Fear-avoidance beliefs are associated with exercise compliance: Secondary analysis of a randomised controlled trial among female healthcare workers with recurrent low back pain

Background Exercise is recommended for the treatment and management of low back pain (LBP) and the prevention of chronicity. Compliance with exercise has proved to be only modest in intervention studies among people with musculoskeletal pain. Fear-avoidance beliefs (FABs) are known to affect exercise adherence.The purpose of the study was twofold: to examine which bio-psycho-social factors contributed to exercise compliance during a 6-month neuromuscular exercise intervention among female healthcare workers with recurrent LBP, and to investigate how exercising affects FABs at 6 and 12 months’ follow-up. Methods A total of 219 healthcare workers aged 30–55 years with mild-to-moderate re-current nonspecic LBP were originally allocated to four groups (exercise, counselling, combined exercise and counselling, control). In the present study, the exercise groups (exercise only and exercise + counselling) and non-exercise groups (counselling only and control) were merged. Baseline factors of the exercise compliers (exercising ≥ 24 times over 24 weeks; n=58) were compared to those of the non-compliers (exercising <1 time/week, 0–23 times; n=52). The effects of the exercise programme on FABs were analysed by a generalised linear mixed model according to the intention-to-treat principle (exercisers; n= 110 vs non-exercisers; n=109) at three measurement points (baseline, 6, and 12 months). A per-protocol analysis compared the more exercised to the less exercised and non-exercisers. Results A low education level (p=0.026), shift work (p=0.023), low aerobic (p=0.048) and musculoskeletal (p=0.043) tness level, and high FABs related to physical activity (p=0.019) at the baseline contributed to lower exercise compliance. The exercise programme reduced levels of both physical activity- and work-related FABs, and there was a dose response: FABs reduced more in persons with better exercise compliance. exercise programme. Exercising with good compliance reduced levels of FABs, which are known to be linked with prolonged LBP. In exercise interventions, motivational strategies should be targeted at persons with low education and tness levels and high FABs in order to achieve better exercise compliance.


Introduction
Disability from low back pain (LBP) is highest in working-age groups worldwide [1]. Among healthcare workers, LBP is the leading musculoskeletal disorder, and it is also the most costly and common selfreported disease [2]. Major contributors to the high incidence of LBP are physically heavy nursing duties, such as lifting and transferring patients, and working in awkward positions [3][4][5].
In physically demanding work duties, maintaining a healthy back obviously requires a certain tness level [6][7][8]. Low ratings of self-reported physical capacity have been shown to be a predictor for future LBP in female healthcare workers [9]. Female nurses with a recent back injury also show more impairment in lumbar movement control [10], which has also been suggested to play an important role in maintaining a healthy back [11,12].
Thus, achieving and maintaining adherence to exercise in the management of spinal pain is important to realise the bene cial effects of exercise. Adherence is a key link between the process and outcome of exercise interventions, and poor adherence compromises the effectiveness of treatment [18].
Barriers to exercise adherence, such as pain with exercise [19], fear of movement and pain aggravation [20], low levels of physical activity [21], low self-e cacy [19], psychological dysfunction [19], poor social support [19], time management, and uncertainty about the bene ts of exercise [22], have been reported among people with musculoskeletal pain [19][20][21][22]. Fear avoidance is the belief that activities should be avoided to reduce pain [23]. Fear of pain develops as a result of a cognitive interpretation of pain as something threatening, and this fear affects attention processes (hypervigilance) and leads to the avoidance of behaviours, like physical activity and exercise, which are expected to cause pain [23]. Fearavoidance beliefs (FABs) in uence treatment effects and are prognostic to poor outcomes in subacute LBP [24]. Besides FABs, there are probably several other internal factors (like a lack of interest [25], and low self-e cacy [26]) and external factors (like time management, environment, and transfer) that compromise adherence to exercise.
Among people with chronic LBP, improvements in functioning are more strongly associated with adherence to exercise than with the type of exercise [27]. Some 50-70% of people with chronic LBP are non-adherent to prescribed home exercise [28]. Thus, factors associated with adherence to exercise in LBP require more investigation [28].
In a previously reported study, we found that a 6-month modi ed Pilates-type neuromuscular exercise intervention, which focused on controlling the neutral lumbar spine posture and developing the muscle strength and endurance needed in heavy nursing tasks, was effective in reducing lumbar pain and lumbar movement control impairments among a sample of nursing personnel with sub-acute or recurrent LBP [29]. The target was to exercise twice a week for 24 weeks, i.e. 48 times. Compliance with the exercise was only modest; the mean attendance rate was 26.1 (of the targeted 48). The reduction of LBP intensity and lumbar movement control impairment was signi cantly better in those who exercised once a week or more compared to the less exercised and non-exercisers [29].
The purpose of this study was to (1) investigate the effects of baseline bio-psychosocial factors on exercise compliance among female healthcare personnel with sub-acute or recurrent LBP. Furthermore, we sought to (2) examine the effects of the exercise intervention on the development of FABs over time (at 6 and 12 months' follow-up).

Study design
This study is based on the data of a four-arm randomised controlled trial (RCT) among female healthcare personnel (NURSE-RCT, clinical trial registration NCT01465698) [30], in which healthcare personnel with sub-acute or recurrent LBP were randomised to participate in neuromuscular exercise/non-exercise and to receive/not receive back care counselling for 6 months [31]. In the secondary analysis, those receiving exercise (combined exercise + counselling, and exercise only) were merged to be exercisers, and nonexercisers (counselling only and controls) were merged to be the controls [29].
The study was conducted in the form of three identical consecutive sub-studies. The participants were female healthcare workers in physically demanding duties: in old people's homes and geriatric wards (in the rst sub-study in 2011, n = 56); in home service, public healthcare units, and community hospital wards (in the second sub-study in 2012; n = 80); and in university hospital wards (in the third sub-study in 2013; n = 83) in the city of Tampere, Finland. The protocol and time frame of each sub-study are presented in the study protocol [30]. The recruitment of participants, eligibility criteria, and reasons for exclusion have been previously described in detail [31]. Brie y, 30-55-year-old female healthcare workers were eligible if they had worked in their current job for at least 12 months and had experienced LBP of an intensity of 2 or above on a numeric rating scale (NRS; 0-10) [32]. The exclusion criteria were a serious earlier back injury, chronic LBP (pain duration ≥7 months), pregnancy or recent delivery (<12 months), and engagement in a neuromuscular-type exercise (NME) more than once a week.
The power calculations, recruitment process, randomisation, and ethical issues of the NURSE-RCT have been presented previously [30], as have the contents of the exercise intervention [29]. The present study aimed to investigate in detail which background and baseline factors are associated with compliance with exercise and to determine how exercising affects FABs.
The study design and ow of the participants are shown in Figure 1. Footnote, Figure 1: Analysis methods for studying (1) the associations between baseline factors and exercise compliance rate, and (2) the effects of the exercise intervention on fear-avoidance beliefs (FABs).

Participants
The participants were female healthcare workers who engaged in physically demanding work and suffered from sub-acute or recurrent LBP. The mean age of the participants was 46 years, and they had worked in their current job on average for 11 years [7]. Some 87% were nurses or nursing assistants, and 70% did shift work [7]. In the pre-study screening, most of the study subjects (82%) experienced LBP on a few or most days of the week, but not daily, and 18% had LBP daily [7]. At the baseline, the mean of the pain intensity measured on a visual analogue scale (VAS; 0-100) during the previous 4 weeks [33] was 36.2 (SD 22.6) [7]. The majority of the study sample can be described as having sub-acute, mild-tomoderate, recurrent or uctuating non-speci c LBP (4).

Measurements
A wide range of measurements was taken at the baseline. In addition to background factors (age, education level, civil status, occupation, number of working years in the current job, working hours, smoking, perceived health and perceived tness in comparison to persons of the same age and gender, current use of medication, high blood pressure (yes/no), and hormonal status); LBP intensity (VAS; 0-100) during the previous 4 weeks [33]; the frequency of LBP; the number of musculoskeletal pain sites [7]; quality of life (RAND 36) with eight sub-scales [34,35]; depression (using the modi ed Finnish version of the Patient Health Questionnaire; PHQ-9) [36]; the short form of the workability index [37]; physical functioning in nursing tasks [30]; tiredness, sleepiness, and di culties in recovering from work [38]; workinduced exertion in different body parts [39]; and psychosocial factors at work (Finnish work satisfaction questionnaire) [40] were investigated by questionnaires. FABs were measured with a questionnaire assessing FABs related to work (FAB-W) and physical activity (FAB-PA) [41]. Tree questions considering long-term sick leave were removed from the original FABs questionnaire, because the participants were still in work [30].
Physical measurements included body mass index (BMI), movement control of the low back [42,43], and performance tests for physical tness, namely aerobic tness by the 6-minute walk test [44], muscular strength (modi ed push-up [45], one-legged squat [45], vertical jump [45], modi ed sit-ups [46]), agility by running a gure-of-eight [47], exibility by trunk lateral side bending [45], and rhythm coordination [47]. More precise information on the measurements is given in the study protocol article [30], the article on the repeatability of the physical measurements [43], and the baseline analysis of the study sample [7].

Exercise interventions in the NURSE-RCT
The contents of the 6-month exercise intervention have been described previously [29]. The modi ed 6month Pilates-type exercise intervention programme, which focuses on controlling the neutral spine posture, started with light and easier exercises, and it was progressive in terms of demands for coordination, balance, and muscular strength over three stages. The goal was to exercise twice a week; during the rst 2 months (stage I) in supervised neuromuscular exercise (NME) classes (lasting 60 minutes) and during the next 4 months (stages II and III) in one supervised class and one home session with the help of a DVD (lasting 50 minutes) or booklet produced for the study [29]. During stages II and III, the participants were also allowed to exercise in supervised group sessions more than once a week if exercise at home was inconvenient, and also only at home if the group sessions were di cult to attend.
During the progression (stages II and III), the participants were allowed and/or advised to do easier exercises from the previous stage if the more challenging exercises proved too demanding.
The leaders of the neuromuscular exercise groups were all certi ed Pilates instructors with a background in physiotherapy, a master's degree in health sciences, or both [29]. Supervised exercise groups were organised in facilities near the workplaces of the healthcare personnel. Group sessions were provided on weekdays starting 15 minutes after the typical work shifts ended. The exercise classes, videos, and booklets were free for the participants, but they exercised in their own time [31].

Compliance with exercise
The instructors monitored the participation of supervised group exercise, and study subjects kept an exercise diary of their home practice. The structured exercise diaries were returned at the end of stage II (week 16) and stage III (week 24). Attendance of the supervised exercise sessions and the number of home exercise sessions were added together to determine the total exercise attendance rate.

Motivational strategies
All participants in the exercise group received an information letter at the beginning of the exercise intervention about the goals and principles of the exercise programme. During the 4 th week of the rststage exercise period, those who had not participated in any group-based exercise sessions received a telephone call from a research nurse (not involved in the exercise intervention or measurements), who encouraged them to start to exercise. All participants received two material packages (between stages I and II, and between stages II and III), which included an exercise DVD, exercise booklet, exercise diary for home practice, and a letter including information about the study and the importance of regular exercise. They also received two e-mails during stage II in order to encourage exercise, and a letter before the 6month follow-up measurements from the principal investigator (JS).
To avoid any contamination to the back care counselling intervention (in the original four-arm setting), and to ensure exactly the same information to all who were allocated to the exercise group, the exercise instructors focused on instructing the standardised exercise programme (individual modi cations due to musculoskeletal problems other than LBP were allowed). All other kinds of counselling (e.g. lifestyle, pain management, and ergonomics) were avoided in the exercise classes.

Statistical methods:
Power calculations (at least 160 subjects needed) for the original NURSE-RCT have been reported previously [30], as has the randomisation of the participants [31].
Partial correlation analysis was conducted between all background and baseline variables and the compliance rate to determine which of the 60 different factors could have an association with the compliance rate. Those variables showing a statistically signi cant association with the compliance rate were selected for bivariate analysis with the compliance rate; the analytical methods were Spearman's correlation for continuous variables and the Kruskall-Wallis test for the categorical variables.
The exercise group was split into the compliers (those who exercised once a week or more; ≥24 times during the 24 weeks) and non-compliers (those who exercised 0-23 times). We examined the baseline characteristics of the participants randomised to the exercisers by the compliance status for those variables showing statistically signi cant associations with the compliance rate in the bivariate analysis.
The analytical methods were the independent samples t-test, the χ 2 test, or the Mann-Whitney U test as applicable.
To analyse the effects of the exercise programme on FABs, the mean differences in time (at three measurement points: baseline, 6 months, and 12 months) between the two groups (exercisers vs nonexercisers) were tested using a generalised linear mixed model (GLMM) (Fig. 1). To take the interaction between back counselling and exercise into consideration, all analyses were rst adjusted for counselling. Second, the sub-study was included as a random effect in all the GLLM analysis models to indicate the possible heterogeneity between the study sites and study time in the three consecutive sub-studies. Other confounding factors were background variables (age, civil status, education), work-related factors (shift work/regular work, psycho-social factors at work [40], perceived work-induced lumbar exertion [39]), and health-related factors (BMI, hormonal status, perceived health, perceived tness, blood pressure, current medication, self-reported physical activity and tness components). Only those confounding factors that improved the model in the second stage in the sense of Bayesian information criteria were included in the nal adjusted model.
After analysis according to the intention-to-treat (ITT) principle ( Fig. 1), the study sample was assigned into two groups in order to investigate the effectiveness of the exercise on FABs, based on a per-protocol (PP) analysis. The mean difference in time (0, 6, 12 months) of exercise compliers (≥24 exercise sessions) were estimated and compared to the results of a combined group of non-compliers and nonexercisers (0-23 exercise sessions + controls).
The correlation between the change in LBP intensity from the baseline to 6 months [29] and the change in the results of the FAB measurements after the intervention period were calculated by Spearman's correlation coe cient (r s ). Associations between professional status and fear avoidance at the baseline were analysed by analysis of variance (ANOVA).

Compliance
The target was for the participants to exercise twice a week-i.e. 48 sessions over 24 weeks. The mean attendance rate was 26.3 (12.2) exercise sessions. Some 53% of the participants exercised 1-2 times/week. The mean compliance rate was 1.1 times/week during the whole intervention. During the last 8 weeks, the mean attendance rate of the group-based exercise decreased, but the home-based exercise rate increased (total amount remaining 1.1 times /week). Only two persons out of the 110 exercised regularly twice a week during the 6-month intervention period.
Of those who were allocated to the exercise group, 10% did not exercise at all, and another 10% took part in only 1-5 exercise sessions in the 6-month period. Of the whole study sample (n = 219), 80% (n = 176) and 72% (n = 157) participated in the 6-month and 12-month follow-up measurements, respectively [31]. At 6 months, 22 persons had dropped out, and 91% of them (n = 20) belonged to the least exercised group (0-5 exercise sessions). The dropout rate (n = 21) was equal among non-exercisers (Fig.1).

Baseline factors associated with exercise compliance
The bivariate associations between exercise attendance rates and continuous background and baseline variables is presented in Table 1. The associations for categorical variables are shown in Table 2. Between-group differences for those variables that had statistically signi cant correlations with compliance rates in the bivariate analysis are presented in Table 3 for the exercise compliers vs noncompliers. Exercise non-compliers had more often a lower education level (p = 0.03), were employed as assistant nurses (p = 0.05), and did shift work (p = 0.02) compared to the exercise compliers (Table 3). From the baseline variables, higher tness results in the one-legged squat (p = 0.043) and 6-minute walk test (p = 0.048) were detected for exercise compliers (Table 3).  At the baseline, there was a difference in the levels of FABs between occupational groups in the whole study sample (n = 219). Nursing assistants had more FABs related to physical activity (FAB-PA, mean 15.5, SD 6.0, n = 89) than nurses (12.0, SD 5.9, n = 102) and other professionals (11.6, SD 6.8, n = 28) (F = 9.5, p<0.001).  Exercise compliers showed lower values for FAB-PA at the baseline compared to non-compliers (p = 0.02, Table 3). During the exercise intervention, both FAB-PA (p = 0.028, adjusted for perceived occupational physical loading) and also FAB-W (p = 0.007, adjusted for age, shift work, perceived health, tness and occupational physical loading, and push-ups) decreased in the exercise group compared to the nonexercisers ( Fig. 2; ITT analysis). There was a dose-response; both FAB-PA (p = 0.006) and FAB-W (p = 0.016) decreased more in the high exercise compliance group compared to the less exercised and nonexercisers ( Fig. 2; PP analysis). A reduction in FAB-PA (from the baseline to 6 months) did not correlate with a reduction in LBP intensity (r p = 0.03, p = 0.54), but there was a correlation between a reduction in FAB-W and a reduction in LBP intensity (r s = 0.16, p = 0.05). work-related fear-avoidance beliefs (the mean difference in percentage with 95% con dence intervals analysed by general linear mixed models).

Discussion
In this 6-month modi ed, Pilates-type exercise study for female healthcare personnel with sub-acute or recurrent, non-speci c LBP, those possessing a lower basic education level, working as assistant nurses, working shifts, and having lower levels of tness and higher levels of physical activity-related FABs at the baseline had a lower exercise compliance. Exercising during the intervention reduced levels of FAB-PA and FAB-W, and there was a dose-response: the levels of FAB-PA and FAB-W decreased more in more exercised persons.
In exercise interventions, levels of exercise adherence usually drop over time [27,48]. In the present intervention, participation in the supervised groups decreased across time, but the amount of home-based exercise increased commensurately. The exercise videos and booklets probably helped in maintaining the same exercise adherence level throughout the 6-month intervention, because shift work makes attending regular group-based exercise demanding. Nevertheless, the compliance rate was lower than we expected.
It has been suggested that factors associated with exercise adherence among LBP patients can be divided into three categories [49]: 1) physical factors like pain [49] and perceived health status [50]; 2) psychological factors like the fear of pain [51], diagnostic uncertainty [20], self-e cacy [19], and depression and anxiety [19]; and 3) environmental factors, such as di culty in integrating exercise into daily life [20,49], lack of time [20,49,51], and intervention-related variables [28]. This classi cation is partly insu cient: it is di cult to place education level, which is a socio-demographic background factor, into any of those categories. Education level has been shown to affect compliance with exercise progression among people with chronic LBP [52], strength training [50], and leisure-time physical activity [53]. In the present study, exercise compliance was the lowest in the rst sub-study, which included those working in an old people's home and on geriatric wards. The majority of the group had a lower education level and worked as assistant nurses.

Physical factors
In previous studies, LBP intensity [49] and older age [52] compromised compliance with exercise or exercise progression. Contradictory to earlier studies, they had no effect in the present study. A higher physical tness level at the baseline contributed to better exercise adherence. We know from experience that those with greater physical tness are more prone to take part in exercise programmes in workplaces than those with poor tness, but the scienti c evidence is lacking.

Psychological factors
FABs related to physical activity are known to affect exercise adherence [20]; activities or exercises are avoided for fear of increasing pain [51]. If an exercise programme is unlikely to produce an immediate tangible bene t, such as pain relief, it is also di cult for many people-for whom pain relief is their primary concern-to recognise the value of exercise [20]. In the present study, those with higher levels of FAB-PA at the baseline exercised less than those with lower levels of FAB-PA.
Cognitive and psychological interventions [23] and graded activity [54] are usually considered helpful in the management of fear avoidance-related pain. We hypothesise that this modi ed, slowly progressing Pilates-type exercise programme, which was conducted at the pace of the participant's calm breathing tempo, might have given the participants positive experiences of movement. They could move in a way that they could control; the movements were not harmful or dangerous and could even release pain. This might explain the reduction of FAB levels during the exercise intervention. No adverse events were reported during the exercise intervention period [31].
Moderators (or treatment effect modi ers) are baseline characteristics that in uence the outcome of treatment [55].Mediators are factors that change during or as a consequence of an intervention and thereby in uence outcome [55]. Thus, it might be hypothesised that the earlier reported reduction of LBP intensity during the exercise intervention [29] might have been mediated by a reduction in the fear of movement. Nevertheless, there was no correlation between the reduction of LBP intensity and the reduction of FAB-PA, but exercise adherence was the key factor. Those with a lower FAB-PA at the baseline exercised more, and those who exercised more gained more positive results in pain reduction [29]. While we measured pain intensity only at the baseline and after the 6-month exercise interventioni.e. not during the intervention period-we cannot say anything about causality. Exercising more might have reduced the pain levels due to exercise-induced hypoalgesia [56][57][58], or a rapid reduction of pain intensity at the beginning of the exercise intervention might have decreased the levels of FAB-PA, and thus increased the motivation to exercise.
The reduction of FABs during the intervention was statistically signi cant, but we do not know if it is clinically relevant (three questions related to returning to work were removed from the original FAB Questionnaire). All the participants engaged in physically demanding work and had low back troubles, which is a challenging combination and a risk for prolonged LBP. For those healthcare workers with previous LBP, both the physical workload and FABs are important in the development of new episodes of LBP [59].
Previous physical inactivity has been shown to interfere with progression through the di culty levels of Pilates among people with chronic LBP [52]. In the recruitment of participants in the NURSE-RCT, those who were already engaged in neuromuscular-type exercise more than once a week were excluded, but the screening did not include other questions about the participants' previous physical activity levels.

Environmental factors
We knew beforehand by experience that women aged 30-55 years and engaged in shift work form a challenging group. There are several other activities, especially family issues (small children and/or taking care of ageing parents or relatives) that compete for time resources. In our study, placing the exercise classes in locations close to workplaces, starting the classes when the typical work shifts ended, and producing exercise videos were attempts to make participation as easy as possible.
A lack of time is the most frequently reported barrier to leisure-time physical activity or exercise, both among the general population [25,60] and among people with LBP [51]. Those working shifts had a lower exercise compliance than those working at regular times. Among nursing personnel, shift work is also associated with sleeping problems, fatigue, and lack of energy [61,62], which might compromise exercise adherence.
Increasing the attractiveness of an exercise programme has been suggested to increase adherence [63].
In 2012-2014, when the study interventions were conducted, Pilates was quite a popular exercise type in Finland. "Back-friendly" exercise can be performed in many ways, but we reasoned that a Pilates type of exercise intervention would have been more attractive than a typically conventional/traditional neuromuscular exercise form.
The intensity of the training content-both being physically too hard or too easy-is an attendance barrier to exercise programmes among healthcare workers [64]. The use of skilful exercise instructors-who demonstrate exercises, observe exercise practice, give feedback, and make subsequent corrections in technique-has been suggested to increase exercise adherence [20,21]. To ensure skilful exercise instruction, we recruited only certi ed Pilates instructors with a high level of basic education who were able to advise participants to exercise "on their level" during the progression of the exercise programme.
In our study, participants exercised in their leisure time. Financial incentives from the employer in the form of exercise during working hours or subsidies for the time used for exercise play an important role in decision-making also among people with LBP [65]. Among Danish healthcare workers, adherence to group-based supervised exercise at the workplace was greater than adherence to exercise at home [66]. However, even when the exercise is offered to workers during working hours, studies report only moderate compliance at best [67].
Increasing adherence to exercise is an important factor for the longer-term effectiveness of an intervention. According to the literature, the following actions may enhance adherence to exercise: Self-regulation behaviour, like strategic planning [51] and self-monitoring [50]; Integration of educational components in exercise sessions [68], like goal-setting [21,69] and education [21]; Supplementary printed material, motivation strategies, and positive re-enforcement [69]; A favourable environment [51]; Action planning to overcome barriers to exercise [51]; Programme organisation and leadership skills [19], including encouragement [51]; Pleasure associated with exercise [51]; Reduced fear of pain and pain itself [51]; Identi cation of especially those who have a low education level, and targeting motivating efforts at them [50].
Understanding the causality and reasons for exercise adherence is complicated, multidimensional, and di cult to study. People do not always behave in the way they intend to behave. Motivation alone is not su cient to trigger an action, and one is often confronted with obstacles [51]. In the present study, exercise compliance was lower among those with a lower level of basic education working as assistant nurses. The levels of FABs were also higher among this group. In clinical practice, motivational strategies with a focus on decreasing FABs especially among people with a low education level could be bene cial. Unfortunately, there is a lack of measurement methods to identify those who would bene t most from motivational actions.

Limitations
This study was a secondary analysis of the NURSE-RCT. Investigating associations between individual factors at the baseline and exercise compliance was not planned simultaneously with planning the RCT, and it was not written into the study protocol [30]. We arrived at the idea for the study after we detected the dose response of exercising on LBP intensity and movement control impairments [29].

Conclusion
Participants with lower education and tness levels who worked shifts and had high physical activityrelated fear-avoidance beliefs at the baseline had a lower compliance with the 6-month neuromuscular exercise programme. Exercising with good compliance reduced levels of FABs, which are known to be linked with prolonged LBP. In exercise interventions, motivational strategies should be targeted at those with low education and tness levels and high fear-avoidance beliefs to achieve better exercise compliance. In exercise intervention studies, strategies to enhance and/or maintain exercise adherence need to be taken more seriously, because adherence is a key link between intervention and outcomes.

Declarations
Ethics approval and consent to participate   Trial pro le (CONSORT ow chart). Analysis methods for studying (1) the associations between baseline factors and exercise compliance rate, and (2) the effects of the exercise intervention on fear-avoidance beliefs (FABs).