I have explained this question before, so it is good for interested friends to read it patiently.
Accurate measurement of human body composition is very difficult, not as simple as we thought.
The body composition measuring instrument used in the gym belongs to "bioelectrical impedance method". Lean meat and fat have different water content and different resistance. By measuring the resistivity between electrodes, the approximate data of human body composition can be calculated.
The bioelectrical impedance method is also used in the laboratory, with simple operation but average accuracy. The equipment and operation mode of the gym are not comparable to those of the laboratory, so the accuracy is even lower.
Let me talk about the factors that affect the accuracy of bioelectrical impedance method. The number of electrodes is very important. The results of instrument measurement with eight tactile electrodes are more accurate, but the instruments in the gym usually have only six, four or even two electrodes.
The standard detection method is that the subject lies flat on the surface of a non-conductive object, and the electrodes are placed on the back of the hands and feet, between the radius and ulna and at the ankle. Measure the electrical impedance value and calculate the body composition according to Siri equation.
Calculation needs data, and impedance value is only one of them. Generally, data such as height, weight, age, gender, race and obesity are needed. Why do you need so much data? Take race as an example. People of different races have different lean body density and conductivity.
Because human body composition is calculated by resistance measurement, the hydration degree of human body will also affect the accuracy of the test. For the same person, the measured results may be very different before and after light exercise.
Skin temperature will also affect the resistance value, so the test room of the laboratory is generally constant temperature. Similarly, the environmental humidity will also affect the test results, and it is also required to keep the humidity in the test room moderate.
There are enough instruments with electrodes, accurate electrode paste position, non-conductive test bed, constant temperature and humidity test room, enough calculation data and professional operators, which are not available in general gyms. The measured results are definitely not accurate. Moreover, even in the laboratory, bioelectrical impedance method itself is not an accurate method for measuring body composition, and its only advantage is convenience and non-invasive.
In order to be as accurate as possible, bioelectrical impedance method generally requires subjects not to eat or exercise, not to dehydrate or over-hydrate for 3-4 hours. The electrode should not be grasped or trampled, because the strength of grasping or pedaling will also affect the impedance value. In other words, you must lie down, not stand. Most gyms can't do this.
Continuous eating can gradually reduce the bioelectricity of human body and make the measured body fat low. Dehydration will increase the bioelectricity of the body, resulting in low lean body weight and high body fat.
In an ideal state, the body composition measured by bioelectrical impedance method is far less accurate than that measured by underwater weighing method or space capsule method, and even less accurate than that measured by skinfold thickness method. Generally speaking, the body fat of thin people measured by bioelectrical impedance method is easy to be high, and that of obese people is easy to be low. It is also found that bodybuilders and strength athletes cannot derive the percentage of body fat by bioelectrical impedance method. This method is only suitable for ordinary people, not for athletes and senior sports fans.