & power), or tendon thickness & neovascularization. Adherence: diary, 88% (HSRT), 90% (Flywheel).	
Romero-rodriguez et al.42 2011, case series, n=10, patellar tendinopathy	1. Isoinertial flywheel ECCT, maximal effort, leg press, 6 weeks	Sets: 4, Reps: 10, Freq: 1 x WK, Prog: NR, Int: 8-RM, reps 1–2 were used for increasing the
inertial resistance, and reps 3–10 were executed
with maximal effort. Rest: 2 MIN between sets. Session time: 20 MIN.	Leg press device, provided by the manufacturer (YoYo Technology AB, Stockholm, Sweden). The concentric phase was executed from about 90º degrees knee angle to almost full extension about the knee joint.
Subjects were instructed to resist gently during the first two thirds of the eccentric action and then apply maximal force to bring the wheel to a stop at approximately 90º. Thus, eccentric overload was achieved in the last third of any eccentric action.	Pain (VAS), function (VISA-P), lower limb maximal strength and vertical counter-movement-jump (CMJ) height. Surface electromyography (SEMG)	Intervention was effective for improving clinical outcomes. Eccentric strength increased but power (CMJ) did not.
Adherence: NR	
Abat et al.43 2014, cohort, n=33, patellar tendinopathy	1. Intratissue Percutaneous Electrolysis (EPI) + Flywheel training, Isoinertial leg press training machine, 12 weeks	Sets: 3, Reps: 10, Freq: 2 x WK, Prog: always maximum intensity, Int: 10-RM.	Isoinertial resistance machines (YoYoTM Technology AB, Stockholm, Sweden). Each repetition was performed with the concentric phase with both extremities whereas the eccentric phase was only performed with the affected limb at a maximum 60º of knee flexion.	Pain & function (VISA-P), Tegner scale, Roles and Maudsley scale.	Significant improvement in pain & function
Adherence: supervised, %NR	
Abat et al.44 2015, cohort, n=40, patellar tendinopathy	1. Intratissue Percutaneous Electrolysis (EPI) + Isoinertial ECCT, leg press machine, 12 weeks	Sets: 3, Reps: 10, Freq: 2 x WK, Prog: NR, Int: 10-RM	Resistance isoinertial leg-press machine (YoYoTM Technology AB, Stockholm, Sweden). Each repetition was performed with the concentric phase with both extremities, whereas the eccentric phase was only performed with the affected limb at a maximum of 60º of knee flexion.	Pain & function (VISA-P), Tegner score, Blazina’s classification, Roles and Maudsley scale.	Combination was effective for improving clinical outcomes. Adherence: supervised, %NR	
Abbreviations: RM: repetition maximum, MIN: minute, NR: not reported, Int: intensity, Freq: frequency, Prog: Progression, RCT: randomised controlled trial, VAS: visual analogue scale, VISA-P: Victorian Institute of Sport Assessment Patellar, n: number, WK: week, Heavy slow resistance training: HSRT.

RESULTS

Included study characteristics

The literature search yielded 130 articles, reduced to 84 after removing duplicates, of which four met the inclusion criteria and were included in the review, with the search results summarised in the PRISMA flow chart (Figure 1). An overview of the characteristics, IFRT parameters and outcomes of the included studies are provided in Table 1. The four studies investigated the effects of IFRT on patellar tendinopathy, including one randomized controlled trial (RCT),41 one case series,42 and two cohort studies.43,44 No studies investigating the effects of IFRT on other tendinopathies were found in the literature search. The sample sizes of included studies ranged from 10-42, with a total of 125 participants with patellar tendinopathy. All included studies investigated the effects of a IFRT intervention, one in isolation,42 one compared with HSRT,41 and two combined with intratissue percutaneous electrolysis (EPI).43,44 The duration of IFRT interventions ranged from 6-12 weeks. The most common exercises used for the IFRT interventions were, leg press in four studies, squats in one study, and knee extension in one study.

92724 Figure 1. PRISMA study flow diagram

Outcome measures

All four studies assessed patient reported pain and function as an outcome measure with IFRT using the Victorian Institute of Sport Assessment Patellar (VISA-P) for patellar tendinopathy. One study used the VAS scale for pain, two studies used the Tegner scale, two studies used the Roles and Maudsley scale, one study used the Blazina scale, and one study used the Patient Specific Functional Scale (PSFS). One study also assessed quality of life using the EuroQol-5D. Two studies assessed the effects of IFRT on strength and power, using methods such as maximal strength testing and the counter movement jump test. One study used ultrasound to assess patellar tendon thickness and doppler signal to measure tendon neovascularization. One study also used surface electromyography (EMG) to assess muscle function.

Outcomes

The four studies included in this review that assessed the effects of IFRT on clinical outcomes in patellar tendinopathy all found significant improvement in pain and function as measured by the VISA-P. However, only two studies used IFRT in isolation,41,42 whereas the other studies implemented IFRT combined with EPI, which may have affected outcomes.43,44 The two studies that assessed strength outcomes, all found increases in strength and power following the IFRT intervention. The only RCT in patients with patellar tendinopathy,41 found equal improvement compared to HSRT, in terms of pain, function, strength, power, patellar tendon thickness and neovascularization.

Training parameters

The four included studies all used specific IFRT devices in their interventions, with three using leg press machines from the same manufacturer (YoYo Technology AB, Stockholm, Sweden). Whereas these studies focused on one specific exercise and device, the other study41 included three specific devices (Ivolution, Sunchales, Argentina) to allow three different exercises in the intervention; leg press, knee extension and squat. Despite some variances in the sets and repetitions of prescribed exercises, exercise prescription was largely homogenous across the included studies. Two studies prescribed 4 sets of 10 repetitions, once per week, with the first two repetitions used for increasing the inertial resistance, and repetitions 3–10 were always executed with maximal effort.41,42 The studies following this same protocol allowed for two minutes of rest between sets. The other two studies prescribed 3 sets of 10 repetitions, twice per week, all repetitions were performed with maximum intensity for 10 repetition maximum (RM).43,44

DISCUSSION

The main findings from this scoping review are that despite the paucity of research on IFRT as a treatment for tendinopathy, the available albeit preliminary evidence indicates that IFRT is a potentially clinically effective management option for patellar tendinopathy, particularly in athletic populations. All studies found in the literature were conducted on patellar tendinopathy, with two studies findings pain and function improvement with IFRT in isolation, and two studies finding clinical improvement with IFRT combined with EPI. Two studies also found significant strength and power improvement with IFRT, with one study finding positive tendon adaptations on ultrasound. The findings from the RCT included in this review may be of particular significance to the clinical rehabilitation and prevention of patellar tendinopathy, particularly in athletes.41

For several decades, heavy eccentric and more recently, heavy slow resistance training have been considered the conservative cornerstone of manging lower limb tendinopathies, as supported by evidence.45 However, the preeminent HSRT protocol in patellar tendinopathy, known as the Kongsgaard protocol in reference to the authors who developed it, which has shown efficacy in several RCTs,29,46–50 was found to be no more effective than IFRT, for improving pain, function, strength, power, and tendon properties. The RCT highlighted how several clinical and physical outcomes could be improved by IFRT at a comparable level to the gold standard HSRT protocol.41 Somewhat surprisingly, this RCT was the only study included in this review that assessed tendon properties as an outcome measure, finding improvements in tendon thickness and neovascularization as assessed by ultrasound and doppler signal. Despite this, previous studies examining the effects of IFRT on healthy tendons, have shown potential beneficial changes in tendon structure, suggesting the same is true for pathological tendons.33 Although further confirmatory research is required to confirm that IFRT induces positive adaptations in tendinopathy, the preliminary findings from this RCT are encouraging. Further studies with larger sample sizes are required to confirm the efficacy of IFRT as a patellar tendinopathy treatment, with the evidence from studies included in this review suggesting possible clinical benefits. There is also a clear scientific rationale for IFRT having potential clinical benefits in tendinopathy, due to the previously highlighted physiological mechanisms it induces, particularly the muscular and tendon responses induced by eccentric overload training.4,9

Although the focus of this review was on the use of IFRT in the rehabilitation of tendinopathy, findings from other studies have suggested a possible role for IFRT in prevention or in-season management of patellar tendinopathy. In a sample of 81 jumping athletes who were at high risk for developing patellar tendinopathy due to repetitive tendon loading, a once weekly IFRT protocol added to normal in-season sports training led to strength and power improvements, along with no increases in patellar tendon pain or diagnoses of patellar tendinopathy within the sample.51 Given the high prevalence and incidence of patellar tendinopathy in elite jumping athletes,52,53 this evidence of a prophylactic effect of IFRT is a potentially significant finding. Although more studies are required to confirm the efficacy of IFRT in preventing patellar tendinopathy, the evidence is nonetheless encouraging given the lack of preventative patellar tendinopathy interventions available,54 and the high number of associated risk factors identified in jumping athletes.55 Despite the potential efficacy of IFRT as a multifaceted tool in the rehabilitation of patellar tendinopathy, current evidence may be limited to athletic populations and not generalizable to other populations, until further evidence emerges.

No studies have investigated IFRT in rehabilitation with any other tendinopathies, so no recommendations can be made for IFRT with any tendinopathy other than patellar at present. Although IFRT may have potential therapeutic benefits in the management of any tendinopathy, Achilles tendinopathy would seem an obvious choice as an immediate candidate for investigation. While eccentric training is recommended as a treatment for most tendinopathies, this method originated with eccentric calf raise training in Achilles tendinopathy,56 and currently the best evidence of clinical effectiveness of eccentric training is for Achilles tendinopathy, with a plethora of RCTs showing efficacy.57,58 Given IFRT is believed to exert clinical benefit and induce positive muscle and tendon adaptations through training with eccentric overload, it may have benefit as an Achilles tendinopathy treatment and should therefore be investigated in future research. Squat training utilising a flywheel squat device has been found to increase Achilles tendon cross-sectional area and the pennation angle of the medial aspect of the gastrocnemius muscle, suggesting eccentric overload training with flywheel devices is feasible for the Achilles tendon.59 Although the study by Sanz-Lopez et al.59 was conducted with healthy Achilles’ tendons and did not assess clinical benefit in Achilles tendinopathy, it serves to highlight the potential for positive Achilles tendon adaptations with IFRT, which may confer similar benefit to Achilles tendinopathy as seen with IFRT for patellar tendinopathy.

This review has several limitations, particularly the paucity of available research on IFRT for tendinopathy, with only four studies included, and only one RCT, highlighting the need for future high-quality studies with larger sample sizes. Future studies should also investigate the effects on specific subgroups known to be at increased risk for tendinopathy, including running as well as jumping athletes and the general sedentary population. Although the IFRT parameters implemented in studies were largely homogenous, there were some variances in sets, repetitions, and training frequency. Therefore, future studies should consider standardized methods and reporting of IFRT interventions in tendinopathy rehabilitation to enhance clinical translation of the research interventions. The longest follow-up times of included IFRT interventions were 12 weeks, with much longer follow up times required to assess the long-term adaptations and outcomes of IFRT in tendinopathy patients. Methods for monitoring and recording adherence to IFRT should also be emphasised in future studies as several included studies did not report the adherence level to IFRT, which may vary due to individual compliance, which may affect reported clinical outcomes.

CONCLUSION

Despite the paucity of research conducted to date on the effects of IFRT as a treatment option for tendinopathies, a small body of evidence is forming suggesting that IFRT may be an effective multifaceted tool at the clinician’s disposal, for use in the rehabilitation of patellar tendinopathy. While the evidence is preliminary, the included studies in this review all found beneficial clinical, physical, and tendon outcomes with IFRT in subjects with patellar tendinopathy. One high quality RCT has shown comparable efficacy for multiple outcomes to the currently considered gold standard method of HSRT. Further high quality RCTs with larger sample sizes are required to confirm the efficacy of IFRT in patellar tendinopathy and explore its efficacy with other tendinopathies such as Achilles tendinopathy. Evidence from studies included in this review, alongside the known scientific rationale for muscle and tendon adaptations with IFRT, is suggestive of potential clinical benefits as a tendinopathy treatment and possible alternative or compliment to current management methods.

Conflicts of interest/Competing interests

None declared.

Data availability statement

All data relevant to the study are included in the article or uploaded as supplementary information.

Appendix 1: Search strategy

Information sources and search strategy.

Databases: MEDLINE (PubMed), CINAHL, AMED, EMBase, SPORTDiscus CENTRAL of Cochrane Library

Search fields: Title, abstract, key words

Search terms (database subject headings)

1. - “Flywheel” OR “inertial” OR “isoinertial” OR “eccentric overload” 2 - “tendon” OR “tendinopathy” OR “tendon injury”

3. 1 AND 2
==== Refs
A gravity-independent ergometer to be used for resistance training in space Aviat Space Environ Med Berg H.E. Tesch A. 1994
65 8 752 756 7980338
Implementing high-speed running and sprinting training in professional soccer International Journal of Sports Medicine Beato Marco Drust Barry Iacono Antonio Dello 2021
42 4 295 299 0172-4622 10.1055/a-1302-7968 10.1055/a-1302-7968 33291180
The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis British Journal of Sports Medicine Roig M O'Brien K Kirk G Murray R McKinnon P Shadgan B Reid W D 2009
43 8 556 568 0306-3674 10.1136/bjsm.2008.051417 10.1136/bjsm.2008.051417 18981046
Flywheel training periodization in team sports Frontiers in Physiology Beato Marco Maroto-Izquierdo Sergio Hernández-Davó José L. Raya-González Javier 8 11 2021
12 732802 1664-042X 10.3389/fphys.2021.732802 10.3389/fphys.2021.732802 34819871
Characterising overload in inertial flywheel devices for use in exercise training Sports Biomechanics Carroll Kevin M. Wagle John P. Sato Kimitake Taber Christopher B. Yoshida Nobuhisa Bingham Garett E. Stone Michael H. 2019
18 4 390 401 1476-3141 10.1080/14763141.2018.1433715 10.1080/14763141.2018.1433715 29558854
Impact of inertial training on strength and power performance in young active men J Strength Cond Res Naczk M. Naczk A. Brzenczek-Owczarzak W. Arlet J. Adach Z. 2016
30 8 2107 2113 10.1097/JSC.0000000000000217 10.1097/JSC.0000000000000217 27457914
Clinical applications of iso-inertial, eccentric-overload (YoYo) resistance exercise Frontiers in Physiology Tesch Per A. Fernandez-Gonzalo Rodrigo Lundberg Tommy R. 27 4 2017
8 241 1664-042X 10.3389/fphys.2017.00241 10.3389/fphys.2017.00241 28496410
Flywheel resistance training calls for greater eccentric muscle activation than weight training European Journal of Applied Physiology Norrbrand Lena Pozzo Marco Tesch Per A. Springer Science and Business Media LLC 30 7 2010
110 5 997 1005 1439-6319 10.1007/s00421-010-1575-7 10.1007/s00421-010-1575-7 20676897
Skeletal muscle functional and structural adaptations after eccentric overload flywheel resistance training: a systematic review and meta-analysis Journal of Science and Medicine in Sport Maroto-Izquierdo Sergio García-López David Fernandez-Gonzalo Rodrigo Moreira Osvaldo C. González-Gallego Javier de Paz José A. 10 2017
20 10 943 951 1440-2440 10.1016/j.jsams.2017.03.004 10.1016/j.jsams.2017.03.004
Chronic effects of flywheel training on physical capacities in soccer players: a systematic review Research in Sports Medicine Allen William J.C. De Keijzer Kevin L. Raya-González Javier Castillo Daniel Coratella Giuseppe Beato Marco 27 7 2021
2021 1 21 1543-8627 10.1080/15438627.2021.1958813 10.1080/15438627.2021.1958813
The effect of a weekly flywheel resistance training session on elite U-16 soccer players’ physical performance during the competitive season. A randomized controlled trial Research in Sports Medicine Raya-González Javier Castillo Daniel de Keijzer Kevin L. Beato Marco 5 1 2021
29 6 571 585 1543-8627 10.1080/15438627.2020.1870978 10.1080/15438627.2020.1870978
Effects of flywheel training on strength-related variables in female populations. A systematic review Research in Sports Medicine Raya-González Javier de Keijzer Kevin L. Bishop Chris Beato Marco 5 1 2021
2021 1 18 1543-8627 10.1080/15438627.2020.1870977 10.1080/15438627.2020.1870977
Effects of flywheel resistance training on sport actions. A systematic review and meta-analysis Journal of Human Kinetics Raya-González Javier Prat-Luri Amaya López-Valenciano Alejandro Sabido Rafael Hernández-Davó Jose Luis 30 1 2021
77 1 191 204 1899-7562 10.2478/hukin-2021-0020 10.2478/hukin-2021-0020 34168704
Chronic adaptations to eccentric training: A systematic review Sports Medicine Douglas Jamie Pearson Simon Ross Angus McGuigan Mike Springer Science and Business Media LLC 2017
47 5 917 941 0112-1642 10.1007/s40279-016-0628-4 10.1007/s40279-016-0628-4 27647157
Gene expression profiling in human skeletal muscle during recovery from eccentric exercise American Journal of Physiology-Regulatory, Integrative and Comparative Physiology Mahoney D. J. Safdar A. Parise G. Melov S. Fu Minghua MacNeil L. Kaczor J. Payne E. T. Tarnopolsky M. A. 6 2008
294 6 1901 1910 0363-6119 10.1152/ajpregu.00847.2007 10.1152/ajpregu.00847.2007 18321953
Injury prevention programs based on flywheel vs. body weight resistance in recreational athletes Journal of Strength and Conditioning Research Monajati Alireza Larumbe-Zabala Eneko Goss-Sampson Mark Naclerio Fernando 2 2021
35 Suppl1 S188 S196 1064-8011 10.1519/jsc.0000000000002878 10.1519/jsc.0000000000002878
Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload Scandinavian Journal of Medicine & Science in Sports Askling C. Karlsson J. Thorstensson A. 14 7 2003
13 4 244 250 0905-7188 10.1034/j.1600-0838.2003.00312.x 10.1034/j.1600-0838.2003.00312.x
Eccentric overload flywheel training in older adults Journal of Functional Morphology and Kinesiology Kowalchuk Kelsi Butcher Scotty 22 8 2019
4 3 61 2411-5142 10.3390/jfmk4030061 10.3390/jfmk4030061 33467376
Potential benefits of a minimal dose eccentric resistance training paradigm to combat sarcopenia and age-related muscle and physical function deficits in older adults Frontiers in Physiology Harper Sara A. Thompson Brennan J. 30 11 2021
12 790034 1664-042X 10.3389/fphys.2021.790034 10.3389/fphys.2021.790034 34916963
Quadriceps function following ACL reconstruction and rehabilitation: implications for optimisation of current practices Knee Surgery, Sports Traumatology, Arthroscopy Gokeler Alli Bisschop Marsha Benjaminse Anne Myer Greg D. Eppinga Peter Otten Egbert 2014
22 5 1163 1174 0942-2056 10.1007/s00167-013-2577-x 10.1007/s00167-013-2577-x
The effect of autoregulated flywheel and traditional strength training on training load progression and motor skill performance in youth athletes International Journal of Environmental Research and Public Health Westblad Niklas Petré Henrik Kårström Andreas Psilander Niklas Björklund Glenn 27 3 2021
18 7 3479 1660-4601 10.3390/ijerph18073479 10.3390/ijerph18073479 33801621
Implementing eccentric resistance training-part 2: Practical recommendations Journal of Functional Morphology and Kinesiology Suchomel Timothy J. Wagle John P. Douglas Jamie Taber Christopher B. Harden Mellissa Haff G. Gregory Stone Michael H. 9 8 2019
4 3 55 2411-5142 10.3390/jfmk4030055 10.3390/jfmk4030055 33467370
Implementing eccentric resistance training-part 1: A brief review of existing methods Journal of Functional Morphology and Kinesiology Suchomel Timothy J. Wagle John P. Douglas Jamie Taber Christopher B. Harden Mellissa Haff G. Gregory Stone Michael H. 24 6 2019
4 2 38 2411-5142 10.3390/jfmk4020038 10.3390/jfmk4020038 33467353
Tendinopathy Nature Reviews Disease Primers Millar Neal L. Silbernagel Karin G. Thorborg Kristian Kirwan Paul D. Galatz Leesa M. Abrams Geoffrey D. Murrell George A. C. McInnes Iain B. Rodeo Scott A. Springer Science and Business Media LLC 7 1 2021
7 1 2056-676X 10.1038/s41572-020-00234-1 10.1038/s41572-020-00234-1
Investigating Achilles and patellar tendinopathy prevalence in elite athletics Research in Sports Medicine Janssen Ina van der Worp Henk Hensing Sjoerd Zwerver Johannes 2018
26 1 1 12 1543-8627 10.1080/15438627.2017.1393748 10.1080/15438627.2017.1393748
Clinical management of tendinopathy: A systematic review of systematic reviews evaluating the effectiveness of tendinopathy treatments Scandinavian Journal of Medicine & Science in Sports Irby Alyssa Gutierrez Jacqueline Chamberlin Claressa Thomas Stephen J. Rosen Adam B. 18 6 2020
30 10 1810 1826 0905-7188 10.1111/sms.13734 10.1111/sms.13734
Management of patellar tendinopathy: a systematic review and network meta-analysis of randomised studies BMJ Open Sport & Exercise Medicine Challoumas Dimitris Pedret Carles Biddle Mairiosa Ng Nigel Yong Boon Kirwan Paul Cooper Blair Nicholas Patrick Wilson Scott Clifford Chris Millar Neal L 11 2021
7 4 e001110 2055-7647 10.1136/bmjsem-2021-001110 10.1136/bmjsem-2021-001110 34900334
Heavy slow resistance versus eccentric training as treatment for Achilles tendinopathy: A randomized controlled trial The American Journal of Sports Medicine Beyer Rikke Kongsgaard Mads Hougs Kjær Birgitte Øhlenschlæger Tommy Kjær Michael Magnusson S. Peter SAGE Publications 27 5 2015
43 7 1704 1711 0363-5465 10.1177/0363546515584760 10.1177/0363546515584760
Corticosteroid injections, eccentric decline squat training and heavy slow resistance training in patellar tendinopathy Scandinavian Journal of Medicine & Science in Sports Kongsgaard M. Kovanen V. Aagaard P. Doessing S. Hansen P. Laursen A. H. Kaldau N. C. Kjaer M. Magnusson S. P. 12 2009
19 6 790 802 0905-7188 10.1111/j.1600-0838.2009.00949.x 10.1111/j.1600-0838.2009.00949.x
Muscle and tendon adaptations to moderate load eccentric vs. concentric resistance exercise in young and older males Geroscience Quinlan Jonathan Iain Franchi Martino Vladimiro Gharahdaghi Nima Badiali Francesca Francis Susan Hale Andrew Phillips Bethan Eileen Szewczyk Nathaniel Greenhaff Paul Leonard Smith Kenneth Maganaris Constantinos Atherton Phillip James Narici Marco Vincenzo 1 7 2021
43 4 1567 1584 2509-2715 10.1007/s11357-021-00396-0 10.1007/s11357-021-00396-0 34196903
Tendon adaptations to eccentric exercise and the implications for older adults Journal of Functional Morphology and Kinesiology Quinlan Jonathan I Narici Marco V Reeves Neil D Franchi Martino V 20 8 2019
4 3 60 2411-5142 10.3390/jfmk4030060 10.3390/jfmk4030060 33467375
Minimal-dose resistance training for improving muscle mass, strength, and function: A narrative review of current evidence and practical considerations Sports Medicine Fyfe Jackson J. Hamilton D. Lee Daly Robin M. 25 11 2021
52 3 463–479 0112-1642 10.1007/s40279-021-01605-8 10.1007/s40279-021-01605-8
Flywheel training in musculoskeletal rehabilitation: a clinical commentary Int J Sports Phys Ther Wonders J. 2019
14 6 994 1000 31803531
Flexor tendon wound healing in vitro: the effect of lactate on tendon cell proliferation and collagen production The Journal of Hand Surgery Klein Matthew B. Pham Hung Yalamanchi Naveen Chang James 9 2001
26 5 847 854 0363-5023 10.1053/jhsu.2001.26185 10.1053/jhsu.2001.26185 11561237
Tendon and skeletal muscle matrix gene expression and functional responses to immobilisation and rehabilitation in young males: effect of growth hormone administration The Journal of Physiology Boesen A. P. Dideriksen K. Couppé C. Magnusson S. P. Schjerling P. Boesen M. Kjaer M. Langberg H. 23 10 2013
591 23 6039 6052 0022-3751 10.1113/jphysiol.2013.261263 10.1113/jphysiol.2013.261263 24081158
Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults Sports Medicine - Open Bohm Sebastian Mersmann Falk Arampatzis Adamantios 27 3 2015
1 1 2199-1170 10.1186/s40798-015-0009-9 10.1186/s40798-015-0009-9 27747846
Eccentric or concentric exercises for the treatment of tendinopathies? Journal of Orthopaedic & Sports Physical Therapy Couppé Christian Svensson René B. Silbernagel Karin Grävare Langberg Henning Magnusson S. Peter Journal of Orthopaedic & Sports Physical Therapy (JOSPT) 11 2015
45 11 853 863 0190-6011 10.2519/jospt.2015.5910 10.2519/jospt.2015.5910
Eccentric exercise: adaptations and applications for health and performance Journal of Functional Morphology and Kinesiology Harris-Love Michael O. Gollie Jared M. Keogh Justin W. L. 24 11 2021
6 4 96 2411-5142 10.3390/jfmk6040096 10.3390/jfmk6040096 34842737
Physical therapy for tendinopathy: An umbrella review of systematic reviews and meta-analyses Physical Therapy in Sport Girgis Beshoy Duarte José Alberto 11 2020
46 30 46 1466-853X 10.1016/j.ptsp.2020.08.002 10.1016/j.ptsp.2020.08.002
PRISMA extension for scoping reviews (PRISMA-ScR): Checklist and explanation Annals of Internal Medicine Tricco Andrea C. Lillie Erin Zarin Wasifa O'Brien Kelly K. Colquhoun Heather Levac Danielle Moher David Peters Micah D.J. Horsley Tanya Weeks Laura Hempel Susanne Akl Elie A. Chang Christine McGowan Jessie Stewart Lesley Hartling Lisa Aldcroft Adrian Wilson Michael G. Garritty Chantelle Lewin Simon Godfrey Christina M. Macdonald Marilyn T. Langlois Etienne V. Soares-Weiser Karla Moriarty Jo Clifford Tammy Tunçalp Özge Straus Sharon E. American College of Physicians 2 10 2018
169 7 467 473 0003-4819 10.7326/m18-0850 10.7326/m18-0850
Inertial flywheel vs heavy slow resistance training among athletes with patellar tendinopathy: A randomised trial Physical Therapy in Sport Ruffino Diego Malliaras Peter Marchegiani Silvio Campana Vilma 11 2021
52 30 37 1466-853X 10.1016/j.ptsp.2021.08.002 10.1016/j.ptsp.2021.08.002
Efficacy of an inertial resistance training paradigm in the treatment of patellar tendinopathy in athletes: a case-series study Physical Therapy in Sport Romero-Rodriguez D. Gual G. Tesch P.A. 2 2011
12 1 43 48 1466-853X 10.1016/j.ptsp.2010.10.003 10.1016/j.ptsp.2010.10.003
Effectiveness of the intratissue percutaneous electrolysis (EPI(R)) technique and isoinertial eccentric exercise in the treatment of patellar tendinopathy at two years follow-up Muscle Ligaments and Tendons Journal Abat F. Diesel W.-J. Gelber P.-E. Polidori F. Monllau J.-C. Sanchez-Ibañez J.M. 1 2019
4 2 188 2240-4554 10.32098/mltj.02.2014.18 10.32098/mltj.02.2014.18
Clinical results after ultrasound-guided intratissue percutaneous electrolysis (EPI®) and eccentric exercise in the treatment of patellar tendinopathy Knee Surgery, Sports Traumatology, Arthroscopy Abat F. Gelber P. E. Polidori F. Monllau J. C. Sanchez-Ibañez J. M. Springer Science and Business Media LLC 2015
23 4 1046 1052 0942-2056 10.1007/s00167-014-2855-2 10.1007/s00167-014-2855-2
Autoregulation in resistance training for lower limb tendinopathy: A potential method for addressing individual factors, intervention issues, and inadequate outcomes Frontiers in Physiology Burton Ian 5 8 2021
12 704306 1664-042X 10.3389/fphys.2021.704306 10.3389/fphys.2021.704306 34421641
No treatment benefits of local administration of insulin-like growth factor-1 in addition to heavy slow resistance training in tendinopathic human patellar tendons: A randomized, double-blind, placebo-controlled trial with 1-year follow-up The American Journal of Sports Medicine Olesen Jens L. Hansen Mette Turtumoygard Ida F. Hoffner Rikke Schjerling Peter Christensen Jan Mendias Christopher L. Magnusson Peter S. Kjaer Michael SAGE Publications 17 6 2021
49 9 2361 2370 0363-5465 10.1177/03635465211021056 10.1177/03635465211021056
Clinical outcomes, structure, and function improve with both heavy and moderate loads in the treatment of patellar tendinopathy: A randomized clinical trial The American Journal of Sports Medicine Agergaard Anne-Sofie Svensson Rene B. Malmgaard-Clausen Nikolaj M. Couppé Christian Hjortshoej Mikkel H. Doessing Simon Kjaer Michael Magnusson S. Peter SAGE Publications 22 2 2021
49 4 982 993 0363-5465 10.1177/0363546520988741 10.1177/0363546520988741
Platelet-rich plasma for patellar tendinopathy: A randomized controlled trial of leukocyte-rich PRP or leukocyte-poor PRP versus saline The American Journal of Sports Medicine Scott Alex LaPrade Robert F. Harmon Kimberly G. Filardo Giuseppe Kon Elizaveta Della Villa Stefano Bahr Roald Moksnes Havard Torgalsen Thomas Lee Jenny Dragoo Jason L. Engebretsen Lars SAGE Publications 30 4 2019
47 7 1654 1661 0363-5465 10.1177/0363546519837954 10.1177/0363546519837954 31038979
Pain-guided activity modification during treatment for patellar tendinopathy: a feasibility and pilot randomized clinical trial Pilot and Feasibility Studies Sprague Andrew L. Couppé Christian Pohlig Ryan T. Snyder-Mackler Lynn Silbernagel Karin Grävare 25 2 2021
7 1 2055-5784 10.1186/s40814-021-00792-5 10.1186/s40814-021-00792-5 33632313
Non-surgical treatment of patellar tendinopathy: A systematic review of randomized controlled trials Journal of Science and Medicine in Sport Vander Doelen Trevor Jelley Wilma 2 2020
23 2 118 124 1440-2440 10.1016/j.jsams.2019.09.008 10.1016/j.jsams.2019.09.008
Effects of in-season inertial resistance training with eccentric overload in a sports population at risk for patellar tendinopathy Journal of Strength and Conditioning Research Gual Gabriel Fort-Vanmeerhaeghe Azahara Romero-Rodríguez Daniel Tesch Per A. 7 2016
30 7 1834 1842 1064-8011 10.1519/jsc.0000000000001286 10.1519/jsc.0000000000001286 26670989
Prevalence of Jumper's knee among nonelite athletes from different sports: a cross-sectional survey The American Journal of Sports Medicine Zwerver Johannes Bredeweg Steven W. van den Akker-Scheek Inge 7 7 2011
39 9 1984 1988 0363-5465 10.1177/0363546511413370 10.1177/0363546511413370 21737835
Preventive interventions for tendinopathy: A systematic review Journal of Science and Medicine in Sport Peters Janne A. Zwerver Johannes Diercks Ronald L. Elferink-Gemser Marije T. van den Akker-Scheek Inge 3 2016
19 3 205 211 1440-2440 10.1016/j.jsams.2015.03.008 10.1016/j.jsams.2015.03.008 25981200
Prevalence of Achilles and patellar tendinopathy and their association to intratendinous changes in adolescent athletes Scandinavian Journal of Medicine & Science in Sports Cassel M. Baur H. Hirschmüller A. Carlsohn A. Fröhlich K. Mayer F. Wiley 2015
25 3 310 318 0905-7188 10.1111/sms.12318 10.1111/sms.12318
Thirty-seven jump-landing biomechanical variables are associated with asymptomatic patellar tendon abnormality and patellar tendinopathy: A systematic review Physical Therapy in Sport Harris Meaghan Schultz Adrian Drew Michael K. Rio Ebonie Adams Stuart Edwards Suzi 9 2020
45 38 55 1466-853X 10.1016/j.ptsp.2020.03.011 10.1016/j.ptsp.2020.03.011 32622328
Heavy-load eccentric calf muscle training for the treatment of chronic Achilles tendinosis The American Journal of Sports Medicine Alfredson Håkan Pietilä Tom Jonsson Per Lorentzon Ronny SAGE Publications 5 1998
26 3 360 366 0363-5465 10.1177/03635465980260030301 10.1177/03635465980260030301 9617396
The implementation of resistance training principles in exercise interventions for lower limb tendinopathy: A systematic review Physical Therapy in Sport Burton Ian McCormack Aisling 7 2021
50 97 113 1466-853X 10.1016/j.ptsp.2021.04.008 10.1016/j.ptsp.2021.04.008 33965702
Efficacy of heavy eccentric calf training for treating mid-portion Achilles tendinopathy: a systematic review and meta-analysis British Journal of Sports Medicine Murphy Myles Calder Travers Mervyn J Chivers Paola Debenham James Robert Docking Sean Iain Rio Ebonie Kendra Gibson William 13 1 2019
53 17 1070 1077 0306-3674 10.1136/bjsports-2018-099934 10.1136/bjsports-2018-099934
Ultrasound changes in Achilles tendon and gastrocnemius medialis muscle on squat eccentric overload and running performance Journal of Strength and Conditioning Research Sanz-López Fernando Berzosa Sánchez César Hita-Contreras Fidel Cruz-Diaz David Martínez-Amat Antonio 7 2016
30 7 2010 2018 1064-8011 10.1519/jsc.0000000000001298 10.1519/jsc.0000000000001298 26677829
