I believe it would be the bending of the arm, utilizing your muscular ad skeletal system. As the joint in your elbow is cartilage part of the skeletal system, and the voluntary bending comes from the contraction of muscles in the arm.
The muscular system is made up of the various muscles of the human body. The Muscular System has some functions that are fundamental to the human body. Here are these functions: body stability; movement production; body warming (maintenance of body temperature); body fill (support); aid in blood flows.
On the other hand, the skeletal system is made up of bones and cartilage, as well as ligaments and tendons. It is responsible for sustaining and shaping the body. It also protects the internal organs and acts in conjunction with the muscle and joint systems to allow movement. Other functions are the production of bone marrow blood cells and the storage of mineral salts such as calcium.
Although they are different systems, you may be that both the muscular system and the skeletal system are responsible for the movement. For this reason, we can conclude that among the options given in the question, the only one that shows the function that the muscular system and the skeletal system have in common is: Bending of the arm
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