ABSTRACT

Background: We sought to determine if women (65–74 y) can self-select an exercise intensity during walking commensurate with current physical activity recommendations.

Methods: Thirteen healthy older women (age = 68 ± 3 y, body mass index = 25.7 ± 4.9 kg·m−2, peak O2 uptake = 24.1 ± 4.5 mL·kg−1·min−1) performed 4 30-min walking trials (2 × treadmill [TM], 2 × overground [OG]) in a counterbalanced, randomized order. For the first walking trials (i.e., TM1 and OG1), participants self-selected walking pace. Walking speed, heart rate (HR) and ratings of perceived exertion (RPE) were recorded. For the second trials for each mode (i.e., TM2 and OG2), walking speed was controlled to match speeds selected during TM1 and OG1, and pulmonary gas exchange, HR, and RPE were measured.

Results: Exercise intensity was within current guidelines: OG = 70% HRpeak, 95% confidence interval (CI) = 61–75%; TM = 66% HRpeak, 95% CI = 63–74%. Significant increases in HR and walking speed were observed during OG (HR P = 0.005, walking speed P = 0.001) compared with TM; O2 uptake during OG was significantly greater than TM for first 15 min exercise.

Conclusion: Healthy women can self-select intensity during walking commensurate with current physical activity recommendations.

INTRODUCTION

The American College of Sports Medicine (ACSM), American Heart Association (AHA), British Heart Foundation, and Exercise and Sports Science Australia recommend that older adults engage in 150–300 min/wk of physical activity, such as walking, to maintain good health. Less well defined is the intensity at which an individual should perform physical activity, although moderate-intensity exercise has been widely promoted as part of the recommended exercise prescription. The ACSM has classified moderate-intensity cardiorespiratory endurance exercise as 64–76% of an individual's maximum heart rate (HR) or 46–63% of an individual's peak O2 uptake (13), whereas the AHA defined moderate-intensity physical activity as 50–69% of maximal HR or 45–59% of peak O2 uptake (4). While maximal exercise is not in the best interests of the older person due to the increased risk of medical events, including acute cardiovascular events, the estimation of exercise intensity is typically based on percent of maximal HR or peak O2 uptake (5,6).

It may be simple enough to prescribe and achieve the recommended relative exercise intensities equivalent to 64–76% of peak HR and 46–63% of peak O2 uptake in an organized and well-equipped center-based physical activity program. However, many individuals engage in home-based, self-directed physical activity, where exercise intensity (e.g., walking speed) is self-selected. This is particularly evident among older individuals who may exercise in the home or community due to economic, accessibility, and/or other barriers (7). However, without specialized equipment and/or appropriate education, it may be difficult for many individuals to select an appropriate exercise intensity that is safe and ensures beneficial health outcomes. Indeed, achieving the recommended exercise intensity may be more important than exercise duration for the consolidation of long-term health benefits, such as the reduced risk and improved management for many chronic diseases (8).

Two studies reported that self-selected walking speed may result in exercise intensities that approach those outlined by the ACSM (9,10). However, both of these studies have methodological deficiencies that prevent the findings from being broadly adopted for community- or home-based exercise prescription in the elderly: neither study directly measured O2 uptake during overground (OG) walking. Rather, both studies derived O2 uptake estimates from a treadmill (TM) walking test performed at the same speed as that observed during OG walking. Using O2 uptake measurements obtained during TM tests to estimate the exercise intensity of OG walking is problematic, as walking pattern has been demonstrated to differ between these 2 modes of walking (11). Furthermore, the inclusion of men and women, as well as participants from a wide age range, adds to the uncertainty for using self-selected walking speed for exercise prescription purposes in a discrete subgroup of the population.

Women aged 65 y and older are currently the fastest growing portion of the population, the most sedentary (12,13), constitute the greatest proportion of recreational walkers in Australia (14), and potentially have the most to gain from undertaking regular physical activity (15,16). Further research specific to facilitating evidence-based physical activity of older women is therefore warranted. The purpose of our study was to determine if healthy older women (65–74 y) self-select a walking intensity that is within the current recommendations for moderate-intensity endurance exercise.

METHODS

Participants

Thirteen women aged between 65 and 74 y participated in this study. All participants had performed light-moderate physical activity in the past 14 d but were not meeting the national guidelines for physical activity, i.e., 150 min of moderate-intensity exercise per week. Participants were excluded if (1) they were currently taking any medication known to interfere with the exercise response; (2) they were unable to walk unaided for at least 30 min; or (3) their medical history, hematological results, resting electrocardiogram and/or basic spirometry indicated significant risk of a cardiac event or other adverse medical complication. All participants received approval to participate from their family physician. The study was approved by the Human Research Ethics Committees of Griffith University and Bond University, and all participants gave their written consent to participate in the present study.

Experimental Design

Participants were required to attend 1 familiarization session and 5 experimental sessions. Familiarization included (1) practicing mounting and dismounting the TM (Lode Valiant, AN Groningen, Netherlands); (2) using the TM speed controller to determine a comfortable walking speed that was subsequently used for the graded walking test; (3) becoming comfortable wearing the portable metabolic measurement system (K4b2 COSMED, Rome, Italy); and (4) using the Borg rating of perceived exertion (RPE) scale (620). Overall RPE was used, as we did not anticipate arm fatigue during TM walking. We did not anchor RPE, as previous work has shown this to be unreliable (17). During the familiarization session, forced vital capacity maneuver was completed using an open-circuit spirometer (Ultima, Medical Graphics Corp., St Paul, MN) to confirm pulmonary function was within normal limits. The first experimental session was used to determine peak exercise values for walking during a graded TM-walking test to volitional fatigue. Experimental sessions 2, 3, 4, and 5 were completed at least 7 d later and were each separated by at least 4 d. Participants completed 2 30-min OG walking trials, and 2 30-min TM walking trials. The first walking trial for each condition (i.e., OG trial 1 [OG1], and TM trial 1 [TM1]) were randomly ordered and required participants to walk for 30 min at a self-selected speed. If participants questioned the instruction, the statement was again repeated and the term “exercise” emphasized. No further instruction was given.

During the subsequent walking trials for each condition (i.e., OG2 and TM2), participants were required to walk at the predetermined walking speed (measured during OG1 and TM1) wearing the portable metabolic measurement system for the measurement of O2 uptake. The order of OG2 and TM2 was randomized.

Experimental Exercise Tests

Determination of Peak Exercise Values

The graded TM-walking test commenced with 4 min of walking at 3.0 km·h−1 (1.86 mph) and 0% grade. Treadmill speed was then increased by 0.5–1.0 km·h−1 (0.31–0.62 mph) each minute until the participant's predetermined preferred TM walking speed was attained. Thereafter, TM grade was increased by 2% every minute until the participant was unable to maintain the required intensity and requested to stop despite verbal encouragement. Gas exchange and ventilatory parameters were measured breath-by-breath using the portable metabolic measurement system, and cardiac rate and rhythm were monitored continuously using a 12-lead electrocardiogram (CardioPerfect, Welch Allyn Inc., Skaneateles Falls, NY). The breath-by-breath gas exchange and ventilatory parameters were averaged over 30-s intervals, and peak exercise values reported as the average of the highest 2 30-s values recorded during the exercise test. The ventilatory threshold (VT) was determined using the modified V-slope method (18).

Experimental Walking Trials

Participants were able to self-select their walking speed during the first 2 30-min trials. During OG1 and TM1, participants were instructed to “walk at a speed you deem appropriate for 30 min of exercise.” No other encouragement or discussion about walking speed during the trial was provided.

Overground walking was conducted around an 83.5 m (274 ft) elliptical indoor track. Walking speed during OG1 was measured twice per lap over 2 straight sections (16 m; 52.5 ft) of the track using timing lights (SMARTSPEED, Fusion Sport, QLD Australia). The 16-m monitoring sections were specifically chosen as the middle portion of a 26-m straight section of track; thus, each 16-m monitoring section included a 5-m “lead-in” and 5-m (16.4 ft) “lead-out” section of straight walking track.

Treadmill walking during TM1 was performed at 0% gradient, and participants could increase and/or decrease the TM speed as often as desired throughout the trial. Treadmill speed was recorded continuously from an external control panel, which was blinded to participants.

The OG2 and TM2 walking trials were conducted in the identical manner except that participants were required to maintain the walking speed observed in the OG1 and TM1 walking trials while pulmonary gas exchange was measured using the portable metabolic system. Walking speed was achieved by either direct control of the TM belt speed by the investigators or by verbal instructions based on the time taken to complete each 16-m section of the OG track.

Heart rate was measured continuously during all 4 walking trials using a HR belt and recorder (Suunto T6, Vantaa, Finland), and RPE was recorded for both chest (cardiac/respiratory exertion) and legs (local muscle fatigue) every 5 min. Data from the walking sessions were averaged over 5-min periods, with the first 5 min of each exercise trial considered a transition period and excluded from the analysis. Walking intensity was expressed in absolute terms (using O2 uptake, HR, and RPE values) and also expressed relative to the peak exercise values attained during the incremental exercise test (i.e., peak HR% and peak O2 uptake%). Individual walking economy was also calculated by dividing O2 uptake by mean walking speed (i.e., mL·min−1/m·min−1 = mL·m−1).

Statistical Analysis

Statistical analysis was conducted using IBM SPSS statistics 19.0 software suite (SPSS, Chicago, IL). Shapiro-Wilk tests confirmed normality prior to analyses. Full-factorial analysis of variance was used to ascertain if there were any trial differences in walking speed, HR, and/or RPE between trials within each walking mode (i.e., OG and TM). As no differences were found between trials for both modes, all further analyses for walking speed, HR, and RPE were conducted on data obtained from TM2 and OG2. Comparisons between walking modes were also conducted across the following time intervals using analyses of variance (5–10, 10–15, 15–20, 20–25, and 25–30 min). Where significant F values were observed, least significant difference post hoc tests were performed. Statistical significance was set at 5% (α = 0.05). Relations between the variables were assessed using Pearson product-moment correlation coefficient.

RESULTS

Table 1 presents the participant characteristics including their blood pressure and pulmonary function results obtained while at rest. The peak exercise values obtained during the graded TM-walking test are presented in Table 2. Values obtained for lung function were within expected ranges, and peak O2 uptake was commensurate with apparently healthy older women reported elsewhere (19).

TABLE 1.

Participant characteristics, resting blood pressure, and spirometry values of women 65–74 y.

Participant characteristics, resting blood pressure, and spirometry values of women 65–74 y.
Participant characteristics, resting blood pressure, and spirometry values of women 65–74 y.
TABLE 2.

Peak exercise values determined during graded treadmill walking in women 64–75 y.

Peak exercise values determined during graded treadmill walking in women 64–75 y.
Peak exercise values determined during graded treadmill walking in women 64–75 y.

Values obtained during TM and OG walking are listed in Table 3. The 95% confidence intervals (CIs) for self-selected walking intensity, when expressed as a percentage of HRpeak, were 61–75% for OG and 63–74% for TM walking. As a percentage of peak O2 uptake, the 95% CIs were 58–76% for OG walking and 52–71% for TM walking. When the O2 uptake values were calculated as a percentage of the participant's VT, the mean O2 uptake during OG walking was 94% VT (95% CI = 82–107% VT), while during TM walking the mean O2 uptake was 87% VT (95% CI = 74–100% VT).

TABLE 3.

Values obtained during overground (OG) and treadmill (TM) walking in older women aged 65–74 y. Values are mean ± SD.a

Values obtained during overground (OG) and treadmill (TM) walking in older women aged 65–74 y. Values are mean ± SD.a
Values obtained during overground (OG) and treadmill (TM) walking in older women aged 65–74 y. Values are mean ± SD.a

The walking speed that was measured every 5 min for self-selected TM and OG walking is presented in Figure 1A. At each 5-min interval, the self-selected walking speed for OG was significantly faster than TM, whereby the difference in walking speed between the 2 modes during each 5-min exercise period (i.e., P1 to P5) were: P1 = 1.04 km·h−1, P < 0.01; P2 = 0.89 km·h−1, P < 0.01; P3 = 0.76 km·h−1, P < 0.01; P4 = 0.72 km·h−1, P < 0.01; and P5 = 0.63 km·h−1, P < 0.01.

FIGURE 1.

(A) Overground (OG) and treadmill (TM) walking speed, (B) heart rate, (C) O2 uptake, and (D) movement economy (O2 uptake per m walked). Data are presented for each discrete time period, represented by P1 (5–10 min), P2 (10–15 min), P3 (15–20 min), P4 (20–25 min), and P5 (25–30 min). Error bars indicate the standard error of the mean (SEM). The first 5 min of each 30-min walking trial (as indicated by the dashed line) was not included in statistical analysis. * denotes a significant difference between OG and TM walking for a given time period; ‡ denotes a significant difference between time periods during TM walking; † denotes a significant difference between the mean values for OG and TM walking.

FIGURE 1.

(A) Overground (OG) and treadmill (TM) walking speed, (B) heart rate, (C) O2 uptake, and (D) movement economy (O2 uptake per m walked). Data are presented for each discrete time period, represented by P1 (5–10 min), P2 (10–15 min), P3 (15–20 min), P4 (20–25 min), and P5 (25–30 min). Error bars indicate the standard error of the mean (SEM). The first 5 min of each 30-min walking trial (as indicated by the dashed line) was not included in statistical analysis. * denotes a significant difference between OG and TM walking for a given time period; ‡ denotes a significant difference between time periods during TM walking; † denotes a significant difference between the mean values for OG and TM walking.

The variation in walking speed (0.19 ± 0.26 km·h−1) observed during the 30-min walking session for OG (P1 to P5) was not significant (P = 0.43). In contrast, there was a significant increase in the mean walking speed (0.66 ± 0.45 km·h−1, P < 0.01) from the beginning to the end of TM. Analysis of consecutive 5-min intervals revealed that the walking speed during TM walking increased between P1 and P2 by 0.22 km·h−1 (P = 0.045), after which the variation between consecutive intervals (i.e., P2–P5) was not significant.

Mean HR (see Figure 1B) was higher for OG walking, when compared to TM walking (P < 0.01). The overall increase of 8 b·min−1 in HR (P1–P5) was significant for both conditions (P = 0.03), although the magnitude of change for OG walking was not significantly different to that observed for TM; thus, no interaction effect was observed for HR.

There was an interaction effect (P = 0.030) for O2 uptake, with significant differences in mean O2 uptake being detected between OG and TM walking at the time periods P1 (0.12 L·min−1, P < 0.01) and P2 (0.10 L·min−1, P = 0.02). For TM, there was an increase in O2 uptake over the 30-min session between P1 to P5 of 0.8 L·min−1 (P = 0.04), although there was no change in O2 uptake for OG (difference from P1 to P5 = 0.3 L·min−1). We found a difference in economy (O2 uptake per meter walked) between OG walking (11.2 ± 2.5 mL·m−1) and TM walking (12.0 ± 3.0 mL·m−1; P = 0.049). The O2 uptake expressed as a percentage of the participants VT was different for the 2 conditions (P = 0.02), where OG (94 ± 21%) was higher than TM walking (87 ± 22%).

Figure 2 illustrates the relation between peak O2 uptake and the walking speed achieved during OG (left panel) and TM (right panel) trials. The relationship was moderate to strong when peak O2 uptake was expressed relative to body mass and in absolute terms (r = 0.72; P < 0.01) during OG walking. No significant relation existed between peak O2 uptake, expressed relative to body mass (r = 0.25; P = 0.40) or in absolute terms (r = 0.38; P = 0.20), and TM walking.

FIGURE 2.

Peak O2 uptake plotted against self-selected walking speed during overground and treadmill walking. Data points represent the mean walking speed (5–30 min) during Trial 2 for each condition. Solid line represents the linear regression of the data.

FIGURE 2.

Peak O2 uptake plotted against self-selected walking speed during overground and treadmill walking. Data points represent the mean walking speed (5–30 min) during Trial 2 for each condition. Solid line represents the linear regression of the data.

DISCUSSION

The salient findings of the present study are that healthy women aged 65–74 y (1) self-select a walking speed that induces a metabolic load approximating the VT during both OG (~94% VT) and TM (~87% VT) walking; (2) self-select a faster walking speed during OG walking, which induces an elevated cardiovascular and metabolic “load,” when compared with TM walking; (3) demonstrate significantly lower O2 cost per meter walked (i.e., improved economy) during OG walking, when compared with TM; and (4) self-select an OG walking speed that is positively related to their peak O2 uptake (i.e., increased physiological functional capacity was associated with faster preferred OG walking speed), although no significant relationship was observed between preferred TM walking speed and peak O2 uptake. Moreover, no medical events were observed during exercise and/or in the weeks following the walking trials. The collective findings of this study support that older women self-select an exercise intensity during OG and TM walking that is commensurate with current recommendations for daily physical activity.

Regular physical activity across the lifespan contributes to healthy aging. Improved cardiorespiratory physiological functional capacity—perhaps the most evident benefit of endurance exercise training—is closely associated with decreased incidence of cardiovascular diseases, musculoskeletal and metabolic disorders, various cancers, and neurological dysfunction (5). Decreased burden of disease is not simply correlated with increased physiological functional capacity; rather, regular exercise induces beneficial causal adaptations at all levels of biological organization, from the organism to the regulation of gene expression (20,21). The accumulation of consistent findings demonstrating such health benefits has helped promote a worldwide initiative for using exercise prescription as an evidence-based method for the prevention and treatment of many chronic diseases (22). An important, yet often overlooked aspect of prescribing exercise to exercise-naïve (e.g., inactive older) individuals is that the associated health benefits of regular exercise may be achieved even among those who were previously and chronically sedentary. A prospective investigation of a large cohort of women aged over 65 y confirmed “conventional wisdom” that those who were active at study onset and remained active over the 5-y follow-up period had a much lower incidence of mortality from all causes when compared with those who were sedentary; however, the study also revealed that older women who became active between initial and follow-up testing shared remarkably similar reductions in all-cause mortality with those women who were always active (23). There is extensive evidence therefore that regular exercise across the lifespan is associated with healthy aging, and these benefits can be realized even when exercise training is first adopted later in life.

The principal barriers to the uptake and adherence to exercise prescription include a lack of awareness among the elderly of appropriate exercise prescription (i.e., intensity, duration) and the costs associated with using exercise facilities (24,25). Personal communications with our own participants (healthy women aged 65–74 y) provided the additional insight that many of these women felt “out of place” in formal exercise facilities (e.g., gymnasiums, health clubs) and thus had associated negative experiences, which supports earlier findings that the culture of exercise facilities is often a significant barrier to exercise participation (26). Given primary care providers prescribe home-based exercise programs that rely upon limited supervision and equipment (27), selection of an exercise-intensity for older adults based on simple and evidence-based methods is of value.

The present study demonstrated that, when healthy women aged 65–74 y are asked to perform walking at an “appropriate pace” for a 30-min exercise bout, they self-select an intensity that is known to confer the health benefits associated with regular exercise. On average, women in the present study walked OG at an intensity of 67 ± 15% of peak O2 uptake, while TM walking was at a lower intensity (62 ± 15% of peak O2 uptake). This difference in metabolic load was the result of an increased walking speed during OG when compared with TM walking. The OG walking intensity in the present study was higher than previously described as moderate-intensity exercise (46–63 and 45–59% of peak O2 uptake) by the ACSM and AHA, respectively (4,5). When we expressed intensity relative to the VT—the most accurate demarcation between moderate- and heavy-intensity exercise—it was found that both TM and OG walking intensities were at the upper limit of the moderate-intensity domain (both ~90% of VT), which extends a similar finding in middle-aged (43 ± 5 y) and sedentary women (28). The capacity to perform exercise within the moderate-intensity domain without the need for specialized equipment is important for effective exercise prescription given there is minimal associated risk for cardiovascular events (29), and this intensity is reported to promote increased adherence to exercise training (30). These findings collectively suggest that older women self-select a walking speed that induces a metabolic load that is associated with established health benefits, is safe, and may facilitate increased exercise adherence.

The difference in self-selected speeds for TM and OG walking in the present study has possible implications when prescribing physical activity on TM compared to OG walking. Any walking speed or exercise intensity determined on a TM may not be appropriate for prescribing OG walking for health benefits and vice versa. For example, if walking speed was titrated on a TM based on an individual's physiological (HR and/or O2 uptake) responses, the corresponding OG walking speed would need to be faster to elicit the desired response. This finding also questions the validity of extending the outcomes of TM-based exercise training studies to home-based training initiatives that primarily employ OG walking.

The increased walking speed and metabolic load observed during OG when compared with TM in the present study may be related to familiarity of OG walking. Marsh et al. (31) reported that older adults tend to walk at slower speeds on a TM, with an elevated perception of stress, when compared with OG walking. Age appears to be an important determinant of TM walking strategies, given that that small differences in walking pattern during TM and OG are within the normal stride-to-stride variation for younger (32), but not older individuals (11). Indeed, when Watt et al. (11) controlled walking speed for both TM and OG walking, older adults employed a significantly increased step rate and decreased stride length (and thus decreased stride time) during TM walking (11). While we did not examine walking kinematics, such differences in walking patterns among healthy older adults may explain the differences in walking economy in the present study: the O2 cost per meter of TM walking (12.0 ± 3.0 mL·m−1) was significantly increased (i.e., less economical) when compared with OG walking (11.2 ± 2.5 mL·m−1). It would be tempting to use this finding and recommend less economical TM walking for the promotion of weight loss in older adults; however, the lower total O2 cost (i.e., metabolic load) of TM walking, despite the increased O2 cost per distance traveled, may have negative implications for those using exercise training to assist in weight loss; for example, TM walking would require a longer total exercise duration to achieve a given caloric expenditure. The different and apparently “unnatural” walking pattern employed during TM walking, therefore, promotes the benefits of OG walking within the context of home-based and/or community-based exercise prescription for older women.

The present study demonstrated that the self-selected OG walking speed of older women was statistically related to their peak O2 uptake. Our findings in older women support those of Cunningham et al. (33), who reported that self-selected OG walking speed of males aged 19–66 y was related to VO2max. Moreover, our findings extend those reported by Spelman et al. (34), who covertly observed younger (~35 y) and mostly female participants during habitual walking and found that self-selected OG walking speed was significantly associated with VO2max. These findings support that physiological functional capacity is a determinant of self-selected OG walking speed, although we did not observe a similar relationship for TM walking. This finding is initially peculiar, given that peak O2 uptake was determined during TM walking to volitional fatigue; thus, it would be logical to extrapolate that self-selected speed during TM walking would relate to peak O2 uptake in the same mode. Our finding is also at odds with an earlier study that reported preferred TM walking speed of older adults had a significant correlation with peak O2 uptake (35). This incongruent finding is likely due to the different methods employed by Malatesta et al. (35) and the present study (i.e., our participants were able to freely modify the TM speed at any time during the 30-min walk, whereas participants in Malatesta et al. (35) walked at a fixed predetermined speed identified during familiarization). Our findings indicate that physiological functional capacity does not influence comfort nor confidence (e.g., RPE values) during submaximal walking on a motorized TM by older women, although self-selected OG walking speed is significantly determined by peak O2 uptake.

The tendency for participants to increase their walking speed during the TM walking trials, compared to little or no change in the OG walking speed, is intriguing. While we stopped the participants walking at 30 min, it would now be important to determine if, given a longer walking time, the difference in walking speeds became negligible, or whether the difference would be still apparent following long-term use of a motorized TM. It may be that, with prolonged acclimation to TM walking, there would be less of an increase in self-selected walking speed across an exercise period.

A limitation of this work was the relatively small number of participants; study recruitment was limited by the previous exercise history. A second limitation were the cues given; specifically, participants may have been advised to exercise at an intensity that may benefit their health, but this walking speed may not have been the same as if they were not participating in the study. Finally, outcomes might have been different had OG walking been conducted in a neighborhood where ground conditions and inclines are less controlled.

In conclusion, we found that when older women are given the opportunity to perform 30 min of walking, they inherently choose a walking speed that approximates the optimal intensity for inducing the health-derived benefits of regular exercise training, which agrees with current exercise guidelines by major medical and health associations. The VT has a strong influence on the self-selected exercise intensity employed by older women, which is the upper limit of exercise that is known to have very low risk of adverse events. Moreover, self-selected walking speed during OG walking appears to provide an indirect and low-cost indication of the intensity associated with the VT independent from expensive and specialized equipment and therefore may serve useful when selecting walking speeds for exercise prescription. Given that access to specialized equipment and personnel represent significant barriers for exercise prescription, in addition to the associated cost which may often be prohibitive to older individuals, these findings reinforce the validity of clinician and clinical exercise physiologist directed, but unsupervised, exercise programs for achieving optimal health management for older women.

Acknowledgments

The authors thank Ms. Tracey Norling for her efforts in patient recruitment and project organization, Dr. Laurie Kelly for supervision during maximal exercise testing, and our participants. The results of the present study do not constitute endorsement by ACSM.

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Author notes

1School of Allied Health Sciences, Griffith University, Gold Coast QLD, Australia

2University of New England, Amdiale, NSA, Australia

Conflicts of Interest: None.

Source of Funding: This project received funding from the National Health and Medical Research Council of Australia (Project ID: 376601).