Which muscle is responsible for changes in body posture?

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

What is a sarcomere?

The central part of the thick filament which is not overlapped by thin filaments is called as:

Select the correct option for skeletal muscle.
$(I)$ In human body,it is made of a number of muscle bundles.
$(II)$ Muscle bundles contain a number of muscle fibers and each fiber is lined by the sarcolemma.
$(III)$ In a muscle fiber,the sarcoplasm contains only one nucleus.
$(IV)$ In a muscle fiber,the sarcoplasmic reticulum is the storehouse of calcium ions.

Choose the correct option for the following:
Biceps $\quad$ Iris $\quad$ Heart