Robert, 3D joint torques in Ingress-Egress motions

A 3-D dynamics analysis of driver's Ingress-Egress Motion T. Robert*†, J. Causse†‡, Lisa Denninger‡, Xuguang Wang† † LBMC, Université de Lyon, IFSTTAR – UCBL, F-69675 Bron ‡ PSA Peugeot-Citroën, Vélizy-Villacoublay, France

Abstract Ingress-egress (I/E) is a major ergonomic challenge for car manufacturers. I/E motions have been largely studied in kinematics, but very rarely in dynamics. However, the joint loads developed during car I/E motions, and in particular the motor torques (torques acting along the degrees of mobility of the joints), might be an important parameter to explain the difficulties perceived by drivers while getting in and out of cars. In order to fill this gap, we propose in this study: 1/ to estimate and describe the joint torques developed during ingress-egress motions 2/ to analyze the influence of the car geometry and subject anthropometry on the torques developed. An experiment was performed to record the kinematic and contact loads of subjects getting in and out a car mock-up. Three groups of subject, based on their anthropometry, and three vehicle configurations were tested. Net joint loads were then extracted using an inverse dynamics analysis. Motor torques were obtained by projecting these net joint torques along the degrees of mobility. Motor torques were stereotypical across subjects and conditions. Both ingress and egress motions were primarily characterized by large extension torques corresponding to the seating/rising phase. These torques are used to elevate (egress) or control the lowering (ingress) of the center of mass. They are similar to those observed in Sitto-Stand maneuvers. On overall, the taller a subject and the lower the seat of the vehicle is, the larger the peak torques are. Moreover, peak torques were higher for egress than ingress. These results could be related to previous studies showing that getting in and out is easier for minivans than for smaller cars, and that egress is more difficult than ingress. Keywords: Ingress-Egress, Joint torques, 3D Human motion analysis, Biomechanics, Digital Human Model.

1. Introduction Vehicle accessibility is a major ergonomic challenge for car manufacturer: vehicles should be designed so that the targeted drivers are able to comfortably get in and out of it. However, ingressegress (I/E) involves complex 3-D motions in a highly restrained environment dealing with equilibrium constraints. It also involves relatively high physical efforts. Not surprisingly I/E movements were identified as a particularly difficult task by 25% (ingress) to 33% (egress) of the elderly drivers (Herriotts, 2005). Many researches have been dedicated to describe this motion and the influence of parameters related to the subject or the vehicle's characteristics. Experimental studies focused on identifying the intrinsic (anthropometry, functional capacities…) and extrinsic (e.g. car geometry) parameters associated with I/E difficulties (Giacomin and Quattrocolo, 1997; James, 1985; Petzäll, 1995). However these studies did not quantify the motion in terms of biomechanical factors, and thus did not allow for a better understanding of possible causes of the difficulties encountered by the subjects. Very few studies combined biomechanical factors and discomfort (Andreoni et al., 2004; Causse et al.,

*Corresponding author. Email: [email protected]

2012). Moreover, they focused primarily on the roof height and neck/torso kinematics, and thus remained relatively limited. Complete and detailed descriptions of the 3-D kinematics have been proposed, notably by Chateauroux and Wang (2010) and Ait El Menceur et al. (2008) who identified different kinematic strategies used by volunteer subjects. Regarding the dynamics, a preliminary analysis of the left hip abduction for one subject can be found in in (Debril et al., 2007). Causse et al. (2009) proposed an analysis of the contact loads between subjects and car environment, and showed the feasibility of the joint loads estimation for these motions. However a complete description of the joint loads observed during car I/E motions is still missing, although it might be an important parameter to explain the difficulties encountered. In order to fill this gap, we propose in this study: 1/ to estimate and describe the joint torques developed during ingress-egress motions 2/ to analyze the influence of the car geometry and subject anthropometry on the torques developed.

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Robert, 3D joint torques in Ingress-Egress motions

2. Materials and Methods 2.1. Experiments Data of 15 young volunteers, out of the 26 who participated in this experiment, were analyzed in this study. They were free from any disorder and routinely driving cars. Subjects were spread in three groups of five according to their anthropometry: Short (S), Medium (M) or Tall (T) (see Table 1). Table 1: Characteristics of the three groups of subjects.

Group Stature (m) Weight (kg) Age S 1.59 51.0 28.4 M T

(1.57 – 1.61)

(41.6 – 56.8)

(26 - 32)

1.68

63.8

25.8

(1.63 – 1.75)

(53.7 – 80.3)

(23 - 29)

1.85

74.2

29.2

(1.81 – 1.87)

(66.2 – 82.0)

(21 - 36)

Sex 5F 2F–3 M

2.2. Data Processing 5M

An adjustable mock-up was used to represent the car (see Figure 1). It consists of a seat, a steering wheel, pedals, a door frame and a door kept open at 70°. The main dimensions were varied so that they corresponded to three types of car: a small car (VS), a medium-sized car (VM) and a minivan (VV). These three configurations are mainly differentiated by their seat height (see Table 2). Table 2: Main dimensions of the car configurations (mm)

VS

VM VV

Seat height above ground 470 550 700 Sill height above ground

360 360 420

Sill height above floor

130 100 70

Doorway width

900 850 850

Roof width from SW

220 220 250

Sill width from SW

470 460 450

The longitudinal position of the seat relative to the pedals and the longitudinal and vertical position of the steering wheel were adjusted by the subjects once at the beginning of the experiment. In addition, for each car configuration, subjects were asked to identify the first uncomfortable and last acceptable roof height and sill width. In this study we analyzed only the data sets for the roof and sill set at mid-distance between these extreme positions.

x

z

The mock-up was equipped with four 6-D forces sensors: two Bertec force plates embedded in the car floor under the steering wheel and in the ground next to the doorframe, a specific force plate located under the seat and a load sensor (Denton) behind the steering wheel (see Figure 1). In addition, subjects were equipped with 44 reflective markers, whose trajectories were recorded using a 10 cameras optoelectronic system (Vicon MX). The task consisted in getting in the car, adopting a driving posture for five seconds, getting out of the car and moving away from the door frame. Subjects were instructed not to use the door. Detailed experimental protocol can be found in Causse et al. (2012).

Ingress motions were defined between the instant when the right foot leaves the ground moving into the car and the instant where the left foot touches the floor. Egress motions were defined between the instant when the left foot leaves the floor and the instant when the right foot touches the ground (see Figure 3). Joint angles were estimated from marker trajectories using a global optimization procedure (Wang et al., 2005). An example of reconstructed motions is displayed in Figure 3. An inverse dynamics procedure was used to compute the net joint torques developed during these motions. The procedure is based on a Newton-Euler recursive approach using the homogeneous (4x4) matrices formalism (Doriot and Chèze, 2004). 3-D inertial parameters of the 16 rigid bodies considered were estimated from regressions (Dumas et al., 2007). According to Desroches et al. (2010), the motor torques were computed by projecting the Net Joint Torques orthogonally onto the 37 mobility axes of the model, i.e. the axes of the non-orthogonal Joint Coordinate Systems, defined according to ISB's standard (Wu et al. 2002). Lastly, motor torques were made unitless (normalized by Body Height × Body Weight, see Hof, 1996) and time normalized over 101 frames (0 to 100 % of the motion). Influence of subject's anthropometry and vehicle's dimension on particular peaks of joint torques (see section Results) was evaluated using an ANOVA with factors Stature ("S", "M", "T") and Vehicle ("VS", "VM", "VV"). Comparison of joint torques between ingress and egress was performed using a paired-sample t-test procedure on the peak extension torques in the left and right knees, right hip and lumbar joint, and the peak abduction torques in the right hip.

y

Figure 1: The adjustable car mock-up

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Robert, 3D joint torques in Ingress-Egress motions

3. Results 3.1. General overview All participants went in and out of the vehicle using the "one foot" strategy (Ait El Menceur et al., 2008; Chateauroux and Wang, 2010). In some cases, different sub-strategies were observed. E.g. three subjects from the "Medium" group got in the car using the backward motion sub-strategy while the two others used the lateral sliding sub-strategy (Ait El Menceur et al., 2008; see Figure 2). However, there is a continuous transition between these substrategies, and it is difficult to distinguish them. In addition, the observed motions and joint torque profiles are, on overall, relatively stereotypical. Consequently we chose in this study to present in this study the results per group of subject and type of vehicle, regardless of sub-strategies. 3.2. Description of the joint torque developed during the Ingress motions Figure 4 provides an example of the torques profiles (mean and standard deviation across subjects) developed by subjects of the "Medium" group while entering the VM vehicle configuration. Three main phases, of about equal duration, can be identified. At first, subjects entered their right foot into the car. It implied flexion and adduction torques in the right hip. Then, during a period which is about the second third of the motion, subjects translated and lowered their pelvis into the seat and moved their torso inside. This phase was mainly characterized by large extension torques in the lower limb joints, to control the lowering of the pelvis. To avoid the collision with the roof, the torso had to be maintained bent laterally and flexed, leading to extension and rightward bending torques in the lumbar joint. Note that the bending torques appeared during the first phase, i.e. while the right foot was being placed into the car. One can notice the large inter-subject variability in the left hip internal rotation torques during this phase. It can be explained by the fact that the subjects of this group used two different sub-strategies to move into the car (see Fig. 2). The third phase consisted of moving the left leg into the car, i.e. moving it above the sill. It is characterized by left hip flexion torque and a leftward bending torque of the lumbar joint. This later tended to tilt the pelvis and to help moving the left leg upward.

Figure 2: two sub-strategies used by subjects of the "medium" group to move into the car. Left: backward motion sub-strategy, implies almost no internal rotation torque in the left hip. Right: lateral sliding sub-strategy, implies internal rotation torque in the left hip.

3.3. Description of the joint torque developed during the Egress motions The joint torques (mean and standard deviation across subjects) developed by subjects of the "Medium" group while exiting the VM vehicle configuration are displayed in Figure 5. According to the joint torque profiles, egress motion can be divided in two main phases of about equal duration. The beginning of the motion was characterized by the transport of the left leg outside the vehicle and the rotation/transfer of the body to the left. In terms of joint torques it implied: 1) flexion torque in the left hip for moving the left foot above the sill; 2) right ankle abduction, right hip external rotation and abduction torques, corresponding to the body rotation and transfer to the left by the right leg. The second phase was characterized by high extension peaks: left ankle plantarflexion, and extension of the left and right knee left hip and lumbar joint, used to elevate the Center of Mass. They were similar to those observed in Sit-to-Stand motions (Bahrami et al., 2000). Extension torques in the right knee tended to transfer the body to the left and facilitate the exit. Right bending lumbar torques corresponded to the straightening up of the trunk, bent to the left during the first phase in order to pass under the roof. In their kinematic description of egress motions, Chateauroux and Wang (2010) identified a third phase corresponding to the motion of the right foot over the sill. However it could not be identified from joint torque curves.

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Robert, 3D joint torques in Ingress-Egress motions

Ingress Motion

Start

25 %

50 % Egress Motion

75 %

End

Start

25 %

50 %

75 %

End

Figure 3: Example of reconstructed Ingress and Egress motions

Right Knee Flexion

Right Hip Flexion

Right Hip Abduction

Right Hip Internal Rotation

Left Knee Flexion

Left Hip Flexion

Left Hip internal rotation

Lumbar Flexion

Lumbar Lateral Bending

Figure 4: Motor torques developped during the ingress motion: Net joint torques (from proximal to distal segment) are projected on mobility axes (axes of the JCS) for the "medium" group of subjects and the VM vehicule configuration ("medium-sized car"), averaged (thick line) ± one standard deviation (shaded area) across subjects. Torques are unitless (normalized by Body Height × Body Weight). For clarity, only the most remarkable torque profiles are plotted.

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Robert, 3D joint torques in Ingress-Egress motions

Left Ankle Flexion

Left Knee Flexion

Left Hip Flexion

Lumbar Flexion

Lumbar Lateral Bending

Right Ankle Abduction

Right Knee Flexion

Right Hip Axial Rotation

Right Hip Abduction

Figure 5: Motor torques developped during the egress motion: Net joint torques (from proximal to distal segment) are projected on mobility axes (axes of the JCS) for the "medium" group of subjects and the VM vehicule configuration ("medium-sized car"), averaged (thick line) ± one standard deviation (shaded area) across subjects. Torques are unitless (normalized by Body Height × Body Weight). For clarity, only the most remarkable torque profiles are plotted.

Table 3: Effect of factors Stature and Vehicle on the peak torques for ingress motions

Phase Joint 1 R Hip 1-2

Torques Flex Abd Lumbar R Bend

2

L Knee L Hip R Knee R Hip Lumbar

3

L Hip Lumbar

Sig. Para. Ø* Stature Vehicle Stature R Ax Rot. Stature Ext Vehicle Stature Ext. Stature Int. Rot. Vehicle Ext Vehicle Stature Ext Ø Int. Rot Vehicle Ext. Vehicle Stature Flex Ø L. Bend. Ø

Groups Ø (S, T) < M VS < VV S< (M, T) S
Table 4: Effect of the Stature and Vehicle on the peak torques for egress motions

Phase Joint 1 RHip 2

LAnkle LKnee LHip Lumbar

RAnkle RKnee RHip

Torques Sig. Para. Groups Ext. Rot. Vehicle VS < (VV) Stature T < (S, M) Dflex Vehicle VV < (VS,VM) Ext Vehicle VV < VM < VS Stature S < (M, T) Ext. Vehicle VV < VS Stature S < T Ext. Vehicle VV < (VS,VM) Stature S < M < T R. Bend Ø Ø Add Vehicle VV < VS Ext. Vehicle VV < (VS,VM) Int. Rot. Vehicle VV < (VS,VM) Stature (S, M) < T

*: Ø indicates the absence of significant effect

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Robert, 3D joint torques in Ingress-Egress motions

3.4. Influence of the stature and vehicle Table 3 and 4 summarize the ANOVA results with factors Stature ("S", "M", "T") and Vehicle ("VS", "VM", "VV") performed on peak torques for ingress and egress motion respectively. Influence of these two factors was mainly observed during the second phase of the motions (lowering of the pelvis for the ingress, rising up for the egress). During this phase, larger torques were observed for smaller vehicles configuration and taller subjects. A notable exception is the influence of Stature on the right knee extension torque for ingress motion: taller subjects developed smaller torques than other groups. It could be explained by the fact that taller subjects used more frequently and more intensively the steering wheel during this lowering phase (Causse, 2011), thus reducing the torques in the right lower limb. Influence of Stature and Vehicle during the other phases was smaller. For ingress, right hip abduction torques during the first phase (entering the right foot) were higher for the medium group of subjects. It could be explained by the fact that subjects of this group tended to slide laterally their right foot into the vehicle. Still during the first phase of ingress, lumbar right bending torque was lower for smaller vehicles (VS
descriptions of the I/E motions (Chateauroux and Wang, 2010). Although subjects used different sub-strategies, the resulting joint torques for a given configuration (group of subjects and vehicle) were, on overall, relatively stereotypical. Both ingress and egress motions were primarily characterized by large extension torques corresponding to the seating/rising phase. On the opposite, joint torques during the initial phase (moving the right foot inside / the left foot outside) and terminal phase (moving in the right foot / moving out the left foot) were specific to ingress or egress. 4.2. Effects of Vehicle and Stature During the rising/seating phase, the car geometry had systematically the same effect on the peak torques: VV configuration induced lower torques than VM and VS. This decrease in extension torques may be directly linked to the increase of the seat height, as already observed in Sit-to-Stand motions for example (Su et al., 1998). A similar effect was found for the Stature: taller subjects tended to develop large torques. Due to torque normalization, Stature effects cannot be reasonably explained by differences in body mass or segment lengths between groups of subject. They rather highlight the interaction of subjects with different anthropometries and vehicles with fixed dimensions. The same seat height is relatively lower for taller subjects than for smaller ones. Results for the other phases are more complex. In particular, subjects may adapt their motion (use a different sub-strategy) when the constraints (e.g. the roof clearance) become too strong, thus leading to lowering the torques developed instead of increasing them. For example, subject entering smaller vehicles, with smaller roof clearance, developed smaller lumbar bending torques. A kinematic analysis shows that, for smaller vehicles, subjects tend to rotate before entering the vehicle, thus flexing more the trunk than bending it. 4.3. Link with ergonomic evaluation Evaluation of the difficulties encountered by the subjects during ingress/egress tasks was not directly the topic of this study. Nevertheless, this study focuses on the motor torques developed by the subject. These are linked to the efforts to be produced during the motion, and, as such, to the difficulty to perform it. Consequently we can discuss the results of this study in regards of known results about ingress/egress discomfort. In particular, we found that car configurations with a higher seat induce lower joint torques. It can be related to previous studies showing that getting in and out is easier for minivans than for smaller cars (Causse, 2011). Similarly, we showed that peak joint torques developed during egress were larger than during ingress. It matches results from Herriotts

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Robert, 3D joint torques in Ingress-Egress motions

(2005) showing that egress is found more difficult than ingress, in particular for older drivers. However, joint torques by themselves cannot explain all the difficulties encountered by the subjects. As an example, moving the left foot in (the right foot out) the vehicle during ingress (egress) was specifically mentioned as a difficulty by Herriotts (2005). However, the corresponding motor torques observed in this study remained relatively small. An explanation could be that the force production capacity (the maximal motor torque that one can produce) depends on the posture. Thus relative motor torques (motor torques normalized by their maximum values) may be a more appropriate candidate than the motor torques to explain the perceive discomfort. Another plausible explanation could be that the causes of the discomfort are not the effort to produce, but rather the proximity to joint limits for example (Cruse et al., 1990; Kee and Karwowski, 2001). 5. Conclusion This study proposes the first description of the joint torques developed during ingress-egress motions. We particularly focused on the motor torques, i.e. the net joint torques projected on the mobility axes. They were stereotypical across subjects and conditions. Both ingress and egress motions were primarily characterized by large extension torques corresponding to the seating/rising phase. These torques are used to elevate (egress) or control the lowering (ingress) of the center of mass. They are similar to those observed in Sit-to-Stand maneuvers. In this study we also analyze the influence of the car geometry and subject anthropometry on the torques developed. On overall, the taller the subject and the smaller the vehicle, the larger the peak torques. Moreover, peak torques were higher for egress than ingress. These results could be related to previous studies showing that getting in and out is easier for minivans than for smaller cars, and that egress is more difficult than ingress. References Ait El Menceur, M. O., Pudlo, P., Gorce, P., Thevenon, A., Lepoutre, F.-X., 2008. Alternative movement identification in the automobile ingress and egress for young and elderly population with or without prostheses. International journal of industrial ergonomics 39 (11-12), 1078–1087. Andreoni, G., Rabuffetti, M., Pedotti, A., 2004. Kinematics of head-trunk movements while entering and exiting a car. Ergonomics 47 (3), 343– 359. Bahrami, F., Riener, R., Jabedar-Maralani, P., Schmidt, G., 2000. Biomechanical analysis of sitto-stand transfer in healthy and paraplegic subjects. Clinical Biomechanics 15 (2), 123 – 133.

Causse, J., Chateauroux, E., Monnier, G., Wang, X., Denninger, L., 2009. Dynamic analysis of car ingress/egress movement: an experimental protocol and preliminary results. SAE International Journal of Passenger Cars-Mechanical Systems 2 (1), 1633–1640. Causse, J., 2011. Analyse cinématique et dynamique du mouvement d’accessibilité à une automobile (kinematic and dynamic analysis of car accessibility movements). Ph.D. thesis, University Claude Bernard Lyon1. Causse, J., Wang, X., Denninger, L., 2012. An experimental investigation on the requirement of roof height and sill width for car ingress and egress. Ergonomics 55, 1596–1611. Chateauroux, E., Wang, X., 2010. Car egress analysis of younger and older drivers for motion simulation. Applied Ergonomics 42 (1), 169 – 177. Cruse, H., Wischmeyer, E., Brüwer, M., Brockfeld, P., Dress, A., 1990. On the cost functions for the control of the human arm movement. Biological Cybernetics 62, 519–528. Debril, J.-F., Pudlo, P., Ait El Menceur, M., Gorce, P., Lepoutre, F., 2007. Human articulation efforts estimation in the automobile vehicle accessibility movement - a pilot study. In: Duffy, V. (Ed.), Digital Human Modeling. Vol. 4561 of Lecture Notes in Computer Science. Springer Berlin / Heidelberg, pp. 23–32. Desroches, G., Cheze, L., Dumas, R., 2010. Expression of joint moment in the joint coordinate system. Journal of Biomechanical Engineering 132 (11), 114503. Doriot, N., Chèze, L., 2004. A three-dimensional kinematic and dynamic study of the lower limb during the stance phase of gait using an homogeneous matrix approach. IEEE Transactions on Biomedical Engineering 51 (1), 21–27. Dumas, R., Cheze, L., Verriest, J.-P., 2007. Adjustments to mcconville et al. and young et al. body segment inertial parameters. Journal of Biomechanics 40, 543–553. Giacomin, J., Quattrocolo, S., 1997. An analysis of human comfort when entering and exiting the rear seat of an automobile. Applied Ergonomics 28, 397–406. Herriotts, P., 2005. Identification of vehicle design requirements for older drivers. Applied Ergonomics 36 (3), 255–262. Hof, A. L., 1996. Scaling gait data to body size. Gait & Posture 4 (3), 222 – 223. James, J., 1985. Problems experienced by disabled and elderly people entering and leaving cars. Institute for Consumer Ergonomics and Transport and Road Research Laboratory. Kee, D., Karwowski, W., 2001. The boundaries for joint angles of isocomfort for sitting and standing

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males based on perceived comfort of static joint postures. Ergonomics 44 (6), 614–648. Petzäll, J., 1995. The design of entrances of taxis for elderly and disabled passengers: an experimental study. Applied Ergonomics 26, 343– 352. Su, F. C., Lai, K. A., Hong, W. H., 1998. Rising from chair after total knee arthroplasty. Clinical Biomechanics 13 (3), 176–181. Wang, X., Chevalot, N., Monnier, G., Ausejo, S., Suescun, A., Celigueta, J., 2005. Validation of a model-based motion reconstruction method developed in the realman project. SAE Transactions Journal of Passenger Cars - Electronic and Electrical Systems 114, 873–879. Wu, G., Siegler, S., Allard, P., Kirtley, C., Leardini, A., Rosenbaum, D., Whittle, M., D’Lima, D. D., Cristofolini, L., Witte, H., Schmid, O., Stokes, I., 2002. ISB recommendation on definitions of joint coordinate system of various joints for the reporting of human joint motion - part 1: ankle, hip, and spine. Journal of Biomechanics 35, 543–548.

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