The metabolic equivalents of one-mile walking by older adults; implications for health promotion

Background: Instructions for older adults regarding the intensity of walking may not elicit an intensity to infer health gains. We recorded the metabolic equivalents (METs) during a 1-mile walk using constant and predicted values of resting MET in older adults to establish walking guidelines for health promotion and participation. Methods: In a cross-sectional design study, participants (15 men, 10 women) walked 1-mile overground, in a wooden floored gymnasium, wearing the Cosmed K4b2 for measurement of energy expenditure. Constant or predicted values for resting MET were used to calculate the number of 1-mile walks to meet 450-750 MET∙min∙wk-1. Results: Participants had MET values higher than 3 for both methods, with 29% and 64% of the participants higher than 6 for a constant and predicted MET value, respectively. The METs of the1-mile walk were (mean ± SD) 6 ± 1 and 7 ± 1 METs using constant and predicted resting MET,and similar for men (constant: 6 ± 1 METs; predicted: 7 ± 1 METs) and women (constant: 5±1METs; predicted: 6 ± 1 METs) (P > 0.05). Conclusion: Older adults that are instructed to walk 1-mile at a fast and constant pace meet the minimum required intensity for physical activity, and public health guidelines. Health professionals, that administer exercise, could encourage older adults to accumulate between six and nine 1-mile walks per week for health gains.


Introduction
Health promotion guidelines suggest that healthy adults benefit from at least 150-minutes of moderateintensity activities per week, achieved by bouts of at least 10 minutes, to reduce risk of chronic diseases and maintain quality of life. 1,2 Walking is a popular form of physical activity and suitable for all population groups. 3 However, instructions regarding the intensity may not elicit an intensity required to infer health gains in older adults. 4 The metabolic equivalent (MET), a method of estimating energy expenditure during physical activity is used to prescribe exercise. 5 The 1-MET represents energy expenditure while sitting quietly, 6 with the assumption that it is equal to 3.5 ml•kg -1 •min -1 as reported by Jetté et al, 7 and defined here as the M1 method. However, Byrne et al 8 observed that resting V O 2 was 35% lower than the commonly accepted 1-MET value of 3.5 mL•kg -1 •min -1 . Therefore, 1-MET needs to be individualised with consideration of body mass index, age and gender, 8 a measurement applicable in the community environment, defined here as the M2 method. Hall et al 9 demonstrated that older adults have 1-MET values 31% lower than 3.5 mL•kg -1 •min -1 . Therefore, more research is required to better understand the frequency, intensity and duration of exercise prescription for older adults, and establish more specific public health guidelines for exercise professionals to promote.
The energy cost of physical activities is expressed as multiples of 1-MET, and considered light (i.e. <3-METs), moderate (i.e. 3-6-METs) or vigorous (i.e. >6-METs) intensity. 7 Physical activity guidelines recommend that adults should accumulate 450-750 MET•min•wk -1 . 6 However, the general public may not understand what this means for their exercise practice, making it difficult to identify if they have completed enough exercise. Therefore, M1 and M2 are simple methods that can be used to simplify public health recommendations to achieve 450-750 MET•min•wk -1 with 1-mile walks. According to Fitzsimons et al,4 instructions to walk at a fast pace for 150 meters achieved an intensity of 4.3 METs HPP (moderate intensity), sufficient to infer health gains. It is not known whether instructions to walk 1-mile as fast as possible will allow older adults to achieve a sufficient intensity and allow to complete multiple 1-mile walks to meet physical activity recommendations, which could be a simple and easy health promotion method for older adults to follow and monitor.
Therefore, the aim of the present study was to examine if instructions to walk fast during the 1-mile walk would result in achieving the intensity guidelines that incur health benefits for elderly men and women. Also, how many 1-mile walks would be required per week to meet physical activity of 450-750 MET•min•wk -1 in elderly men and women based on the M1 and M2 method for 1-MET.

Participants and settings
Twenty-five healthy older adults (mean age: 67 [SD 4] years) were recruited within the local community for this cross-sectional study. Participants completed a health history questionnaire and were in good health with four on medications to lower cholesterol. Ethical approval was obtained from the University Research Ethics Committee (Code: REC260697), and written informed consent was obtained after an explanation of the procedures and risks. Procedures followed were in accordance with the Declaration of Helsinki of 1975, as revised in 2008.

Procedures
Height (Holtain Ltd, Crymych, U.K.) and body mass (Seca Model 880, Seca Ltd., Birmingham, UK) were measured to the nearest cm and 0.1 kg, respectively, with participants lightly clothed and shoeless. Body mass index (BMI) was calculated and body fat percentage measured with bioelectrical impedance analysis (BF%; BC418 MA, Tanita, U.K.). Participants completed two indoor timed 1-mile walks individually, one familiarisation 10 and one wearing the Cosmed K4b 2 portable metabolic system, with a maximum of 1-week between. The timed 1-mile walk was used to predict maximal oxygen uptake using end heart rate (Polar FS1, Polar U.K.), age, and gender. 11 Participants completed 28½ laps around a wooden floored gymnasium (19x9 m) with recording of step frequency at minutes 1, 5 and 10. Instructions were to complete the 1-mile walk at a fast constant pace. No encouragement was provided but participants were informed how many laps remained. 10

Energy expenditure
Energy expenditure and heart rate were measured using the Cosmed K4b 2 portable metabolic system throughout the duration of the 1-mile walk (Cosmed K4b 2 , Cosmed, Rome, Italy). Wearing the Cosmed K4b 2 portable metabolic system has no effect on the performance of older adults completing the one-mile walk. 12 Calibration on each testing day was with standard gases of known concentrations (oxygen, O 2 : 15.6%, carbon dioxide, CO 2 : 5.66%; Linde Gas, UK). Initially, a room air measurement calibration was conducted (O 2 : 20.93%, CO 2 : 0.03%) and respiratory volume was calibrated using a 3 L volume syringe. A delay calibration was used in order to match the changes in fractions of expired oxygen (i.e. F E O 2 ) and fractions of expired carbon dioxide (i.e. F E CO 2 ).

Discussion
Findings from the present study extend and simplify the existing knowledge on exercise prescription and public health, indicating that instructions to walk 1-mile as fast as possible allow older adults to achieve a moderate to vigorous intensity. This observation enables us to recommend more explicit and manageable weekly walking exercise to promote and maintain health in older adults, with the completion of between four and nine 1-mile walks per week at a fast, constant pace.
The 1-mile walk is a distance used to estimate aerobic  capacity. 14 Walking is also an exercise modality promoted by many public health campaigns, for example Change for Life and Walking for Health in the United Kingdom, suggesting that achieving the recommendations reduces the risk of chronic diseases and maintains quality of life. 1 The step frequency used to maintain these speeds were different between men (127 [SD 9] steps·min -1 ) and women (135 [SD 6] steps·min -1 ). This could be attributed to the fact that men were taller than women, have a longer stride length and therefore fewer steps to cover 1-mile. 16 General recommendations suggest that a generic stride rate of 103 steps per minute equates to an intensity of 3-METs, 16,17 a much lower stride rate than both men and women in the present study, and indicating an intensity higher than 3 METs. Using M1, participants were walking at 6 METs using a step rate of 130 per minute and 7 METs for the M2 method. Rowe et al 17 suggest 4 METs and 5 METs require 120 steps per minute and 140 steps per minute, respectively, and for every 1 MET increase there is an increase in stride rate of 16-24 steps per minute. If these findings were applied to the participants in the present study, to achieve the 6-7 METs they would have a stride rate of 156-164 or 172-188 steps per minute, respectively. Such a step-rate would be difficult to achieve for the general population, and only a few participants were able to achieve a vigorous intensity walk within the study by Rowe et al. 17 Utilising a step frequency of 130 steps per minute achieved a higher MET value for older adults in the present study (M2 7-METs for 1-mile), 40 steps per minute less than the suggestion by Rowe et al. 17 One reason for this was that Rowe et al 17 used the same method as M1 to calculate 1-MET. This was much higher than M2, and when divided into oxygen uptake, MET values were higher indicating the walk was more intense. Further to this, the MET values for both M1 and M2 could be higher than that calculated by Rowe et al 17 because measured V O 2 values during the walk were higher in the present study (19.4 mL·kg -1 ·min -1 vs Rowe et al 17 : 14.7 mL·kg -1 ·min -1 ). This could be the result of differences in walk duration, 6-minutes as opposed to 12 to 19-minutes. These findings could be utilised by health and exercise practitioners when prescribing exercise intensity, providing older adults with clear instructions to maintain a step-rate of 127-135 steps per minute when walking, to elicit an appropriate intensity, facilitating meeting physical activity recommendations.
Heart rate during the walk also indicated a moderatevigorous intensity (men: 116 beats•min -1 , 72% maxHR; women:134 beats•min -1 , 83% maxHR). Men completed the walk quicker than women, with a lower heart rate and percentage of their maximum, but this was reflected in their higher levels of physical fitness (36.6 vs. 25.4 mL•kg -1 •min -1 ). This is similar to the results obtained by Bazzano et al, 14 were men had a lower heart rate than women during the 1-mile walk. However, relative V O 2 during the walk for both men and women was a similar intensity, suggesting a similar overall effort around 70%-75% of their V O 2 max. 14 As mentioned before, 1-MET differed between M1 and M2 and caused the maximal METs and 1-mile walking METs to be greater for M2. These methods of predicting 1-MET were used, as opposed to a direct measurement, because they can be easily employed in the public domain by practitioners. Many participants exercised at a vigorous intensity during the 1-mile walk when M2 was used (64%), with 29% achieving vigorous intensity exercise when M1 was used. However, all individuals were 4 METs and above for both methods. This indicates that instructions to walk at a fast pace for 1-mile will at least achieve a moderate intensity, and facilitate the instructions given to older adults by exercise professionals to ensure sufficient physical activity participation and resultant physiological adaptations. 6 The difference between M1 and M2 resting MET values suggest that the walk is more intense using M2, therefore, if they complete the walk in the same time, the MET·min -1 per walk is higher for M2. And, to achieve the recommended 450-750 MET•min•wk -1 , M1 requires the accumulation of more 1-mile walks per week (6-9 vs. 4-7).
When comparing the proposed walking protocol to that completed by younger adults in Murphy et al, 18 they walked at a speed of 1.64 (SD 0.71) m·s -1 with an oxygen uptake of 59% (SD 1.2) of V O 2 max and an average heart rate of 119 (SD 4) beats•min -1 for 10-minutes. The walking speed and heart rate for the 1-mile bouts was greater for the older adults in the present study, i.e. 1.86 m·s -1 and 123 (SD 16) beats•min -1 respectively. However, older adults in the present study exercised at a similar percentage of their V O 2 max (62 (SD 12.8) %) to those in Murphy et al, 18 which concluded that brisk walking undertaken in accumulated sessions throughout the day can reduce post-prandial plasma triacylglycerol concentrations and increase fat oxidation. A later intervention study by this group used a similar protocol over the duration of 6-weeks, 10-minute bouts of brisk walking, 3-times per day, 5 d·wk -1 at 70%-80% predicted maximal heart rate. 3 This intervention improved predicted V O 2 max, sum of skin folds, waist, hip circumference and blood lipid profiles of middle-aged men and women (44.5 [SD 6.1] years). 3 The overall concept is similar to the suggestions made in the present study, and therefore an intervention of similar duration could elicit similar results. However, participants in Murphy et al 3 completed the recommended 150 min·wk -1 of moderate intensity exercise. Using M1 as a recommendation tool for older adults, they would have to complete between 90 and 133 min·wk -1 of exercise and 60-103 min·wk -1 for M2. This is less than those in Murphy et al, 3 however it should be noted that a majority of individuals demonstrate vigorous intensity activity with M2. Recommendations suggest older adults can complete a minimum of 75 min·wk -1 vigorous activity per week, therefore a minimum of five 1-mile walks per week would reach this recommendation. 1 Sedentary, overweight men and women (aged 40-65 years) benefitted from walking 12-miles, in approximately 3-sessions per week at 40%-55% or 65%-80% of V O 2 max. 19 The later intensity prescription is similar to the current recommendations from M2, vigorous intensity, however more miles in fewer repetitions (3 per week). Slentz et al 19 suggested that the number of calories expended is an important part of exercise prescription. Time required to expend calories depends on the fitness levels, with the lower fit individuals requiring more time and higher fit individuals requiring less time to expend a given number of calories. With our results, we observed a negative correlation between time to complete 1-mile and walking METs, indicating those that walk the 1-mile quicker will have a higher walking intensity. When aligned to the suggestion by Slentz et al, 19 those more fit individuals are able to walk faster at a higher intensity, therefore require less walks, which is also reflective of the differences between M1 and M2 for calculating 1-MET. Public health exercise professionals should therefore promote the accumulation of more 1-mile walks to those older adults with a lower aerobic capacity to ensure they achieve the threshold for improving health.

Conclusion
Many different parameters, i.e. frequency, intensity, duration and type of exercise, have to be considered in exercise guidelines for older adults, with some difficult to measure in the general public. Our observations suggest that if exercise/health professionals provide older adults instructions to walk 1-mile as fast as possible, they can achieve an intensity that is in line with recommendations for % HRmax, V O 2 max, and step frequency. In accordance to M2, older adults walk at a vigorous intensity for METs and exercise professionals working within public health, should encourage them to complete between four and seven 1-mile walks per week. M1 however requires moderate intensity walks of between 6 and 9, therefore when prescribing exercise to individuals with a lower aerobic capacity, who are unable to maintain a higher intensity for 1-mile, they should be encouraged to accumulate more 1-mile walks. Both methods do not directly measure 1-MET for individuals, but rather predict this, which is practical for use by the general public. Further research should directly measure 1-MET to ensure the predicted MET intensity is transferrable, in addition to determining the application of this across various settings. This should lead to future prospective studies in larger samples, encouraging older adults to progressively accumulate between seven and nine 1-mile walks per week.

Ethical approval
Ethical approval for this study was provided by the University of Chichester Research Ethics Committee prior to its initialisation.

Competing interests
There are no funding conflicts or other conflicts of interest to declare.

Authors' contributions
MLG and METW proposed the study. Data collection and analysis was performed by MLG, with manuscript drafting and data interpretations by both authors.