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I have read
"Flexion… decreases the angle… Extension is the opposite of flexion, describing a straightening movement that increases the angle between body parts"
I am clear on the directions of flexion and extension
So all agree on direction, but if I look up degrees of flexion, for example
and videos showing usage of a goniometer to measure shoulder flexion, which echo that picture, so 180 degrees is the arm up, and 0 degrees is the arm down.
What I don't quite understand, is that flexion according to wikipedia, should decrease the angle. Whereas looking at the degrees in that diagram, and in videos of usage of a goniometer, the more flexion you have, the higher the degree.
I can think of two possible resolutions
A) that wikipedia has missed out that in fact, for the case of the arm, flexion increases the angle. Whereas for bending movement, it decreases it.
B) the planes that describe degree of flexion, are different to the ones shown on a goniometer, perhaps, one field uses a gynometer, the other uses a measure with the angles going the other direction
I'm guessing that it's "A" , that wikipedia is misleading and in the case of the "shoulder"(the arm movement considered shoulder flexion and extension), flexion doesn't decrease the angle, it increases it(though wikipedia doesn't say that). But i'd be interested if that can be confirmed.
There are two criteria to differ between flexion and extension:
1) Decreasing or increasing the angle between the involved bones. By this criteria, it should be clear that flexion means bending of the elbows and knees and lateral bending of the spine (the neck and torso) toward the bent side.
2. The anterior/posterior direction of the movement. All anterior (ventral) movements in the neck or torso part of the spine, shoulder, elbow, wrist, fingers, hip, ankle and toes are called flexion and all posterior (dorsal) movements (except in the knee) are called extension.
You should not use the both criteria at the same time, because they can contradict each other.
There are some specifics:
- In the wrist, you can have palmar flexion, dorsiflexion (extension), ulnar flexion (abduction) and radial flexion (adduction) (Teachmeanatomy).
- In the ankle, you can have plantar flexion, dorsiflexion (extension), inversion (inward rotation, adduction) and eversion (outward rotation, abduction). (ScienceDirect).
- In the shoulder and hip, raising a limb to the same side as the limb is, is abduction (lateral extension) and raising it to the opposite side is adduction.
- Moving the thumb toward the palm (in the same plane as palm) is flexion (adduction) and moving it away from it is extension (abduction).
You can read about flexion and extension and other movements here: Types of Body Movements (BCcampus)
Flexion and extension are movements that take place within the sagittal plane and involve anterior or posterior movements of the body or limbs. For the vertebral column, flexion (anterior flexion) is an anterior (forward) bending of the neck or body, while extension involves a posterior-directed motion, such as straightening from a flexed position or bending backward. Lateral flexion is the bending of the neck or body toward the right or left side.
In the limbs, flexion decreases the angle between the bones (bending of the joint), while extension increases the angle and straightens the joint. For the upper limb, all anterior-going motions are flexion and all posterior-going motions are extension. These include anterior-posterior movements of the arm at the shoulder, the forearm at the elbow, the hand at the wrist, and the fingers at the metacarpophalangeal and interphalangeal joints. For the thumb, extension moves the thumb away from the palm of the hand, within the same plane as the palm, while flexion brings the thumb back against the index finger or into the palm. These motions take place at the first carpometacarpal joint. In the lower limb, bringing the thigh forward and upward is flexion at the hip joint, while any posterior-going motion of the thigh is extension. Note that extension of the thigh beyond the anatomical (standing) position is greatly limited by the ligaments that support the hip joint. Knee flexion is the bending of the knee to bring the foot toward the posterior thigh, and extension is the straightening of the knee. Flexion and extension movements are seen at the hinge, condyloid, saddle, and ball-and-socket joints of the limbs (see Figure 1).
Now, to answer the question about the goniometer.
Again, the confusion about interpretation of this goniometer arises from two different criteria about flexion. The one mentioned in the Wikipedia and other anatomical articles says that in flexion the angle between the involved bones decreases. When you raise your arm, this criteria does not aply because the angle between the bones (humerus/scapula) in the shoulder joint does not really change (unlike in the elbow, for example), so here the criteria that all anterior movements are considered flexion applies. When you have an arm straight down, there is no flexion, so it's considered zero degrees and when it's straight up, it's considered 180 degrees, so the angle is increasing, but this is not an angle between the arm and other bones or the body but between the starting and ending position of the arm.