Limb rotation: lateral upper limb versus medial lower limb
The upper limb rotates laterally (outwards, away from the midline) and the lower limb rotates medially (inwards, towards the midline), so comparable muscle groups end up on opposite surfaces.
Remember
After lateral rotation of the upper limb the thumb is lateral and the extensor muscles (the group that straightens the elbow, wrist and fingers) lie mainly posteriorly; after medial rotation of the lower limb the great toe is medial and the extensors lie mainly anteriorly.
Memory aid
arms turn out (lateral) so thumbs go out; legs turn in (medial) so big toes go in
Use it to place the thumb and the great toe: the arm turns out and the leg turns in.
Early in development, the upper limb sticks out sideways with its thumb side pointing towards the head. It then undergoes lateral rotation, which carries the thumb to the outer side. The same turn brings the extensor muscles mainly to the back, or posterior surface, and leaves the flexor muscles (which bend the elbow, wrist and fingers) mainly at the front, or anterior surface.
The lower limb turns the opposite way. Medial rotation brings the great toe to the medial side. Its extensor muscles (for example the quadriceps and the tibialis anterior, which straighten the knee or lift the foot) face mainly anteriorly, while its flexor muscles (for example the hamstrings and calf muscles, which bend the knee and push the foot down) lie mainly posteriorly.
The two limbs rotate in opposite directions, and that explains why comparable muscle groups face opposite surfaces. In both limbs a dorsal muscle mass broadly becomes the extensor group and a ventral muscle mass broadly becomes the flexor group. Early on, the dorsal masses face outwards in both limbs. Lateral rotation then carries the extensors to the posterior surface in the upper limb, and medial rotation carries them to the anterior surface in the lower limb.
Rotation also twists the skin map. The dermatomes of the limbs (strips of skin supplied by one spinal root) finish as curved or spiral bands, not straight horizontal rings.
To check a proposed limb-rotation map, use two clues together: the border that carries the thumb or great toe, and the face on which the extensor muscles end up. Both follow from the same rotation, so they are consistent with each other.
Key facts
- Upper limb rotation
- lateral, outwards and away from the midline.
- Upper limb map
- the thumb is lateral, the flexors are mainly anterior and the extensors are mainly posterior.
- Lower limb rotation
- medial, inwards and towards the midline.
- Lower limb map
- the great toe is medial, the extensors are mainly anterior and the flexors are mainly posterior.
- Early layout
- each limb has a dorsal muscle mass that broadly becomes the extensors and a ventral mass that broadly becomes the flexors.
- Limb dermatomes
- they end up as curved or spiral bands, not straight horizontal rings.
- Checking a limb-rotation map
- use the thumb or great toe side and the extensor face together.
Exam traps
- Medial rotation is the movement of the lower limb, not the upper limb. The upper limb rotates laterally, which leaves its extensors mainly posterior rather than anterior.
- A medial great toe does not mean the muscle groups copy the upper limb. The lower limb rotates the other way, so its extensors end up mainly in front and its flexors mainly behind.
- Flexors and extensors are not fused into one mass on a single medial surface after upper limb rotation. They remain separate groups and are carried to different surfaces.
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