Physical Activity and Energy Balance: Basic Concepts, Classifications, and Physiological Effects of Weight Loss Exercise
Energy balance is a crucial foundation for maintaining a healthy weight. Physical activity accounts for approximately 15% of total energy expenditure in adults. Overweight and obesity result from energy intake exceeding energy expenditure, leading to excessive fat accumulation in the body. Therefore, reducing dietary energy intake, increasing physical activity to boost energy expenditure, and controlling energy balance are fundamental conditions for maintaining health, supported by substantial and credible scientific evidence. Physical activity refers to all activities that increase energy expenditure relative to the body's resting state due to skeletal muscle contraction. In other words, physical activity includes all activities except sleep and absolute rest. Physical activity is not the same as the commonly understood concepts of sports or exercise. Sports refer to activities conducted according to a set of rules as part of recreational or competitive activities. Sports typically include team or individual activities supported by an organization, such as a sports organization. Exercise, also commonly referred to as "training," refers to planned, repetitive activities with specific content aimed at improving or maintaining one or more aspects of physical fitness. In addition, the words “exercise” and “training” are often used interchangeably, usually referring to exercise done during leisure time, with the main purpose of improving or maintaining physical fitness, athletic ability or health.
The basic elements of physical activity, according to the basic principles of physical activity, include frequency, intensity, timing, and type, abbreviated as the FITT four elements. (1) Frequency: The frequency of physical activity refers to the number of times exercise is performed per unit time, usually in "weeks". It is usually expressed as the number of days of activity per week (days/week). Frequency reflects the level of regular exercise and is an important aspect of exercise promotion. Emphasizing a certain regularity is an important prerequisite for the health benefits of exercise. (2) Intensity: The intensity of physical activity refers to the level of energy consumption of physical activity per unit time or the degree of physiological stimulation to the human body. It is usually expressed by metabolic equivalents (METs or METs), subjective exercise intensity scale level, percentage of maximum oxygen consumption (%VO₂max), and percentage of maximum heart rate (%HRmax). (When the human body is engaged in strenuous exercise, the oxygen consumption and heart rate of the human body can reach the limit level. The oxygen consumption at this time is called maximum oxygen consumption VO₂max, and the corresponding heart rate is the maximum heart rate HRmax). It can be divided into three levels: light, moderate, and heavy. (3) Time: Physical activity time refers to the duration of a single activity, usually expressed in minutes or hours. Cumulative exercise time refers to the total duration of certain specific exercises within a certain time frame to achieve a certain exercise goal. For example, 10 minutes of activity 3 times a day, 5 days a week can be expressed as 150 minutes per week. (4) Type: Different types of aerobic exercise, balance exercises, flexibility exercises, and resistance (or strength) exercises have different physiological stimulation effects on the body, and their health benefits also vary. Specific forms of exercise may meet the specific exercise goals of a specific group of people, and specific environments and conditions will also affect the choice of specific forms of exercise.
Classification of physical activity (1) Classification by energy metabolism: Based on the different energy sources in physical activity, physical activity can be divided into aerobic exercise and anaerobic exercise, abbreviated as aerobic exercise and anaerobic exercise. 1) Aerobic exercise: refers to a form of exercise that mainly involves large muscle groups such as the trunk and limbs, is rhythmic, lasts for a relatively long time, can be maintained in a stable state, requires oxygen to participate in energy supply, and uses aerobic metabolism as the main energy supply pathway. It is also called endurance exercise. Aerobic activities include walking at a moderate speed of 4 km/h and cycling at a speed of 12 km/h. 2) Anaerobic exercise: refers to a form of exercise that uses anaerobic metabolism as the main energy supply pathway. It is generally a strong contraction activity of muscles. Anaerobic activity can only be maintained for 2-3 minutes. The energy of the muscle groups exerted during exercise is mainly supplied by anaerobic glycolysis. For example, 100m sprint is almost entirely powered by anaerobic metabolism. Furthermore, anaerobic exercise can also occur at the end of aerobic exercise such as 5000m long-distance running, and is also the main form of lifting heavy objects, push-ups, and resistance muscle strength training. (2) Classification by physiological effects: According to different physiological effects, physical activities can also be divided into: 1) Flexibility training: also called stretching exercises, which refers to exercising the flexibility and agility of joints through the stretching, flexion and rotation of the body or limbs, such as the dynamic stretching of Tai Chi, Qigong and Yoga, as well as static stretching. 2) Muscle-strengthening activities: refers to repetitive movements of muscles against resistance, which can maintain or improve muscle strength (how much resistance can be overcome), endurance (how many times or how long the resistance can be overcome), or strength (how fast the resistance can be overcome). This includes daily activities such as carrying heavy objects, shoveling snow, carrying children or climbing stairs, as well as using fitness equipment such as weightlifting machines, free weightlifting or rubber bands. When exerting force against resistance, anaerobic energy supply is mainly relied upon (the whole process of exercise also contains oxygen energy supply components). 3) Balance and Coordination Training: This refers to a combination of activities that improve the body's balance and coordination. Regular practice, whether walking, standing, or sitting, can enhance the body's resistance to internal or environmental forces that could cause falls. Specific forms of exercise include standing on one leg, walking from heel to toe, balance walking, and using a rocking board. 4) Bone-Strengthening Activities: These are exercises that exert impact on bones and load on muscles. These forces act on the bones, adapting them by changing their structure (shape) or mass (mineral content), thereby increasing their resistance to fractures. Jumping, hopping on one leg, skipping rope, and dancing are all beneficial activities for bone strengthening, as are activities for muscle strengthening. 5) Mind-Body Activities such as Yoga, Tai Chi, and Qigong: These traditional, comprehensive practices typically combine muscle strengthening, balance training, low-intensity aerobic exercise, and flexibility training. Relaxation, meditation, or spirituality are also emphasized in these activities.
The scope of physical activity Physical activity occurs at any time of day, for different purposes, in different places, and in different forms. Occupational physical activity was the focus of most epidemiological studies on physical activity and health in the early days. With the development of science and technology and social progress, the content of human physical activity has changed significantly. As occupations requiring high-intensity physical activity have gradually decreased, the focus of research has gradually shifted to leisure or recreational physical activity. According to the difference in purpose and place, people's physical activities in modern society can generally be divided into four categories: occupational activity, transportation activity, housework activity, and leisure activity. (1) Occupational physical activity: refers to physical activities that are paid or obligatory for the purpose of work. For example, putting things on the shelves in a store, delivering packages in the office, preparing or serving food in a restaurant, or carrying tools in a workshop. (2) Transportation physical activity: physical activities that move from one place to another for the purpose of transportation. Such as walking or cycling, taking a car, driving, etc. to and from the workplace, school, or vegetable market, shopping center, etc. (3) Household physical activity: This refers to activities performed at home or in the yard, including housework such as cooking, cleaning, repairing the house, tidying the yard or gardening. (4) Leisure-time physical activity: This refers to activities undertaken without working, traveling, or doing housework, including exercise, walking, playing games, basketball, etc.