The posterior bellies of digastric muscle are especially active during
Swallowing and speech
Swallowing and smiling
Swallowing and chewing
Swallowing and frowning
Digastric

Origin – The posterior bellay arises from the mastoid, or digastric, notch immediately behind the mastoid process of the temporal bone.
Insertion – The posterior belly passes downwards and forwards towards the hyoid bone where it becomes the intermediate digastric tendon and joints with the anterior belly.
Nerve supply – Derived from the digastric branch of the facial nerve.
Vasculature – Arterial blood supply from the posterior auricular and occipital arteries.
Action – The muscle depresses the mandible and can elevate the hyoid bone. The posterior bellies act in unison and are particularly active during swallowing and chewing.