This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
The following acts as a muscle spring 
Select and explain. 
A. Titin, B. Alfa actinin, C. Both, D. None
A: Titin
Answer: A: Titin 
Explanation: Titin is the muscle spring It is a suppoive protein It extends between zline and m line Ref:Guyton and Hall textbook of medical physiology 12th edition,page number: 86,87