Anatomy and Physiology of the Musculoskeletal System
Why this domain matters
Socket and orthosis design is applied anatomy: the device must load tolerant areas and unload sensitive ones. The exam tests bony landmarks, joint types, muscle action, lever mechanics and normal ranges.
Core anatomy
- Bone: cortical bone is stiff and carries load; cancellous bone absorbs energy. In the leg the tibia bears most weight; the fibula transmits roughly 10–15% at the ankle and its head is a key landmark for socket trim lines.
- Joints: elbow and knee are modified hinges; hip and shoulder are ball-and-socket; the thumb CMC is a saddle; the distal radioulnar joint is a pivot.
- Muscles act in groups. For a transtibial limb, gastrocnemius–soleus and tibialis anterior control the residual segment; for transfemoral, the adductor group and hamstrings drive prosthetic control.
- Lever classes: first (nodding head), second (calf raise), third (biceps curl, most limb actions). Third-class levers sacrifice force for speed and range.
Worked example
A 5 kg mass is held 0.30 m from the elbow. The biceps inserts 0.04 m from the joint. Load moment = 5 × 9.81 × 0.30 = 14.7 N·m. Muscle force = 14.7 ÷ 0.04 ≈ 368 N. The muscle must pull about 7.5 times the load because its moment arm is short - a third-class lever. This explains why loads held near the hand stress the elbow.
Traps
Treating the fibula as fully weight-bearing; memorising muscle names without actions; confusing the sagittal, frontal and transverse planes; recording ROM without checking for compensatory movement.