Scientific Measurement of Physical Activity Intensity: Application of Metabolic Equivalent (MET), Heart Rate, and Subjective Fatigue Scales

The body's energy expenditure includes four aspects: basal metabolism, physical activity, the thermic effect of food, and the needs of growth and development. For the average adult, daily energy expenditure mainly consists of basal metabolism (60%–70%), the thermic effect of food (5%–10%), and physical activity (15%–30%). It is evident that physical activity is the most variable part of the human energy metabolism pathway and a key factor influencing energy balance. In other words, the balance between food intake and physical activity is one of the two main factors in maintaining energy balance and a healthy weight. Simply put, the energy expenditure from physical activity is approximately the product of the intensity of physical activity (metabolic equivalent) and the duration of activity. Among these, the intensity of physical activity is a crucial element, and scientifically assessing the intensity of physical activity is one of the core components of estimating the energy expenditure level of physical activity and providing guidance for energy balance-related interventions.

Physical activity intensity is divided into two categories: absolute intensity (also known as "physical intensity") and relative intensity (also known as "physiological intensity"). The absolute intensity of the same type of exercise is consistent, while the relative intensity, such as the feeling of fatigue, may vary greatly among individuals under different physiological conditions. (1) Absolute intensity: This is usually the result of a specific physical activity test performed on ordinary, healthy adults. The commonly used indicator is metabolic equivalent (MET). Metabolic equivalent refers to the energy metabolism level relative to the resting state during exercise. 1 MET is equivalent to 3.5 ml of oxygen consumed per kilogram of body weight per minute. Metabolic equivalent is currently the most commonly used indicator in the world to reflect the absolute intensity of exercise. (2) Relative intensity: This falls under the category of physiological intensity and reflects the subjective fatigue level and physiological load of an individual during physical activity. The commonly used indicators are the self-perceived exercise intensity scale (i.e., RPE scale, Borgs scale), percentage of maximum heart rate (%HRmax), percentage of maximum oxygen consumption (%VO₂max), target heart rate, etc.

Measurement of physical activity (1) Measurement of aerobic exercise intensity 1) Metabolic equivalent (MET): When assessing or assigning a value to the MET of an individual's activity, it is usually based on an existing metabolic equivalent database. Due to differences in social culture, there are certain differences in the forms and basic characteristics of physical activity among different populations. The globally widely used adult physical activity metabolic equivalent database is a summary of physical activity published and updated by the American College of Sports Medicine, which assigns values ​​to the metabolic equivalent of hundreds of types of exercise in people's lives, work, transportation, entertainment, etc. This database has been widely recognized and used worldwide. At present, there is a lack of measured database of metabolic equivalents for Chinese residents. Therefore, when using the above database to assign values ​​to the intensity (MET) of physical activity in the Chinese population, the specific exertion level and characteristics of physical activity should be considered to assign values ​​reasonably. Since the health benefits of moderate to high intensity physical activity have been widely recognized, the WHO and many countries have classified various types of physical activity according to MET levels. Among them, ≥6 MET is considered high intensity; 35.9 MET is moderate intensity; <3 MET is low intensity; and activities below 3 MET, primarily involving quiet lying, reclining, or sitting positions, are called resting behaviors. 2) Heart rate: Heart rate during exercise can be directly measured by palpating the carotid or limb arteries for 10 seconds. A more convenient method is to use wired or wireless instruments to monitor heart rate. The target heart rate for exercise is obtained by multiplying the individual's maximum heart rate by a percentage. Maximum heart rate can be determined by a progressive exercise test, or more simply by estimating it based on age, i.e., maximum heart rate HRmax = 220 - age (years). Moderate intensity heart rate is generally defined in a 60% 20-point scale. According to the subjective fatigue level classification, moderate intensity is usually within the range of 11-14. The specific measurement method is as follows: the subjective fatigue level is set as 6 as the lowest level (maximum ease, no load) and 20 as the highest level (extreme fatigue). Then, the individual's fatigue level is subjectively estimated based on the fatigue of the specific activity (such as running). There may be significant differences in the feelings of different individuals. For example, jogging may feel very easy for professional athletes, with a score of 7 or 8, while adults who rarely exercise may feel quite tired, with a score of 14. In conducting physical activity guidance, self-perceived exercise intensity is safer and more practical. The self-perceived feeling of moderate intensity activity is that the heart rate and breathing are faster, effort is exerted but not strenuous, and one can talk continuously in rhythm with breathing, but cannot sing loudly. Generally healthy people can also feel and control the intensity based on the heart rate during the activity, but for the elderly and those with poor physical condition, the intensity should be determined in combination with their physical condition and feelings. (2) Measurement of muscle strength activity: Traditionally, strength is tested with a load that can be repeated less than 3 times, and endurance is tested with a load that can be repeated more than 12 times. Muscle strength tests: ① Static or isometric strength: Tests are limited to specific muscle groups and joint angles, and cannot fully reflect muscle strength. Peak exertion is often expressed as maximum active contraction (MVC). ② Dynamic tests: The maximum resistance (1-RM) resisted by a controlled, well-posture, full-range joint movement. The measured value is a specific indicator of a particular muscle or movement. Measurement methods and procedures: ① Use several submaximal RMs as a warm-up; ② Determine 1-RM or m-RM through 4 trials, with 35-minute intervals between each trial; ③ The initial test weight should be a weight that the subject feels capable of completing; ④ Gradually increase the resistance, with increases between 2.5 and 20 kg, until the subject fails. All tests should maintain a consistent movement pattern and speed; ⑤ The maximum weight successfully completed is the 1-RM. Muscle endurance tests: Given a frequency and the number of repetitions of a resistance movement, such as the number of squats. The measurement of muscle endurance in the test should comprehensively consider three indicators: resistance (weight), time (frequency), and number of repetitions.

Estimating Physical Activity and Energy Consumption: 1 MET is equivalent to consuming 3.5 ml of oxygen per kilogram of body weight per minute, or approximately 1 kcal per kilogram of body weight per hour. In other words, for a healthy adult weighing 60 kg, the energy consumption from physical activity is equivalent to the amount of physical activity, which can be estimated using the formula: Physical Activity Energy Consumption (kcal) ≈ Physical Activity Amount (MET-min) = Physical Activity Intensity (MET) × Activity Time (min) For example, the metabolic equivalent of a healthy adult walking at 4 km/h is 3.0 MET. Therefore, the activity amount and energy consumption of a 60 kg healthy adult walking for 10 minutes would be: Physical Activity Amount (MET-min) = 3.0 MET × 10 min = 30 MET-min Physical Activity Energy Consumption ≈ Physical Activity Amount = 30 kcal Physical activity amount can be estimated by cumulative summation. A healthy adult could walk for 50 minutes at a speed of 4 km/h daily, 5 days a week, and cycle twice a week for 20 minutes each time. Therefore: Daily walking activity (MET-min) = 3.0 MET × 50 min = 150 MET-min; Weekly walking activity (MET-min) = 3.0 MET × 50 min × 5 = 750 MET-min; Weekly cycling activity (MET-min) = 4.0 MET × 20 min × 2 = 160 MET-min; Total weekly activity = 750 MET-min + 160 MET-min = 910 MET-min. For a healthy adult weighing 60 kg, the physical activity (MET-min) is equivalent to energy expenditure (kcal). The heavier the adult, the higher the energy expenditure. That is, a 60 kg adult will consume approximately 910 kcal per week from walking and cycling, while a 72 kg adult will consume approximately 1.2 times more energy (72 kg ÷ 60 kg = 1.2), or 1092 kcal.