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Study: New simple method determines rate at which we burn calories walking uphill, downhill, and on level ground

New method uses three variables of speed, load carried and slope to improve on the accuracy of existing standards for predicting how much energy people require for walking — a method beneficial to many, including military strategists to model mission success

When military strategists plan a mission, one of many factors is the toll it takes on the Army’s foot soldiers.

A long march and heavy load drains energy. So military strategists are often concerned with the calories a soldier will burn, and the effect of metabolic stress on their overall physiological status, including body temperature, fuel needs and fatigue.

Now scientists at Southern Methodist University, Dallas, have discovered a new more accurate way to predict how much energy a soldier uses walking.

The method was developed with funding from the U.S. military. It significantly improves on two existing standards currently in use, and relies on just three readily available variables.

An accurate quantitative assessment tool is important because the rate at which people burn calories while walking can vary tenfold depending on how fast they walk, if they carry a load, and whether the walk is uphill, downhill or level.

“Our new method improves on the accuracy of the two leading standards that have been in use for nearly 50 years,” said exercise physiologist Lindsay W. Ludlow, an SMU post-doctoral fellow and lead author on the study. “Our model is fairly simple and improves predictions.”

The research is part of a larger load carriage initiative undertaken by the U.S. Army Medical Research and Materiel Command. The average load carried by light infantry foot soldiers in Afghanistan in April and May 2003 was 132 pounds, according to a U.S. Army Borden Institute report.

“Soldiers carry heavy loads, so quantitative information on the consequences of load is critical for many reasons, from planning a route to evaluating the likelihood of mission success,” said SMU biomechanist and physiologist Peter Weyand, @Dr_Weyand.

“The military uses a variety of approaches to model, predict and monitor foot-soldier status and performance, including having soldiers outfitted with wearable devices,” Weyand said. “There is a critical need with modern foot soldiers to understand performance from the perspective of how big a load they are carrying.”

Weyand is senior author on the research and directs the Locomotor Performance Laboratory in the SMU Simmons School of Education, where subjects for the study were tested.

The researchers call their new method the “Minimum Mechanics Model” to reflect that it requires only three basic and readily available inputs to deliver broad accurate predictions. They report their findings in “Walking economy is predictably determined by speed, grade and gravitational load” in the Journal of Applied Physiology.

The necessary variables are the walker’s speed, the grade or slope of the walking surface, and the total weight of the body plus any load the walker is carrying.

“That’s all it takes to accurately predict how much energy a walker burns,” Ludlow said.

While the measurement is a critical one for foot soldiers, it’s also useful for hikers, backpackers, mall-walkers and others who are calorie conscious and may rely on wearable electronic gadgets to track the calories they burn, she said.

Muscle and gait mechanics tightly coupled across speed, grade, load
Existing standards now in use rely on the same three variables, but differently, and with less accuracy and breadth.

The new theory is a departure from the prevailing view that the mechanics of walking are too complex to be both simple and accurate.

“Ultimately, we found that three remarkably simple mechanical variables can provide predictive accuracy across a broad range of conditions,” Ludlow said. “The accuracy achieved provides strong indirect evidence that the muscular activity determining calorie-burn rates during walking is tightly coupled to the speed, surface inclination and total weight terms in our model.”

By using two different sets of research subjects, the researchers independently evaluated their model’s ability to accurately predict the amount of energy burned.

“If muscle and gait mechanics were not tightly coupled across speed, grade and load, the level of predictive accuracy we achieved is unlikely,” Weyand said.

First generalized equation developed directly from a single, large database
The two existing equations that have been the working standards for nearly 50 years were necessarily based on just a few subjects and a limited number of data points.

One standard from the American College of Sports Medicine tested only speed and uphill grades, with its first formulation being based on data from only three individuals.

The other standard, commonly referred to as the Pandolf equation is used more frequently by the military and relies heavily on data from six soldiers combined with earlier experimental results.

In contrast, the generalized equation from SMU was derived from what is believed to be the largest database available for human walking metabolism.

The SMU study tested 32 adult subjects individually under 90 different speed-grade and load conditions on treadmills at the SMU Locomotor Performance Laboratory, @LocomotorLabSMU.

“The leading standardized equations included only level and uphill inclinations,” Weyand said. “We felt it was important to also provide downhill capabilities since soldiers in the field will encounter negative inclines as frequently as positive ones.”

Subjects fast prior to measuring their resting metabolic rates
Another key element of the SMU lab’s Minimum Mechanics Model is the quantitative treatment of resting metabolic rate.

“To obtain true resting metabolic rate, we had subjects fast for 8 to 12 hours prior to measuring their resting metabolic rates in the early morning,” Ludlow said. “Once at the lab, they laid down for an hour while the researchers measured their resting metabolic rate.”

In separate test sessions, the subjects walked on the treadmill for dozens of trials lasting five minutes each, wearing a mouthpiece and nose clip. In the last two minutes of each trial, the researchers measured steady-state rates of oxygen uptake to determine the rate at which each subject was burning energy.

Adults in one group of 20 subjects were each measured walking without a load at speeds of 0.4 meters per second, 0.7 meters per second, 1 meter per second, 1.3 meters per second and 1.6 meters per second on six different gradients: downhill grades of minus six degrees and minus three degrees; level ground; and uphill at inclines of three degrees, six degrees and nine degrees.

Adults in a second group of 20 were each tested at speeds of 0.6 meters per second, 1 meter per second and 1.4 meters per second on the same six gradients, but they carried loads that were 18 percent of body weight, and 31 percent of body weight.

Walking metabolic rates increased in proportion to increased load
As expected, walking metabolic rates increased in direct proportion to the increase in load, and largely in accordance with support force requirements across both speed and grade, said Weyand and Ludlow.

Weyand is Glenn Simmons Professor of Applied Physiology and professor of biomechanics in the Department of Applied Physiology and Wellness in SMU’s Annette Caldwell Simmons School of Education and Human Development. He also is lead scientist for the biomechanics and modeling portion of the Sub-2-Hour marathon project, an international research consortium based in the United Kingdom. — Margaret Allen, SMU

Categories
Health & Medicine Plants & Animals

Good news! You’re likely burning more calories than you thought

Leading standardized equations used to predict or estimate walking energy expenditure — calories burned — count too few calories in nearly all cases on level surfaces, study finds. New method improves accuracy.

Jennifer Nollkamper and Dr. Lindsay Ludlow assist Dr. Takeshi Fujii in a treadmill test that captures volume of oxygen, volume of expired air and the levels of oxygen and carbon dioxide, all variables that help measure energy expenditure during walking. (Hillsman Jackson, SMU)
Jennifer Nollkamper and Dr. Lindsay Ludlow assist Dr. Takeshi Fujii in a treadmill test that captures volume of oxygen, volume of expired air and the levels of oxygen and carbon dioxide, all variables that help measure energy expenditure during walking. (Hillsman Jackson, SMU)

Walking is the most common exercise, and many walkers like to count how many calories are burned.

Little known, however, is that the leading standardized equations used to predict or estimate walking energy expenditure — the number of calories burned — assume that one size fits all. The equations have been in place for close to half a century and were based on data from a limited number of people.

A new study at Southern Methodist University, Dallas, found that under firm, level ground conditions, the leading standards are relatively inaccurate and have significant bias. The standards predicted too few calories burned in 97 percent of the cases researchers examined, said SMU physiologist Lindsay Ludlow.

A new standardized equation developed by SMU scientists is about four times more accurate for adults and kids together, and about two to three times more accurate for adults only, Ludlow said.

“Our new equation is formulated to apply regardless of the height, weight and speed of the walker,” said Ludlow, a researcher in the SMU Locomotor Performance Laboratory of biomechanics expert Peter Weyand. “And it’s appreciably more accurate.”

Ludlow and her colleagues report the new equation in the Journal of Applied Physiology, “Energy expenditure during level human walking: seeking a simple and accurate predictive solution.” The article is published in the March 1, 2016 issue, and available online at this link.

“The economy of level walking is a lot like shipping packages – there is an economy of scale,” said Weyand, a co-author on the paper. “Big people get better gas mileage when fuel economy is expressed on a per-pound basis.”

The SMU equation predicts the calories burned as a person walks on a firm, level surface. Ongoing research is expanding the algorithm to predict the calories burned while walking up- and downhill, and while carrying loads, Ludlow said.

SMU’s research is funded by the U.S. Department of Defense Medical Research and Materiel Command. The grant is part of a larger DOD effort to develop load-carriage decision-aid tools to assist foot soldiers.

The research comes at a time when greater accuracy combined with mobile technology, such as wearable sensors like Fitbit, is increasingly being used in real time to monitor the body’s status. The researchers note that some devices use the old standardized equations, while others use a different method to estimate the calories burned.

New equation considers different-sized people
To provide a comprehensive test of the leading standards, SMU researchers compiled a database using the extensive walking metabolism data available in the existing scientific literature to evaluate the leading equations for walking on level ground.

“The SMU approach improves upon the existing standards by including different-sized individuals and drawing on a larger database for equation formulation,” Weyand said.

The new equation achieves greater accuracy by better incorporating the influence of body size, and by specifically incorporating the influence of height on gait mechanics. Specifically:

  • Bigger people burn fewer calories on a per pound basis of their body weight to walk at a given speed or to cover a fixed distance;
  • The older standardized equations don’t account for size differences well, assuming roughly that one size fits all.

Accuracy of standardized equations had not previously undergone comprehensive evaluation
The exact dates are a bit murky, but the leading standardized equations, known by their shorthand as the “ACSM” and “Pandolf” equations, were developed about 40 years ago for the American College of Sports Medicine and for the military, Ludlow said.

The Pandolf method, for example, draws on walking metabolism data from six U.S. soldiers, she said. Both the Pandolf and ACSM equations were developed on a small number of adult males of average height.

The new more accurate equation will prove useful. Predicting energy expenditure is common in many fields, including those focused on health, weight loss, exercise, military and defense, and professional and amateur physical training.

“Burning calories is of major importance to health, fitness and the body’s physiological status,” Weyand said. “But it hasn’t been really clear just how accurate the existing standards are under level conditions because previous assessments by other researchers were more limited in scope.”

Energy expenditure estimates could assist with monitoring the body’s physiological status
Accurate estimations of the rate at which calories are burned could potentially help predict a person’s aerobic power and likelihood for executing a task, such as training for an athletic competition or carrying out a military objective.

In general, the new metabolic estimates can be combined with other physiological signals such as body heat, core temperature and heart rate to improve predictions of fatigue, overheating, dehydration, the aerobic power available, and whether a person can sustain a given intensity of exercise.

Military seeks solutions to overburdened soldier problem
The military has a major interest in more accurate techniques to help address their problem of over-burdened soldiers.

“These soldiers carry incredible loads — up to 150 pounds, but they often need to be mobile to successfully carry out their missions,” said Weyand, a professor of Applied Physiology and Wellness in the SMU Simmons School of Education.

Accurately predicting how many calories a person expends while walking could supply information that can help soldiers avoid thermal stress and fatigue in the field, especially troops deployed to challenging environments.

“Soldiers incur a variety of physiological and musculoskeletal stresses in the field,” Weyand said. “Our metabolic modeling work is part of a broader effort to provide the Department of Defense with quantitative tools to help soldiers.” — Margaret Allen

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