The basic factor that plays a decisive role is the size of muscle physiological cross section. Generally speaking, it is the size of the muscle mass, the thickness of the muscle and the circumference. The size of muscle strength is positively correlated with the cross section of muscle. The greater the circumference of the muscle, the greater the muscle strength. This is easy to understand. There is a saying that "strength is not lost", which means that height and strength are naturally not small. The absolute strength of muscle is directly proportional to the physiological cross section of muscle. Physiologically, it is generally believed that the absolute muscle strength of the physiological cross-sectional area is 3.6kg/cm2, which means that a bundle of muscle fibers per square centimeter of cross-sectional area can produce 3.6kg of strength when contracting.
Because of this, we can indirectly evaluate muscle strength by measuring muscle circumference. During exercise, we can know the effect of exercise and how much muscle has grown by changing the circumference of limbs before and after exercise. During the exercise of rehabilitation function after injury surgery, we can compare the girth of limbs and healthy sides to get a general idea of how much muscle strength has recovered and whether it is still much worse. You see, this is the source of physical strength, so if the strength of a person's whole body or a certain part decreases, the muscles in this part will decrease or atrophy.
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