Which ions must be present for the binding of cross-bridges during muscle contraction?

Analogy type questions:
$(1)$ The portion between two successive $Z$-lines is the functional unit of contraction,which is called the sarcoplasmic reticulum / sarcomere.
$(2)$ The thick filaments in the $A$-band are held together in the middle of this band by a thin fibrous membrane. It is called the $Z$-line / $M$-line.

The largest muscle in the human body is:

The Achilles tendon is associated with which of the following muscles?

In a skeletal muscle fibre,the nuclei are located:

Which of the following is the contractile protein of a muscle?