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jwyoung32
11-03-2007, 06:06 AM
Dear Biomch-L subscribers -

I am working on a project examining the scaling of limb joint moments
in an ontogenetic sample of small arboreal primates (squirrel
monkeys). One of the more interesting findings is that as monkeys
age and become larger, limb segments become relatively longer (i.e.,
increase with positive allometry) and elbow joint postures become
more flexed at mid-stance. Together, these changes lead to positive
allometry of substrate reaction force (SRF) moment arms at the
elbow. However, SRF magnitudes, scaled to body weight, decrease with
negative allometry. Together, these changes (longer moment arms *
smaller SRF magnitudes) lead to isometry of elbow joint moments as
animals age.

I'd like to use path analyses/structural equation modeling to dissect
the independent effects of body size and speed on joint postures,
moment arms, SRF magnitudes and joint moments. Also, path analysis
would allow for a clearer representation of the contrasting effects
of body mass on joint moment arm lengths and SRF magnitudes.
However, I've been told that path analysis is not appropriate for
this problem, particularly since joint moments are literally the
mathematical product of moment arms and SRF magnitudes. Here's my
basic model (all kinematic and kinetic analyses were limited to the
sagittal plane):

Speed Body mass
Speed -> SRF magnitudes
Speed -> SRF angle (i.e., 2D angle in the sagittal plane)
Speed -> joint angle
Body mass -> SRF magnitude
Body mass -> joint angle
Body mass -> relative limb length
SRF angle -> SRF joint moment arm
Joint angle -> SRF joint moment arm
Relative limb length -> SRF joint moment arm
SRF magnitude -> SRF joint moment
SRF joint moment arm -> SRF joint moment

Can anyone provide some insight on this problem? Is path analysis
really inappropriate? Is there a better way to construct the path
diagram to make the model amenable to path analysis?

thank you,

Jesse

______________________________________________
Jesse W. Young
Postdoctoral Fellow
Department of Anthropology
University of Texas at Austin
1 University Station C3200
Austin, TX 78712
Phone: (512) 232-6380

Email: jesse.young@mail.utexas.edu
URL: http://gibbon.anat.sunysb.edu/IDPAS/index.php?page=students/young