6.4 c&d Reading

Direction of Muscle Fibers: Refers to an imaginary line (midline/long axis);
- Rectus (straight): fibers parallel to the line (e.g., rectus femoris).
- Oblique: fibers run at a slant to the line.
Relative Size:
- Maximus (largest), Minimus (smallest), Longus (long) (e.g., gluteus maximus).
Location: Named for associated bone (e.g., temporalis, frontalis).
Number of Origins:
- Biceps, Triceps (two/three origins).
Shape: Distinctive shape (e.g., Deltoid means triangular).
Action: Named for their action (e.g., Flexor, Extensor, Adductor).

Skeletal muscles consist of fascicles, arranged in various patterns (refer to figures such as $6.1$ and $6.15$ ).
Common Patterns:
- Circular: Fascicles in concentric rings, surrounding and closing external openings (sphincters, e.g., orbicularis muscles).
- Convergent: Fascicles converge towards a single insertion tendon; triangular or fan-shaped (e.g., pectoralis major).
- Parallel: Fascicles run parallel to the long axis of the muscle; strap-like (e.g., sartorius).
- Fusiform: A modification of parallel; spindle-shaped with an expanded belly (e.g., biceps brachii).
- Pennate: Short fascicles attach obliquely to a central tendon.
  - Unipennate: Fascicles insert on one side of the tendon (e.g., extensor digitorum).
  - Bipennate: Fascicles insert on opposite sides of the tendon.
  - Multipennate: Fascicles insert from several different sides.
Muscle Function based on Fascicle Arrangement:
- Range of motion and power are determined by fascicle arrangement.
- Longer, more parallel fascicles = greater shortening, but less power.
- Muscle power depends on total number of muscle fibers.
- Stocky bipennate and multipennate muscles = little shortening, but very powerful (e.g., rectus femoris).