Head, Neck & Trunk 

Masseter

The masseter is a major muscle of mastication (chewing).

Sternocleidomastoid (SCM) 

The sternocleidomastoid is a lot of people’s favorite muscle because it is fun to say. You will often see it abbreviated as SCM. Sterno- refers to the sternum. Cleido- refers to the clavicle. Mastoid refers to the mastoid process of the temporal bone.

Trapezius

The trapezius is named for its shape. When looking at both the right and left trapezius muscles together, they form a trapezoid, hence their name. It is common for people to get muscle pain in this muscle, especially if they are stressed. The trapezius has many functions, but we will not explore too many of them here.

Breathing Muscles: Diaphragm, Internal & External Intercostals

The diaphragm, internal intercostals, and external intercostals play a role in breathing. You will remember from Unit 1 that the diaphragm separates the thoracic cavity from the abdominal cavity. The diaphragm is active for inhalation and relaxes for exhalation. The intercostals are muscles that run between the ribs. They play a role in either exhalation or inhalation as noted below by either elevating or depressing the ribs. We will study these muscles more in the respiratory system (Unit 17).

Abdominals

The abdominal muscles perform a variety of functions, but for this unit, we just want you to be familiar with their names and basic functions.

Erector Spinae 

The erector spinae are a grouping of three muscles: spinalis, longissimus, iliocostalis. You do not need to know them individually, just collectively. Together, they are spinal extensors. They play a major role in maintaining upright posture.