Overview
This lecture covers the major muscles of the back, their locations, functions, and relationships as synergists or antagonists.
Trapezius Muscle
- The trapezius is a large muscle with descending, transverse, and ascending parts.
- Main functions: elevates the scapula, extends the neck, and retracts the scapula.
Rotator Cuff and Shoulder Muscles
- The rotator cuff includes supraspinatus, infraspinatus, teres minor, and subscapularis (SITS).
- Infraspinatus and teres minor perform lateral (external) rotation at the shoulder; are synergists.
- Supraspinatus abducts the arm at the shoulder joint.
- Teres major performs extension, adduction, and medial rotation at the shoulder; is a synergist with latissimus dorsi for these actions but an antagonist to infraspinatus and teres minor.
Latissimus Dorsi
- Latissimus dorsi extends, adducts, and medially rotates the shoulder; shares these functions with teres major.
- Originates partly from the thoracolumbar fascia.
Rhomboids
- Rhomboid minor and rhomboid major retract the scapula.
- Rhomboids are synergists to trapezius for scapular retraction but antagonists to serratus anterior and pectoralis minor, which protract the scapula.
Erector Spinae Group
- Consists of spinalis (medial), longissimus (intermediate), and iliocostalis (lateral).
- Extends the vertebral column and neck.
- Antagonists to abdominal wall muscles (rectus abdominis, internal/external oblique) that flex the vertebral column.
Additional Back Muscles
- Serratus posterior inferior is a thin muscle on the lower back.
- Quadratus lumborum helps extend the lower back and fix the ribs from below.
- Thoracolumbar fascia is a connective tissue aponeurosis and origin for latissimus dorsi.
Key Terms & Definitions
- Scapula — shoulder blade bone.
- Abduction — movement away from the body’s midline.
- Adduction — movement towards the body’s midline.
- Synergist — muscles that assist each other in performing the same action.
- Antagonist — muscles that perform opposite actions.
Action Items / Next Steps
- Review muscle locations and functions.
- Memorize the SITS muscles of the rotator cuff.
- Study synergist and antagonist muscle relationships for the shoulder and back.