COMFORT AND INTERFACE FORCES IN

Paula Silva (1), Miguel Silva (2), Jorge Martins (2)

1. IPSetúbal-ESTSetúbal, Portugal; 2. IDMEC/IST, Inst. Sup. Técnico, Portugal

Introduction

The use of orthoses can lead to discomfort due to

the abnormal loading of skin in areas that are

unaccustomed to support forces. To promote the

comfort of the users a standard recommendation for

the design of these devices, is to lower these forces.

Different strategies can be applied: increase the

area of contact and lower the pressure or limit the

area of contact to prescribed locations. Although

pressure pain thresholds and indications for suitable

locations to apply force are available [Pons, 2008],

still it is essential to calculate the forces to infer

about comfort. The present methodology allows the

calculation of the interface forces, in this case

between the lower limb and an Ankle-Foot Orthosis

(AFO), using a two-dimensional computational

multibody dynamics model. The input of this model

encompasses the patient’s pathological data from a

lab gait. With the ability to calculate the interface

forces in the indicated locations, it is possible to

design the orthotic device to the required function

and comfort.

Methods

A multibody formulation with natural coordinates

is developed in MATLAB for the description and

simulation of the human and device integrated

model [Silva, 2010]. The system integrates a 2D

whole-body model of the human body described by

twelve rigid segments interconnected by

geometrically ideal revolute joints and a model of

the AFO described by two rigid segments

interconnected by a revolute joint. The contact and

friction forces generated at the human-device

interface are simulated using the Hunt and Crossley

continuous contact model with hysteretic damping

for the normal forces and the Coulomb and viscous

friction models for the tangential forces. A cloud of

80 contact points was applied on the boundary lines

of the lower limb using equally spaced spheres with

radius of 10 mm, as shown in Figure 1. These

points can be active or not depending on the design

of the device. Non-pathological gait motion data is

used as input to the model. The experimental data

was obtained from the gait lab by acquiring the

kinematic and kinetic data of a stride of a healthy

female subject. The kinematic data is used to

prescribe the whole-body movement and the kinetic

data to prescribe the ground reaction forces to be

applied to the foot and AFO. The interface forces

and the joint moments of the human model are

calculated, as a result of the simulation.

�3

�2

�1

0

1

2

3

0 20 40 60 80 100

Frictio

n�Forces��[N�]�

Stride��[%]

Figure 1: Cloud of contact points and plot of

friction forces for 10 points.

Results and Discussion

Figure 1 shows an example of the friction forces

developing in a set of 10 adjacent points on the

interface. Several shifts in direction can be spotted

clearly indicating possible sliding of the orthosis, a

typical source of discomfort. Similar results are

obtained for normal forces, with which pressure can

be calculated and compared with pressure pain

thresholds. Although not shown, the plots for

normal forces indicate strong variations in

behaviour for adjacent locations. They also point

out where these forces peak, and force

redistribution is needed. Restricting or enlarging the

possible contact regions alters significantly the

force distribution and may eliminate the peaks. This

in turn will allow a better design of the orthosis,

avoiding discomfort.

Generalizing the model for three dimensions and

applying optimization methods will increase the

accuracy of the model and allows applying the

model to other gait pathologies that occur in the

transverse and frontal plane.

Acknowledgements

The authors would like to thank FCT for the

support in the project DACHOR (MIT-Pt/BS-

HHMS/0042/2008) and the PhD grant SFRH/BD/

47662/2008 of the first author.

References

Pons JL, Wearable Robots, Wiley Blackwell, 2008.

Silva PC et al. Multibody System Dynamics,

24(3):367-388, 2010.

ANKLE–FOOT ORTHOTICS

Presentation 1364 − Topic 36. Orthotics and prosthetics S515

ESB2012: 18th Congress of the European Society of Biomechanics Journal of Biomechanics 45(S1)