clinical bio mechanics

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Biomechanical analysis of sit-to-stand transfer in healthy andparaplegic subjects

Fariba Bahrami a,*, Robert Riener b, Parviz Jabedar-Maralani a, Gunther Schmidt b

a Department of Electrical and Computer Engineering, Faculty of Engineering, Building No. 2, North Kargar Avenue, Tehran University, Tehran 14399,

Iranb Institute of Automatic Control Engineering, Technical University of Munich, Munich, Germany

Received 12 March 1998; accepted 18 May 1999

Abstract

Objective. An experimental study of the sit-to-stand transfer in healthy adults with/without arm-support and in paraplegic pa-

tients with/without electrical stimulation of the quadriceps muscles was performed. The study was aimed to compare the joint

torques, momentum transfer hypothesis, and stability of the sit-to-stand transfer in the healthy and paraplegic subjects.

Methods. A planar 3-linkage rigid body model was used to compute the body-segmental linear momentum and the reaction

forces and torques at the joints from measured data.

Results. In healthy subjects the arm-support enlarged the support base of the body and thus, increased the postural stability.

Strong arm-assistance reduced the maximum hip and knee joint torques by more than 50%. It was observed that the healthy

participants rising with arm-support used momentum transfer to facilitate the transition from sitting to standing. The paraplegic

participants did not apply the momentum transfer strategy and the sit-to-stand transfer was accomplished in a quasi-static manner.

Stimulating the quadriceps, the legs could participate partly in the movement dynamics.

Conclusion. Our results indicate that some signicant dierences exist between the maneuver applied by the paraplegic patients to

stand up and the strategies used by the healthy adults rising with arm-support.

RelevanceAnalysis of the biomechanical factors underlying the sit-to-stand activity is essential in the design of competent closed-loop

neuroprosthesis controllers which assist paraplegic patients during rising. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Biomechanics; Standing up; Spinal cord injury; Functional electrical stimulation

1. Introduction

Sit-to-Stand (STS) transfer is one of the most com-

mon daily movements, which is also a pre-requisite for

many other activities. STS transfer is mechanically a

demanding task [1]. It requires adequate torques bedeveloped at each joint, while spatial and temporal

motion of the body segments are coordinated. It has

been demonstrated that persons with spinal cord injuries

can re-obtain the capability of standing up [2,3], stan-

ding [35] and walking [6] by means of Functional

Electrical Stimulation (FES) of the lower extremity

muscles. Although FES is a useful and feasible method,

there are still many problems which must be solved to

enhance the eciency of the FES-controllers [3,7]. For

example, in most FES systems the voluntary movement

of the upper body (intact limbs) causes external forces

and torques disturbing the FES-induced motion of the

paralyzed lower extremities. To enhance the perfor-mance of the FES-controllers, it has been proposed to

integrate the eect of the voluntary movement of the

trunk and arm-support in the controller [810]. Biome-

chanical analysis of the factors which describe the ability

or disability of a given movement (e.g., standing up) in

dierent healthy and paraplegic subjects, will lead us to

nd an eective solution for the last posed problem.

Many previous studies have analysed dierent fun-

damental aspects and biomechanical factors of the STS

movement. Coghlin and MacFadyen [11] have reported

that two dierent strategies were used by their test

Clinical Biomechanics 15 (2000) 123133

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* Corresponding author.

E-mail address: [email protected] (F. Bahrami).

0268-0033/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 8 - 0 0 3 3 ( 9 9 ) 0 0 0 4 4 - 3


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groups to stand up without arm-support. They called

these schemes knee and hip-trunk strategies. In the hip-

trunk strategy the hip and ankle joint torques are greater

than the corresponding joint torques in the knee strat-

egy. Thus, Doorenbosch et al. [12] have suggested that,

the persons with muscle weakness of the knee extensors

should use additional compensatory mechanisms, like

the arm-assistance. Applying a semi-dynamic calcula-

tion, Arborelius et al. [13] have shown that the arm-

support during standing up reduces the mean maximum

hip joint torque by 50%. Schultz et al. [14] have com-

pared the joint torque requirements at the seat-o in-

stant with the maximum voluntary joint strengths.

Whence, they have inferred that except for the people

who have some disabilities in their lower extremities,

joint torque requirements may not be the only factor

limiting the ability to lift o. Riley et al. [1] have eval-

uated two possible strategies in healthy adults during

standing up without arm-support. Their results indicate

that, the upper body exion before leaving the seat iseither to garantee the postural balance just after seat-o

(strategy I) or it is to generate a suitable horizontal

linear momentum (HLM). The HLM is then trans-

formed to the vertical linear momentum (VLM) to fa-

cilitate the dynamic transition from sitting to standing

(strategy II). In another study Riley et al. [15] have

shown that the sit-back failures occur by insucient

momentum and torque generation, and justied the role

of momentum control in a stable STS transfer without

arm-support.

However, to the best of our knowledge, few com-

prehensive analysis of the arm-supported STS transfer

in healthy adults has been accomplished. In addition,

there are few works on the voluntary functions of the

trunk and arms during STS transfer in paraplegic pa-

tients [10,16]. Given the existing gaps, this study has

been conducted with the following purposes in mind:

(a) Accomplishing a dynamic analysis ofarm-support-

ed chair rise in healthy adults in order:

(a.1) to evaluate the stability of arm-supported

standing up;

(a.2) to investigate the momentum control hypoth-

esis; and

(a.3) to study the eect of arm-support in the joint

torque distributions during STS transfer.(b) Investigating the function of the intact limbs in

the paraplegic patients during STS transfer without/

with FES and to nd out if the paraplegic subjects ap-

ply also the momentum control strategy to stand up.

According to Riley et al. [1,15,17] the displacements

of the Center of Pressure (CoP) under the support base

of the feet and the body Center of Mass (CoM) relative

to the ankle may be used as criteria for dynamic (pos-

tural stability) and static stability (postural balance) of

the movement, respectively. Thus, the time history of the

spatial displacement of the body CoM, the CoP under

the feet, and the LMs will be used to analyze the stability

of the movement. The horizontal and vertical LMs and

the kinetic energy will clarify the momentum transfer

hypothesis. Further, the vertical ground reaction forces,

the joint torques, and the forces at the shoulder will

explain how the healthy and paraplegic participants

distributed the movement loads between their hands

(shoulders) and legs during STS transfer.

2. Methods

2.1. Subjects

The experimental data were collected from 10 healthy

participants and two paraplegic patients with lesions at

T8 level. Informed consent was obtained for each sub-

ject prior to data collection. The age, sex, body weight

and height, and segmental lengths of all subjects are

listed in Table 1. The mean and standard deviationvalues of the anthropometric data for healthy partici-

pants are also given in the same table. Shank length was

measured from the medial malleolus to the visually de-

termined approximate knee center of rotation; thigh

length was measured from the knee center of rotation to

the great trochanter, and the length of the trunk was

measured from femur to the center of rotation of the

shoulder, to which the lengths of neck and head were

later added to obtain the length of the upper body.

Masses, mass centers and inertia tensors for each seg-

ment were calculated from regression equations [18],

and scaled for each subject to her/his body weight (BW)and body height (BH).

2.2. Protocols

All subjects were asked to sit comfortably in a self-

selected body state on a chair without back and arm-

rests, while keeping their back upright. Both feet were

placed symmetrically and parallel to each other on a

forceplate. To stand up with arm-support, all partici-

pants have used two xed bars located in front of them.

Locating the bars in front of the healthy participants,

enlarged considerably the support base of the body. In

our measurements Mean (Xwrist Xankle) was 27.97 cm(SD, 4.5 cm). For the paraplegic participants to be able

to rise, the seat had to be located almost between two

bars such that Mean (Xwrist Xankle) 4.8 cm (SD, 4.1cm). The height of the chair and bars as well as the

distance between the chair and bars have been adjusted

for each subject separately so that the subjects could rise

from the chair in a natural and comfortable manner

while the feet were completely kept on the force plate

and the hands were holding the bars (Fig. 1). It was

encountered that the average preferred height of the

chair and bars were 99.4% of subject's shank length (SD,

124 F. Bahrami et al. / Clinical Biomechanics 15 (2000) 123133


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7.6% SL) and 50.58% of the subjects' BH (SD, 2.02%

BH), respectively.

The healthy subjects were asked to stand up in 3 dif-

ferent manners and then remain motionless: (a) 2 times

with crossed arms on the chest, (b) 5 times with normal

assistance of the arms, and (c) 5 times with strong arm-

support. In all cases the subjects were instructed to rise at

their own natural speed and the way they felt would be

the most usual manner for that situation. In case (a) the

arms should not move. In case (b) the function of arms

was principally to support the stability of the movement.

In case (c) the relative distance between chair and bars

for each subject was the same as in case (b). In case (c) the

subjects were asked to use their hands as the main sup-

port of the body and to regard their legs to be too fa-

tigued. For each trial 4 s of data were collected. The

paraplegic patients have performed the STS task: (a) 5

times only with the help of their arms, and (b) 5 times

with additional surface stimulation of quadriceps mus-

cles. A two-channel stimulator was used and the stimu-

lating pulses were modulated by a ramp signal with a

duty-cycle of 2 s. For each paraplegic participant 9 s of

data have been recorded.

2.3. Data and equipment

Infrared-reecting markers were xed on the ankle,

knee, hip, shoulder, elbow, and wrist joints from the

right side of the body, and the 3-dimensional (3D)

Cartesian coordinates of each joint (with an accuracy of

1 mm for position) have been recorded by two CCDcameras of an ELITE system (BTS, Milan, Italy). The

vertical seat reaction force (SRF) was obtained from an

instrumented laboratory chair. A Kistler piezoelectric

force plate type 9284 (Winterthur, Switzerland) was

used to measure the 3D ground reaction forces (GRF)

under both feet (with an accuracy of 1% of the full

Table 1

Data about the ten healthy adults and two paraplegic subjects participated in the measurements. (In the table, HA stands for Healthy Adults)

No. Name Sex Diag-

nosis

Age We ight

[kg]

Height

[cm]

Trunk

[cm]

Fore-

arm

[cm]

Upper

arm

[cm]

Thigh

[cm]

Shank

[cm]

Foot

[cm]

Height

of

chair

[cm]

Height

of bars

[cm]

1 MM f T8 39 46 158 56 25 30 40 40 24 36 742 UK m T8 30 74 183 58 32 30 46 50 30 45 88

3 BR f HA 26 60 169 51 25.5 31 44.5 45.5 24.5 40.5 84

4 HH m HA 33 74 187 55 30 34 46.5 50 28 49.5 95.5

5 II f HA 27 64 164 53.5 26.5 27.5 39.5 43.5 24 42.5 86

6 JD m HA 26 65 181 55 30.5 33.5 45 49.5 26 49.5 95.5

7 OR m HA 31 81 180 53.5 27 29.5 42 42 27 49.5 94.5

8 RR m HA 28 83 185 53.5 29 31.5 44 47.5 26 49.5 94.5

9 SE f HA 26 65 165 52 26 31.5 41.5 43.5 24 40.5 85.5

10 SV m HA 27 72 184 55 30 33 43 50 29 49.5 94.5

11 TE m HA 29 75 180 56.5 30 34.5 41.5 44.5 26 42.5 88.5

12 UW m HA 26 70 175 55 29 32.5 39 47 26 46.5 93.5

Mean

(HA)

29 70.9 177 53.98 28.35 31.85 42.65 46.3 26.05 46 91.2

SD 3.88 7.55 8.35 1.64 1.9 2.15 2.4 2.93 1.64 4.03 4.64

Fig. 1. Schematic representation of the experimental set-up used

during the measurements. Since the movement was assumed to besymmetrical, a 3DoF planar rigid body model has been used to des-

cribe the dynamics of the STS transfer in the healthy and paraplegic

subjects. The origin of the coordinate system was assumed to be on the

ankle joint. The positive directions for joint angles, displacement and

velocity of dierent components of the CoM and LM as well as re-

action forces are according to the coordinate system shown in the

gure.

F. Bahrami et al. / Clinical Biomechanics 15 (2000) 123133 125


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scale) and the corresponding CoP. The 3D reaction

forces under each hand (HRF) were measured with two

laboratory bars instrumented with strain gauges. All

kinematic and kinetic data have been recorded simul-

taneously and sampled at 100 Hz. The measured data

were ltered with a 9th order forwardreverse Butter-

worth lter with 4 Hz cut-o frequency [19].

2.4. Model

The movement was assumed to be symmetrical [20].

The head and neck were also assumed to remain along

the trunk. Thus, a 3-linked rigid body model (see Fig. 1)

with 3 degrees of freedom (DoF) was used to describe

the dynamics of the upper body and lower extremities in

the sagittal plane [14,21]. In arm-supported standing up

the function of arms has been integrated into the model

as external loads at the shoulders which could vary with

time during the movement. It has been assumed that the

chair exerts forces at the hip joint onto the femur. Thechair reaction forces were described by its horizontal

and vertical components. Thus, two nonlinear spring

dashpot pairs were used to model the visco-elastic

characteristics of bodychair contact in each direction.

The parameters of the springdashpot pairs have been

identied for each subject separately so that the forces

applied by the chair become zero at the seat-o instant.

Some of the previous studies have used only quasi-

dynamic models to evaluate the total joint forces and

torques [13,14]. According to the more recent analysis of

Hutchinson et al. [22] although the static loads dominate

the joint forces and torques, the dynamic analysis can

especially improve the accuracy of the calculations at the

upper-most joints like back and shoulders. Thus, the

dynamic equations of the system have been recursively

derived by applying NewtonEuler equations [23,24] to

each link starting from the feet:

sankle mfoot lmfoot CoP pygrY

w gY q

tHwsT

wsh

2 3 tHchpchair t

HgrpgroundY

1

pxsh mshank xshank mthigh xthigh

mtrunk xtrunk pxgr pxchY

pysh mshank yshank mthigh ythigh

mtrunk ytrunk pygr pychY

2

where Y and are relative joint angular displacement,

velocity and acceleration vectors, M(q) the inertia ma-

trix of the system, gY the term comprising the co-riolis and centrifugal forces, G(q) the gravitational term,

sT the total joint torque vector at ankle, knee and hip

joints, Msh, Fxsh and Fysh the reaction torques and forces

(horizontal and vertical) at the shoulder, respectively,

and JT, Jch and Jgr Jacobians, mapping the external

torques and forces to the joint space; m, lm, ax and ay are

segmental masses, distances of segmental CoM from

distal joints and horizontal and vertical velocities of

segmental CoMs, respectively. Fchair is the vector of ex-

ternal forces exerted by the chair at the hip joint in the

horizontal (X) and vertical (Y) directions according to

the following mathematical description:

pich uich

h fich li

iejli j

1Y i Y 3

where li is the incremental change of the spring length in

X or Y direction; Kich and Bich are the stiness and

damping parameters of the springdashpot pairs, re-

spectively.

2.5. Treatment of data

Applying the sagittal plane components of the mea-

sured kinematic and kinetic data to the 2D/3DoF rigid

body model, the relative joint angles, body CoM, hori-

zontal and vertical LM of the body CoM together with

the joint torques, forces and the external loads at the

shoulder have been computed. To calculate the angular

velocity and acceleration for each joint, a fth order

polynomial has been tted to each ve consecutive

samples of the joint angle.

To allow comparisons, the collected and calculated

data have been normalized according to the following

procedure. The total duration of the movement has been

dened to be 100, which began at the instant of forward

movement of the trunk until the moment when standingfully upright. The CoP and XCoM have been normalized

to the units of percent of the foot length, and the vertical

displacement of the CoM (YCoM) to the body height

(%BH). The horizontal and vertical LMs together with

the reaction forces have been normalized to the units of

percent of the body weight (%BW), and torques to the

units of percent of the BW BH. The same normaliza-

tion techniques for the kinematic and kinetic data are

also suggested by Hof [25]. Later the results have been

averaged for all the healthy subjects. For each paraple-

gic patient participating in this study, only two typical

trials from ve performed tasks in each case have beenchosen, and then the results have been averaged and

normalized. The criteria for trial selection were: (1) none

of the markers were hidden from any of the CCD

cameras at any moment, (2) the feet remained xed on

the force plate all the time during rising, and (3) no

spasticity occurred during the motion.

3. Results

Table 2 lists the total duration of the STS tasks per-

formed by the healthy and paraplegic participants with



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dierent strategies (Mean values and standard devia-tions). The total duration of the movement is divided

into two phases. Phase I is before leaving the chair, and

phase II begins after seat-o until the body reaches the

standing posture. Figs. 26 depict the results. The pos-

itive torques act to extend the joints and the numerical

values are calculated for one joint. In Fig. 6 the knee

joint angle is plotted according to the AAOS system

[26]. The full extension of the knee joint is dened as 0

and the negative values indicate the exion of knee. The

time at which the healthy subjects left the seat was de-

termined as the instant when the horizontal componentof the GRF attained its minimum value [21] and named

seat-o instant. It was not possible to apply the same

denition to determine the exact seat-o event for the

paraplegic participants. For the paraplegic subjects the

seat-o was investigated directly as the instant at which

the body left the contact with the seat. The joint coor-

dinates in the Cartesian space as well as the joint angles

and seat reaction forces were used to determine the

approximate seat-o instant for the two paraplegic

participants. In rising with or without FES, the seat-o

event occurred almost at the same relative time

(when almost 42% of the movement duration is over). In

the gures the seat-o instants for the healthy subjects

are marked by vertical lines of the line style corre-

sponding to the main curves; the approximate seat-o

instants for the paraplegic patients are also shown by

vertical lines.

3.1. Healthy adults

3.1.1. Body center of mass

According to Fig. 2, in rising with arm-support the

horizontal and vertical displacement of CoM occurred

simultaneously from the beginning of the movement. In

rising with normal arm-support leaving the seat, thebody CoM was anterior to the ankle, outside of the

support base of the feet (10.75 cm, SD 4 cm). Instanding up with strong arm-support, the XCoM at seat-

o was located anterior to the ankle (4.1 cm, SD 8cm), under the heel (Mean (Xheel Xankle) 5.7 cm,SD 0.67 cm). The relative high s.d.s for XCoM Xanklein the STS transfer with strong arm-support may be due

to the subjective preferences in establishing postural

stability when leaving the seat. It was observed that in

all cases the CoP at seat-o instant was under the sup-

port bases of the feet and the CoM was displaced from

Fig. 2. Normalized and averaged spatial displacement (XY projec-

tion) of the body Center of Mass (CoM) for the healthy adults (HA)

and paraplegic patients (PP). In each case the horizontal projection of

the CoM at the seat-o instant is also marked.

Table 2

Average duration (standard deviation) of the STS transfer for dierent modes of the task accomplishmenta

(a) Healthy adults

Without arm-support With normal arm-support With strong arm-support

Total Phase I Phase II Total Phase I Phase II Total Phase I Phase II

Duration [s] 1.738 0.706 1.032 1.956 0.707 1.249 2.801 1.085 1.716

(0.46) (0.07) (0.07) (0.34) (0.10) (0.10) (0.67) (0.35) (0.35)

Duration [%] 100 40.63 59.37 100 36.125 63.875 100 38.75 61.25

(b) Paraplegic patients

Without FES With FES

Duration [s] 4.25 (0.57) 5.175 (1.4)

a For healthy adults phases I and II are represented as absolute time and as percentage of the total duration of the movement. For the paraplegic

participants it was not possible to determine the exact seat-o instant.



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the CoP. Table 3 summarizes the average horizontal

distance of the ankle and of the body CoM at seat-oinstant from CoP.

3.1.2. Linear momentum

In standing up with arm-support, the horizontal and

vertical LM were almost everywhere positive and the

maximum HLM occurred about the seat-o event (Fig.

3). The HLM at seat-o was always positive, and in

rising without arm-support it was larger

(HLMjseat-off 30%BW cm/s) than in two other strate-gies (HLMjseat-off 20%BW cm/s). On the other hands,leaving the chair with normal arm-support the VLM

was larger than (20%BW cm/s) VLM in two other cases

(less than 10%BW cm/s). Riley et al. [1] observed that

the kinetic energy of the CoM had two peaks which

occurred almost when HLM and VLM achieved their

maximum values. They dened the time interval be-

tween maximum HLM and maximum VLM as Mo-

mentum Transfer Phase (MTP). According to our results

(Fig. 4), in rising with arm-support, the kinetic energy of

the CoM had also two local maxima which occurred

almost at the same instants as the HLM and VLM

Fig. 3. Normalized horizontal and vertical linear momentum of the

body CoM for the healthy adults (HA) and paraplegic patients (PP).Fig. 4. Averaged kinetic energy of the body CoM for healthy adults

and paraplegic patients. For each case the two successive peaks of the

kinetic energy are marked with vertical lines.



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attained their maximum values, but these peaks were

not as large and distinct as in the STS transfer without

arm-support. The vertical and horizontal LMs in STS

transfer without arm-support have larger maximum

values than those maximum values in the STS with arm-

support. In addition, the MTPs for rising with normal

and strong arm-support (0.14 and 0.28 s, respectively)

were shorter than MTP in standing up without arm

support (0.522 s).

3.1.3. Reaction forces

According to our results, for standing up without

arm-support the vertical GRF (VGRF) had a larger

peak (113%BW, SD

10.8%BW) compared with the

VGRF in the other strategies. In rising with normal

arm-support, the VGRF had a smaller peak (104%BW,

SD 6.8%BW) than the VGRF in the STS without arm-support. Since in the former, short after seat-o the

velocity of the body ascension was began to decrease. In

standing up with strong arm-support, the bars carried a

part of the subject's BW. However, the positive peak in

the VGRF (68%BW, SD 11%BW) occurred shortlyafter seat-o, shows that the legs shared the dynamics of

the movement with hands.

3.1.4. Joint torques

According to Table 4, in rising with normal/strongarm-support maximum torques at the knee and hip

joints were smaller (up to 50%) than in the STS transfer

without arm-support, but applying normal arm-support

the ankle joint torque was relatively larger than the

ankle joint torque during STS movement without arm-

assistance. In all cases the total knee and hip joint tor-

ques attained their maximum shortly before seat-o.

3.1.5. Shoulder loads

Table 4 summarizes also the average maximum values

of the torques and forces at one shoulder for healthy

Fig. 5. Normalized average external loads at shoulder for healthy

adults and paraplegic patients.

Fig. 6. Knee joint angles averaged over the data of the two paraplegic

patients.



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subjects rising with arm-support. Fig. 5 shows that in

the STS transfer with arm-support the vertical forces at

shoulder assisted the upward movement of the body. It

was mentioned that in rising with normal arm-support,

the subjects lifted from the chair before locating the

horizontal projection of the CoM under the feet (Fig. 2).

The horizontal forces at shoulder together with larger

torque at ankle supported the body in this case and

prevented it from sit-back failure. In standing up with

strong arm-support, the negative external torques at the

shoulder were mainly due to the rotation of the trunk

about the shoulder joint. The positive horizontal force

(Fig. 5) at the shoulder indicates that applying strong

arm-support, the subjects pulled the bars with a con-

siderable horizontal force (maximum 18%BW) to move

forward and to locate their trunk between two hands.

This maneuver facilitated separation from the seat and

reduced notably the maximum forces and torques at thelower extremity joints about seat-o instant (up to 50%).

3.2. Paraplegic patients

In standing up without FES the patients launched

their body forward (see Fig. 6) to be able to leave the

chair (launching maneuver). In the FES-assisted STS

transfer the patients did not use the launching maneuver

to leave the seat. Moreover, at the end of phase II the

knee joint was more extended than in the standing up

without FES.

3.2.1. Body center of mass

The spatial displacement of the CoM (Fig. 2) indi-

cates that before leaving the chair, the paraplegic pa-

tients located their body CoM near or between two

hands. Launching maneuver implied the additional

downward displacement of the CoM in the STS transfer

without FES. After establishing the CoM in the support

base of the body, the patients rose almost vertically

trying to keep the trunk between two hands.

3.2.2. Linear momentum

Fig. 3 shows that, in standing up without/with FES

the VLM about seat-o was almost zero. In rising

without FES the positive peaks in the HLM (20%BW

cm/s) about and short after seat-o are mainly due to

the launching maneuver. After standing up, the patients

could not stand properly and the arms and trunk ut-

tered, whence, the HLM at the end of movement wasnot zero. In the FES-assisted STS transfer, the HLM

reached its maximum before seat-o, and had a smaller

averaged maximum value (15%BW cm/s) than the HLM

of the healthy subjects rising with strong arm-support.

3.2.3. Reaction forces

In rising without FES, after seat-o the hands toler-

ated all dynamics of the upward movement (more than

85%BW) and thus, the VGRF decreasedafter seat-o to

less than 15% of patient's BW. Applying FES, at the end

of phase II the knee joints were fully stretched and the

Table 3

The average horizontal distances of the CoM from ankle and CoP under the feet at seat-o (standard deviation)


Xankle CoP [cm] +1.1 (0.75) +4.15 (1.5) 3.1 (2.5)XCoM CoP [cm] +4.1 (10.8) 6.6 (3.8) 7.8 (8.5)

Table 4

The averaged maximum values (standard deviation) of the joint torques and the horizontal and vertical forces at the shoulders for healthy subjects

rising with three dierent strategiesa


Max (sankle) 1.88 1.98 1.24

[%BWBH] (0.79) (0.8) (0.67)

Max (sknee) 5.07 4.41 2.76

[%BWBH] (0.8) (1.1) (1.29)

Max (ship) 3.66 3.03 1.02

[%BWBH] (1.69) (1.28) (1.82)

Max (sshoulder) 4.55 10.8[%BWBH] (0.14) (0.63)

Max (Fxshoulder) 9.48 7.11[%BW] (6.01) (4.63)

Max (Fyshoulder) 6.54 23.65

[%BW] (6.01) (12.7)

a The numerical values are calculated for one joint.



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feet could carry up to 60% of the patient's BW. In this

case, the loads were transferred from hands to the legs

when actually 6570% of the movement duration was

passed.

3.2.4. Joint torques

In the FES-assisted standing up, the maximum total

knee joint torque was 2.0% BW BH and thus, the active

knee joint torque which was generated by the electrical

stimulation of the quadriceps was signicant. Due to the

stimulation pattern, the FES-induced knee joint torque

of the patients attained its maximum value after seat-o,

when almost 60% of the movement duration was over.

3.2.5. Shoulder loads

The vertical forces at the shoulder (Fig. 5) indicates

that the patients used their arms about and after seat-o

to compensate for the limited joint torques at the lower

extremities. In addition, after seat-o, the torque at the

shoulder has remained almost constant. This indicatesthat the reaction torque at the shoulders was most

probably produced to adjust the orientation of the trunk

relative to the horizon and to stabilize the posture. It

should be noticed that the torques at the patients'

shoulder are not completely comparable with that of the

healthy subjects, since the relative positioning of the

bars and seat for healthy and paraplegic test groups

were dierent. The horizontal forces at the shoulder

might be generated to control the forwardbackward

uctuation of the body during rising. In the STS transfer

with FES at the end of phase II, the vertical forces at the

shoulder decreased (Fig. 5), which was on account of the

load sharing of the legs with the hands.

4. Discussion

According to Riley et al. [1] a number of variables

(e.g., chair height or speed of rising) eect the perfor-

mance of a STS task. However, when the task perfor-

mance is restrained, even with constraints which are

within the range of normal conditions, one must be

careful in the generalization of the results. We believe

that the STS transfer, like most of other human tasks

(e.g., reaching [27]) exhibits a number of propertieswhich are almost independent of the initial body state or

speed of task performance, etc. Thus, to investigate the

invariance properties of the STS task no constraint was

established on the participants in performing the de-

manded tasks.

4.1. Healthy adults

4.1.1. Stability of the movement

Using the arm-assistance, the support points of the

body and thus the stabilizing factors of the movement

were increased. Thus, the healthy test group rising with

arm-support did not try to transfer the body CoM from

the chair to the support base of the feet before leaving

the seat. However, in all cases at the seat-o instant the

CoP was under the support base of the feet (see Table 3).

In rising with arm-support the HLM at seat-o was

positive but smaller than HLM in standing up without

arm-support, whereas, in rising with normal arm-sup-

port the ascension velocity of the body CoM at the seat-

o instant was larger than in the other cases. This fact

together with considerable gravity moment arms

(XCoM CoP) at seat-o instant indicate that, the forcesand torques at the shoulder (or the forces and torques

applied by the hands at the bars) supported the stability

of the movement in the STS transfer with arm-assis-

tance.

4.1.2. Momentum control

Table 2 indicates that the STS task without arm-

support was accomplished more quickly than otherstrategies. This fact together with the positive gravity

moment arm at the seat-o instant, suggest that in

standing up without arm-support the motion had to

be performed with more dynamics to ensure the pos-

tural stability. Since in this case the support base of

the body after seat-o is smaller than in rising with

arm-assistance. In the STS transfer with normal arm-

assistance the MTP was shorter than in the two other

strategies and as mentioned, the two peaks in the ki-

netic energy were less distinct. The horizontal and

vertical LM's at seat-o were almost equal and thus,

momentum transfer between two phases was less sig-

nicant. In rising with strong arm-support the HLM

at seat-o was greater than VLM, and the MTP was

longer than in the STS transfer with normal arm-

support. These observations along with the spatial

displacement of the body CoM suggest that in rising

with strong arm-support, the healthy participants ap-

plied a combination of the two strategies described by

Riley et al. [1] to control the upper body exion and

its velocity prior to seat-o (emphasizing more on the

strategy I).

4.1.3. Joint torque distribution

In the STS transfer with normal arm-assistance thearms supported the stability of the movement and

thus, reduced the maximum knee and hip joint torques

which occurred shortly before seat-o instant. The

vertical reaction forces indicate that in this case the

legs compensated for the static loads as well as for a

part of the dynamic loads due to the inertial, coriolis

and centrifugal forces. Applying stronger arm-support

by the healthies, the arms not only shared with the

legs in the dynamics of the motion, but also they

carried a part of the body weight (the static loads)

which in turn resulted in the smaller joint torques at



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the lower extremities compared with the normal arm-

support.

4.2. Paraplegic patients

We do not intend to generalize all of our results about

the paraplegic patients because, only two paraplegic

subjects participated in this experimental analysis, and

therefore no statistical data were presented. Although

the relative positions of the chair-bars were dierent for

the paraplegic and healthy test groups, nevertheless,

comparing the results with the data obtained from the

healthy participants, it is still possible to deduce quali-

tative general information.

4.2.1. Momentum control

It was noticed that in our paraplegic participants the

VLM at seat-o was almost zero and the HLM in phase

I was controlled to ensure the postural balance and to

facilitate the separation from the seat. In rising withoutFES, the patients used the launching maneuver to leave

the seat, which resulted in a higher maximum HLM

than rising with FES. These observations together with

the time history of the kinetic energy of the body CoM

indicate that for the paraplegic participants the mo-

mentum transfer was negligible and the STS transfer was

accomplished in a more static manner than in the

healthier rising with strong arm-support. After leaving

the seat, the patient controlled his vertical velocity

(VLM) such that he reached the standing stance with a

zero vertical velocity while adjusting his trunk orienta-

tion. Thus, not only the position of the CoM but also its

velocity in the horizontal and vertical directions were

controlled.

4.2.2. Load distribution

The maximum total knee joint torque generated by

the stimulation of the quadriceps (2% BW BH) was

comparable with the maximum knee joint torque of the

healthy subjects rising with strong arm-support (2.76%

BW BH). Nonetheless, the vertical GRF and the LMs

indicate that applying FES, the legs participated eec-

tively in the dynamics of the movement rst when the

VLM has reached its maximum and the HLM was al-

most zero (almost 65% of the movement duration wasover).

5. Conclusion

The results of this study suggest that, locating the

support-bars in front of the healthy subjects enlarged

the support base of the body and thus, the postural

stability was increased. Applying strong arm-support,

the hip and knee joint torques were reduced by 50%. The

healthy participants rising with strong arm-support also

used momentum transfer maneuver to facilitate the

transition from sitting to standing. In the paraplegic test

group it was observed that the momentum transfer

strategy was not applied. With/without FES the HLM

was used mainly to facilitate the separation of the body

from the seat. The forward exion of the trunk was to

provide the postural balance before leaving the seat. In

the FES-induced standing up the legs could participate

partly in the movement dynamics, but the load sharing

of lower extremities was complex and most probably

subject-dependent. These observations suggest that the

healthy participants used the constant information from

the actual state of the entire body to distribute the loads

between their hands and legs, and to control the whole

body movement. For the paraplegic patients only a part

of these information was available and thus, some sig-

nicant dierences were observed between the maneuver

they have used compared with the strategy applied by

the healthy adults rising with strong arm-support.

Acknowledgements

A part of this work was accomplished during the stay

of the rst author at the Technical University of Munich

under grants of the Deutscher Akademischer Aus-

tauschdienst (DAAD) and SFB462 project Sensomotorik

sponsored by the German Research Council DFG.

Those parts of this project carried out at the University

of Tehran have been nancially supported by that uni-

versity. The authors are also indebted to Prof. Parn-

ianpour from the Ohio State University for his valuableremarks.

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