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Bend arm in supine position
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last update: 30.5.2021
Name: Arm bend in supine position
Instructions
- The exploration consists of two parts.
- Firstly, lie on your back with your arms turned out at a slight angle (approx. 10°) next to your body, palms facing upwards. The right upper arm lies on an elevation a few centimetres high so that the elbow joint cannot be bent without the forearm being raised adequately. The shoulder blade is resting heavily on the floor. The right hand is holding a (rather light) dumbbell.
- Slowly bend the right elbow joint and note the force exerted and also the extent to which further lifting becomes easier.
- Next, perform the same bending of the elbow joint while standing upright, with the arm hanging vertically at the start and the elbow leaning against the body, again so that bending the elbow does not lead to it moving backwards, but rather to the forearm being lifted. Again, observe the force required to bend the elbow and the change over the further lift.
details
- It is regularly observed that lifting from the supine position is perceived as significantly more difficult than lifting from a standing position. This is due to the fact that from the supine position you have to work against the maximum gravitational effect ofthe dumbbell, because the first lift in the limit position is exactly in the direction of gravity. The more the elbow joint is flexed, the easier it becomes. In contrast, lifting the dumbbell in the standing position until the forearm is horizontal becomes more and more difficult after starting out as easy as you like, as the direction of the lifting movement was perpendicular to the direction of the dumbbell’s gravity at the start in the limit. Furthermore, it is easy to observe that the tendon of the biceps is still concealed in the forearm when it is first tensed to initiate the lift. With each degree of biceps flexion of the elbow joint, this tendon emerges further to the surface and finally to such an extent that it not only becomes easily palpable but also rises visibly above the contour of the forearm. This shows how the distance of the tendon to the centre of rotation and thus the force arm of the lifting movement changes favourably up to the horizontal. Conversely, it means that the synergists of the biceps have to perform a much greater proportion of the work at the beginning of flexion of the elbow joint, mainly the brachialis and brachioradialis. Both have a more favourable distance from the centre of rotation, i.e. the power arm, when the elbow joint is extended. In both tests, it can be observed that the (subjectively perceived, but also objectively exerted) force is constantly changing. Overall, we are dealing with three important influences:
- The change in the direction of movement compared to the direction of gravity already discussed above, whose angle at zero means maximum effect of an external weight or body part. The effect of gravity decreases with increasing angle between 0° and 90° from 100% to 0° according to the cosine.
- The change in the force arm, i.e. the distance from the muscle or its tendon to the centre of rotation, which has also already been mentioned, the greater this is, the more load can be moved with the same tendon force of the muscle – naturally with a longer distance to be covered.
- The force-length function, according to which a muscle has no idealised force at minimum and maximum sarcomere length. At maximum length, this is because in this case myosin cannot yet engage with actin, so that synergists must first bring the joint into a more favourable position; at minimum length, this is because actin and myosin have already fully engaged with each other and no further concentric contraction is possible. In between, the curve resembles an upside-down parabola, except that it is actually polygonal and not round.