James J. McCormick, M.S., Christine Mermier, Ph.D. and Len Kravitz, Ph.D.
The scientific literature indicates that the cumulative effects of walking may reduce the risk for developing coronary heart disease, help in the treatment of hypertension, improve insulin/glucose metabolism for the prevention or management of type 2 diabetes, and aid in the treatment of some musculoskeletal diseases (Hu et al., 1999; Lee, Rexrode, Cook, Manson, & Buring, 2001; Morris and Hardman, 1997). Walking programs are a most common form of exercise advocated by exercise professionals due to the simplicity of the exercise, low participation cost, and numerous health benefits. While walking programs may have many positive health benefits for a wide range of populations, one of the primary aims for many exercise enthusiasts is increasing caloric expenditure (kilocalories per minute) for weight loss goals. However, individuals with above average fitness levels or those suffering from ambulatory physical limitations may not to be able to realize their desired health or weight management goals from traditional walking methods. This review will highlight research understandings on energy expenditure and present several evidence-based ideas for incorporating newer strategies for walking exercise program designs.
Calorie Expenditure 101: The Science of Calorimetry
The science of measuring caloric expenditure is referred to as calorimetry. The word calorimetry comes from the Latin word calor, meaning heat, and metry from the Greek word metron which refers to measurement. This science utilizes the understanding that cellular energy (in the form of ATP from carbohydrates and fats) is about 30% efficient for muscular work, with 70% of the energy degrading to heat. Since this heat production is equivalent to the rate of cellular reactions for work, the direct or indirect measurement of heat proves to be accurate measurement energy expenditure.
The science of calorimetry dates back to the 19th century with the use of devices called bomb (nothing to do with the military) calorimeters, which are sealed metal chambers surrounded by a container of known volume of water. Heat flow from the combustion of food in the chamber crosses the wall and heats the container of water, which is used to determine the measurement of heat change. Thus, bomb calorimeters are a direct measure of calorimetry, as scientists can ignite a food source directly within an oxygen-rich environment to measure the heat released. A kilocalorie is the amount of heat required to raise the temperature of 1 kilogram (which is about 33.8 ounces or 1 liter) of water by one degree from 14.5°Celsius (58°Farenheit) to 15.5°Celsius (60°Farenheit). Thus, food is burned under controlled conditions in the bomb calorimeter, breaking chemical bonds, and releasing free energy and heat.This burning is chemically similar to the metabolic breakdown of food in cellular respiration in the human body.
From the efforts of this early research, scientists quantified the calories derived from fats, carbohydrates, proteins, and alcohol. This method of measurement was later adapted to measure oxygen consumption, carbon dioxide production and heat production in humans, by having an individual sit or exercise in a large, enclosed, insulated chamber. Due to the impractical limitations of using a big chamber to measure energy expenditure, indirect methods were developed (to measure kilocalorie expenditure) based on the expired measurements of oxygen, carbon dioxide and ventilation (air moved in and out of the lungs). This is called indirect calorimetry, and is the widely used technique for measuring energy expenditure of exercise in exercise physiology laboratories.
What is An Adult's Natural Walking Pace and Why?
Most healthy adults tend to naturally select a walking pace of approximately 2.8 mph (Willis, Ganley, & Herman, 2005). Prior research has hypothesized this preferred walking pace is the result of a minimal energy phenomenon, wherein, the central nervous system selects the person's preferred walking speed as a way to lessen the body's energy expenditure (Martin, et al., 1992). Willis and colleagues have found that this preferred walking speed might also be due to changes in fuel utilization. In most adults, fat is the primary fuel source at speeds equal to and below 2.8 mph, which serves as a metabolic walking threshold speed (Willis et al., 2005). Above this speed, carbohydrate oxidation (breakdown) abruptly increases resulting in an increased perception of effort, due to carbohydrates being a limited fuel source as compared to fat. As a result, preferred walking speed appears to be naturally selected due to the most economical fuel conditions in the muscle, in which fat oxidation is the primary fuel source.
With aging and inactivity, there is often a diminished capacity of musculoskeletal functioning of the lower body gait muscles (Martin et al, 1992). This may necessitate the recruitment of additional motor units and perhaps an additional proportion of less economical fast twitch muscle fibers (which use predominantly carbohydrate as their fuel source) to generate necessary forces for walking. Thus, there is a decline in walking speed and a change in gait characteristics in the elderly.
Focus Point. How Does the Body Choose it's Natural Walking Speed?
A person's natural walking speed is habitually determined by the central nervous system's valuation of the most economical walking gait for optimal fat fuel utilization (Willis et al, 2005).
What is Brisk Walking?
The American College of Sports Medicine (ACSM, 2014) recommends that most adults accumulate 30-60 minutes/day of moderate intensity exercise on at least 5 days/week; or vigorous intensity exercise 20-60 minutes/day on at least 3 days/week (or a combination of both). However, brisk walking is open to interpretation. A brisk walk for some may be a leisurely walk for others. So how do walking exercisers determine a moderate intensity walk to meet ACSM guidelines? Scientifically, walking at an intensity of 3-6 METS (metabolic equivalent; a physiological measure expressing the energy cost of physical activities) is considered moderate intensity exercise. Fortunately, Marshall et al., 2009 determined that walking at a pace of &Mac179;100 steps/minute has been shown to equate to moderate intensity exercise as recommended by ACSM. At a rate of 100 steps/minute, current recommendations for moderate intensity physical activity would equate to walking at least 3000 steps in 30 minutes on at least 5 days/week. This can easily be tracked with any pedometer or pedometer 'app' (application) on a mobile device. A walker could also 'accumulate' three daily walks of 1000 steps in 10 minutes on 5 days each week.
Murtagh and associates (2002) examined 82 recreational walkers at a self-selected brisk walking pace. The average walking speed for the participants was approximately 3.5 mph, with subjects able to accurately meet moderate-intensity exercise levels by self-selecting their pace. Similar goals were also reached in older adults (60-85 years); however, the average self-selected walking pace was just a little slower (3.3-3.5 mph) (Parise, Sternfeld, Samuels, & Tager, 2004). This indicates the term "brisk" is an accurate term for attaining a moderate-intensity walk, though the actual walking speed may vary depending on age and individual fitness levels.
How Does Load Placement Affect Caloric Expenditure of Walking?
There are a number of methods and combinations of adding weight to the body, such as, wearing a backpack, ankle weights, carrying hand weights, or wearing a weighted vest; however, not all methods of adding load have the same effect on energy expenditure. Much of the energy cost of walking is a result of the activation of muscles that act to perform work on the body's center of mass, swinging the legs relative to the center of mass, and supporting body weight (Griffin, Roberts, & Kram, 2003). Changes in load position can alter the rotational torque functioning around the body's center of mass resulting in differences in muscle activation and metabolic cost; thus, not all forms of a given mass will equate to equal amounts of energy expenditure (Watson et al., 2008). Thus exercise professionals will need to educate clients that different forms of loaded exercise vary widely in their caloric expenditure demands on the body. And, some forms of load distribution on the body are very unsafe. For instance, weights placed on the feet (i.e., heavy boots) and ankle weights have been shown to be five to six times less efficient (meaning higher caloric expenditure) than hand weights carried close to the body while walking (Knapik, Reynolds, & Harman, 2004). However, weight placed on the feet and ankle while walking may lead to metatarsalgia (an overuse injury to the forefoot), a serious repetitive stress foot injury, or even a stress fracture. As well, carrying a weight (up to 18% body weight) in one hand is more energy demanding than carrying the same total weight distributed between both hands. However, this asymmetrical loading is a very complex behavior in terms of balance (Watson et al, 2008), and thus not a safe option during sustained walking (perhaps O.K. with short &Mac178;1-minute spurts of walking in a circuit or metabolic training class) for most exercise enthusiasts.
Have You Heard of the 'Free-Ride' Walking Phenomenon?
The "free-ride" walking phenomenon was first observed and coined in the late 1980's when energy-cost was measured in African women carrying loads under 20% of their body mass on their head (called headloading). Researchers noted no significant incremental difference in metabolic cost as compared to non-headloading (Charteris, Scott, & Nottrodt, 1989). This energy saving effect was observed when carrying loads of less than or equal to 20% body mass; however, this free-ride effect diminishes when carrying weight above 20% of body mass. Charteris, Scott and Nottrodt suggest this free-ride energy saving phenomenon may be due to changes in both step frequency and stride length that change biomechanical mechanisms of walking. Abe and colleagues (2004) observed a similar energy-saving effect while carrying a backpack at a load equal to 15% of body mass during slow walking (<3.35 mph); however, this effect also ceases with walking speeds above 3.35 mph. This energy saving effect is also observed when loads are carried in the hands (6.5 to 13.5 lbs in each hand) during slower walking speeds. Abe and colleagues summarize the total energy cost to the body from the hand and arm muscles holding weight during slow walking are negligibly small. Additionally, and perhaps more importantly (from a safety standpoint), with traditional types of dumbbells (or kettlebells) it becomes problematic to even grip/hold the weights for a sustained period of time (such as 30-minute walk), and thus this is not a worthy option for exercise professionals to recommend to clients.
Does Wearing a Weighted Vest While Walking Increase Caloric Expenditure?
Weighted vests are a type of exercise equipment that is gaining attention from exercise professionals and fitness enthusiasts. Weighted vests (typically 5% to 20% of a person's body weight) can be used in many different types of workouts and most vests are adjustable to add more or less weight as needed. Additionally, weighted vests are worn over the shoulders making them a more natural addition to an exerciser's center of gravity.
A study conducted by Puthoff and associates (2006) examined walking energy expenditure with incremental treadmill speeds ranging from 2.0 mph to 4.0 mph and vest weights ranging from 10% to 20% of body mass. They found that energy expenditure increases as vest weight and walking speed increase; however, the relationship between vest weight and walking speed is not entirely linear. As walking speed increased, the effect of wearing a weighted vest had a more pronounced impact on energy expenditure. These findings have many practical implications in the design of walking programs. For instance, walking at slow speeds may require the use of a heavier vest to achieve the aspired increases in energy expenditure, while walking at faster speeds will see a more pronounced increase in energy expenditure with less weight needed to facilitate the increase. Additionally, weighted vest walking may be beneficial for those persons with an inability to walk briskly, as adding just 10% of body mass at a slower walking speed (~2 mph) may produce a similar relative exercise intensity of a faster walking speed without added mass. Thus, weighted vest walking and exercise is a viable training strategy to increase exercise intensity for all fitness levels and is a strategic approach exercise professionals can incorporate with clients. In fact, much research is currently being completed to better identify the optimal vest weight, walking speed and treadmill grade combinations for greater energy expenditure. So stay tuned for more research findings in this area.
What About Inclined Treadmill Walking?
Increasing grades while walking on a treadmill is a common way to increase the intensity of walking exercises, especially in those unable to reach faster walking speeds or in obese populations where injuries associated with excess joint loading is of concern. Ehlen and colleagues (2011) found that obese individuals were able to achieve an adequate exercise stimulus for weight management with speeds as low as 1.7 mph with the addition of inclines between 6-9%. Additionally, slower walking speeds at a moderate incline reduced the load placed on the lower extremity joints in comparison to faster walking speeds (~3.35 mph). Incline walking may also be appropriate for older populations or those suffering from joint problems. Currently, no standardized recommendations are available for the use of incline during walking bouts. Therefore, exercise professionals are encouraged to individualize the incline to the fitness level and perceived effort for each client.
Walking vs. Running: Impact on Caloric Expenditure
A fundamental principle of physics is that movement of a specific mass over a given distance requires the same amount of energy; thus, in theory, to walk or run a given distance should require the same amount of energy regardless of speed (Hall et al. 2004). While this principle is sometimes observed in quadripeds running a mile compared to a leisurely pace (Kram & Taylor, 1990), humans tend to expend a greater amount of energy when running (~30% higher depending on intensity) than walking the same distance (Hall, et al., 2004). More research is needed to better clarify this comparison.
Adapting High Intensity Interval Training Programs to Walking
Numerous high intensity training (HIIT) research studies have been completed utilizing jogging, running and cycling modes of exercise. Walking programs may be readily adjusted to these programs. With the programs presented below, the intensity of intervals has been adapted for walking using the Ratings of Perceive Exertion scale. Presented are 5 HIIT research-based interval programs revised for walking. Personal trainers should modify each program accordingly to the fitness level of each client. Remind clients to always complete an appropriate warm-up and cool-down with each walking workout.
1) High Intensity Aerobic Interval Walking (Perry et al., 2008)
Protocol: Complete up to 10 high intensity walking intervals lasting 4 minutes interspersed with 2-minute relief walking intervals.
Intensity: Perform the 4-minute high intensity intervals at a Hard (16 RPE) to Very Hard (18 RPE) intensity and the relief interval at a Light (11 RPE) level. Use walking speed or treadmill grade (or a combination of both) to vary the intensity of walking.
Duration: This total workout takes close to one hour to complete.
2) Sprint Interval Walking (Burgomaster et al., 2008).
Protocol: Complete 4-6 sprint walking intervals lasting 30 seconds interspersed with 4.5 minutes of light walking at a self-selected pace.
Intensity: Perform the sprint walks at a near maximal walking intensity, which would suggest about a Very Hard (18-20 RPE) rating. During the self-selected 4.5 minute walking recovery exercise period a RPE of 8-9 units is appropriate.
Duration: This total workout takes 20 to 30 minutes.
3) Step-Wise Interval Walking (Jacobs and Sjodin, 1985).
Protocol: Start at a relatively easy walking workload for 5 minutes of exercise and then increase intensity about 15 percent for 4 minutes and continue to increase exercise intensity every 4 minutes. This program can be halted when a particular intensity level is reached or a specific duration is attained, and then a cool-down walk is completed.
Intensity: The initial walk intensity should be about an RPE of 11. Then, increase the intensity roughly 1 RPE with each subsequent 4-minute stage by increasing the speed of walking or incline on a treadmill, or a combination of both. For example, this program starts at a RPE of 11; after 4 minutes the intensity becomes a 12 RPE; after 4 minutes the intensity becomes a 13 RPE; after 4 minutes the intensity becomes a 14 RPE. This continues until a specific time or intensity level is attained.
Duration: Duration should follow ACSM (2014) guidelines, which recommend 20-60 minutes of continuous cardiorespiratory exercise.
4) Near-Maximal Interval Walking (Gormley et al., 2008).
Protocol: Perform a 5-minute near-maximal intensity walk to be followed by a 5-minute recovery walk and repeat.
Intensity: The near-maximal walking interval is around 17-18 on the RPE scale. The recovery interval is in the region of an 11-12 RPE. Utilize walking speed or treadmill incline to vary intensity.
Duration: Duration should follow ACSM (2014) guidelines, which recommend 20-60 minutes of continuous cardiorespiratory exercise.
5) Supramaximal Interval Walking (Gosselin et al., 2012)
Protocol: Complete 7-10 sprint walking intervals lasting 90 seconds interspersed with 30 seconds of walking at self-selected pace.
Intensity: Perform the sprint walks at a near maximal walking pace, which would suggest about an 18-20 RPE rating. During the self-selected 30-second walking relief a RPE of 8-9 units is appropriate.
Duration: This total workout takes 20 to 30 minutes.
Rating of Perceived Exertion Scale
6 No exertion at all: This would be analogous to sitting and relaxing
7 Extremely light: This is very easy standing movement.
9 Very light: This is similar to casual walking.
11 Light: This is comparable to the intensity of a light warm-up.
13 Somewhat hard: This is a workout intensity that feels mildly challenging.
15 Hard: This is a workout intensity that feels difficult.
17 Very hard: This is a very demanding workout intensity.
19 Extremely hard: This is a rigorous intensity that cannot be maintained.
20 Maximal exertion: This is an all-out exercise exertion.
Rating of Perceived Exertion Scale
Adapted from: Borg, G.A. (1982). Psychophysical bases of perceived exertion. Medicine & Science in Sports & Exercise, 14(5), 377-381.
Side Bar 1: Joint Stress With and Without Load
When engaging in any weight bearing activity involving repetitive joint movement, stress on those joints is always of concern, whether exercising with or without additional load. Articular cartilage (found in knee joints) is a lubricated surface that absorbs and transfers load to allow joint movement without friction. Moderate joint loading (30 minutes of 60% of 1 repetition maximum) has been shown to actually benefit articular cartilage by decreasing markers of inflammation (Franciozi et al., 2013); however, excessive exercise, such as seen in ultra endurance runners can lead to deterioration of articular cartilage similar to that seen in osteoarthritis patients (Helmark, et al., 2012). Therefore, exercise professionals should be aware that moderate joint loading (as in walking) can be beneficial, but extreme overuse can lead to detrimental joint harm.
Key points on Walking:
1. Walking at at rate of 100 steps/minute equates to moderate intensity exercise.
2. Weighted vests (5% to 20% of a person's body weight) meaningfully increases the energy expenditure of walking.
3. Carrying asymmetrical weight (weight in one hand) is a complex balance challenge, and not recommended for sustained periods of walking.
4. Adapting HIIT programs from other modes of exercise is an excellent strategy to vary walk programs.
5. Feet and ankle weights increase energy expenditure, but may also result in several harmful overuse and repetitive stress injuries.
6. Slower walking speeds at a moderate incline (up to 10%) reduces the load placed on the lower extremity joints and markedly increases energy expenditure.
Carring weights in the hands (up to13.5 lbs in each hand) during slower walking speeds (<3.4 mph) is ineffective for increasing energy expenditure.
With proper modifications, walking programs can be tailored to meet the needs of all fitness levels with the added benefit of less joint stress in comparison to high impact exercises. There are many ways to alter walking programs to meet the target intensity of exercisers of all fitness levels. These may include walking at different speeds, wearing weighted vests while walking, adjusting the grade while on a treadmill, and adapting existing interval training programs to walking. Exercise professionals are encouraged to incorporate and combine any of these modifications to meet their client's needs and goals. Keep on Walking!
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James J. McCormick, is completing his Master's degree in exercise science at the University of New Mexico, Albuquerque, where he also got his bachelor's degree in Exercise Science. His research interests include human performance, exercise testing and prescription, and environmental training adaptations.
Christine Mermier, Ph.D. is an assistant professor and exercise physiology laboratory director in the exercise science Program at UNM. Her research interests include the effect of exercise in clinical patients, women, and aging populations, and high altitude physiology.
Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and American Council on Exercise Fitness Educator of the Year.