Body fat measuring machines in gyms usually use the bioresistive resistance method. The principle is that body fat is almost non-conductive, while body components such as muscle and water are easily conductive. Therefore, a body fat detector can calculate the ratio of body fat, water and other tissue components by measuring the body's bioelectrical impedance.
This measurement method is theoretically valid, but there are so many uncontrollable factors, from the design, installation, location and status of the machine, to the time of day when the person takes the measurement, the amount of water in the body, the physical state, and even how the person is connected to the machine, the part of the body that is connected, and the difference in the connection, all of which may have an impact on the measurement results.
Expanded Information:
Cautions on the use of the fat measuring machine
Do not measure the use of the measurement in the following special circumstances, otherwise the measurement results may not be Accurate:
1. After intense exercise.
2. After a sauna or shower.
3. After excessive alcohol consumption.
4. After consuming a lot of water and 1-2 hours after meals.
5. For pregnant women.
6. People who wear or have implanted medical electronic devices (such as pacemakers) must also avoid using these devices.
Baidu Encyclopedia - Fat Measuring Instruments