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Effects of combined cognitive and resistance training on physical and cognitive performance and psychosocial well-being of older adults ≥65: study protocol for a randomised controlled trial
  1. Deniz Aminirakan,
  2. Björn Losekamm,
  3. Bettina Wollesen
  1. Department of Human Movement Science, Universität Hamburg, Hamburg, Germany
  1. Correspondence to Professor Bettina Wollesen; bettina.wollesen{at}


Introduction With increasing life expectancy of older adult population, maintaining independence and well-being in later years is of paramount importance. This study aims to investigate the impact of three distinct interventions: cognitive training, resistance training and a combination of both, compared with an inactive control group, on cognitive performance, mobility and quality of life in adults aged ≥65 years.

Methods and analysis This trial will investigate healthy older adults aged ≥65 years living independently without cognitive impairments. Participants will be randomly assigned to one of four groups: (1) cognitive training, (2) resistance training, (3) combined cognitive and resistance training, and (4) control group (n=136 participants with 34 participants per group). The interventions will be conducted over 12 weeks. The cognitive training group will receive group-based activities for 45–60 min two times a week. The resistance training group exercises will target six muscle groups and the combined group will integrate cognitive tasks into the resistance training sessions. Primary outcomes are: Short Physical Performance Battery, Sit-to-Stand Test, Montreal Cognitive Assessment, Trail Making Test and Stroop Test combined with gait on a treadmill (dual task). Life satisfaction will be measured by the Satisfaction With Life Scale. Secondary outcomes encompass hand grip strength and the Functional Independence Measure.

Ethics and dissemination Ethical approval was provided by the local Ethics Committee at the University of Hamburg (no. 2023_009). Informed consent will be obtained from all study participants. The results of the study will be distributed for review and discussion in academic journals and conferences.

Trial registration number DRKS00032587.

  • aging
  • multitasking behavior
  • randomized controlled trial
  • physical therapy modalities

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  • The randomised controlled trial compares four groups: cognitive-only training, strength-only training, combined cognitive and strength training, and a waitlist-control group.

  • Primary outcome measures include physical performance, cognitive function and satisfaction with life.

  • Secondary outcomes are muscle strength (hand dynamometer) and functional independence measurement.

  • Innovative insights are expected from the unique combination of cognitive and resistance training as a multifaceted intervention.

  • Generalisation of findings to the broad population needs to be estimated after analysing the various outcome measures and their effect sizes.


With increasing life expectancy, staying healthy and independent becomes more important for older adults.1 Independence in older age is related to physical fitness and functional mobility.2 However, for older adults, a decline in muscle strength and mass along with impaired physical performance and mobility are significant age-related processes3 leading to increased risk of falls.4 Consequently, an appropriate level of strength in the muscles, for example, hip and leg extensors, is required to ensure proper function of walking for engaging in activities of daily living (ADLs).5

The observed decrease in muscle strength in older adults ranges from 2% to 4% per year with higher declines observed with increasing age.6–9 Low muscle mass and strength have also been related to brain mass atrophy,10 next to the increasing physical limitations and immobility. The combination of decreasing mobility and declining cognitive abilities leads to accelerated reduction of both mental and overall well-being in older adults11 12 as well as decreased engagement in ADLs.13 For instance, a deterioration in cognitive function results in functional restrictions on basic physical and cognitive tasks such as walking, standing balance or interacting with the environment.14–16 Hence, daily activities such as walking require higher levels of cognitive control17 and, in turn, declines in cognitive functioning often result in irregular walking performance causing a host of health issues like injuries, falls and hospitalisation.18

Recognising the importance of cognitive abilities, studies highlighted that executive functions (EFs) are crucial to interact with the environment in daily activities.19 EF can be subdivided into three fundamental elements: (1) cognitive flexibility, (2) inhibitory control and (3) working memory.20 Cognitive flexibility is a concept that involves shifting attention between different tasks or mental sets which is equally important in adjusting to changing situations.20 Inhibitory control describes the ability to suppress prepotent responses which play a role in self-regulation and control of behaviour contributing to older adults’ self-efficacy and independence.21 Working memory comprises temporarily storing and manipulating information which is a crucial element for decision-making and learning.22 Furthermore, other cognitive factors such as visual-spatial processes, audio-spatial processes and basic cognitive functions, for example, reaction time and processing speed, are significant contributors to ADLs in older adults.10

Growing evidence has sparked increasing interest in the potential role of physical exercise as a treatment to improve cognition next to physical fitness among older adults.23 Findings strongly suggest that several significant risk factors for cognitive decline, dementia, impaired gait, falls and decreased quality of life are interconnected and could be modified by physical or cognitive training.24 25 Resistance exercise can especially counteract age-related muscle loss and improve muscle function26 primarily through two mechanisms: muscle hypertrophy and neuromuscular adaptation.27 28

There is evidence that resistance training contributes to the preservation of muscle mass and strength, improves mobility and balance, enhances functional abilities and potentially benefits cognitive performance (eg, working memory, inhibitory control, cognitive flexibility).29 30 In this context, it is worth highlighting the positive impacts related to the lower limb functions, for example, expressed by the Short Physical Performance Battery (SPPB). Moreover, interventions lead to notable increases in strength levels among older adults.14–16 31–33 These findings highlight the potential benefits of tailored interventions for older adults’ well-being emphasising the importance of personalised exercise and cognitive training programmes to enhance both physical and mental aspects of health.34

Besides the potential benefits of one isolated type of exercise, there is evidence that combined motor–cognitive training (ie, dual-task (DT) training) improves motor–cognitive function for ADLs as well as the quality of life of older adults. Prior research shows that physical and cognitive exercises both separately and in combination can reinforce cognitive performance in older individuals. Cognitive–motor training which merges mental and physical training helps to maintain the capacity for multitasking daily activities as well as other cognitive abilities.35–39

Cognitive–motor training incorporates, for example, resistance, balance or aerobic exercises along with cognitive tasks simultaneously.40 This combination challenges and facilitates cognitive processes and abilities.41 42 In addition to engaging in cognitive–motor exercises, older adults can stimulate neuroplasticity leading to improved brain health and cognitive resilience.43 44 One explanation is the ‘guided plasticity facilitation’ theory which suggests that combined physical and cognitive activities produce superior effects that surpass the sum of their benefits. Through the mechanism where physical actions harmonise with cognitive engagement, attentional faculties are honed, enabling improved multitasking capabilities and cognitive control.42 The physical exercises trigger neurophysiological mechanisms that drive neuroplasticity through the possible release of neurotrophic factors such as the brain-derived neurotrophic factor (BDNF). BDNF associated with synaptogenesis and neurogenesis could lead to improved cognition. While physical exercises instigate fundamental neurophysiological changes necessary for neuroplasticity, cognitive stimulation is believed to ‘guide’ this process. The release of growth factors and anti-inflammatory cytokines to the bloodstream during exercise is hypothesised to activate some signalling pathways that facilitate neuroplasticity,45 leading to motor or cognitive improvements when motor or cognitive task is performed in proximity to exercise.46

It is important to note that the specific design and implementation of cognitive-resistance training programmes may vary. However, the selection of cognitive tasks, the types of resistance exercises and the intensity of training can influence the outcomes42 and might improve the quality of life.47 However, to date, most cognitive–motor training studies have focused on cognitive training combined with balance, or aerobic, or a combination of different physical training, but the benefits of resistance training combined with cognitive training in cognitively healthy adults remain largely inconclusive.24 48 More research is needed to discover which types of cognitive-resistance training yield optimal outcomes.49

In summary, improving cognitive and motor performance related to strength is crucial for older adults to maintain an independent lifestyle to promote independent living among older adults.50–52 This study aims to examine the potential impacts of a 12-week combined cognitive and resistance training programme hypothesising that a combination programme shows the highest benefits on older adults’ cognitive performance, physical performance and quality of life. Moreover, one can assume that the intervention programme will improve muscle strength (hand grip test) and Functional Independence Measure (FIM).


Accordingly, the present study will attempt to answer the following research questions:

  1. Does cognitive training improve older adults’ cognitive performance, mobility and quality of life?

  2. Does resistance training improve older adults’ cognitive performance, mobility and quality of life?

  3. What is the difference between resistance training combined with cognitive training in comparison with only resistance or only cognitive training in older adults’ cognitive performance, mobility and life satisfaction?

Method and analysis

The reporting of this study protocol follows the guidelines put forth by the Standard Protocol Items: Recommendations for Interventional Trials statement.53

Study design

This four-arm randomised controlled trial includes three training groups and a waitlist-control group: (1) single-task (ST) cognitive training, (2) ST strength training, (3) combined strength training and cognitive training with (4) a waitlist-control group. It will examine the effectiveness of three exercise intervention programmes (12 weeks, twice a week, 45–60 min) in motor and cognitive performance of independently living older adults aged ≥65 years and above (N=136 participants; n=34 participants per group) compared with an inactive control group, as shown in table 1.

Table 1

Schedule of enrolment, intervention and assessment


The study will be conducted in a university setting with participants providing written informed consent following the Declaration of Helsinki.54 Participants will receive information on the study aims, duration, possible side effects, methods used and their right to withdraw. Furthermore, information on data protection will be included (online supplemental file 1). The study protocol has been approved by the local Ethics Committee of the University of Hamburg (approval number: 2023_009) and registered at (registration number: DRKS00032587) on 28 August 2023.

Supplemental material

Recruitment of participants

Via advertisements and social media, participants will be recruited, informed of the study details and screened for eligibility.

Participants will be selected based on the following inclusion and exclusion criteria:


Included will be generally healthy persons aged ≥65–85 years living independently.


Excluded will be persons younger than 65 years and over 85 years of age, not living independently and affected by (1) brain injuries or cognitive impairments, (2) physical impairments (eg, using a cane or walker) and (3) chronic illnesses (eg, multiple sclerosis or Parkinson’s disease). Furthermore, people who regularly take medication that has an impact on physical and mental performance will be excluded. In addition, according to the commonly reported cut-off score for the Montreal Cognitive Assessment (MoCA), people who score below 26 will be excluded. Apart from the inclusion and exclusion criteria mentioned, there are no further restrictions on participation.

Assignment of interventions

The eligible participants will be divided into four study groups: (1) ST cognitive training, (2) ST resistance training, (3) integrated resistance and cognitive training, and (4) waitlist-control group.

The principal investigator (PI) will employ a stratified randomisation approach to ensure balanced participant allocation by lot, considering gender, age and cognitive performance. Data collection will be performed by blinded assessors, strictly using pseudonymised forms to ensure blinded data analysis, safeguarding data integrity and participant privacy throughout the study. The intervention programme adheres to a standardised protocol to prevent performance bias. The passive control group will follow their normal activities. Personal data will be pseudonymised and later anonymised with a coding list, using only participant pseudonyms during testing. The allocation sequence, pseudonymised codes, and baseline data will be confidentially and separately stored, with documented restrictions independently minimising predictability.


The pretest and post-test will be conducted on two separate days: (1) cognitive assessment and (2) physical assessment. The physical assessment will be performed the day after the cognitive assessment to avoid influence on the cognitive outcomes. The test sequence is as follows: cognitive pretest (1 day), physical pretests (the following day), 1 day of rest, 12-week training intervention according to groups, 1 day of rest after the last session of training, cognitive post-tests (1 day) and physical post-test (the following day) (see figure 1).

Figure 1

Flow diagram of the study procedure.


The exercise programmes consist of 24 sessions two times per week for 45–60 min over 12 weeks in three groups with 34 participants each (see figure 2).

Figure 2

Schematic description of (A) the study design, (B) the outcome measures and (C) the intervention characteristics.

The participants will undergo a 10-minute warm-up before the training to prevent injuries. The warm-up includes exercises such as light cardiovascular activity, stretching as well as flexibility and joint mobility. At the end of the training session, 10-minute cool-downs including static stretching, deep breathing and relaxation will be implemented.

To prevent injuries during training, the exercises are individualised to match the capabilities of the participants and consider their actual physical performance. The programme incorporates gradual challenges through modifications and progressions in exercises closely supervised by a trainer to ensure correct form and techniques.

Sports science experts specialised in exercise and cognitive training methodologies will supervise the interventions. These supervisors are trained and experienced in exercise management and evaluation. Precautions including equipment setup, routine communication and feedback help to maintain a safe and effective training environment.

Cognitive training

The cognitive training group will receive only cognitive training in group-based activities or individually two times a week (eg, a memory task inhibitory control task, reaction tasks, visual-spatial tasks) (see table 2).

Table 2

Description of the cognitive training intervention

Resistance training

Resistance training involves six muscle groups: shoulder, arms, abdomen, back, leg extensors and leg flexors (see table 3). The training consists of strength exercise sessions with strength machines two times a week. Training intensity will be adapted to the muscle strength of participants regularly and the programme will follow the principle of progressive overload.

Table 3

Description of a training plan for the strength group as well as the strength and cognitive training group for 2 weeks of training sessions

Combined resistance and cognitive training

The combined training involves the same resistance training as described above and will be combined with the cognitive training between sets in the rest time (see table 3).

Waitlist-control group

The control group will be asked to continue their normal regular daily activities without an additional intervention for 12 weeks. These participants will receive the intervention type which was randomly assigned after the second measurement phase (see figure 1).

Outcome measures

To assess the effectiveness of the intervention programme, a battery of standard assessments will be employed to measure the primary and secondary outcomes. Next to the primary and secondary outcomes, data on age, gender, height, body mass, self-reported mental status, fitness status, training frequency, previous injuries and current state of health will be collected with an anamnestic questionnaire (see figure 1).

Primary outcomes

The assessment will focus on three key domains: physical functioning, cognitive performance and psychosocial well-being. For the assessment of the primary outcomes, physical functioning (SPPB (for the lower extremity physical performance), Sit-to-Stand Test (StS), walking performance (step length, step width) under ST and DT conditions (visual-verbal Stroop Task)), cognitive performance (MoCA, Trail Making Test (TMT) A+B) and psychosocial well-being (Satisfaction With Life Scale (SWLS)) will be conducted.

Physical functioning

The SPPB is one of the most commonly used instruments for measuring physical performance in studies on the ageing population.55 The SPPB consists of three subtests: the hierarchical test of balance, gait speed and leg strength. Low scores on the SPPB have a high predictive value for a wide range of health consequences including disability in ADLs.9

The StS is a functional evaluation of lower body muscle strength. The test requires the participant to sit upright on an armless chair around 43 cm high with their feet grounded and arms crossed on their chest. On command, the participant stands up and then sits back down for either 30 or 60 s. The number of these repetitions accomplished in the assigned time frame is recorded as the test’s score. This test is recognised as a reliable and valid measure of lower body strength among typically active, community-dwelling older adults.56

Cognitive performance

The MoCA is a screening tool for global cognition and to identify the beginning of dementia. The test is mainly used by medical professionals to identify impairments in brain performance. The assessment has proven excellent reliability. A test score of 26 points or above (from a total of 30 points) is determined as normal cognitive functioning.57

The TMT is a reliable and valid neuropsychological test to assess cognitive flexibility. The test comprises two sections (A and B) of 25 circles spread over a sheet. In part A, circles are numbered 1–25 and the objective is to link the numbers in ascending order. In part B, circles contain numbers (1–12) and letters with the task focusing on alternating connections (1-A-2-B-3-C, etc). The completion time for each part constitutes the test’s score particularly assessing cognitive flexibility and attention-switching abilities in part B.

The Stroop Test, combined with a walking task, will be incorporated. The test is a valid method for this target group to assess the DT performance.58–60 Participants will practise 5-minute treadmill walking and become familiar with ST and DT walking. Performance will be assessed during a 30-second walking test with gait parameters measured. The DT condition comprises a visual-verbal Stroop Test involving naming font colours while inhibiting word reading as a cognitive task projected on a wall in front of them. Word insertions’ time intervals will be varied with participants given three out of four Stroop Test versions in a random process.

Psychosocial well-being

The SWLS has proven to be a valid and reliable measure of life satisfaction suited for use with a wide range of age groups and applications which saves interview time and resources compared with other measures of life satisfaction.61

Secondary outcomes

Secondary outcomes will be considered including muscle strength measured by a hand dynamometer and FIM.

The hand grip strength will be measured by a Jamar hydraulic hand grip dynamometer (serial number: FS2425) while sitting in an upright position with the shoulder adducted and elbow flexed at 90° and the forearm allowed to rest lightly on the arm of the chair or the subject’s thigh. Three contractions will be performed overall to verify an objective measurement of the isometric muscle strength of the hand and forearm.62 63

The FIM instrument is a minimal dataset designed to assess functional independence. The FIM includes 18 items each with a maximum score of 7 points and a minimum score of 1 point. Possible scores range from 18 to 126 points. Each level of scoring is defined. The areas examined by the FIM include motor and cognitive domains. The motor domain includes 13 items in the areas of self-care, sphincter control, transfers and locomotion. The cognitive domain contains five items from the communication and social cognition subscales.64

Participant timeline

The participant timeline outlines the schedule of enrolment, interventions and assessments (pretest, post-test; see table 1). This schematic diagram provides a clear and comprehensive overview of the chronological flow of participant involvement in the study.

Sample size calculation

Based on a priori sample size calculation using G*Power V.3.1, an analysis of variance with repeated measures within/between interaction factors with four groups of two measurements, an effect size of 0.25, an alpha error rate of 0.05 and a statistical power of 0.95 was conducted. Calculations resulted in a total sample size of 112 participants and 28 participants per group. Assuming a dropout rate of 20%, the final sample size would be N=136 participants with 34 participants per group. This sample size adjustment was made with the suggestion and guidance provided by the ethics board.

Statistical analysis

The intervention’s impact on all types of outcomes, including quantitative, qualitative and ordinal, using appropriate statistical methods such as repeated measures or mixed models, t-tests, Kruskall-Wallis or Χ2 tests, depending on the outcome type and its distribution, will be assessed. The primary analysis will be a mixed-model between-group comparison of the SPPB, MoCA, TMT, StS, gait variable (Stroop DT) and SWLS. The Bonferroni correction will be used to appropriately adjust the overall level of significance for multiple comparisons. Between-group differences for all primary and secondary outcomes will be adjusted for baseline values, age and sex. Secondary outcomes will be analysed with a similar methodology. Anticipation of at least an 80% completion of training sessions by participants is expected but in case of participant dropouts or insufficient participation, per-protocol and intention-to-treat analyses will be used. The handling of missing data will involve multiple imputation techniques including estimating missing values for a more thorough analysis of the gathered data. All statistical analyses will be performed using SPSS Statistics for Windows (V.27.0, IBM). The statistical significance level is set at p<0.05.

Data management

The study prioritises data confidentiality and security through initial pseudonymisation followed by anonymisation, adhering to the University of Hamburg data protection guidelines. Personal and research data will be separately stored, with access limited to the PI and authorised staff and no third-party transfers allowed. Personal details will be filed under pseudonyms and retained for 10 years following the erasure of the coding list, in line with scientific standards. Participants within the study can be identified by pseudonyms linked to their personal details, permitting them the opportunity to request access to personal scores via the secure coding list. Only the research team will have access to this list, and participants can request personal data removal up until its deletion. After deletion, all data become untraceable to participants. The coding list will be maintained until the data evaluation completion. Moreover, to enhance participant retention, we will make sure to keep them engaged and informed by sharing the study results.

Data monitoring

The study encompasses regular data accuracy and quality assessments through scheduled data reviews by the authors including interim analyses after 6 weeks to evaluate the study’s progress. This procedure covers participant safety and intervention efficacy checks. Project staff are integral to monitoring participant safety during evaluations and intervention training.

Adverse events

Adverse effects are not expected but might result out of muscle soreness due to the strength intervention part and possible mental fatigue shortly after the cognitive interventions. Within pilot studies (also with more vulnerable groups), none of these effects had negative consequences for the health of the participants.65 Participants are advised to always inform the test management or trainer in case of such incidents. All adverse events, negative reactions or unintended effects will be swiftly addressed, documented and reported following adverse event reporting procedures.

Patient and public involvement



Upon the conclusion of the trial, the research findings will be disseminated through a variety of carefully selected methods with the aim of reaching the broadest audience and having a meaningful impact on the quality of life of older adults. The main research paper will be submitted to a peer-reviewed journal that is indexed in a well-respected database, ensuring academic credibility and accessibility (eg, open access). A summary of the results will be presented at conferences and seminars that focus on the welfare of older adults, allowing for direct engagement with professionals in the field. Additionally, we plan to create and distribute easy-to-understand informational materials, such as fact sheets or briefs, to, for example, healthcare providers. Finally, we will transfer the results and published papers to social media platforms and online forums related to ageing. These multifaceted steps are part of a concerted effort to ensure that the research outcomes are communicated effectively and contribute to the enhancement of health promotion for the ageing population.


The main purpose of this study is to investigate the benefits of separated and combined cognitive and resistance training to older adults’ cognitive and physical abilities. An improvement in strength, cognitive performance, mobility and quality of life among participants was expected.

Previous research underscored the effectiveness of cognitive–motor intervention to maximise cognitive and motor benefits in older populations66 67 using mostly aerobic or balance exercises. The combination of both training regimes notably improved cognitive–motor coordination and provides unique challenges to the brain that may not be achievable through ST interventions.68 The cognitive–motor exercises urge participants to perform DTs and switch their attention between cognitive and physical assignments.41 As a result, this approach engages different brain networks simultaneously due to the variations of physical exercises. This engagement could be an influential factor enabling its effectiveness.52 Further supporting the present study, previous research indicates that the combination of resistance and cognitive exercises may result in improved cognitive performance.40 69 Given these findings, this research explores an integrated cognitive-resistance training regimen.

The innovative aspect of this study is the focus on cognitive-resistance training. Resistance exercise has been demonstrated to mitigate age-related muscle loss, improve muscle function, and enhance overall mobility and balance.70 Importantly, it can also contribute to the preservation of cognitive function.69 This preservation is achieved through muscle hypertrophy and neuromuscular adaptation which are mechanisms that not only strengthen muscles but also enhance neural drive to skeletal muscles.71 In addition, cognitive exercises likely activate cellular adaptation processes in newly generated neurons contributing to their functional integration within brain circuits. This integration is vital for maintaining the induced neuroplastic changes and ensuring effective brain function.16 33 When these two types of training are integrated, individuals may experience greater improvements in cognitive and physical abilities compared with when these interventions are administered separately.72 73 Cognitive training challenges the brain’s cognitive processes while resistance training enhances neuromuscular adaptation. Overall, the combination of resistance and cognitive exercises may lead to synergistic effects where the benefits of each type of training enhance and complement each other.42 However, it should be noted that the combination proposed here still lacks a systematic investigation.

In conclusion, this study unravels the potential of cognitive training, resistance training and cognitive-resistance training to enhance strength cognitive performance, functional mobility and quality of life among older adults. Possible advantages of the cognitive-resistance training group can advocate for multifaceted approaches to address the complex challenges of ageing comprehensively. Such findings provide a solid foundation for further research and the development of tailored interventions to promote the well-being and independence of older adults.

Trial status

This study protocol was designated as V.1.0 dated 3 November 2023; the revised version is dated 28 February 2024. All future versions will be sequentially numbered and dated to track protocol revisions. This study was conducted independently and did not receive any financial support.

Consent or assent

Upon receiving all necessary study information, participants will be encouraged to read and sign a consent form. This document confirms their understanding of the study information, their awareness of their rights to withdraw at any time and their voluntary agreement to partake in the study. The document also includes data protection information.


Participants are assigned a serial number linked to their personal information, accessible only to specific personnel. Codes can be given to participants for score access. Participants may request data deletion at any time until the coding list is erased, after which data are anonymised. The list will be kept secured until the evaluation is completed on 31 December 2025.

Consent for publication

Personal information about the study participants will not be published.

Ethics and dissemination

All participants give written informed consent before the study enrolment. The Ethics Committee of the Faculty of Psychology and Human Movement Science of the University of Hamburg, Germany has approved the study protocol (2023_009). The results of the study will be distributed for review and discussion in academic journals and conferences.

Ethics statements

Patient consent for publication


The Deutscher Akademischer Austauschdienst (DAAD) awarded a 7-month scholarship to DA to pursue advanced studies/research in sport psychology at the Universität Hamburg, Faculty of Psychology and Human Movement Science.



  • Contributors This study protocol was collaboratively developed by the authors DA, BL and BW. BW and DA jointly contributed to the design of the study protocol and played a key role in refining the study content. The intervention was conceptualised and developed by BW and DA. The initial draft of the manuscript was prepared by DA, and supervised by BW. Sample size calculations and the data analysis plan were undertaken by BW, while DA and BL authored the statistical section of the manuscript. All authors, BW, DA and BL, were involved in the drafting and contributed significantly to the revision of this manuscript and have given approval for the final manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.