Zvi Ladin

06-01-1994, 06:26 AM

In recent postings Brian Davis and Paul Devita discussed the role of

the inertial parameters in the determination of joint moments, suggesting

that even large errors in the inertial parameters will lead to relatively

small errors in the calculated moments. I would like to address two

issues that are related to this discussion - the overall magnitude of

the inertial loads (i.e. forces and moments that arise from the motion

of the body segments), and the role of the inertial parameters in the

determination of the inertial loads.

In a study that Ge Wu (who is now at Penn State) and I conducted a few

years ago, we used the kinematometers (devices that combine position

markers, linear accelerometers and angular velocity sensors) to monitor the

kinematics of the lower limb during physical activities ranging from

slow walking to jumping. We then compared the magnitudes of inertial

and static components of the joint loads (forces and moments). The

results were presented last year, during the Second International

Symposium on 3-D Analysis of Human Movement (Poitiers, France), and

in summary we found that the inertial effects are largest in the

transverse plane and that they increase in magnitude in the proximal

direction and with an increase in the speed of the activity.

We compared the ratios of the maximum inertial forces to the maximum

static forces (i.e. the forces needed to maintain equilibrium).

In SLOW WALKING the maximum inertial forces for the ankle were on the

order of 3-6% (of the maximum static forces) in the vertical direction,

16% in A-P direction and 60-80% in the M-L direction.

The corresponding values for the hip were 20% in the vertical direction,

100-110% in the A-P direction and 200-300% in the M-L direction.

As the speed of the activity increased, we saw the expected increase in

the inertial effects, leading to inertial forces that were either HIGHER

than or similar to the static components in the A-P and M-L directions,

and are on the order of 10-20% in the vertical direction. These results

would be linearly affected by errors in the segmental masses.

The corresponding values for the moments were generally smaller, however

even there we saw values that were on the order of 20-40% in the medial

direction for running.

As both Devita and Davis suggest, the results of the moments will

be very sensitive to errors in the locations of the joints, however,

since the accepted practice of the inverse dynamic solution of the moment

equations invovles conducting the calculations in a Body Coordinate

System centered at the segmental center of mass, any errors in the location

of the COM would have just as grave consequences as errors in the locations

of the joint centers (since the equations contain terms that are the radii

vectors from the segmental COM to the joint centers).

References:

Ge Wu and Zvi Ladin. The effect of inertial load pm human joint force

and moment during locomotion. Proceedings of the Second International

Symposium on Three-Dimensional Analysis of Human Movement, 6/30-7/3/1993,

Poitiets, France, pp 106-107.

Zvi Ladin

Biomedical Engineering Department

Boston University

e-mail: ZL@buenga.bu.edu

the inertial parameters in the determination of joint moments, suggesting

that even large errors in the inertial parameters will lead to relatively

small errors in the calculated moments. I would like to address two

issues that are related to this discussion - the overall magnitude of

the inertial loads (i.e. forces and moments that arise from the motion

of the body segments), and the role of the inertial parameters in the

determination of the inertial loads.

In a study that Ge Wu (who is now at Penn State) and I conducted a few

years ago, we used the kinematometers (devices that combine position

markers, linear accelerometers and angular velocity sensors) to monitor the

kinematics of the lower limb during physical activities ranging from

slow walking to jumping. We then compared the magnitudes of inertial

and static components of the joint loads (forces and moments). The

results were presented last year, during the Second International

Symposium on 3-D Analysis of Human Movement (Poitiers, France), and

in summary we found that the inertial effects are largest in the

transverse plane and that they increase in magnitude in the proximal

direction and with an increase in the speed of the activity.

We compared the ratios of the maximum inertial forces to the maximum

static forces (i.e. the forces needed to maintain equilibrium).

In SLOW WALKING the maximum inertial forces for the ankle were on the

order of 3-6% (of the maximum static forces) in the vertical direction,

16% in A-P direction and 60-80% in the M-L direction.

The corresponding values for the hip were 20% in the vertical direction,

100-110% in the A-P direction and 200-300% in the M-L direction.

As the speed of the activity increased, we saw the expected increase in

the inertial effects, leading to inertial forces that were either HIGHER

than or similar to the static components in the A-P and M-L directions,

and are on the order of 10-20% in the vertical direction. These results

would be linearly affected by errors in the segmental masses.

The corresponding values for the moments were generally smaller, however

even there we saw values that were on the order of 20-40% in the medial

direction for running.

As both Devita and Davis suggest, the results of the moments will

be very sensitive to errors in the locations of the joints, however,

since the accepted practice of the inverse dynamic solution of the moment

equations invovles conducting the calculations in a Body Coordinate

System centered at the segmental center of mass, any errors in the location

of the COM would have just as grave consequences as errors in the locations

of the joint centers (since the equations contain terms that are the radii

vectors from the segmental COM to the joint centers).

References:

Ge Wu and Zvi Ladin. The effect of inertial load pm human joint force

and moment during locomotion. Proceedings of the Second International

Symposium on Three-Dimensional Analysis of Human Movement, 6/30-7/3/1993,

Poitiets, France, pp 106-107.

Zvi Ladin

Biomedical Engineering Department

Boston University

e-mail: ZL@buenga.bu.edu