Static Biomechanical Modeling in Manual Lifting - Fundamentals and Assessment Tools for Occupational Ergonomics - page 932

where V 1 is the forearm length with link unit vectors, V 1X , V 1Y , V 1Z are, respectively, the X, Y, Z com-
ponents of the forearm unit vector, and LeLink is the magnitude of forearm length from anthropometric
data.

V 1cg ¼ (V 1X i þ V 1Y j þ V 1Z k) (cglink)

where V 1cg is the cg distance from elbow to forearm center of gravity vector expressed in unit vector form,
multiplied by the cglink, which is the magnitude of proximal distance to cg of forearm from anthropo-
metric data.

M 1 ¼ (M 1X i þ M 1Y j þ M 1Z k)

where M 1 is the elbow resultant moment with X, Y, Z unit vector components, and M 1X , M 1Y , M 1Z , are,
respectively, the elbow moment about X, Y, Z axes and

M 1 ¼ (V 1 F RHAND þ V 1cg W 1 )

where W 1 ¼ 0i þ 0j W 1Z k, (which is the forearm weight Vector), and

R 1 ¼ (R 1X i þ R 1Y j þ R 1Z k)

where R 1 is the elbow joint reaction force vector with X, Y, Z unit vector components, and

M 2 ¼ M 1 þ V 2cg W 2 þ V 2 ( R 1 )

where M 2 is the right shoul der resultant moment with V 2cg the upperarm center of gravity vector, V 2 the
upperarm link vector, and W 2 the upperarm weight vector.

M 2 ¼ M 2X i þ M 2Y j þ M 2Z k

where M 2X is the right shoulder movement with X, Y, Z, unit vector components, M 2X , M 2Y , and M 2Z ,
are respectively, the shoulder moment about the X, Y, and Z axes.
A recursive computational procedure is used to continue the analysis to compute external load
moments and forces at the elbow and shoulder of the arm or arms doing the exertion, the lumbosacral
joint, hip joints, and knee and ankle joints.
The size and mass of the person (linkage size) is most often specified as a select stratum of the popu-
lation (i.e., a percentile of specific anthropometric dimensions is selected from population surveys).
Thus, a small, medium, or large man or woman can be specified, or specific link anthropometry can
be used if available. Link length-to-stature ratios from Drillis and Contini (1966) and link mass-to-
bodyweight ratios from Dempster (1955) and Clauser et al. (1969) are used to simplify this procedure,
if specific anthropometry is not available on a subject. Most often an average male or female anthropo-
metry is chosen for assessing the strength requirements of a given task in industry.
The strength moment values used as population limit values in the program were measured by Stobbe
(1982) for 25 men and 22 women employed in manual jobs in three different industries. These values
have been combined with the earlier values from Chaffin and Baker (1970) and Schanne (1972) to
form the statistical data for the population joint moment limits.
Once the size of the person has been specified or selected from a known anthropometric data source,
the posture is entered with reference to either photographs or videos (or by manipulating a computer
generated hominoid) and then the hand forces of interest are entered. The program then computes
the load moments at each joint of the linkage, and compares each to the corresponding strength