The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Skeletal estimates of upper limb effective mechanical advantage do not predict joint strength or speed in living humans


Department of Anthropology, Washington University in Saint Louis

Saturday 1:15-1:30, Ballroom C Add to calendar

Effective mechanical advantage (EMA) is often estimated in fossil hominins as a measure of joint rotational force and velocity. To estimate EMA in skeletal remains, the muscle moment arm (r) is measured as the distance between the center of rotation of the joint and the line of action of the muscle, and the external load arm (R) is assumed to be proportional to the distal limb segment length. The relationship between EMA and joint mechanics is often cited on theoretical principles, but it has not been demonstrated empirically. Furthermore, the correlation between R and distal segment length has not been demonstrated.

Here, the effects of variation in EMA on elbow joint isometric strength and joint angular velocity are explored in a sample of 30 athletes. EMA at the elbow was measured using MRI, isometric strength was assessed with a load cell, and maximum joint angular velocity was assessed during throwing with a high-speed infrared camera system. Additionally, R measured during throwing was compared with ulna length to determine if skeletal measures of R are relevant to living behaviors.

Contrary to predictions, independent of muscle size, EMA does not significantly correlate with elbow isometric strength, or angular velocity during throwing. Furthermore, R measured during throwing is not correlated with ulna length. Instead, R varies within and between individuals due to elbow angle and arm position. These results strongly suggest that skeletal estimates of EMA should not be used to infer joint strength or angular velocity in fossil hominins.

This research was partially funded by a Grant in Aid of Research from the Society for Integrative and Comparative Biology.

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