yogabook / movement physiology / extension deficit
Extension deficit
The inability to bring a joint into full extension, as would be physiologically possible, or the angular dimension of the discrepancy. Just as, but less frequently, a joint such as the knee joint or the elbow joint can overstretch, there is also the case that straight extension cannot be achieved in the first place. This is referred to as an extension deficit. The extent in degrees is referred to as an „extension deficit“, as is the mere possibility. To test for an extension deficit, the influence of biarticular and polyarticular muscles spanning the joint on the test must be excluded. For example, the upper arm must not be in retroversion when testing for extensibility of the elbow joint, as otherwise a biceps that is not particularly flexible could simulate an extension deficit. To exclude its influence, it is better to bring the arm into a frontal abduction of just under 90° and then try to extend the elbow joint. If the knee joint in the standing leg (this does not apply if a free leg is being examined) is examined for an extension deficit, the hip joint must be clearly flexed so that the influence of the hip flexors pulling the thigh ventrally does not produce a false positive result. However, the flexion angle must not be too large, as otherwise the biarticular part of the hamstrings can also cause a false positive result. On the side of the joint adjacent to the knee joint caudally, the ankle joint, it must also be ruled out that biarticular and polyarticular muscles as the gastrocnemius falsify the result. For this purpose, the ankle joint must only be able to achieve a standard anatomical position-compliant angle of 0° of dorsiflexion. With the exception of a few pathological situations, this should always be the case because the movement of walking, which is familiar to humans as a means of locomotion, with its own plantar flexion, which typically contributes to propulsion, should regularly maintain sufficient flexibility in direction of plantar flexion. The situation could be different if the activity of walking is completely eliminated due to quadriplegia, for example, and the foot is also regularly placed in a dorsiflexed position for long periods of time. In this case, three contractures based on negative longitudinal muscle adaptation could prevent the knee joint of the supporting leg from extending when attempting to stand:
- in the ankle joint with its contracture acquired through prolonged dorsiflexion, which prevents an standard anatomical position-compliant position of the ankle joint
- in the knee joint, especially with its biceps femoris caput breve, which is shortened due to prolonged flexed nee position
- the hip joint with its shortened hip flexors due to mainly hip-flexed sitting
The first and third effects must be ruled out by significant but not excessive (15° should be sufficient, even 30° is too much in extreme cases) flexion in the hip joint and slight plantar flexion in the ankle joint in order to be able to test for a genuine extension deficit in the knee joint. If the test result is positive for an extension deficit (the knee joint cannot be extended either actively or passively with external force), it is then necessary to find out whether the extension deficit is muscular in nature or not. In the former case, a carefully progressive attempt to stretch the joint by applying external force would result in a clear stretching sensation in the shortened muscle, i.e. in the example above in the biceps femoris caput breve on the outer back of the knee and thigh, radiating laterally from the knee towards the pelvis. If there is no stretching sensation at all in the muscles spanning the joint, it is reasonable to suspect that the stretching deficit is non-muscular. If non-muscular discomfort occurs, the attempt to stretch the joint should be stopped and a specialist examination should be initiated. In analogy to the extension deficit, there is also a flexion deficit.