What are the long strings showed which flex or, extend the hand?

Related questions

• What the chemical messengers that travel in the blood?
• What are the different body systems in human body and what are their functions?
• What major roles does RNA play in the body?
• Where exactly is DNA found in the body? What are the biggest difference between DNA and RNA?
• How are the mind and body related?
• How many molecules are in the human body?
• What are five different ways the body maintains homeostasis?
• What organs are located on the left side of your body below the rib cage?
• How can the heart beat when it is outside of the body?
• What is the "trunk" of the body? Where is it located?