A treadmill allows you to walk or run without changing location because you’re ambulating on a moving belt that rotates to the rear to offset your movement forward. Motorized treadmills have an electrical motor that provides the impetus for belt movement. These treadmills also typically have another motor that elevates the machine’s front end to change the grade at which you are working.
Speed and Grade
A motorized treadmill allows you to manually enter the speed and grade at which you want to work. The speed dictates the rate at which the belt spins and the grade dictates the degree to which the front end is raised. These factors provide input to a computer chip in the console that integrates the information to perform various calculations. Total distance is calculated by taking into account your speed and the time for which you walked or ran, which is monitored by a digital timer.
Caloric Expenditure
The central computer in a treadmill’s console also contains software for calculating the energy you expend during your exercise session. This estimation is based on regression equations that predict the amount of oxygen required to work aerobically at a given steady rate. Once the oxygen cost of a particular exercise work rate is known, the energy equivalent can be estimated because consumption of one liter of oxygen results in liberation of approximately 5 calories of energy.
Energy Cost of Walking
To calculate the energy cost of walking, the speed and grade at which you are walking and your body’s weight must be taken into account. The American College of Sports Medicine’s "Guidelines for Exercise Testing and Prescription" says when walking, it requires 0.1 milliliters of oxygen to transport one kilogram of your body weight 1 meter horizontally. In addition, it costs 1.8 milliliters of oxygen per kilogram of body weight for each meter of vertical distance you ascend. Another variable that must be included is your resting metabolic rate, which is the amount of oxygen you consume to support your body’s metabolism at rest. This requirement is still present during exercise, so it must be factored in. Resting metabolic rate can be estimated based on your body weight.
Walking vs. Running
According to the laws of physics, moving a specific mass a specific distance should require a given amount of energy. However, it has been shown that humans use more energy when running compared to walking. For example, Cameron Hall and colleagues at Syracuse University showed that running 1,600 meters required 40 percent more energy than walking the same distance. This means that equations that predict the energy cost of walking must be altered for running. The treadmill’s computer should be programmed to take this into account by switching from the walking to running equation when the speed typically associated with the walk-run transition is surpassed.
Metabolic Equivalents
In addition to energy expenditure in calories, some treadmills also calculate the value in metabolic equivalents. A MET represents multiples of the resting metabolic rate, which is generally estimated as 3.5 milliliters of oxygen per kilogram of body weight per minute. For example, a MET value of 10 during exercise means that you’re consuming 35 milliliters of oxygen for each kilogram of your body weight each minute you're sustaining that pace. Walking generally requires 2 to 7 METS, and MET values greater than 10 characterize vigorous exercise.
References
- “ACSM’s Guidelines for Exercise Testing and Prescription, Sixth Edition”; American College of Sports Medicine; 2000
- “Medicine and Science in Sports and Exercise”; Cameron Hall, et al.; 2004
- “Exercise Physiology: Human Bioenergetics and Its Application”; G.A. Brooks, T.D. Fahey and K.M. Baldwin; 2005
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