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In a recent posting, H. Hatze and A.Baca suggested a "discussion forum on
the accuracy required in determining human body segment parameters"
(BSP's). I would like to address this issue in the context of using the
Inverse Dynamic Approach (IDA) to calculate joint moments. (In this
approach, kinematic, Ground Reaction Forces (GRF's) and BSP data are all
utilized.) The question that has interested me (and others) for some time
is, "How important are uncertainties in each type of data set--e.g., does
an error of 1% in GRF data degrade the accuracy of hip moments as much as
say a 10% error in BSP data?
If one uses an approach described by Cappozzo, Leo and Pedotti (Equation
17 in their article in J. Biomechanics, v.8, 307-320, 1975), one can assess
the contribution of different errors on the overall uncertainty of a
calculated value for a joint moment. I have used the sagittal plane data
described in Winter's 1979 book on gait and, as a starting point, assumed
the following uncertainties;
1. BSP and acceleration data: 10% of nominal values
2. GRF data: 1% of nominal value (based on forceplate specifications)
3. Position of resultant GRF vector: 6.3 mm (based on Bobbert and
Schamhardt's article in J. Biomechanics, v23, 7, 705-710, 1990)
4. Coordinates of joints: 10mm
The results of this exercise showed that uncertainties in BSP and
acceleration data had little effect on the overall uncertainty of the hip
moment during the stance phase of gait, accounting for less than 2% of the
total uncertainty. Errors in Center of Pressure (CoP) location, and joint
axes had much greater ramifications, accounting for over 64% of the
uncertainty. Percentage-wise, the contributions of BSP and
accelaration errors became greater during swing (accounting for up to 50%
of the average uncertainty), but this must be seen in light of the fact
that moments during swing are extremely small. (The only people who
placed great importance on the moments during the swing phase were Braune
and Fischer who at the turn of the century were testing the theory of the
Weber brothers that the leg acted as a pendulum!)
The above statements are directly in line with the findings of Capozzo, Leo
and Pedotti who stated "Errors in determining the position of the
structure, with respect to the vector of ground reactions, are major
determinants of the errors on the muscular moments".
In summary, one could make a 50% error in one's estimates of BSP
parameters and hardly effect the accuracy of calculated values for joint
moments. So, in the context of human gait, it makes little sense in
striving to get perfect BSP data. It makes more sense to obtain reliable,
accurate data for joint centers of rotation and GRF data.
Regards,
Brian L. Davis, Ph.D.
Dept. Biomedical Engineering (Wb3)
Cleveland Clinic Foundation
9500 Euclid Avenue
Cleveland, Ohio 44195, U.S.A
E-Mail: davis@bme.ri.ccf.org
Ph: (216) 444-1055 (Work)
Fax
216) 444-9198 (Work)
the accuracy required in determining human body segment parameters"
(BSP's). I would like to address this issue in the context of using the
Inverse Dynamic Approach (IDA) to calculate joint moments. (In this
approach, kinematic, Ground Reaction Forces (GRF's) and BSP data are all
utilized.) The question that has interested me (and others) for some time
is, "How important are uncertainties in each type of data set--e.g., does
an error of 1% in GRF data degrade the accuracy of hip moments as much as
say a 10% error in BSP data?
If one uses an approach described by Cappozzo, Leo and Pedotti (Equation
17 in their article in J. Biomechanics, v.8, 307-320, 1975), one can assess
the contribution of different errors on the overall uncertainty of a
calculated value for a joint moment. I have used the sagittal plane data
described in Winter's 1979 book on gait and, as a starting point, assumed
the following uncertainties;
1. BSP and acceleration data: 10% of nominal values
2. GRF data: 1% of nominal value (based on forceplate specifications)
3. Position of resultant GRF vector: 6.3 mm (based on Bobbert and
Schamhardt's article in J. Biomechanics, v23, 7, 705-710, 1990)
4. Coordinates of joints: 10mm
The results of this exercise showed that uncertainties in BSP and
acceleration data had little effect on the overall uncertainty of the hip
moment during the stance phase of gait, accounting for less than 2% of the
total uncertainty. Errors in Center of Pressure (CoP) location, and joint
axes had much greater ramifications, accounting for over 64% of the
uncertainty. Percentage-wise, the contributions of BSP and
accelaration errors became greater during swing (accounting for up to 50%
of the average uncertainty), but this must be seen in light of the fact
that moments during swing are extremely small. (The only people who
placed great importance on the moments during the swing phase were Braune
and Fischer who at the turn of the century were testing the theory of the
Weber brothers that the leg acted as a pendulum!)
The above statements are directly in line with the findings of Capozzo, Leo
and Pedotti who stated "Errors in determining the position of the
structure, with respect to the vector of ground reactions, are major
determinants of the errors on the muscular moments".
In summary, one could make a 50% error in one's estimates of BSP
parameters and hardly effect the accuracy of calculated values for joint
moments. So, in the context of human gait, it makes little sense in
striving to get perfect BSP data. It makes more sense to obtain reliable,
accurate data for joint centers of rotation and GRF data.
Regards,
Brian L. Davis, Ph.D.
Dept. Biomedical Engineering (Wb3)
Cleveland Clinic Foundation
9500 Euclid Avenue
Cleveland, Ohio 44195, U.S.A
E-Mail: davis@bme.ri.ccf.org
Ph: (216) 444-1055 (Work)
Fax
216) 444-9198 (Work)