The human skeletal structure resembles, in many ways, our ancestral quadrupedal heritage. You may be thinking, "What does he mean by that?" Well, the human upper limb is structurally designed as a supportive appendage, capable of absorbing shock during locomotion. Many quadrupeds, dogs, horses, and deer, have no bony attachment between the forelimb and axial skeleton. This allows the proximal end of the pectoral limb (scapula) to "float" on the thoracic wall.

Human upright posture has forced some structural modifications in the upper limb. We have a clavicle that "connects" the upper limb (appendicular skeleton) with the sternum (axial skeleton). The clavicle does several things.

• Projects the upper limb away from the body to more effectively carry objects and climb.
• Provides a weak but direct bony connection that transmits energy to and from the axial skeleton.
• Allows a wide range of limb movement.

The lower or pelvic limb serves as the propulsive appendage. These limbs are designed to transfer energy from the axial skeleton through the appendicular skeleton (lower limb) to the ground for propulsion. This means the limbs must be firmly attached to the axial skeleton. We'll discuss the lower limb in more detail in Exercise Thirteen.