The functional significance of olecranon process morphology: measuring the triceps
moment arm via tendon excursion.
Caley M. Orr1, 3, Lisa M. Raleigh2,3, Marc Jacofsky1,3
Dept. of Anthropology and Institute of Human Origins
Dept. of Bioengineering
IGERT Program in Neural and Musculoskeletal Adaptation in Form and Function
The olecranon process of the ulna forms the bony aspect of the elbow and marks
the insertion of the triceps brachii muscle, which is the sole extensor of the forearm.
Variation in the length and angulation (whether it is aligned with the long axis of the ulna
or angled dorsally) of the olecranon is frequently used to reconstruct locomotor or
manipulative behaviors in fossil primates. Two assumptions are made in such studies.
First, it is assumed that olecranon length reflects the maximum length of the triceps
moment arm (MA). The MA is the perpendicular distance from the joint center of
rotation to the line of action of the muscle, with a longer MA resulting in a greater
mechanical advantage for the muscle lever system. Second, it is assumed that angulation
of the olecranon affects the joint angle of maximal MA, such that primates with dorsally
angled olecranons have triceps MAs that are maximal at more extended joint postures.
These assumptions are tested using a tendon excursion method to measure the triceps MA
in primate cadavers. MA is calculated as the first derivative of the tendon excursion
versus joint angle curve, allowing determination of the MA throughout the range of
motion. The results are compared with bony measurements of the olecranon process
from those specimens. Preliminary results suggest that olecranon length is reflective of
relative MA length, but might overestimate the actual maximum MA. Angulation of the
olecranon also appears to influence the joint angle at which maximum MA length occurs,
supporting the second assumption.