In disciplines and industries such as anatomy, kinematics and fitness, it is necessary to make a standardized definition of human joint movement, which includes the definition of joint movements such as flexion, stretching, internal rotation, external rotation, varus, valgus, adduction, and abduction.
Inside, outside, and up and down movements are easier to understand, but the concepts of "flexion" and "stretching" are often elusive. The concept of joint flexion and extension is mainly aimed at the movement of the sagittal surface of the human body. The so-called "sagittal movement" is to put it bluntly, the movement of the human body in both directions, through the movement of the knee joint and hip joints before and after swinging the legs, the head and neck back and forth movements, reaching out to open the door and other actions are typical sagittal (anterior and posterior) joint flexion and extension movements.
Seeing this, some people may think that the concept of joint flexion and extension is easy to understand, but things don't end there. The crux of the matter is that flexion and extension do not correspond to the front and rear directions.
For example, moving the calf forward by bending and extending the thigh is called "flexing the hip", but if the calf is moved forward by flexing the knee joint, it is called "extending the knee". The same is the calf forward, if it is achieved through the hip joint, it is called "flexion", and the knee joint is called "extension". Some people may say, so easy, I will die and remember it. The hip is called flexion forward and extension backward, as are the wrists, spine (including the cervical, lumbar and thoracic vertebrae), shoulders, and elbows, but the knee joint is an exception. Well, you're smart, but there's a question: do you know which is flexion and which is extension when the ankle is moved forward and backward? Few people probably know the standard answer: the ankle movement, whether forward or backward, can be called "flexion", the difference is that the forward movement is called "ankle dorsal flexion" and the backward movement is called "ankle plantar flexion".
Therefore, we still have to figure out the definition criteria of the two concepts of joint flexion and extension. This criterion mainly has two conditions, one is the joint movement on the sagittal surface (joint neutrality, front and back direction), as long as this condition is met, it can be called flexion or extension; the second is the angle of the adjacent segment of the joint, if this angle is getting smaller and smaller during the movement and can be minimized, it is called "flexion", otherwise it is "extension". To give a few examples: when moving the knee joint so that the calf is forward or backward, the adjacent two segments are the calf and the thigh, when the calf is backward, the angle between the calf and the thigh is getting smaller and smaller and can be minimized, so it is called "flexion", when the calf is moved forward, the angle between the calf and the thigh cannot reach the minimum state, so it is "stretched". The same is true of the spine, adjacent segments are the vertebral bodies of the spine, when moving forward, the range of motion between the spine is larger, and the angle between each other can reach a minimum state, called "flexion", and when moving backwards, it is "extended". The same is true for hips, elbows, wrists and other joints. But the ankle joint is more special, because the two segments of the foot and the calf are perpendicular to each other, the toe and heel are one in the front, one is in the back, and the lower joint is centered, whether it is the movement of the toes up or the movement of the heel down, the two segments can be close to each other, and the amplitude of the proximity is not much different, in this case, only the definition of "plantar flexion" and "dorsal flexion" will be more appropriate.