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egress of the vehicle
ARTICLE IN PRESS
JSAE Review 24 (2003) 335–339
Quantitative analysis of muscular stress during ingress/egress of the vehicle Kazuhiko Namamoto, Bunji Atsumi, Haruyuki Kodera, Hitoshi Kanamori Department No. 13, Vehicle Evaluation and Engineering Division 1, Vehicle Development Center 1, Toyota Motor Corporation, Toyota-cho, Toyota-shi, Aichi, 471-8572 Japan Received 19 November 2002; received in revised form 24 February 2003
Abstract Recently elderly users have been increasing in number, and the ease of ingress/egress of the vehicle becomes an important issue. In this paper, a method of evaluating the muscular stress during ingress/egress using several MVC (maximum-voluntary-contraction) was proposed. Further MVC for elderly people was measured as well as for young people, and the increasing ratio of muscular stress for elderly people was made clear as compared with young people. Thus the necessity of an assisting tool for elderly people was confirmed quantitatively. r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction With the arrival of the elderly people’s society, it is desired to develop vehicles which are easy to use for those whose physical functions are deteriorated. For elderly users, there are increasing needs for realizing ingress/egress performance with less muscular stress and developing means for quantitatively indicating such stress. Conventional means mostly quantify the ease of ingress/egress by estimating the stress indirectly from the vehicle dimensions and articular angle [1–2]. In recent years, a new method has been proposed to directly evaluate the muscular stress by measuring myoelectricity during the ingress/egress [3]. This method, however, expresses the stress level according to the reading of a section having the greatest stress only, failing to evaluate the overall physical stress. In addition, in the studies of the ingress/egress of elderly people, their characteristic ingress/egress patterns have been qualitatively analyzed [4] but there is no case where the elderly people’s muscular stress was directly measured for analysis. In the present report, we propose a quantitative analytical method of muscular stress of not only the most stressed section but also the principally used muscles (representative muscles) and demonstrate its effectiveness, simultaneously observing the conditions when elderly people feel that they have major stress.
According to the conditions, we also obtained the performance phase and section of particularly large stress. In addition, to prove the needs for assistance, we obtained the level of greater muscular stress the elderly people have compared with younger people under the same vehicle conditions. At the same time, we discuss the cause of the elderly people’s particular pattern of performance from the viewpoint of muscular stress.
2. Proposal of a muscular stress quantifying method 2.1. Setting of hypotheses On quantifying the muscular stress during the ingress/ egress, we have set up two hypotheses. The first hypothesis reads ‘‘if the stress of a section (muscle) exceeds a given level, the people feel that they have large stress, (hypothesis 1).’’ In the present report, we have used a maximum-voluntary-contraction ratio calculated with the surface myoelectric measurement as the index for the muscular stress level, where the myoelectricity refers to potential generated during muscular contraction, amplitude of which is approximately proportional to the strength (tension) of muscular contraction. Also, the maximum muscular power ratio refers to the ratio of a Fig. 1 measured amplitude (B) to the amplitude (A; Fig. 1) when a maximum-voluntary-contraction is generated for each
0389-4304/03/$30.00 r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. doi:10.1016/S0389-4304(03)00038-9
JSAE20034258
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
336
B
A Maximum power
MVC% =
B A
Platysma Rectus abdominis
Ingress/Egress
Fig. 1. MVC (maximum-voluntary-contraction).
section of muscles expressed in percentage (B/A). This ratio is used in the conventional field of labor science as the index of the muscular stress. As the means for predicting the level of stress, we thought that ‘‘we can express the level of stress of ingress/egress performance with a sum of the stress of the agonists (hypothesis 2)’’ in addition to hypothesis 1. This is because we thought that since we always use multiple sections of muscles simultaneously during ingress/egress performance, it is not enough to pay attention to only one muscle having most stress to express the general stress of ingress/egress. However, in the case the performance pattern and phase are different (when the representative muscles are different), it is insignificant to compare the sums of the stress levels of different representative muscles. Therefore, hypothesis 2 is limited to the comparison of performance of the same representative muscle (performance pattern and phase). 2.2. Clarification of representative muscles Upon verifying the hypotheses, we began by discussing which section is the representative muscle for the ingress and/or egress motion. We measured the myoelectricity at 13 points of the lower limb and trunk (Fig. 2) using one unit each of the sedan and the RV series vehicles. We found the representative muscle for each motion phase where a series of motions was analyzed (Table 1). The ingress/egress patterns include sitting down (standing up) with both feet side by side, entering (stepping out) head first and other motions. In this report, we determined the representative muscle for the most general motion pattern of sitting down on the seat by entering a maximum-voluntary-contraction from obliquely behind (reverse motion for alighting). We discussed the motion phases of ‘‘placing inner foot’’, ‘‘sitting’’ and ‘‘standing up’’ where we confirmed each representative muscle has the major stress. 2.3. Verification of hypothesis 1 Here we state the result of verification of hypothesis 1 by using the case of ‘‘sitting’’ motion. We studied the motion under the following test conditions using a mock-up for evaluation: 1. Mock-up (Fig. 3): Hip point (HP) in sitting position measured from the ground level, height (HP height
Inner leg (Same points as outer leg) Rectus femoris
Erector spinae
Vastus medialis Biceps femoris
Tibialis anterior
Gastrocnemius
Outer leg
Fig. 2. EMG measurement points. Table 1 Representative muscles during ingress/egress
Ingress
Motion phase
Representative muscle
Place inner foot
Rectus abdominis Biceps femoris Gastrocnemius Vastus medialis Gastrocnemius Platysma Rectus abdominis Tibialis anterior Rectus abdominis Tibialis anterior Gastrocnemius Platysma Erector spinae Vastus medialis Rectus abdominis Biceps femoris
Sit Enter head . Enter outer foot Egress
Step out outer foot Place outer foot to GL Step out head Stand up Step out inner foot
Hip Point HP height (variable)
Floor height(variable) Ground
Fig. 3. Mock-up for evaluation.
from the ground) and the floor height (from the ground) are variable. Side sill height from the floor, the side sill lateral position from HP and the roof side rail height can be set to the average positions of vehicles in the market according to the floor height and HP above the ground. No step was provided for the present test, however. 2. Subject of the test: 10 persons, male and female in good health and physical condition (age: 20–50s: 8 persons; 60s: 2 persons).
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
Sum of three MVC Vastus medialis
100
(inner leg)
MVC(%)
Vastus medialis Gastrocnemius
(outer leg)
50
(outer leg)
0 low
high HP Height/Stature
Subjective evaluation Hard ← acceptable→ Easy
Fig. 4. Muscular stress at sitting.
r = 0.79
0
30
50
100
Maximum of %MVC among three muscles Fig. 5. Influence of maximum of %MVC at sitting.
Maximum of MVC is less than 30% (B) Maximum of MVC is more than 30% (A) Subjective evaluation Hard ← acceptable→ Easy
3. Test method: Disposable electrodes are attached to the surfaces of the muscular sections of the representative muscles (gastrocnemius, vastus medialis of inner/outer legs) for the sitting motion on the seat to measure the myoelectricity during the ingress/egress performance, simultaneously conducting the sensory evaluation. Also, to calculate a maximum-voluntarycontraction ratio, myoelectricity of the maximumvoluntary-contraction generated after the test was measured. Regarding the amplitude, time constant of 0.1 s was integrated and the peak value was rendered as the measured value. 4. Test results: Fig. 4 shows the results of a polynomial regression (broken lines) of the maximum-voluntarycontraction ratio of the 3 sections of the representative muscles plotted to the HP ground height. To eliminate the effects of different heights of the subjects, the transverse axis represents the values obtained by dividing the HP ground height with the stature. We think that the maximum-voluntarycontraction ratio of gastrocnemius increases as the subject tends to sit down with the heel of the outer leg raised as the HP ground height rises. On the other hand, we think that vastus medialis increases the maximum-voluntary-contraction ratio because the muscle applies extra braking force to check the impact of sitting as the HP ground height rises. Next, to verify hypothesis 1, we compared the maximum-voluntary-contraction ratio of the muscle having the severest stress among the three representative muscles with the subjective (sensory) evaluation (Fig. 5). In Fig. 5, the result of a discriminant analysis of the cases where ‘‘stress is felt (above the tolerance limit)’’ and ‘‘no stress is felt’’ indicates that the two cases can be classified at the borderline of 30.4%. That is, the subjects feel a major stress if the maximum-voluntary-contraction ratio should exceed about 30% at any one point.
337
r = 0.84
0
100
200
Sum of three %MVC Fig. 6. Influence of sum of three %MVC at sitting.
2.4. Verification of hypothesis 2 We then compared the sum of the maximumvoluntary-contraction ratio of the three representative muscles (solid line in Fig. 4 representing polynomial regression value) with the subjective (sensory) evaluation (Fig. 6). Correlation coefficient when fitted to linear regression r=0.84, which is higher than the correlation coefficient (r=0.79) is shown in Fig. 5. Consequently, the presence of stress feeling is expressed with the size of the maximum-voluntary-contraction ratio of the representative muscles. To make general quantification of the stress feeling, consideration should be given to other
muscular stress than those of the maximum stressed muscles (hypothesis 2 stands). In Fig. 6, the test results are classified into case (A) where there is at least a section having the maximum-voluntary-contraction ratio of over 30% and the case (B) where there is no such section. According to the figure, (A) tends to have the lower sensory evaluation if the sum of the maximum-voluntary-contraction ratio is equivalent (in the vicinity of 50%). We found that it is appropriate to predict the muscular stress by combining hypotheses 1 and 2. Based on the findings thus far obtained, we have been able to verify that ‘‘the subject feels the stress is major if
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
MVC (%)
100
Erector spinae Vastus medialis 50
Vastus medialis
0
low → high HP Height/Stature Fig. 7. Muscular stress at standing up.
100
MVC(%)
Rectus femoris (inner leg) Sum of three MVC
50
Biceps femoris 30
(outer leg) Gastrocnemius
0 low
→ high
(outer leg)
Side Sill Height/Stature
Sum of three MVC (%)
Fig. 8. Muscular stress at placing inner foot.
100
Standing up 50
20%
Elderly(60s) Young
0 low → high HP Height/Stature Sum of three MVC (%)
100
Placing inner foot 50
20%
Elderly(60s) Young
0
low → high Side Sill Height/Stature →
We have thus far analyzed the subjects of 20s to 60s of age as a group. Now, to analyze the characteristics of the elderly subjects, we separated the elderly people (60s) from the young. The test results indicate that the elderly people have the greater muscular stress with MVC ratio greater by approx. 20% than the younger people under the same vehicle conditions (vehicle measurements/ stature) (‘‘Standing up’’ and ‘‘Placing inner foot’’ examples of Fig. 9). Initially, the maximum-voluntarycontraction is defined using the ‘‘muscular strength of the subject’’ as the denominator and the ‘‘stress during ingress/egress’’ as the numerator (Fig. 1). In the case of the same vehicle conditions (that is, the same stress), the
(inner leg)
→
4. Comparison between young and elderly people
(outer leg)
30
3. Clarification of muscles having major stress We measured the maximum-voluntary-contraction ratio in a similar test for the ‘‘stand up’’ motion and ‘‘place inner leg’’ motion (Figs. 7 and 8). In the ‘‘stand up’’ motion, the maximum-voluntarycontraction ratio increases for any of the representative muscles (erector spinae, vastus medialis of inner leg and vastus medialis of outer leg) as the HP ground height lowers. It is assumed that as the HP ground height lowers, the center of gravity shift is greater. Also, with a vehicle of lower HP, test result indicates that the maximum-voluntary-contraction ratio exceeds 30% and the subject feels that the stress is major. In the following test for the ‘‘place inner leg’’ motion, the maximum-voluntary-contraction ratio increases for any of the representative muscles (rectus abdominis of inner leg, biceps femoris of outer leg and gastrocnemius) as the side sill ground height rises. It is assumed that as the side sill ground height rises, the lift of the inner leg increases, simultaneously the muscular strength increases to support the body with a leg (outer). With a vehicle of higher side sill (in the range having no steps), no subject felt a major stress. On the basis of the findings thus far stated, we have found that the motion and muscle having particularly large stress (maximum-voluntary-contraction ratio of over 30%) are gastrocnemius of outer leg (Fig. 4) during sitting motion in the higher HP vehicle and elector spinae and vastus medialis of outer leg (Fig. 7) during standing motion in the lower HP vehicle.
Sum of three MVC
→
the maximum-voluntary-contraction ratio exceeds 30% even at one point’’ and ‘‘if the representative muscles are the same (motion pattern and phase are the same), the general feeling of muscular stress is closely correlated with the sum of the maximum-voluntary-contraction ratio of the representative muscles.’’
→
338
Fig. 9. Comparison between young and elderly people.
numerator for the elderly and the young is the same, while the denominator is different (muscular Fig. 10 strength of the people of 60s is approx. 80% of the young; Fig. 10 [5], which, we assume, is the cause of this result. We have thus verified quantitatively the necessity of an assist tool for the elderly people. In addition, in the
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
Muscular power (%)
100
20% *
50
0 20
30
40
50
60
Age Fig. 10. Muscular power ratio.
‘‘place inner foot’’ motion (placing a foot from obliquely behind), the elderly people have higher muscular stress than the younger people while they stand with the outer leg only. To avoid the stress, we assume that they stand with both feet, then lower their waist first in a motion pattern we observe with the elderly people.
339
the sum of the maximum-voluntary-contraction ratio of the representative muscles. Elderly people have higher muscular stress (approx. 20% higher for the people in their 60s) compared with the young people even under the same vehicle conditions, having major needs for an assist tool.
In addition, we can verify the rate of increase of muscular stress from the rate of decrease of muscular power. 2. On the basis of the above-mentioned human characteristics, we have found the motion (muscles) that requires a major stress: * Standing motion (elector spinae, vastus medialis of outer leg) in the low HP vehicle. * Sitting motion (gastrocnemius of outer leg) in the high HP vehicle.
References 5. Conclusions By measuring the myoelectricity during ingress/egress of a vehicle using a mock-up, we have: 1. Observed the following human characteristics: * Users feel that the stress is major when the maximum muscular power ratio exceeds 30% for at least one muscle. * In the case the representative muscles are the same (same motion pattern and phase), sensory evaluation of muscular stress is closely correlated with
[1] Y. Inuzuka, et al., Ergonomic considerations of vehicle workplace design by using 3-dimensional man mode (in Japanese with English Summary), Proceedings of JSAE, No. 9535963, 1995. [2] T. Shida, et al., Ergonomic consideration for the ease of entry & exit (in Japanese with English Summary), Proceedings of JSAE; No. 9539851, 1995. [3] R. Kogori, et al., Analysis of muscular load during ingress/egress motion, Mazda Tech. Rev. 18 (2000) 98–103. [4] Y. Nakahama, et al., An analysis of motion sequence in vehicle ingress/egress patterns of older people (in Japanese with English Summary), Proceedings of JSAE, No. 20015456, 2001. [5] E. Grandjean, Fitting the Task to the Man, 2nd Edition, Taylor & Francis, London, 1971.
JSAE Review 24 (2003) 335–339
Quantitative analysis of muscular stress during ingress/egress of the vehicle Kazuhiko Namamoto, Bunji Atsumi, Haruyuki Kodera, Hitoshi Kanamori Department No. 13, Vehicle Evaluation and Engineering Division 1, Vehicle Development Center 1, Toyota Motor Corporation, Toyota-cho, Toyota-shi, Aichi, 471-8572 Japan Received 19 November 2002; received in revised form 24 February 2003
Abstract Recently elderly users have been increasing in number, and the ease of ingress/egress of the vehicle becomes an important issue. In this paper, a method of evaluating the muscular stress during ingress/egress using several MVC (maximum-voluntary-contraction) was proposed. Further MVC for elderly people was measured as well as for young people, and the increasing ratio of muscular stress for elderly people was made clear as compared with young people. Thus the necessity of an assisting tool for elderly people was confirmed quantitatively. r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction With the arrival of the elderly people’s society, it is desired to develop vehicles which are easy to use for those whose physical functions are deteriorated. For elderly users, there are increasing needs for realizing ingress/egress performance with less muscular stress and developing means for quantitatively indicating such stress. Conventional means mostly quantify the ease of ingress/egress by estimating the stress indirectly from the vehicle dimensions and articular angle [1–2]. In recent years, a new method has been proposed to directly evaluate the muscular stress by measuring myoelectricity during the ingress/egress [3]. This method, however, expresses the stress level according to the reading of a section having the greatest stress only, failing to evaluate the overall physical stress. In addition, in the studies of the ingress/egress of elderly people, their characteristic ingress/egress patterns have been qualitatively analyzed [4] but there is no case where the elderly people’s muscular stress was directly measured for analysis. In the present report, we propose a quantitative analytical method of muscular stress of not only the most stressed section but also the principally used muscles (representative muscles) and demonstrate its effectiveness, simultaneously observing the conditions when elderly people feel that they have major stress.
According to the conditions, we also obtained the performance phase and section of particularly large stress. In addition, to prove the needs for assistance, we obtained the level of greater muscular stress the elderly people have compared with younger people under the same vehicle conditions. At the same time, we discuss the cause of the elderly people’s particular pattern of performance from the viewpoint of muscular stress.
2. Proposal of a muscular stress quantifying method 2.1. Setting of hypotheses On quantifying the muscular stress during the ingress/ egress, we have set up two hypotheses. The first hypothesis reads ‘‘if the stress of a section (muscle) exceeds a given level, the people feel that they have large stress, (hypothesis 1).’’ In the present report, we have used a maximum-voluntary-contraction ratio calculated with the surface myoelectric measurement as the index for the muscular stress level, where the myoelectricity refers to potential generated during muscular contraction, amplitude of which is approximately proportional to the strength (tension) of muscular contraction. Also, the maximum muscular power ratio refers to the ratio of a Fig. 1 measured amplitude (B) to the amplitude (A; Fig. 1) when a maximum-voluntary-contraction is generated for each
0389-4304/03/$30.00 r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. doi:10.1016/S0389-4304(03)00038-9
JSAE20034258
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
336
B
A Maximum power
MVC% =
B A
Platysma Rectus abdominis
Ingress/Egress
Fig. 1. MVC (maximum-voluntary-contraction).
section of muscles expressed in percentage (B/A). This ratio is used in the conventional field of labor science as the index of the muscular stress. As the means for predicting the level of stress, we thought that ‘‘we can express the level of stress of ingress/egress performance with a sum of the stress of the agonists (hypothesis 2)’’ in addition to hypothesis 1. This is because we thought that since we always use multiple sections of muscles simultaneously during ingress/egress performance, it is not enough to pay attention to only one muscle having most stress to express the general stress of ingress/egress. However, in the case the performance pattern and phase are different (when the representative muscles are different), it is insignificant to compare the sums of the stress levels of different representative muscles. Therefore, hypothesis 2 is limited to the comparison of performance of the same representative muscle (performance pattern and phase). 2.2. Clarification of representative muscles Upon verifying the hypotheses, we began by discussing which section is the representative muscle for the ingress and/or egress motion. We measured the myoelectricity at 13 points of the lower limb and trunk (Fig. 2) using one unit each of the sedan and the RV series vehicles. We found the representative muscle for each motion phase where a series of motions was analyzed (Table 1). The ingress/egress patterns include sitting down (standing up) with both feet side by side, entering (stepping out) head first and other motions. In this report, we determined the representative muscle for the most general motion pattern of sitting down on the seat by entering a maximum-voluntary-contraction from obliquely behind (reverse motion for alighting). We discussed the motion phases of ‘‘placing inner foot’’, ‘‘sitting’’ and ‘‘standing up’’ where we confirmed each representative muscle has the major stress. 2.3. Verification of hypothesis 1 Here we state the result of verification of hypothesis 1 by using the case of ‘‘sitting’’ motion. We studied the motion under the following test conditions using a mock-up for evaluation: 1. Mock-up (Fig. 3): Hip point (HP) in sitting position measured from the ground level, height (HP height
Inner leg (Same points as outer leg) Rectus femoris
Erector spinae
Vastus medialis Biceps femoris
Tibialis anterior
Gastrocnemius
Outer leg
Fig. 2. EMG measurement points. Table 1 Representative muscles during ingress/egress
Ingress
Motion phase
Representative muscle
Place inner foot
Rectus abdominis Biceps femoris Gastrocnemius Vastus medialis Gastrocnemius Platysma Rectus abdominis Tibialis anterior Rectus abdominis Tibialis anterior Gastrocnemius Platysma Erector spinae Vastus medialis Rectus abdominis Biceps femoris
Sit Enter head . Enter outer foot Egress
Step out outer foot Place outer foot to GL Step out head Stand up Step out inner foot
Hip Point HP height (variable)
Floor height(variable) Ground
Fig. 3. Mock-up for evaluation.
from the ground) and the floor height (from the ground) are variable. Side sill height from the floor, the side sill lateral position from HP and the roof side rail height can be set to the average positions of vehicles in the market according to the floor height and HP above the ground. No step was provided for the present test, however. 2. Subject of the test: 10 persons, male and female in good health and physical condition (age: 20–50s: 8 persons; 60s: 2 persons).
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
Sum of three MVC Vastus medialis
100
(inner leg)
MVC(%)
Vastus medialis Gastrocnemius
(outer leg)
50
(outer leg)
0 low
high HP Height/Stature
Subjective evaluation Hard ← acceptable→ Easy
Fig. 4. Muscular stress at sitting.
r = 0.79
0
30
50
100
Maximum of %MVC among three muscles Fig. 5. Influence of maximum of %MVC at sitting.
Maximum of MVC is less than 30% (B) Maximum of MVC is more than 30% (A) Subjective evaluation Hard ← acceptable→ Easy
3. Test method: Disposable electrodes are attached to the surfaces of the muscular sections of the representative muscles (gastrocnemius, vastus medialis of inner/outer legs) for the sitting motion on the seat to measure the myoelectricity during the ingress/egress performance, simultaneously conducting the sensory evaluation. Also, to calculate a maximum-voluntarycontraction ratio, myoelectricity of the maximumvoluntary-contraction generated after the test was measured. Regarding the amplitude, time constant of 0.1 s was integrated and the peak value was rendered as the measured value. 4. Test results: Fig. 4 shows the results of a polynomial regression (broken lines) of the maximum-voluntarycontraction ratio of the 3 sections of the representative muscles plotted to the HP ground height. To eliminate the effects of different heights of the subjects, the transverse axis represents the values obtained by dividing the HP ground height with the stature. We think that the maximum-voluntarycontraction ratio of gastrocnemius increases as the subject tends to sit down with the heel of the outer leg raised as the HP ground height rises. On the other hand, we think that vastus medialis increases the maximum-voluntary-contraction ratio because the muscle applies extra braking force to check the impact of sitting as the HP ground height rises. Next, to verify hypothesis 1, we compared the maximum-voluntary-contraction ratio of the muscle having the severest stress among the three representative muscles with the subjective (sensory) evaluation (Fig. 5). In Fig. 5, the result of a discriminant analysis of the cases where ‘‘stress is felt (above the tolerance limit)’’ and ‘‘no stress is felt’’ indicates that the two cases can be classified at the borderline of 30.4%. That is, the subjects feel a major stress if the maximum-voluntary-contraction ratio should exceed about 30% at any one point.
337
r = 0.84
0
100
200
Sum of three %MVC Fig. 6. Influence of sum of three %MVC at sitting.
2.4. Verification of hypothesis 2 We then compared the sum of the maximumvoluntary-contraction ratio of the three representative muscles (solid line in Fig. 4 representing polynomial regression value) with the subjective (sensory) evaluation (Fig. 6). Correlation coefficient when fitted to linear regression r=0.84, which is higher than the correlation coefficient (r=0.79) is shown in Fig. 5. Consequently, the presence of stress feeling is expressed with the size of the maximum-voluntary-contraction ratio of the representative muscles. To make general quantification of the stress feeling, consideration should be given to other
muscular stress than those of the maximum stressed muscles (hypothesis 2 stands). In Fig. 6, the test results are classified into case (A) where there is at least a section having the maximum-voluntary-contraction ratio of over 30% and the case (B) where there is no such section. According to the figure, (A) tends to have the lower sensory evaluation if the sum of the maximum-voluntary-contraction ratio is equivalent (in the vicinity of 50%). We found that it is appropriate to predict the muscular stress by combining hypotheses 1 and 2. Based on the findings thus far obtained, we have been able to verify that ‘‘the subject feels the stress is major if
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
MVC (%)
100
Erector spinae Vastus medialis 50
Vastus medialis
0
low → high HP Height/Stature Fig. 7. Muscular stress at standing up.
100
MVC(%)
Rectus femoris (inner leg) Sum of three MVC
50
Biceps femoris 30
(outer leg) Gastrocnemius
0 low
→ high
(outer leg)
Side Sill Height/Stature
Sum of three MVC (%)
Fig. 8. Muscular stress at placing inner foot.
100
Standing up 50
20%
Elderly(60s) Young
0 low → high HP Height/Stature Sum of three MVC (%)
100
Placing inner foot 50
20%
Elderly(60s) Young
0
low → high Side Sill Height/Stature →
We have thus far analyzed the subjects of 20s to 60s of age as a group. Now, to analyze the characteristics of the elderly subjects, we separated the elderly people (60s) from the young. The test results indicate that the elderly people have the greater muscular stress with MVC ratio greater by approx. 20% than the younger people under the same vehicle conditions (vehicle measurements/ stature) (‘‘Standing up’’ and ‘‘Placing inner foot’’ examples of Fig. 9). Initially, the maximum-voluntarycontraction is defined using the ‘‘muscular strength of the subject’’ as the denominator and the ‘‘stress during ingress/egress’’ as the numerator (Fig. 1). In the case of the same vehicle conditions (that is, the same stress), the
(inner leg)
→
4. Comparison between young and elderly people
(outer leg)
30
3. Clarification of muscles having major stress We measured the maximum-voluntary-contraction ratio in a similar test for the ‘‘stand up’’ motion and ‘‘place inner leg’’ motion (Figs. 7 and 8). In the ‘‘stand up’’ motion, the maximum-voluntarycontraction ratio increases for any of the representative muscles (erector spinae, vastus medialis of inner leg and vastus medialis of outer leg) as the HP ground height lowers. It is assumed that as the HP ground height lowers, the center of gravity shift is greater. Also, with a vehicle of lower HP, test result indicates that the maximum-voluntary-contraction ratio exceeds 30% and the subject feels that the stress is major. In the following test for the ‘‘place inner leg’’ motion, the maximum-voluntary-contraction ratio increases for any of the representative muscles (rectus abdominis of inner leg, biceps femoris of outer leg and gastrocnemius) as the side sill ground height rises. It is assumed that as the side sill ground height rises, the lift of the inner leg increases, simultaneously the muscular strength increases to support the body with a leg (outer). With a vehicle of higher side sill (in the range having no steps), no subject felt a major stress. On the basis of the findings thus far stated, we have found that the motion and muscle having particularly large stress (maximum-voluntary-contraction ratio of over 30%) are gastrocnemius of outer leg (Fig. 4) during sitting motion in the higher HP vehicle and elector spinae and vastus medialis of outer leg (Fig. 7) during standing motion in the lower HP vehicle.
Sum of three MVC
→
the maximum-voluntary-contraction ratio exceeds 30% even at one point’’ and ‘‘if the representative muscles are the same (motion pattern and phase are the same), the general feeling of muscular stress is closely correlated with the sum of the maximum-voluntary-contraction ratio of the representative muscles.’’
→
338
Fig. 9. Comparison between young and elderly people.
numerator for the elderly and the young is the same, while the denominator is different (muscular Fig. 10 strength of the people of 60s is approx. 80% of the young; Fig. 10 [5], which, we assume, is the cause of this result. We have thus verified quantitatively the necessity of an assist tool for the elderly people. In addition, in the
ARTICLE IN PRESS K. Namamoto et al. / JSAE Review 24 (2003) 335–339
Muscular power (%)
100
20% *
50
0 20
30
40
50
60
Age Fig. 10. Muscular power ratio.
‘‘place inner foot’’ motion (placing a foot from obliquely behind), the elderly people have higher muscular stress than the younger people while they stand with the outer leg only. To avoid the stress, we assume that they stand with both feet, then lower their waist first in a motion pattern we observe with the elderly people.
339
the sum of the maximum-voluntary-contraction ratio of the representative muscles. Elderly people have higher muscular stress (approx. 20% higher for the people in their 60s) compared with the young people even under the same vehicle conditions, having major needs for an assist tool.
In addition, we can verify the rate of increase of muscular stress from the rate of decrease of muscular power. 2. On the basis of the above-mentioned human characteristics, we have found the motion (muscles) that requires a major stress: * Standing motion (elector spinae, vastus medialis of outer leg) in the low HP vehicle. * Sitting motion (gastrocnemius of outer leg) in the high HP vehicle.
References 5. Conclusions By measuring the myoelectricity during ingress/egress of a vehicle using a mock-up, we have: 1. Observed the following human characteristics: * Users feel that the stress is major when the maximum muscular power ratio exceeds 30% for at least one muscle. * In the case the representative muscles are the same (same motion pattern and phase), sensory evaluation of muscular stress is closely correlated with
[1] Y. Inuzuka, et al., Ergonomic considerations of vehicle workplace design by using 3-dimensional man mode (in Japanese with English Summary), Proceedings of JSAE, No. 9535963, 1995. [2] T. Shida, et al., Ergonomic consideration for the ease of entry & exit (in Japanese with English Summary), Proceedings of JSAE; No. 9539851, 1995. [3] R. Kogori, et al., Analysis of muscular load during ingress/egress motion, Mazda Tech. Rev. 18 (2000) 98–103. [4] Y. Nakahama, et al., An analysis of motion sequence in vehicle ingress/egress patterns of older people (in Japanese with English Summary), Proceedings of JSAE, No. 20015456, 2001. [5] E. Grandjean, Fitting the Task to the Man, 2nd Edition, Taylor & Francis, London, 1971.