Metabolic Equivalents (METs): Understanding, Calculation, and Application

How to Calculate a MET?

While individuals typically do not calculate a MET value for a specific activity, understanding the concept of a Metabolic Equivalent of Task (MET) is crucial for estimating energy expenditure and applying established values to quantify physical activity intensity and caloric burn.

Understanding the MET Concept

The Metabolic Equivalent of Task, or MET, is a physiological measure expressing the energy cost of physical activities. It represents the ratio of the metabolic rate during a specific physical activity to the resting metabolic rate. Essentially, it quantifies how much more energy an activity requires compared to simply sitting still.

• What is 1 MET? One MET is defined as the energy expended while sitting quietly. Physiologically, 1 MET is approximately equivalent to an oxygen uptake of 3.5 milliliters per kilogram of body weight per minute (3.5 mL O2 · kg⁻¹ · min⁻¹). This serves as the baseline for all activity-specific MET values.
• Why are METs Used? METs provide a standardized, simple, and convenient way to express the intensity of physical activities and estimate the energy expenditure associated with them. They allow for easy comparison across different activities and individuals, making them invaluable tools in exercise science, public health, and clinical settings for exercise prescription and tracking.

The Calculation (and Misconception) of METs

It's important to clarify a common misconception: individuals typically do not "calculate" a MET value for an activity in real-time. Instead, MET values for thousands of activities have been scientifically determined and compiled into extensive databases. When you want to use METs, you reference these established values.

• How MET Values Are Determined (Scientific Context): Scientists determine MET values primarily through indirect calorimetry. This involves measuring a person's oxygen consumption and carbon dioxide production during specific activities in a controlled laboratory setting. By comparing the oxygen consumed during an activity to the resting oxygen consumption (1 MET), the MET value for that activity is established. These values are then averaged across a representative population.
• The Compendium of Physical Activities: The most widely recognized and utilized resource for established MET values is the "Compendium of Physical Activities." Developed by Ainsworth and colleagues, this compendium provides MET values for a vast array of common daily activities, sports, and exercises, ranging from sleeping (0.9 METs) to running (e.g., 8.0 METs for 7.0 mph).

Applying METs to Estimate Energy Expenditure

While you don't calculate the MET value itself, you use the established MET value to calculate the estimated caloric expenditure for a given activity. The formula is straightforward:

Estimated Calories Burned (kcal) = METs × Body Weight (kg) × Time (hours)

This formula is derived from the understanding that 1 MET is roughly equivalent to burning 1 kilocalorie (kcal) per kilogram of body weight per hour.

Let's break down the steps with an example:

Example: Estimating Calories Burned During Brisk Walking

Assume:

• Activity: Brisk Walking
• Duration: 45 minutes
• Body Weight: 160 lbs

1. Identify the Activity's MET Value:

   • From the Compendium of Physical Activities, brisk walking (e.g., 3.5 mph or 5.6 km/h) has an approximate MET value of 3.5 METs.

2. Determine Body Weight in Kilograms:

   • Convert pounds to kilograms: 160 lbs ÷ 2.2046 kg/lb ≈ 72.57 kg

3. Determine Activity Duration in Hours:

   • Convert minutes to hours: 45 minutes ÷ 60 minutes/hour = 0.75 hours

4. Perform the Calculation:

   • Calories (kcal) = 3.5 METs × 72.57 kg × 0.75 hours
   • Calories (kcal) ≈ 190.4 kcal

5. Interpret the Result:

   • Based on these values, a 160-pound individual would burn approximately 190.4 calories during 45 minutes of brisk walking.

Practical Applications for Fitness Professionals and Enthusiasts

METs offer significant utility for anyone interested in understanding and optimizing physical activity:

• Exercise Prescription: Fitness professionals can use METs to prescribe exercise intensity. For instance, moderate-intensity activity is generally defined as 3.0 to 6.0 METs, while vigorous-intensity activity is > 6.0 METs.
• Tracking Progress and Goals: Individuals can track their weekly physical activity in "MET-minutes" (METs × minutes of activity) to ensure they meet public health guidelines. For example, 150 minutes of moderate-intensity activity (e.g., 3 METs) equates to 450 MET-minutes per week.
• Weight Management: By estimating caloric expenditure, individuals can better understand the energy balance equation (calories in vs. calories out) crucial for weight loss, maintenance, or gain.
• Public Health Reporting: METs are fundamental to large-scale epidemiological studies and public health recommendations regarding physical activity.

Limitations and Considerations

While METs are a powerful tool, it's essential to acknowledge their limitations:

• Individual Variability: MET values are averages. Actual energy expenditure can vary significantly based on an individual's fitness level, age, sex, body composition, efficiency of movement, and even genetics.
• Environmental Factors: External conditions like temperature, humidity, altitude, and terrain (e.g., walking uphill vs. downhill) can influence the true energy cost of an activity, which may not be fully captured by a single MET value.
• Accuracy: The formulas provide an estimation of caloric expenditure, not a precise measurement. Wearable devices and fitness trackers often use algorithms that incorporate METs but also other physiological data (like heart rate) to improve accuracy.
• Activity Specificity: While the Compendium is extensive, not every unique movement or sport has a precisely determined MET value, requiring some estimation based on similar activities.

Conclusion

The concept of the Metabolic Equivalent of Task (MET) is a cornerstone of exercise science, providing a standardized and accessible method to quantify the intensity and energy cost of physical activity. While you won't typically calculate a MET value from scratch, understanding how to apply established MET values from resources like the Compendium of Physical Activities allows you to estimate caloric expenditure, prescribe exercise effectively, and track progress towards fitness and health goals. METs empower both fitness professionals and enthusiasts to make informed decisions about their physical activity endeavors, fostering a deeper understanding of energy balance and exercise physiology.