The ulna is one of two bones in your forearm, sitting on the medial (inside) aspect, parallel to the radius, when your arm is in anatomical position. Your arm is in anatomical position when your palms are facing forward, thumb to the outside – this ensures that no bones are crossed. When you pronate your forearm (rotate your forearm so that your thumb is more medial) the radius crosses over the ulna. In the image below you can see the ulna and radius in both positions.
The hinge joint of the elbow is formed by the humerus, ulna, and radius (unlike at the knee joint where only the femur and tibia articulate, leaving the fibula out). The semilunar notch (see image to the right) of the ulna hooks around the distal end of your humerus. The ulna actually does not contribute to the true wrist joint, which is formed by the distal end of the radius with your carpals, there is an articular disk between the end of the ulna and the carpal bones.
The ulna is one of the long bones that has been used in studies that investigate whether handedness can be observed skeletally. Some studies have demonstrated a difference in bone mineral density and length of the ulna, humerus, and radius of the dominant side (Steele 2000). These findings, however, are not always consistent and a certain amount of asymmetry is normal.
The idea of handedness has also been tested in the animal kingdom. While foraging, walruses preferred their right flipper 89% of the time – granted this was on a study of about five walruses but measurements from 23 walrus skeletons demonstrated longer humeri, scapulae, and ulnae on the right (Levermann et al. 2003).
That’s a wrap on the ulna and the handedness of walruses – come back tomorrow for some fun facts about the radius to finish out the forearm.
REFERENCES & MORE TO EXPLORE
The Ulna. TeachMe Anatomy Series. 2020
The evolution of handedness in humans and great apes: a review and current issues. Cashmore et al. Journal of Anthropological Sciences. 2008
Are Walruses Right-handed? Science Daily. October 2003
Feeding behaviour of free-ranging walruses with notes on apparent dextrality of flipper use. Levermann et al. BMC Ecology. 2003
Handedness in past human populations: Skeletal markers. Steele. Laterality: Asymmetries of Body, Brain and Cognition. 2000
[…] the lateral aspect in anatomical position but crosses over the ulna when you pronate your forearm (see yesterday’s post). The radius articulates with the humerus to help form the hinge joint of the elbow and articulates […]