6
Clarification of Muscle Synergy Structure During Standing-up Motion of Healthy Young, Elderly and Post-Stroke Patients N. Yang 1 , Q. An 1 , H. Yamakawa 1 , Y. Tamura 1 , A. Yamashita 1 , K. Takahashi 2 , M. Kinomoto 2 , H. Yamasaki 3 , M. Itkonen 3 , F. S. Alnajjar 3 , S. Shimoda 3 , H. Asama 1 , N. Hattori 2 and I. Miyai 2 Abstract— Standing-up motion is an important daily activity. It has been known that elderly and post-stroke patients have difficulty in performing standing-up motion. The standing-up motion is retrained by therapists to maximize independence of the elderly and post-stroke patients, but it is not clear how the elderly and post-stroke patients control their redundant muscles to achieve standing-up motion. This study employed the concept of muscle synergy to analyze how healthy young adults, healthy elderly people and post-stroke patients control their muscles. Experimental result verified that four muscle synergies can represent human standing-up motion. In addition, it indicated that the post-stroke patients shift the weights of muscle synergies to finish standing-up motion comparing to healthy subjects. Moreover, different muscle synergy structures were associated with the CoM and joint kinematics. I. INTRODUCTION Since 1950, the world’s elderly population has been in- creasing sharply. According to United Nation, the proportion of the world’s population aged 60 years or over increased from 8% in 1950 to 12% in 2013, and it is estimated that it will increase more rapidly to reach 21% in 2050 [1]. In order to improve the quality of life of the elderly, the standing- up motion is focused as one of the most common daily activities, which usually followed by other activities. For the elderly with lower extremity muscle weakness, it is difficult for them to rise from a chair. Alexander et al. [2] found that the elderly generated less knee extension torque than young adults. However, the elderly need to use more knee extension torque to stand-up from a chair than that of young adults. Muscle strength was further impaired in post-stroke patients comparing to the elderly [3]. It is also found that compared to the healthy elderly people, the post-stroke patients need more time to do standing-up motion [4][5]. Hence, the standing-up motion is retrained by therapists to maximize independence of the elderly and post-stroke patients. To retrain the elderly and the post-stroke patients effi- ciently, it is important to understand the mechanism of the standing-up motion. Hughes et al. defined three strategies (momentum tansfer, stabilization, and hybrid) used in hu- mans standing-up motion which generate different move- ments [6]. They found that younger persons tend to use the 1 N. Yang, Q. An, H. Yamakawa, Y. Tamura, A. Yamashita, and H. Asama are with Department of Precision Engineering, Gradu- ate School of Engineering, The University of Tokyo, Tokyo, Japan [email protected] 2 K. Takahashi, M. Kinomoto, N. Hattori, and I. Miyai are with Mori- nomiya Hospital, Osaka, Japan 3 H. Yamasaki, M. Itkonen, F. S. Alnajjar, and S. Shimoda are with RIKEN Brain Science Institute, Japan momentum transfer strategy which utilizes the momentum to lift up their body. On the other hand, the elderly per- sons tend to use the stabilization strategy in which they carry their center of mass (CoM) first on their feet and then they move upward. The hybrid strategy is the middle between momentum transfer and stabilization. Other studies also found that post-stroke patients have delayed muscle activation compared to healthy elderly people [7]. However, they could not reveal how humans coordinate muscles to generate different standing-up movements. Essentially human movement is varied because human body is a redundant system that there are more muscles than the number of joints to be controlled. In order to clarify how humans coordinate their redundant number of muscles to generate different standing-up movements, the concept of muscle synergy has been employed in the area of human motor control theory. Muscle synergy was firstly proposed by Bernstein, which suggested that human movements could be generated from a limited number of modules (called muscle synergy) [8]. It claimed that humans did not control individual muscles, but they controlled modules of muscles. Muscle synergy theory decomposes the complex control of individual muscles into modular organization. Previous stud- ies showed human locomotion could be explained by a small number of muscle synergies [9][10]. Ivanenko et al. found that muscle synergy structure were similar when healthy humans walked with different speeds and gravitational loads, but they adaptively changed the weighting coefficient of muscle synergy to achieve the different locomotion [9]. These findings suggested that humans might utilize different combinations or different ways to activate the limited number of muscle synergies to accomplish adaptive movements. For human standing-up movements, both forward dynamic simulation and measurement experiment were employed to clarify the muscle synergy structure in our previous studies [11]. However, the measurement experiment was based on healthy young adults. It is still unclear how the healthy elderly people and post-stroke patients coordinate redundant muscles to achieve standing-up motion. Therefore, we perform the standing-up motion experiment of healthy elderly people and post-stroke patients in this study. We aim at clarifying the muscle synergy structure in standing- up motion of the healthy elderly people and post-stroke patients. The differences among the healthy humans and post-stroke patients were compared. In addition, muscle synergy structure of the post-stroke patients were especially focused. 2017 International Conference on Rehabilitation Robotics (ICORR) QEII Centre, London, UK, July 17-20, 2017. 978-1-5386-2296-4/17/$31.00 ©2017 IEEE 19

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Page 1: Clarification of Muscle Synergy Structure During Standing

Clarification of Muscle Synergy Structure During Standing-up Motionof Healthy Young, Elderly and Post-Stroke Patients

N. Yang1, Q. An1, H. Yamakawa1, Y. Tamura1, A. Yamashita1, K. Takahashi2, M. Kinomoto2, H. Yamasaki3,M. Itkonen3, F. S. Alnajjar3, S. Shimoda3, H. Asama1, N. Hattori2 and I. Miyai2

Abstract— Standing-up motion is an important daily activity.It has been known that elderly and post-stroke patients havedifficulty in performing standing-up motion. The standing-upmotion is retrained by therapists to maximize independence ofthe elderly and post-stroke patients, but it is not clear howthe elderly and post-stroke patients control their redundantmuscles to achieve standing-up motion. This study employedthe concept of muscle synergy to analyze how healthy youngadults, healthy elderly people and post-stroke patients controltheir muscles. Experimental result verified that four musclesynergies can represent human standing-up motion. In addition,it indicated that the post-stroke patients shift the weights ofmuscle synergies to finish standing-up motion comparing tohealthy subjects. Moreover, different muscle synergy structureswere associated with the CoM and joint kinematics.

I. INTRODUCTION

Since 1950, the world’s elderly population has been in-creasing sharply. According to United Nation, the proportionof the world’s population aged 60 years or over increasedfrom 8% in 1950 to 12% in 2013, and it is estimated that itwill increase more rapidly to reach 21% in 2050 [1]. In orderto improve the quality of life of the elderly, the standing-up motion is focused as one of the most common dailyactivities, which usually followed by other activities. For theelderly with lower extremity muscle weakness, it is difficultfor them to rise from a chair. Alexander et al. [2] found thatthe elderly generated less knee extension torque than youngadults. However, the elderly need to use more knee extensiontorque to stand-up from a chair than that of young adults.Muscle strength was further impaired in post-stroke patientscomparing to the elderly [3]. It is also found that compared tothe healthy elderly people, the post-stroke patients need moretime to do standing-up motion [4][5]. Hence, the standing-upmotion is retrained by therapists to maximize independenceof the elderly and post-stroke patients.

To retrain the elderly and the post-stroke patients effi-ciently, it is important to understand the mechanism of thestanding-up motion. Hughes et al. defined three strategies(momentum tansfer, stabilization, and hybrid) used in hu-mans standing-up motion which generate different move-ments [6]. They found that younger persons tend to use the

1N. Yang, Q. An, H. Yamakawa, Y. Tamura, A. Yamashita, andH. Asama are with Department of Precision Engineering, Gradu-ate School of Engineering, The University of Tokyo, Tokyo, [email protected]

2K. Takahashi, M. Kinomoto, N. Hattori, and I. Miyai are with Mori-nomiya Hospital, Osaka, Japan

3H. Yamasaki, M. Itkonen, F. S. Alnajjar, and S. Shimoda are with RIKENBrain Science Institute, Japan

momentum transfer strategy which utilizes the momentumto lift up their body. On the other hand, the elderly per-sons tend to use the stabilization strategy in which theycarry their center of mass (CoM) first on their feet andthen they move upward. The hybrid strategy is the middlebetween momentum transfer and stabilization. Other studiesalso found that post-stroke patients have delayed muscleactivation compared to healthy elderly people [7]. However,they could not reveal how humans coordinate muscles togenerate different standing-up movements.

Essentially human movement is varied because humanbody is a redundant system that there are more muscles thanthe number of joints to be controlled. In order to clarifyhow humans coordinate their redundant number of musclesto generate different standing-up movements, the concept ofmuscle synergy has been employed in the area of humanmotor control theory. Muscle synergy was firstly proposedby Bernstein, which suggested that human movements couldbe generated from a limited number of modules (calledmuscle synergy) [8]. It claimed that humans did not controlindividual muscles, but they controlled modules of muscles.Muscle synergy theory decomposes the complex control ofindividual muscles into modular organization. Previous stud-ies showed human locomotion could be explained by a smallnumber of muscle synergies [9][10]. Ivanenko et al. foundthat muscle synergy structure were similar when healthyhumans walked with different speeds and gravitational loads,but they adaptively changed the weighting coefficient ofmuscle synergy to achieve the different locomotion [9].These findings suggested that humans might utilize differentcombinations or different ways to activate the limited numberof muscle synergies to accomplish adaptive movements.

For human standing-up movements, both forward dynamicsimulation and measurement experiment were employedto clarify the muscle synergy structure in our previousstudies [11]. However, the measurement experiment wasbased on healthy young adults. It is still unclear how thehealthy elderly people and post-stroke patients coordinateredundant muscles to achieve standing-up motion. Therefore,we perform the standing-up motion experiment of healthyelderly people and post-stroke patients in this study. Weaim at clarifying the muscle synergy structure in standing-up motion of the healthy elderly people and post-strokepatients. The differences among the healthy humans andpost-stroke patients were compared. In addition, musclesynergy structure of the post-stroke patients were especiallyfocused.

2017 International Conference on Rehabilitation Robotics (ICORR)QEII Centre, London, UK, July 17-20, 2017.

978-1-5386-2296-4/17/$31.00 ©2017 IEEE 19

Page 2: Clarification of Muscle Synergy Structure During Standing

II. METHOD

A. Subjects

In this experiment, six healthy young adults (25±3.0 yearsold), five healthy elderly participants (66.8±8.5 years old)and three post-stroke patients (58.3±11.4 years old) wereasked to stand up from their own comfortable feet location.Three months have passed after the stroke for two patients,and five months have passed for one subject. All of thesubjects could stand up from a chair by themselves withoutany support. Average Fugl-Meyer Assessment score was72.7±6.6. There were 15 trials for each healthy young andelderly participants, and 10 trials for each post-stroke patientsin the experiment. The chair height was adjusted to the heightof lower leg. The participants finished the motion withoutmoving their feet in all the trials. The informed consent wasobtained from all participants according to the protocol ofthe Institute Review Board of The University of Tokyo andMorinomiya Hospital, Japan.

B. Experiment Setting

In this experiment, the motion capture system (MotionAnalysis. Corp.) was used to record the movements of jointsin 100 Hz. This motion capture system has eight infraredcameras. Human joint positions were measured based onHelen Hayes marker set. Joint angle data was calculatedusing SIMM (Musculographics, Inc) based on the jointposition data and the jiont angle data was used to calculatethe trajectories of CoM. Surface EMG device (DL-141,S&ME Corp.) was used in this experiment to get muscleactivities data in 2,000 Hz. Ten muscles related to standingup motion were measured according to their contribution toextend or flex the ankle, knee, and hip joints: tibialis anterior(TA), gastrocnemius (GAS), soleus (SOL), rectus femoris(RF), vastus lateralis (VAS), biceps femoris long head (BFL),biceps femoris short head (BFS), gluteus maximus (GM),rectus abdominis (RA) and erector spine (ES). The namesof the ten muscles are shown in Fig. 1. Two force platedevices (TechGihan. Corp.) were used to record the reactionforce data in 2,000 Hz. The force data was used to definethe start time of phases.

C. Muscle Synergy Model

Bernstein suggested that human movement was composedof the small sets of modules (called synergy). It assumeshumans do not control individual muscles but they controlgroup of muscles. The muscle synergy model has beenwidely used to analyze human movements [9][10]. For themuscle synergy model, muscle activation can be expressedas a linear summation of spatiotemporal patterns in a math-ematical expression, as in Eq. (1),

M = WC, (1)

where matrices M, W, and C indicate muscle activation, spa-tial pattern and temporal pattern matrices respectively. Matrix

RA

RF

TA

VAS

(a) Front

ES

GMA

BFL

BFS

GAS

SOL

(b) Back

Fig. 1. Measured Muscles.

M consists of the muscle activation vectors mi(i=1,2,...,n) torepresent n different muscle activations (Eq. (2)).

M =(m1(t) m2(t) . . . mn(t)

)T

=

m1(1) . . . m1(tmax)...

. . ....

mn(1) . . . mn(tmax)

. (2)

The components of the vector are mi(t) to represents discretei-th muscle activation at time t (1≤ t ≤ tmax). Variable nrepresents the number of muscles. Spatial pattern W is usedto represent the relative activation level of muscle. Variable Nindicates the number of muscle synergies. Its column showsN different spatial pattern vectors w j( j=1,2,...,N). The vectorw j consists of wi j which represents relative activation levelof muscle i included in j-th muscle synergy (Eq. (3)).

W =(w1 w2 . . . wN

)=

w11 . . . w1N...

. . ....

wn1 . . . wnN

. (3)

Temporal pattern C is used to indicate the time-varyingweighting coefficient of N muscle synergies (Eq. (4)). Itsrow shows N different temporal pattern vectors c j( j=1,2,...,N),which indicate temporal pattern corresponded to the spa-tial patterns w j. Its components are c j(t), which representweighting coefficient of j-th muscle synergy at time t.

C =(c1(t) c2(t) . . . cN(t)

)T

=

c1(1) . . . c1(tmax)...

. . ....

cN(1) . . . cN(tmax)

. (4)

Figure 2 shows a schematic design of muscle synergymodel. Three muscle synergies are used to express n muscleactivations. They are composed of spatial and temporal pat-terns. Spatial patterns (w1,2,3) show relative muscle activation

20

Page 3: Clarification of Muscle Synergy Structure During Standing

Muscle Activation

(a) Spatial Pattern (b) Temporal Pattern (c) Muscle Activation

Time [s]Act

ivat

ion

Wei

gh

t

Fig. 2. Muscle synergy model.

level. Temporal patterns (c1,2,3) show the relative weightingcoefficients. During the motion, spatial patterns are constant,but temporal patterns change according to the time. Muscleactivations are generated from linear summation of spatialand temporal patterns of muscle synergies. Muscle activa-tions are shown in gray areas and muscle synergies 1, 2, and3 are described in solid lines, dashed lines and solid lineswith circles in 2.

To calculate elements of the matrices W and C, non-negative matrix factorization (NNMF) [12] was used. Afterobtaining the spatiotemporal patterns of muscle synergy, thedetermination of coefficient R2 was calculated to test if themuscle synergy can well represent muscle activation. R2

was also used in previous studies to analyze the ability toactivate muscle synergy independently [14]. This study con-ducts a measurement experiment of stroke patient performingstanding-up motion and it would be analyzed how the musclesynergy structure differs among different types of standing-up motion. It was found that there are fewer muscle synergiesin the walking of post-stroke patients [10]. The reduction inmuscle synergy numbers suggested that post-stroke patientshad lower locomotor output complexity which associatedwith poor walking performance.

D. Center of Mass CalculationIn order to calculate the center of mass (CoM) position,

human body is expressed in four link model: 1. HAT (head,arm and trunk), 2. pelvis, 3. thigh and 4. shank. This studyfocuses on sagittal movement and horizontal and verticalCoM positions CoMx(t) and CoMy(t) at time t are calculatedfrom Eqs. (7)-(8). In the equations, Mk, px

k(t) and pyk(t)

indicate the mass, horizontal and vertical CoM positions ofthe k-th segment. px

k(t) and pyk(t) are calculated from the

ratio of CoM to the segment length and joint angle data ofankle, knee, hip and lumbar was used [15].

CoMx(t) =∑4

k=1 Mk pxk(t)

∑4k=1 Mk

, (5)

CoMy(t) =∑4

k=1 Mk pyk(t)

∑4k=1 Mk

. (6)

E. Kinematic Events of Standing-up MotionIn the experiment, the duration time of the standing-

up motion is different among trials and subjects. In order

Fig. 3. Definition of phases.

to compare different trials, the duration time needs to benormalized. To normalize the duration time, the standing-upmotion is divided into four phases [13], as shown in Fig. 3.

III. RESULTS AND DISCUSSION

A. Trajectory of Center of Mass

Figure 4 shows the trajectory of CoM during the standing-up motion. The black solid circled lines, solid lines anddashed lines show the CoM trajectories of the healthy youngadults, the elderly and post-stroke patients respectively. Thedisplacements of CoM on both horizontal and vertical direc-tions were normalized based on the height of each subject.The gray part shows the feet location. Comparing to thehealthy young adults and the elderly, the post-stroke patientsfirst moved the CoM to their feet before they moved upward.As shown in the result of temporal patterns, the post-strokepatients used longer time to move the CoM forward beforeextended the body to move upward. This result of CoMalso showed that the post-stroke patients firstly moved theCoM to the feet to make the standing-up motion more stable.Figures 4 (a) and (b) also show that healthy young adults andthe elderly moved the CoM upward before the CoM reachingthe feet position. Therefore, the young and the elderly neededto decelerate the horizontal CoM movement to keep balanceat the end of the motion. This is related to the results oftemporal pattern of muscle synergy 4. The healthy youngadults and the elderly reached the peak time ealier than thatof the post-stroke patients.

B. The Number of Synergies

Figure 5 shows the coefficient of determination R2 ofdifferent numbers of muscle synergies. The black solid cir-cled lines, solid lines and dashed lines respectively representhealthy young adults, the elderly and post-stroke patients.For the healthy young adults and the elderly, the coefficientsof determination R2 were 94±2% and 88±3% when thenumber of muscle synergies was four. This indicate that fourmuscle synergies can represent most of the muscle activationduring the standing-up motion. In contrast, R2 was 88±4%for the post-stroke patients and when four muscle synergieswere used to represent muscle activation. In addition, morenumbers of muscle synergies increased the value of R2 little.To compare the characteristics of muscle synergy structurebetween the healthy participants and post-stroke patients, the

21

Page 4: Clarification of Muscle Synergy Structure During Standing

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.00 0.05 0.10 0.15 0.20 0.25No

rmal

ized

Ver

tica

l D

ispla

cem

ent

[%]

Normalized Horizontal Displacement [%]

Feet Location

(a) CoM of young adults

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.00 0.05 0.10 0.15 0.20 0.25No

rmal

ized

Ver

tica

l D

ispla

cem

ent

[%]

Normalized Horizontal Displacement [%]

Feet Location

(b) CoM of the elderly

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.00 0.05 0.10 0.15 0.20 0.25No

rmal

ized

Ver

tica

l D

ispla

cem

ent

[%]

Normalized Horizontal Displacement [%]

Feet Location

(c) CoM of post-stroke patients

Fig. 4. Trajectory of CoM.

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10Co

effi

cien

t o

f D

eter

min

atio

n

The Number of Muscle Synergy

(a) Healthy young adults

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10Co

effi

cien

t o

f D

eter

min

atio

n

The Number of Muscle Synergy

(b) Healthy elderly people

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8 9 10Co

effi

cien

t o

f D

eter

min

atio

n

The Number of Muscle Synergy

(c) Post-stroke patients

Fig. 5. Coefficient of determination.

same number of muscle synergies were also used to representthe muscle activation level of the patients [14].

C. Spatial and Temporal Patterns of Muscle Synergies

The result of muscle synergy structure were shown inFig. 6. Figures 6 (a), (c), (e), and (g) show the spatial patternsof four muscle synergies, which represent muscle activationof the ten selected muscles. Black, white and gray bars showrelative muscle activation of healthy young adults, the elderlyand the post-stroke patients respectively.

Each spatial pattern had particular contribution to bodymovements according to human anatomy. In Fig. 6 (a), itshows that muscle synergy 1 mostly activated RA whichflexed the lumbar and produced momentum necessary forthe standing-up motion. Muscle synergy 2 mostly activatedTA which dorsiflexed the ankle joint to move center of mass(CoM) forward, and activated VAS and RF to extend theknee as in Fig. 6 (c). In Fig. 6 (e), muscle synergy 3 mainlyactivated ES, VAS, BFL, BFS to extend trunk and knee,which contribute to extend the whole body. Figure 6 (g)shows that muscle synergy 4 activated GAS, SOL and GMA,which planterflexed the ankle and extend the hip to deceleratemovement of CoM and control the standing posture to staystable. The young, the elderly and the post-stroke patientsshow the same characteristic muscle activations. This impliesthat spatial patterns of muscle synergies would be preservedeven in the post stroke patients as well. However, furtherstudy will be necessary for the wider range of subjects.

Current study employed the post stroke patients who com-paratively had better motor function (FMA scor is larger than70), and different spatial patterns can be observed from thepatients who had lower mobility.

Figures 6 (b), (d), (f) and (h) show the temporal pattern ofthe four muscle synergies. The vertical solid lines, solid cir-cled lines and dashed lines show the start time of the phases1–4 of healthy young adults, healthy elderly people and post-stroke patients respectively. The start time of each phase isshown in Table I. The duration time of the whole standing-upmotion is normalized to 100% from the beginning of phase1 to the end of phase 4. The solid lines with circle markers,solid lines and dashed lines show the temporal patterns ofthe elderly, the healthy young and post-stroke patients.

In Fig. 6 (b), muscle synergy 1 was firstly activated duringthe phase 1. It shows that the post-stroke patients had longertime to bend their trunk forward. This difference results inthe downward movement of CoM at the beginning of theirmovement as in Fig. 4 (c) whereas downward movement wasnot observed for the young and the elderly as in Figs. 4 (a)

TABLE IONSET TIME OF FOUR PHASES. BELOW SHOWS START TIME OF EACH

PHASES.

phase 2 phase 3 phase 4Healthy young 33% 46% 80%

Healthy old 31% 48% 80%Stroke Patient 38% 65% 80%

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Page 5: Clarification of Muscle Synergy Structure During Standing

0.0

0.5

1.0

TA GAS SOL RF VAS BFL BFS GMA RA ES

Activation L

evel

Muscle Name

Elderly Young Stroke Patients

(a) Spatial pattern of muscle synergy 1

0.0

0.5

1.0

0 20 40 60 80 100

Weig

hting C

oeff

icie

nt

Motion Progress [%]

Phase 2Elderly

Young

Stroke Patients

(b) Temporal pattern of muscle synergy 1

0.0

0.5

1.0

TA GAS SOL RF VAS BFL BFS GMA RA ES

Activation L

evel

Muscle Name

Elderly Young Stroke Patients

(c) Spatial pattern of muscle synergy 2

0.0

0.5

1.0

0 20 40 60 80 100

Weig

hting C

oeff

icie

nt

Motion Progress [%]

Phase 2Elderly

Young

Stroke Patients

(d) Temporal pattern of muscle synergy 2

0.0

0.5

1.0

TA GAS SOL RF VAS BFL BFS GMA RA ES

Activation L

evel

Muscle Name

Elderly Young Stroke Patients

(e) Spatial pattern of muscle synergy 3

0.0

0.5

1.0

0 20 40 60 80 100

Weig

hting C

oeff

icie

nt

Motion Progress [%]

Phase 3Elderly

Young

Stroke Patients

(f) Temporal pattern of muscle synergy 3

0.0

0.5

1.0

TA GAS SOL RF VAS BFL BFS GMA RA ES

Activation L

evel

Muscle Name

Elderly Young Stroke Patients

(g) Spatial pattern of muscle synergy 4

0.0

0.5

1.0

0 20 40 60 80 100

Weig

hting C

oeff

icie

nt

Motion Progress [%]

Elderly

Young

Stroke Patients

Phase 4

(h) Temporal pattern of muscle synergy 4Fig. 6. Spatiotemporal patterns of four muscle synergies.

and (b).

The characteristic difference can be observed in the tem-poral patterns of muscle synergy 2 (Fig. 6 (d)). This showsthat the peak times comes in the order of the young, theelderly, and the stroke patients. Duration time of phase 2 isalso the longest for the patients (28%) compared to the young(13%) and the elderly (17%). Similarly the gradient after thepeak value differs for three types of the subjects. The patientshad the most gentle gradient, the elderly had the second mostgentle one, and the young had the most steep gradient. TakingCoM trajectory into account, this phenomena is related to thehorizontal CoM movement. At the time of upward movementof CoM, the post-stroke patients moved his CoM closest to

the feet. On the other hand, CoM of the young subjects isthe farthest from the feet position, and the CoM trajectoryof the elderly was the middle between the young and thepost-stroke patients. This implies that humans controls theactivation timing and duration of the muscle synergy 2 tocontrol horizontal CoM movement.

In Fig. 6 (f), temporal patterns of the post-stroke patientsdiffered from the young and the elderly. Since the start timeof the phase 3 (body extension phase) is the latest for thepost-stroke patients, the peak time was delayed accordingly.As it has been discussed above, the patients firstly movethe CoM on the feet and start moving upward. Delayedactivation of the muscle synergy 3 resulted in the late upward

23

Page 6: Clarification of Muscle Synergy Structure During Standing

movement.At last, the muscle synergy 4 was activated as in Fig. 6

(h). This muscle mainly activate the muscle in the back sideof human body (GAS, SOL, BFL, BFS, GMA) to deceleratethe horizontal movement. As well as other synergies, thegradient became more gentle in the order of the post-stroke patients, the elderly, and the young. This phenomenacould be interpreted as the compensation of the forwardhorizontal movement of the muscle synergy 2. When themuscle synergy 2 activated latter and its gradient becamegentle, the same trend was observed for the muscle synergy4 accordingly. This compensated activation is considered tobe important because this maintains the CoM on their feet.If the post stroke patients cannot control the start time ofthe muscle synergy 4 properly, the movement would resultin falling down.

IV. CONCLUSIONS

In this study, muscle synergy model was employed toanalyze how the healthy young adults, the healthy elderlypeople and post-stroke patients controlled their muscles todo standing-up motion. Experimental result verified thatfour muscle synergies could represent human standing-upmotion. Our findings showed that the spatial pattern of themuscle synergies were common among the young, the elderlyand the patients. However, the post-stroke patients couldutilize the same synergies to achieve more stable strategy ofstanding-up motion in which they firstly move their CoM ontheir feet and extend their body upward. The results showedthe possibility that the humans changed the temporal patternsof the muscle synergy to realize the different strategies of thestanding-up motion.

In the future study, wider range of post-stroke patients willbe studied. If the muscle synergy structure, including peaktime or gradient of the temporal patterns , differs accordingto the severity of the stroke, this can be used for evaluationof the patients, or it can provide useful information forrehabilitation methodology.

ACKNOWLEDGMENT

This work was supported by JSPS KAKENHI GrantNumber 15K20956, 26120005, 16H04293, JST RISTEX Ser-vice Science, Solutions and Foundation Integrated ResearchProgram.

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