Dosages of swallowing exercises in stroke rehabilitation: a systematic review

Purpose To investigate the dosages of swallowing exercises reported in intervention studies on post-stroke dysphagia through systematic review. Methods Five electronic databases were searched from inception until February 2022 with reference tracing of included studies. Studies were included, where adults with post-stroke dysphagia received rehabilitative, behavioural swallowing exercises, pre/post outcomes were reported, and intervention dosage was described in detail, including frequency, intensity, time, and type of exercise. Two reviewers independently screened studies and rated quality using ASHA Levels of Evidence tool. Data was tabulated and narratively described. Results 54 studies were included with a total 1501 participants. Studies included 28 randomised controlled trials, 8 non-randomised controlled trials, 12 pre/post studies, 3 retrospective case controls and 3 case studies. Results showed inconsistent reporting of intervention dosage, with intensity the least consistently reported dosage component. While swallowing intervention was most commonly provided five times per week for four weeks, there was a wide breadth of type, frequency, intensity and duration of swallowing exercises reported. Dosage under-reporting and variation was particularly observed in “standard care” co-interventions or control groups. Study strengths included following PRISMA guidelines, providing a comprehensive review of swallowing exercise methodology and dosages, and including non-English studies. The limitation was lack of meta-analysis due to the heterogeneity of included studies. Conclusions Dosages of swallowing exercises are inconsistently reported and vary significantly in post-stroke dysphagia studies. Results indicate the need for consistent and comprehensive dosage reporting in dysphagia studies, and for further research into evidence-based principles to optimise swallowing exercise dosages. Systematic review registration number 131294 Supplementary Information The online version contains supplementary material available at 10.1007/s00405-022-07735-7.


Introduction
Dysphagia is a common and significant symptom following stroke. Dysphagia, or swallowing difficulties, affect a third to over two thirds of patients after stroke [1,2]. Dysphagia causes medical complications, including increased hospitalisation, morbidity, and risk of aspiration pneumonia [3]. It is associated with poor psychosocial health outcomes, such as reduced nutrition, hydration and quality of life [4]. Patients with dysphagia have longer lengths of hospital stay and higher healthcare costs [5,6].
Current management of dysphagia involves compensation and rehabilitation. Compensatory techniques-such as chin tuck or modifying diet and fluid consistencies-enable safe swallowing but do not alter long-term function [7]. Rehabilitative exercises, however, can improve swallowing function and resumption of oral intake or normal food and drink [7]. Rehabilitative exercises can be indirect (motor without swallow) or direct (motor with swallow) [8]. Indirect exercises aim to strengthen muscles involved in swallowing, and include the Shaker head-lift and tongue strengthening exercises [9]. Direct exercises involve the action of swallowing and include the Mendelsohn manoeuvre and effortful swallow [9]. Studies have shown the positive effects of rehabilitative exercises on reducing the severity and symptoms of post-stroke dysphagia [10].
However, there is limited understanding of the optimal way to conduct swallowing exercises, particularly, the optimal dosages of swallowing exercises. Dosage is an important factor which can impact on intervention efficacy and efficiency [11]. According to the American College of Sports Medicine's (ACSM) FITT framework, dosage consists of Frequency (how often), Intensity (how hard), Time (how long) and Type (what kind) of exercise [12]. Altering or increasing these components of dosage can optimise exercise or intervention outcomes, as seen in sports medicine and stroke rehabilitation [13,14]. However, there is limited knowledge on what dosages to use for swallowing exercises. Previous scoping and literature reviews have highlighted the paucity of data regarding dosage recommendations in dysphagia intervention [9,15]. This is reflected in surveys of speech pathologists which indicate variability in the exercises and dosages used to treat dysphagia [16].
The aim of this systematic review was to investigate what dosages of swallowing exercises are reported in studies in post-stroke dysphagia. To our knowledge, only one scoping review has specifically examined the dosages of swallowing exercises [15]. Our review was conducted to systematically update the search with new studies. Given that intervention dosage may vary depending on diagnosis, this review focused on one of the most common causes of acquired dysphagia: stroke [17]. This systematic review aimed to investigate dosage reporting in research and describe current swallowing exercise dosages in intervention studies to guide clinicians when considering dosage prescription. The findings can be used to identify areas for future research in optimising dosage of swallowing exercises to facilitate more cost-effective intervention, increased patient engagement and improved outcomes.

Methods
This systematic review was conducted according to PRISMA guidelines [18]. Prior to conducting the study, a protocol was registered on PROSPERO (https:// www. crd. york. ac. uk/ PROSP ERO/, registration number: 131294).

Eligibility criteria
Studies were included if: (i) they included adult participant/s with dysphagia due to stroke, (ii) they examined rehabilitative, behavioural swallowing exercises, (iii) they were a published intervention study, where pre/post outcomes were reported, and (iv) they provided a detailed description of the dosage of the rehabilitative exercise. Only full-text published studies accessible through online databases were included in this review. For the purpose of this review, the ACSM FITT framework was used to specify the minimal elements required in a detailed description of dosage [12]. Dosage description needed to include the frequency (or number) of sessions, intensity or dose (at a minimum, the number of repetitions of each exercise), intervention duration and type of exercise. Stating the type of exercise required the name and reference of a well-known exercise, or a detailed description of the materials, procedures, activities and/or processes involved in the exercise (as per points 1-4 of the Template for Intervention Description and Replication checklist) [19]. Studies were included even if only a subset of participants matched inclusion criteria (e.g., participants with dysphagia due to stroke and other conditions). This review focused on methodology reporting, not intervention effect, so type of outcome measure was not an inclusion criterion. Outcome measures were collected as reported, without limit. Studies in all languages were included. Studies were excluded if they only applied passive interventions, such as acupuncture, thermal-tactile stimulation, compensatory strategies, or electrical stimulation (i.e., when not combined with active exercise), as these require different dosages to behavioural exercises and were not the focus of this review.

Search strategy and selection process
A comprehensive search of studies was conducted from inception until 10 February 2022 using the electronic databases: MEDLINE, Embase via Ovid, CINAHL, Web of Science and SpeechBITE. The Medical Subject Heading terms: "Deglutition, Deglutition Disorders, Pharynx OR Pharyngeal Muscles" were combined with "Stroke OR Cerebrovascular Disorders"; and "Exercise, Exercise Therapy, Neurological Rehabilitation, Stroke Rehabilitation OR Rehabilitation" along with free key word searches of specific swallowing exercises. The search strategy was developed in conjunction with a university librarian using candidate search terms from two relevant studies. See Online Appendices 1-3 for full search strategies for each database. The reference lists of included studies were hand searched to identify further studies.
Covidence software was used to remove duplicates and double checked by the lead author [20]. Two reviewers independently screened titles, abstracts, and eligible full text articles against inclusion criteria using Covidence software. Conflicts were resolved through discussion with a third reviewer. Abstracts or full text studies which were not in English were 1 3 translated by bilingual speakers. All members of the research term agreed on the final studies included for review.

Data collection
Data was extracted from included studies by the lead author using an Excel form and checked by a second reviewer. The following data was extracted: • Study author, year, and source of publication. • Participant demographics (sample size, age, sex, inclusion/exclusion criteria, length of time since stroke) and participant diagnoses (cause of dysphagia, stroke type and severity). • Setting and study design (including study aims and intervention groups). • Dosage of swallowing exercises (type of exercises, any reported intensity, frequency of sets/sessions and duration). • All outcomes pre and post intervention (excluding follow-up timepoints).

Risk of bias assessment
Included studies were assessed for quality by two independent reviewers using the American Speech-Language-Hearing Association (ASHA) Levels of Evidence framework on an online spreadsheet tool [21,22]. The framework involved rating studies against eight quality markers: blinding of assessors, random sampling/allocation, group/participant comparability, treatment fidelity, validity and reliability of an outcome measure, whether significance was reported, precision of effect size and/or confidence interval and analysis by intent-to-treat. Each quality marker contributed to one point in an overall quality score. A quality score of 7-8 was considered high quality, 5-6 good quality and ≤ 4 low quality [21]. Conflicts were resolved through discussion with a third reviewer.

Summary measures
Information about participants, swallowing exercises, reported dosages and outcome measures was collated into two summary tables. Tables were organised alphabetically by exercise type to allow comparison of swallowing exercise dosages. When summary data was missing or in a different form, means and standard deviations were estimated using Hozo et al. or Wan et al.'s methods [23,24]. Effect sizes (Hedges' g) were calculated for pre-post changes in continuous data for swallowing intervention groups using an online effect size calculator [25]. Hedges' g effect sizes can be interpreted as 0.2 = small effect, 0.5 = medium effect, 0.8 = large effect [26]. Meta-analysis could not be conducted due to heterogeneity of study designs, interventions, dosages, and outcome measures, and was not necessary to address study objectives.

Results
The initial search resulted in 7263 studies. After duplicates were removed, 4835 studies were screened for inclusion. Of these, 54 studies passed full text review (Fig. 1).

Study characteristics
There were 28 randomised controlled trials, eight non-randomised controlled trials, three retrospective case controls, twelve pre/post case series and three case studies. Studies were published from 2002 [38] to 2022 [46]. See Online Appendix 4 for study designs.

Exercise type
Fourteen different swallowing exercises and twelve different swallowing programs were described in the included studies.

3
Eleven studies investigated indirect oral exercises, including lip exercises, tongue exercises and an orofacial exercise program. Twenty-eight studies investigated indirect pharyngeal exercises, including Shaker head lift, expiratory muscle strength training and chin tuck against resistance. Ten studies investigated direct swallowing exercises, most commonly the Mendelsohn manoeuvre and effortful swallow. Twelve studies examined a combined swallowing program. Some studies examined more than one intervention. The most reported exercise was Shaker head lift (investigated in ten studies). See Table 1 for single swallowing exercises in included studies (including swallowing exercises, dosages, outcome measures and effect sizes), Table 2 for combined  swallowing programs and Table 3 for definitions of common exercises.
In general, exercise type was well-reported but varied between studies. Studies named specific exercises or provided detailed descriptions of how to conduct exercises (as per inclusion criteria). Consistent descriptions were mostly used when replicating the same exercises. For example, studies examining Shaker head lift all involved patients lifting their heads to look at their feet while in supine position. However, there was variation in devices and variation between combined exercise programs. In studies investigating single exercises, such as chin tuck against resistance, jaw opening and lip training, the main variations were in the use of devices. For example, the jaw opening exercise was described with four different types of resistance: none, against a trainer's hand, against a jaw opening device, or against a ball. In studies investigating combined intervention programs, there was variation in which exercises were included. While all combined programs included orofacial exercises, they varied in whether they included pharyngeal exercises, swallowing with or without real boluses and breathing exercises.

Frequency
Frequency of intervention was well-reported but varied between studies. Frequency was consistently described by the number of sets per day and/or number of days per week of intervention. Most studies conducted intervention five times per week (30 out of 54 studies). However, frequency ranged from one to ten sets per day, and from one time per fortnight to seven times per week [31,36]. Reports assessed for eligibility (n = 6) Reports excluded: Non-dysphagic adults (n = 5) Dysphagia not due to stroke (n = 1) Studies included in review (n = 54)

Intensity
Studies varied in intensity and in how they reported intensity. All studies reported the number of repetitions of each exercise (as per inclusion criteria). Nine studies reported both number of repetitions and length of sessions [28, 34, 39-42, 70, 80, 88]. Total exercise repetitions ranged from 3 to 411 repetitions per session [48,57,75,89] and from 7 to 500 repetitions per day [50,60]. Seventeen studies reported intensity as a percentage of one-repetition maximum (with one-repetition maximum [1-RM] defined as the maximum resistance that can be applied to one exercise repetition through full range of motion) [12]. Indirect strengthening or accuracy exercises using devices-most commonly expiratory muscle strength training and tongue strengthening exercises-were more likely to report a specific intensity level. All expiratory muscle strength training studies specified exercise intensity, but targets ranged from 30% to > 70% of 1-RM. Six tongue strengthening exercises specified a target intensity of either 60% or 80% of 1-RM for strengthening and between 25% and 85% of 1-RM for accuracy training. For chin tuck against resistance, game-based biofeedback, jaw opening, Mendelsohn and Intensive Dysphagia Rehabilitation, one study per intervention specified a target intensity, e.g., 70% of 1-RM with a device.
Intensity of swallowing exercises was otherwise reported and measured in different ways. Many studies used qualitative descriptions of effort (e.g., "as hard as possible", "to maximum extent") [39,73]. Biofeedback (e.g., surface electromyography) with set targets was used in four studies to set specific intensity levels [28,66,72,78]. External resistance was used in around 12 studies with varying forms of resistance depending on the exercise type. Intensity or task difficulty was increased in some studies through increasing length of holds, number of repetitions or changing the amount or substance being swallowed [33,36,57]. See Table 4 for different ways swallowing exercise intensity was reported in studies.

Time
Time, or duration, of intervention was reported as the number of days, weeks, or months of intervention. Most studies had an intervention duration of 4 weeks (22 studies), or 6 weeks (12 studies). However, across all studies, duration ranged from 6 days to 12 weeks [41,42,69,70]. Three studies reported duration based on performance (e.g., once patients were no longer tube feeding) [36,43,63]. See Tables 1 and 2 for detailed exercise dosages reported in included studies.

Standard care
Around half (27) of included studies provided some form of standard care additional to their experimental intervention, either given to a control group for comparison or used as a co-intervention in both groups. Synonymous terms were used to describe this baseline intervention, including "conventional dysphagia therapy", "traditional dysphagia therapy", "regular" or "routine" training. While all included studies described the dosage of the experimental intervention in detail, the same level of detail was not used when describing the dosage of interventions in standard care. Most studies used general terms (e.g., orofacial muscle exercises, therapeutic manoeuvres) to describe what was involved in standard care rather than naming specific exercises. Most studies stated the length of intervention time provided in standard care groups but not the number of exercise repetitions and no other measures of intensity. Standard care was typically provided for 30 min, 5 days per week for 4 weeks. However overall, in studies that described exercises, there was variation in which exercises were included in "standard care" and in their dosages. See Table 5 for details on interventions and dosages used in "standard care" groups.

Reported rationales for dosage prescription
Only five studies specifically described an evidence-based rationale for their dosage. The recommended dosage for strength training drawn from limb rehabilitation research (i.e., ten repetitions, three times per day, 3 days per week for 8 weeks) [90] was applied to tongue strengthening exercises [27,68] and effortful swallowing [62]. Two studies provided a high dosage of intervention, derived from principles of neural plasticity and/or exercise physiology [33,78]. One crossover study investigating a high-intensity program with Mendelsohn manoeuvre showed improved outcomes on treatment weeks compared to non-treatment weeks, and with 2 weeks of intervention compared to 1 week [78]. No other studies in this review specifically compared different dosages of the same intervention.

Outcome measures
There was a wide range of different outcome measures used. Within 54 studies, 52 different outcome measures were used. The most commonly reported outcome measure was the Penetration-Aspiration Scale [91] (used in 23 studies) which rates depth and severity at which food or drink is aspirated into the airway. The next most commonly used measures were the Functional Oral Intake Scale [92] (12 studies) which rates level of oral intake, hyoid displacement/ elevation (ten studies) and maximal tongue strength (nine studies). See Tables 1 and 2 for outcome measures used in included studies.

Risk of bias
There were 12 studies of high quality, 25 good quality and 17 low quality. Studies generally performed well on having similar groups and participants, using a valid and reasonable outcome measure, and reporting significance, effect size and confidence interval. The quality markers that were least often observed were blinding of assessors, randomised sampling, treatment fidelity and intention-totreat analysis. See Online Appendix 4 for quality ratings for each study.

Discussion
This systematic review found that overall, the dosages of swallowing exercises in post-stroke dysphagia studies were poorly reported, and when reported, varied significantly. Most post-stroke dysphagia studies were excluded from review due to under-reporting of exercise dosage, particularly intensity. While swallowing intervention was often provided five times per week for 4 weeks, there was a wide breadth of different exercises, frequencies, durations and intensities of intervention, with a range of different study designs and outcome measures. Variation between studies may have been due to arbitrary selection of intervention dosage, with few studies reporting a rationale for their dosage. Due to the heterogeneity of results, it was difficult to determine optimal dosages of swallowing exercises. Despite a growing awareness of the importance of dosage, more work is needed to improve consistency of dosage reporting and identify evidence-based principles to optimise prescription of swallowing exercise dosages.
To investigate dosages of swallowing exercises, we needed to identify if dosages were reported in studies. One hundred and forty-six studies were excluded due to not providing a detailed description of swallowing exercises. Only 27% of the 200 studies which matched the first three inclusion criteria (i.e., rehabilitative intervention studies in post-stroke dysphagia) reported frequency, intensity, time, or type of intervention in detail, and were included for review. Even within included studies, most studies only described the dosage of the experimental intervention in detail but not the dosage of the non-experimental or control intervention. These findings reveal an under-reporting of dosage in poststroke dysphagia studies. Poor reporting of interventions and dosages prevents reliable implementation, comparison and replication of interventions [19]. Evidence suggests that this is an issue that extends beyond dysphagia research [93,94]. The disproportionate under-reporting of control group interventions has also been highlighted in stroke rehabilitation studies [95]. While checklists exist to improve reporting of interventions [19,96], specific guidelines around comprehensive dosage reporting would further improve study reporting.
Better reporting and measurement of intensity of swallowing exercises is needed. Most studies excluded for not providing a detailed description of dosage did not report intensity. Within included studies with dosage descriptions, exercise type, frequency and duration were relatively well-reported compared to intensity. Intensity is defined as how hard or how much effort an exercise involves [12]. The methods used by studies in this review may provide a starting point when considering how to measure, change and report on swallowing exercise intensity (see Table 4). Number of exercise repetitions provides some indication of intensity but does not describe amount of effort [97]. Providing both exercise repetitions and session length (as reported in nine studies) allows calculation of dosage rate, which contributes to intensity [98]. Qualitative descriptors (e.g., "as hard as you can") or patient rating scales [99] can indicate subjective level of effort. Exercises using devices can set specific intensity levels as percentages of 1-RM, in a similar format to limb training. However, there was still significant variation in how intensity was measured, and many swallowing exercises did not have routine ways to set or measure intensity, such as a Masako manoeuvre, or head lift exercise (see Table 3 for explanations of these exercises). To properly quantify dosages, further work is needed to identify consistent methods to set and report exercise intensity for the wide range of swallowing exercises.
Examining studies that did report dosage showed significant variation in dosages of swallowing exercises in both experimental and standard care interventions. A wide range of different exercises were used, and there was variation in dosage across different swallowing exercises, similar to findings in a previous review [15]. This variation was also observed in standard care, or "conventional" or "traditional dysphagia therapy". While a similar repertoire of exercises was used, the combination of exercises and dosages involved in standard care varied between studies. This variation in exercise selection and dosage is reflected in surveys of speech pathologists, which show no true "standard care" in dysphagia management [16]. While some variation is inherent to dysphagia intervention, using different forms of standard care in studies impacts on the ability to compare and determine relative intervention efficacies [96,100]. Consistency in "standard care" is needed for a stable baseline in research. Evidence-based guidelines and rationales Not reported whether results were significant or not for setting intervention dosages are also needed to reduce unwarranted variation in clinical care. Studies in this review appeared to have different rationales for dosage prescription. Only five studies specifically provided a rationale for their dosage prescription, drawn from strength-training in limb exercises and neural plasticity principles. Most commonly, studies appeared to replicate dosages from previous studies. This was seen in the Shaker exercise, where seven out of nine studies used the same 30 isotonic and three 60-s isometric head lifts used in the pioneering article [38]. Convenience also appeared to be a contributing factor. Most studies provided intervention five times per week, likely catering to typical working days rather than rehabilitation need. Overall, there was lack of evidence-based rationales for dosage prescription, similar to findings in stroke rehabilitation literature [101]. Transparent reporting of the rationale for dosage prescription in studies could facilitate better consideration of dosage selection and allow readers to understand the reasoning of researchers. The use of strength-training and neural plasticity principles offer some direction when considering rationales for swallowing exercise dosages. These principles include specificity (targeting swallow-specific exercises), increasing the volume of intervention and introducing resistive loading to swallowing exercises [102][103][104].
This study had several strengths and weaknesses. The strengths included the systematic approach following PRISMA guidelines. The review followed ethical guidelines, including pre-registering a protocol. The comprehensive search strategy, and inclusion of studies without limitations on language or study design, generated a high number of studies. Study selection included blinded screening and quality assessment of studies by two independent authors to reduce bias. Further, findings included measures of effect size despite the heterogeneity of results. The limitations in this systematic review were related to the quality and heterogeneity of studies. To capture all swallowing exercises and dosages used in stroke rehabilitation, all study designs and quality ratings were included and there was no specification of outcome measures. This allowed a more thorough investigation of the topic, but may have introduced biases [105]. Using a less well-known quality appraisal tool to cater for various study designs may have also impacted on quality assessment. Finally, the focus on intervention methodology  [67] Blow with force to generate high expiratory pressures against adjustable resistance with an EMST device Strengthen expiratory muscles and suprahyoid muscles to improve cough strength and hyoid excursion Jaw opening exercise Open the jaw with/without resistance Strengthen suprahyoid muscles to improve hyoid excursion Oral-motor or orofacial muscle exercises Exercises involving moving oral muscles (typically lips, tongue and jaw) as far as possible, as strongly as possible with/without resistance or as quickly as possible Improve the range-of-movement, strength and/ or co-ordination of oral muscles

Shaker head lift [38]
Raise the head to look at the toes while in supine position Strengthen suprahyoid muscles to improve upper oesophageal sphincter opening Direct exercises Effortful swallow [84] Swallow as hard as possible Improve hyoid excursion, tongue base retraction and pharyngeal constriction Masako [85] Protrude the tongue and hold it between the teeth while swallowing Improve anterior movement of the posterior pharyngeal wall Mendelsohn maneuver [86] Prolong the elevation of the larynx at the peak of a swallow Increase hyolaryngeal elevation and duration of upper oesophageal sphincter opening Supraglottic swallow [87] Take a breath and hold it, swallow while holding the breath and cough immediately post-swallow Improve airway protection rather than outcome precluded in-depth statistical analysis or data synthesis.

Conclusions
Dosage is important in exercise-based intervention. There is increasing awareness of the importance of intentional dosage prescription and reporting in research. However, this review indicates that further work is needed to improve consistent dosage reporting and evidence-based dosage prescription in post-stroke dysphagia studies. Uniform terminology and frameworks are needed to improve consistent and comprehensive dosage reporting across the field. Current frameworks can be used when prescribing and reporting on dosage [11,12,15,106]. Methods used by studies in this review could help guide setting and reporting of intensity in clinical practice. Given the variable reporting of exercise types and dosages, clinicians should pay careful attention to the descriptions of exercises and dosages in studies when replicating or evaluating new interventions. More consistent dosage reporting within studies will improve quality and useability of studies and facilitate reproducibility, comparison, and synthesis of research.
Further work is also needed to improve evidencebased dosage prescription. Current evidence (such as strength training and neural plasticity principles) can be considered along with clinical reasoning to guide dosage prescription. However, more research is needed to examine which principles are applicable to dysphagia rehabilitation. Future research could investigate the impact of altering different components of dosage, such as comparing similar interventions provided at different dosages. Improved dosage reporting, and evidence-based dosage prescription has the potential to improve intervention efficacy and outcomes for patients with post-stroke dysphagia. Chin tuck against resistance (CTAR) Verbal description, e.g., "as far as possible" [31], "as strongly as possible" [52] Use of external resistance, e.g., device [52,71], resistance bar [72], ball [31] Game-based feedback at certain percentage of 1-RM [72] Effortful swallowing Verbal description, e.g., "as hard as possible" [73], "as forcefully as possible" [62] EMST/breathing exercises Expiratory pressure set on device [50,64,65,67] Verbal description, e.g., "deep and forceful breaths" "fast and forcefully" Jaw opening Against external resistance (e.g., ball, device) [54,58,74] Against trainer's hand at set percentage of maximum voluntary contraction using sEMG feedback [66] Verbal description, e.g., to "maximum extent" [35] Lip exercises Gradually increasing external resistance, e.g., pulling force on oral screen [75], oral device [59] Percentage of maximum pressure with IOPI [76] Mendelsohn Increasing length of laryngeal excursion [36] Use of sEMG feedback [78] Verbal description, e.g., swallow "long and strong" [78] PNF-based exercises Tester providing resistance in opposite direction to jaw or neck movement [79] Swallowing Percentage of maximum pressure and different lengths of swallow hold using game-based biofeedback [28] Against resistance from kinesiology taping [60] Gradually increasing amount of food to be swallowed [57] Tongue strengthening Percentage of maximum pressure with IOPI or MOST, ranging from 60% to 80% [27,49,51,68,69] Verbal description, e.g., "press strongly" [62] or "as hard as possible" against roof of mouth [29] Therapeutic programs Use of external resistance, e.g., external force pushing in opposite direction [56,82] Increasing training target goals, repetitions, or duration [33] Move from passive to active exercise and/or increasing amount of activity [57] Verbal description, e.g., "as much as possible" [39], "very hard" [34,44]