Anatomical Overview of the Human Muscular System

Muscles Acting on the Shoulder and Upper Limb

The upper limb is designed for a wide range of motion, from heavy lifting to fine motor manipulation.

Scapular Movers and the Shoulder

Movement of the scapula (rotation, elevation, protraction, etc.) is driven by several key muscles.

• Scapular Movers: The trapezius (elevates/rotates/depresses), levator scapulae (elevates), rhomboids (retract), and the serratus anterior and pectoralis minor (protract).
• Large Arm Movers: The deltoid is a major mover for flexion, extension, and abduction. The pectoralis major and latissimus dorsi contribute to adduction and medial rotation.

The Rotator Cuff (SITS)

A critical group of four muscles, the rotator cuff, works to hold the head of the humerus firmly within the glenoid cavity of the scapula. These muscles are:

1. Supraspinatus
2. Infraspinatus
3. Teres minor
4. Subscapularis

Clinical Note: The supraspinatus tendon is particularly prone to injury and damage.

Elbow, Forearm, Wrist, and Hand

Movement at the elbow is primarily driven by the brachialis, biceps brachii, and triceps brachii. The brachioradialis also assists in this region.

• Forearm Rotation: Managed by the pronator teres and supinator.
• Wrist and Hand: These are functionally categorized into groups of flexors and extensors. The hand contains both extrinsic muscles (arising from the forearm) and intrinsic muscles (contained entirely within the hand).

Muscles Acting on the Hip and Lower Limb

The lower limb is specialized for weight-bearing and locomotion.

Hip and Thigh

• Hip Movers: The iliopsoas group (iliacus and psoas major) and the tensor fasciae latae act on the hip.
• Gluteal Muscles: The gluteus maximus, medius, and minimus provide power and stability to the hip.
• Adductors: The adductor longus, adductor magnus, and gracilis pull the thigh toward the midline.
• Quadriceps Femoris: A large group on the anterior thigh consisting of the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius; these muscles extend the knee.
• Hamstrings: Located on the posterior thigh, this group includes the biceps femoris, semitendinosus, and semimembranosus.

Knee, Leg, and Foot

• Knee and Leg: The sartorius is a key muscle of the thigh. In the lower leg, the tibialis anterior is a primary muscle acting on the ankle. The posterior compartment includes the gastrocnemius (calf muscle) and the soleus.
• Ankle and Foot: The leg is organized into compartments (anterior, lateral, and posterior). The foot contains complex layers of intrinsic muscles, including the abductor hallucis and various interosseous muscles.

Summary of Functional Muscle Groups

When studying these muscles, it is vital to understand how they work in concert:

• Prime Mover (Agonist): The muscle providing the most force for a movement.
• Synergist: A muscle that aids the prime mover.
• Antagonist: A muscle that opposes the prime mover.
• Fixator: A muscle that stabilizes a bone to prevent unwanted movement.

Exam Tip: When identifying muscles on a cadaver or model, always look for the origin (the more stationary attachment) and the insertion (the moving attachment) to determine the direction of the muscle's action.