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Dynamic responses to landing impact at different key segments in selected body positions
Aerospace Science and Technology 12 (2008) 331–336 www.elsevier.com/locate/aescte
Dynamic responses to landing impact at different key segments in selected body positions Bingkun Liu a,∗ , Honglei Ma b , Shizhong Jiang b a Department of Aviation and Space Medicine, The Fourth Military Medical University, Xi’an, China b Institute of Space Medico-engineering, Beijing, China
Received 28 October 2006; received in revised form 27 August 2007; accepted 27 August 2007 Available online 31 August 2007
Abstract The purpose of this study was to observe the acceleration responses of the key body segments to the landing impact in selected body positions. 5 young male subjects in 45 experiments were voluntarily exposed to the peak from 4 to 10g and duration from 50 to 80 ms acceleration pulses at 20◦ supine angle and the peak 10g and duration 50 ms impact at the supine angles from 20 to 60◦ . The acceleration responses on the dropping platform of the impact tower, the seat, the subject’s head, shoulder, chest and ilium, as well as ECG of the subject were measured. The results demonstrated that the acceleration peaks of these key body segments in the chest-back direction had a highly significant positive correlation with the impact level. But their correlation in the head-foot direction was lower than that in the chest-back direction except that of the head. The acceleration peaks of these key body segments in the chest-back direction had a negative correlation with the supine angle. But the acceleration peaks of ilium in the head-foot direction had a positive correlation with the supine angle, and that of the chest almost bore no correlation. There were nonlinear relations between acceleration peak and supine angle at the head and the shoulder respectively, and the acceleration peaks of the head and shoulder in the head-foot direction reached minimum at about 40◦ supine angle. It is concluded that the acceleration responses of the key body segments demonstrate different properties between the chest-back direction and the head-foot direction. It is recommended that the angle of the seat back be adjusted about 40◦ before the spacecraft landing in order to prevent potential head injuries. © 2007 Elsevier Masson SAS. All rights reserved. Keywords: Landing impact; Dynamic responses; Impact acceleration; Human safety
1. Introduction When a manned spacecraft descends to an Earth landing by parachute, the crewmen are exposed to the ground impact forces because of an abrupt deceleration. Our previous studies have demonstrated that human body can tolerate certain levels of landing impact forces, but the high-level impact forces may cause disadvantageous influence on the human body, and even threaten the lives of crew in case of emergencies occurring during a spacecraft landing, such as the failure of the braking rockets ignition [3]. In order to study human responses to impact forces in certain body orientations likely to occur during the landing of Apollo command model, Brown et al. [1] simulated 24 body positions and conducted 288 experiments * Corresponding author.
E-mail address: [email protected] (B. Liu). 1270-9638/$ – see front matter © 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2007.08.004
in which subjects were exposed to the acceleration peak from 5.5 to 30.7g and the onset from 300 to 2500 g/sec. It was observed that impact forces produced effects to the nervous, cardio-respiratory and musculoskeletal systems; and that man can endure certain predicted Apollo landing impact forces in different body orientations without significant incapacitation or undue pain. Weis et al. [9] exposed 20 different volunteer subjects in 75 experiments to six different impact configurations in seven different body orientations by means of a vertical drop tower decelerating with a water inertia-piston and cylinder brake. The drop velocity at brake entry ranged from 4.28 to 8.47 m/s, the peak from 13.5 to 26.6g, the onset from 386 to 1380 g/sec, and the duration from 56 to 75 ms. Neither the subjective reports nor clinical findings indicated that the tolerance end point had been reached in Weis’ series. Stapp et al. [4,5] summarized the human tolerance to the deceleration and reported a series of impact experimental results by using the
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rocket sled in which 58 human volunteer subjects in 146 experiments were exposed to 16 body positions, 7 configurations of the onset (1000, 1500 and 2000 g/sec), magnitude (10, 15, 20 and 25g) and duration from 60 to 130 ms. The results indicated that all the body positions and impact configurations were within voluntary tolerance limits except the forward facing 45◦ reclining position at 25.4g measured on the sled with the onset of 1000 g/sec and the duration of 60 ms, in which compression of soft tissues around 6th, 7th, 8th thoracic vertebrae caused pain and stiffness for 60 days. Head fixation, automation retraction of harness before impact, and energy attenuation to keep the impact force magnitude below 20g and pulse duration less than 60 ms were recommended. Wang Yulan, Cheng Zilong et al. [7,8] conducted many experiments on men, dummies and animals by means of a vertical drop tower, and presented the human tolerance limitations to deceleration. Zhuang Xiangchang et al. [10] summarized the application of the mechanical model in the analysis of human responses to impact forces. In a word, we have known that human responses have relation to the peak g, onset and duration as well as orientations [6]. However, the previous researches had been in general limited to the pathological and physiological effects of the impact. This study was initiated to observe the acceleration responses of the key body segments to the landing impact. This information would furnish guidelines to be used in setting human body position in the spacecraft. 2. Methods 2.1. Test program A total of 45 human impact tests were conducted on 5 young male subjects. All subjects were chosen from male volunteers who met the requirements of a thorough medical evaluation. Selected subjects ranged in age from 19 to 21 years, in height from 166 to 173 cm, and in weight from 59 to 65 kg. There were 4 impact levels for the test of supine angle 20◦ . The acceleration peak ranged from 4 to 10g and the duration from 50 to 80 ms (measured on the dropping platform). The impact level was increased by increment of 2g if the test subject felt little or no effect. The occurrence of adverse reaction, based on an evaluation of the subjective, clinical and physiological responses of the subject by a medical monitor, determined next level of the impact forces tested. Each subject was exposed to 4 impacts in 4 days and to only one impact on the same day lest there would be any accumulated injury effect due to the repetitious impact forces. 5 body positions were selected at the supine angles from 20 to 60◦ . At each body position, the impact peak of 10g and duration of 50 ms (measured on the dropping platform) kept constant. The supine angle was increased by increment of 10◦ if the test subject felt little or no effect. Each subject was exposed to 5 impacts in 5 days and to only one impact on the same day lest there would be any accumulated injury effect due to the repetitious impact forces.
Fig. 1. Acceleration curve on the platform.
2.2. Test equipment The laboratory test equipment used in these experiments was the ISME Vertical Deceleration Tower. It is a guided free-fall device with a controlled deceleration produced by a plunger which displaces water from a cylinder. The entry velocity is controlled by the drop height. The peak of acceleration from 4 to 10g is produced by the rising height of the dropping platform from 1.2 to 1.8 m respectively with the corresponding entry velocities from 4.90 to 6.00 m/s. The deceleration pattern is controlled by the plunger shape. The deceleration pattern (Fig. 1) is readily reproduced. A space seat was installed on the dropping platform of the tower (Fig. 2(a)). The foot-end of the seat was connected to a rigid support frame by a hinge, and the head of the seat was fixed with a metal pole. The angle between the seat back and horizontal plane could be adjusted from 20 to 60◦ (increment of 10◦ ) by increasing the height of the head supporting. The interior profile of the seat was molded closely to the exterior surface of the seat cushion used in these tests. The interior profile of the seat cushion was molded closely to the test subject’s body contour. Before used in these experiments, the seat cushions had been tested to determine their mechanical properties [2]. The seat cushion was made of dacron web and polyurethane foam. The restraint system used is shown in Fig. 2(b). It consists of the chest and leg complex, all of straps 5 cm width dacron webbing and 4000 kg test strength. The chest complex consists of two torso straps and two shoulder belts fastened with a safety belt latch anterior to the sternum. The leg complex consists of a continuous belt on each side which starts at the seat hitch, passes through the knee and connects with the other side seat hitch. 2.3. Instrumentation and measurement Acceleration instrumentation consisted of strain gage accelerometers: one accelerometer (EG&G 3145-20, weight 0.013 kg, measurement range: 20g, frequency responses 0–1050 Hz) mounted in the direction of linear motion on the platform of the impact tower; another same model accelerometer was mounted to the seat lateral frame (approximately to the plane of man–seat system gravity center) in the direction of lin-
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(a)
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(b)
Fig. 2. Installation of seat and the body position, the restraint system and the fixation of the accelerometer on the subject’s head, shoulder, chest and ilium.
ear motion; four accelerometer clusters of two oriented in the subjects chest-back and head-foot directions (EG&G 3145-20, weight 0.046 kg, measurement range: 20g, frequency responses 0–1050 Hz) were secured by strap fasteners over the subject’s head, shoulder, chest and ilium respectively (Fig. 2(b)). The acceleration signals picked up by the accelerometers were amplified and filtered by a voltage amplifier and a low-pass filter with the cut-off frequency of 500 Hz (model A4100), and they were recorded on the magnetic tape by a SONY-1021 magnetic recorder. A DATA-6000 signal analysis instrument processed, displayed and printed these signals. At the same time, the electrocardiogram of the test subject was recorded by a remote ECG-5403 instrument, which was used to monitor the physiological responses pre-run, impact and 5-minute post-impact. In the time domain we made statistical analysis on the peaks of acceleration at the key segments including mean, standard deviation and linear or nonlinear regression analyses. 2.4. Subject protocol The subject was examined and instrumented one hour before the test. After instrumentation checkout in a medical room, the subject put on the “SZ” space-suit (weight 9.8 kg including helmet) and walked to the platform of the impact tower, and then was positioned and strapped into the seat, and briefed about the expected impact by the medical monitor. A 40-second
countdown was begun. ECG data were continuously collected from 40 seconds pre-impact to at least 5 minutes post-impact. The restraints were loosened from the subject immediately after the impact. He was asked about the test and observed for physical signs of the impact. After leaving the impact tower, the subject walked to the checkout room for a medical examination. The examination consisted of checking the vital signs, deep tendon reflexes, ocular reflexes and fundi, heart, lungs, abdomen and gross evaluation of the musculoskeletal system. The subject submitted a written description of his reaction immediately and 24 hours following the impact test. 3. Results 3.1. Dynamic responses at different impact levels The typical acceleration-time curves measured at the head, shoulder, chest and ilium are shown in Fig. 3. Their patterns are similar to damped concussive waves. It can be seen that there are dynamic overshoots at the subject’s head and chest. The results of statistic analyses of acceleration peaks at the head, shoulder, chest and ilium under 4–10g landing impacts are shown in Table 1. It shows that human acceleration responses at different segments vary with the level of landing impact, and there is a certain individual variation. In order to observe the changes in these key segments responses with
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(a)
(b)
(c)
(d)
Fig. 3. Variations of acceleration with time at the subject’s head (a), shoulder (b), chest (c) and ilium (d). (Note: ax denotes acceleration in the chest-back direction; az denotes acceleration in the head-foot direction; aseat denotes acceleration at the seat in the impact direction.) Table 1 Peak acceleration of human body responses at different g levels of impact Position platform seat head z head x shoulder z shoulder x chest z chest x ilium z ilium x
Peak acceleration (mean ± s, n = 5) unit: g g-level 1
g-level 2
g-level 3
g-level 4
4.27 ± 0.12 4.27 ± 0.12 6.38 ± 1.62 8.05 ± 1.68 3.93 ± 1.02 4.84 ± 1.72 6.30 ± 1.87 8.40 ± 1.51 5.48 ± 3.62 8.14 ± 1.44
5.80 ± 0.06 5.43 ± 0.05 9.34 ± 2.93 10.12 ± 2.98 5.92 ± 1.38 5.84 ± 2.31 8.86 ± 3.53 11.19 ± 2.42 5.66 ± 4.69 9.50 ± 1.04
7.43 ± 0.23 7.59 ± 0.29 13.57 ± 4.81 13.74 ± 4.00 8.65 ± 5.43 7.99 ± 2.27 8.45 ± 2.59 13.93 ± 1.56 5.89 ± 2.24 12.20 ± 2.04
9.32 ± 0.17 9.36 ± 0.11 18.07 ± 3.29 18.89 ± 1.85 6.75 ± 1.82 11.92 ± 0.93 10.39 ± 3.97 17.67 ± 2.16 6.00 ± 1.12 15.21 ± 1.17
Table 2 Regression equation between acceleration and impact level Body segment
Direction
Equation
head
Z X Z X Z X Z X
Y Y Y Y Y Y Y Y
shoulder chest ilium
= −3.18548 + 2.25553u (4 u 10) = −1.44832 + 2.12414u (4 u 10) = 2.13763 + 0.62666u (4 u 10) = −1.32584 + 1.34704u (4 u 10) = 4.59038 + 0.59239u (4 u 10) = 1.52128 + 1.69247u (4 u 10) = 5.15827 + 0.09019u (4 u 10) = 1.99133 + 1.39181u (4 u 10)
Correlation coefficient 0.83 0.87 0.39 0.83 0.37 0.87 0.06 0.91
Note: Y , response acceleration (g); u, input acceleration (g). z denotes headfoot direction; x denotes chest-back direction.
Note: z denotes head-foot direction; x denotes chest-back direction.
3.2. Dynamic responses in different body positions the impact level, we conducted the linear regression analyses shown in Table 2. The results show that the peaks at the head, shoulder, chest and ilium in the chest-back direction have a highly significant correlation with the impact level, and increase with the rise of the impact level. But the increasing rate is faster at the head, followed by the chest, ilium and shoulder. In the head-foot direction, the peaks at the head have a higher correlation and increase with the impact level, but the peaks at the shoulder and chest have a lower correlation, and the peaks at the ilium have almost no correlation.
The results of statistic analyses of acceleration peaks at the head, shoulder, chest and ilium under 10g at different supine angles from 20 to 60◦ are shown in Table 3. It shows that human acceleration responses at different segments vary with the supine angle, and there is a certain individual variation. In order to observe the changes in the key segments responses with the supine angles, we conducted the linear and nonlinear regression analyses shown in Table 4. It can be seen that the peaks at the head, chest and ilium in the chest-back direction have a higher correlation, and decrease with the rise of the supine an-
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Table 3 Peak acceleration of human body responses in different body positions Position platform seat head z head x shoulder z shoulder x chest z chest x ilium z ilium x
Peak acceleration (mean ± s, n = 5) unit: g 20◦
30◦
40◦
50◦
60◦
9.32 ± 0.17 9.36 ± 0.11 18.07 ± 3.29 18.89 ± 1.85 6.75 ± 1.82 11.92 ± 0.93 10.39 ± 3.97 17.67 ± 2.16 6.00 ± 1.12 15.21 ± 1.17
9.67 ± 0.20 9.69 ± 0.30 11.81 ± 3.00 16.67 ± 2.17 6.29 ± 0.95 13.43 ± 0.78 15.42 ± 3.61 15.21 ± 1.86 8.11 ± 3.03 13.88 ± 1.45
9.35 ± 0.35 9.55 ± 0.41 10.36 ± 2.44 12.91 ± 0.81 4.93 ± 0.56 12.87 ± 2.49 15.38 ± 3.52 12.07 ± 1.30 12.70 ± 3.76 12.72 ± 2.89
9.61 ± 0.62 10.00 ± 1.02 15.32 ± 3.92 10.91 ± 1.78 8.55 ± 1.78 10.71 ± 3.92 12.60 ± 2.37 10.05 ± 1.86 13.83 ± 1.97 7.79 ± 1.01
9.50 ± 0.10 10.29 ± 0.56 16.66 ± 4.26 7.58 ± 1.18 8.83 ± 1.75 9.24 ± 2.73 12.13 ± 1.53 7.21 ± 1.99 13.04 ± 1.21 6.19 ± 2.11
Note: z denotes head-foot direction; x denotes chest-back direction. Table 4 Regression equation between acceleration and supine angle Body segment
Direction
Equation
head
Z X Z X Z X Z X
Y Y Y Y Y Y Y Y
shoulder chest ilium
Correlation coefficient
= 0.0158α 2 − 1.2559α + 35.76 (20◦ α 60◦ ) = 24.7396 − 0.28378α (20◦ α 60◦ ) = 0.0048α 2 − 0.3291α + 11.402 (20◦ α 60◦ ) = 14.86 − 0.08066α (20◦ α 60◦ ) = 12.9232 + 0.00652α (20◦ α 60◦ ) = 22.8624 − 0.26056α (20◦ α 60◦ ) = 2.8156 + 0.19794α (20◦ α 60◦ ) = 20.8104 − 0.24132α (20◦ α 60◦ )
0.88 0.94 0.79 0.43 0.03 0.91 0.74 0.87
Note: Y , response acceleration (g); α, supine angle (◦ ). z denotes head-foot direction; x denotes chest-back direction.
gle. But the peaks at the shoulder in the chest-back direction have a lower correlation although they decrease with the rise of the supine angle. There are nonlinear relations between acceleration peak and supine angle at the head and the shoulder respectively (Fig. 4), and the acceleration peaks of the head and shoulder in the head-foot direction reached minimum at about 40◦ supine angle. The peaks at the ilium in the head-foot direction increase with the rise of the supine angle, but the peaks at the chest in the head-foot direction have almost no correlation with the supine angle. 3.3. Physiological reactions The subjects complained that they had a feeling of shock at the head and ilium after exposed to the peak from 4 to 10g duration from 50 to 80 ms at 20◦ supine angle and the peak 10g duration 50 ms at the supine angles from 20 to 60◦ , and that they were nervous before the test. The subject’s heart rate speedup was observed from recordings in ECG of the subject during the landing impact because of nervousness. As soon as the platform of impact tower rose, the heart rate of the subject began to speed up. At the moment of landing, the heart rate speeded up again. The maximum heart rate reached 113 per minute. After the impact the heart rate slowed down and recovered one minute later. The ECG of the subject also showed that decreases in the amplitude of R and T wave existed simultaneously with the increase of heart rate during the impact. But the changes could recover within one minute after the impact. Except these changes in ECG of the subject there were no other abnormalities.
4. Discussion When the human body is exposed to the landing impact in supine position, it will make dynamic responses to the impact in both the chest-back and head-foot directions. There are dynamic overshoots at the head and chest, which reflects the viscoelastic behavior of the human body. According to the results of the experiments, human dynamic responses increase with the rise of impact level, but the increase rates are not the same at different parts of the human body. The reason may be related to the body segment’s mechanical properties. With the increase of the angle of body position, human responses demonstrate different properties between the chest-back direction and the head-foot direction. These characteristics may be determined by the biomechanical properties of the human spine. The tolerance of the human body to the overload along the chest-back direction is higher than that along other directions. So astronauts are usually restrained in the seat at about 20◦ supine angle during the spacecraft launch. But what is the suitable body position before the landing in the stage of the spacecraft reentry? The greater a local acceleration is, the worse the damage. So we hope that the acceleration responses of human body, especially at the head, can be minimized during a landing. This experiment indicated that acceleration response at the head reached minimum at about 40◦ body position. Therefore, it may be advantageous that the angle of the seat back is adjusted about 40◦ before the spacecraft landing. Why do the acceleration responses at the head and shoulder reach the minimum at about 40◦ supine angle? We think it may
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(a)
(b)
Fig. 4. Nonlinear regression between acceleration and supine angle. (a) At head in the head-foot direction. (b) At shoulder in the head-foot direction.
be related to the muscle tension near the head-neck or upper limbs. As we know, at different levels of external forces, muscle tissues conduct static or dynamic contraction with different strengths to resist the impact load. When the body position angle is about 40◦ , the muscle near the head-neck or upper limbs may be in the best tension state. In this condition, the stiffness of the head-neck and the shoulder may increase. Therefore, the acceleration responses at the head and shoulder reach the minimum. In addition, we also found that it is very useful that the subjects contract the skeletal muscles actively before and during the landing impact. 5. Summary 45 human impact tests were accomplished. The results indicated that a set of impact conditions was tolerable. The acceleration responses of key body segments were measured. The peaks of the responses of these segments at different impact levels and supine angles were statistically analyzed. The results showed that the acceleration responses of the key body segments demonstrated different properties between the chestback direction and the head-foot direction. It is recommended that the angle of the seat back be adjusted about 40◦ before the spacecraft landing in order to prevent potential head injuries.
References [1] W.K. Brown, J.D. Rothstein, L.P. Foster, Human response to predicted Apollo landing impacts in selected body orientations, Aerospace Med. 37 (4) (1966) 394–398. [2] B. Liu, H. Liu, Experimental research on mechanical properties of seat cushions, AF Tech. Report Z006, 2005. [3] B. Liu, H. Ma, Effect of impact factors on human body and evaluation method of human safety. GF-A0067076M, 2005. [4] J.P. Stapp, Human tolerance to deceleration: Summary of 166 runs, J. Aviat. Med. (1951) 22–42. [5] J.P. Stapp, E.R. Taylor, Space cabin landing impact vector effects on human physiology, Aerospace Med. 35 (1964) 1117–1132. [6] H.E. Von Gierke, J.W. Brinkley, Impact acceleration, in: M. Calvin, O.G. Gazenko (Eds.), Foundations of Space Biology and Medicine, II(1)6, National Aeronautics and Space Administration and Academy of Science of Russia, Washington, DC, 1975, pp. 214–246. [7] Y. Wang, Z. Cheng, Y. Han, Characteristic of human responses to +Gx landing impact, Space Med. Med. Eng. China 5 (2) (1992) 96– 100. [8] Y. Wang, J. Luo, Z. Cheng, An experimental study on +Gx landing impact injury, Space Med. Med. Eng. China 7 (3) (1994) 157–161. [9] E.B. Weis, N.P. Clarke, J.W. Brinkley, Human response to several impact orientations and patterns, Aerospace Med. 34 (1963) 1122– 1129. [10] X. Zhuang, F. Wang, F. Xiao, Application of model in study of impact overload, Space Med. Med. Eng. China 2 (2) (1989) 101–107.
Dynamic responses to landing impact at different key segments in selected body positions Bingkun Liu a,∗ , Honglei Ma b , Shizhong Jiang b a Department of Aviation and Space Medicine, The Fourth Military Medical University, Xi’an, China b Institute of Space Medico-engineering, Beijing, China
Received 28 October 2006; received in revised form 27 August 2007; accepted 27 August 2007 Available online 31 August 2007
Abstract The purpose of this study was to observe the acceleration responses of the key body segments to the landing impact in selected body positions. 5 young male subjects in 45 experiments were voluntarily exposed to the peak from 4 to 10g and duration from 50 to 80 ms acceleration pulses at 20◦ supine angle and the peak 10g and duration 50 ms impact at the supine angles from 20 to 60◦ . The acceleration responses on the dropping platform of the impact tower, the seat, the subject’s head, shoulder, chest and ilium, as well as ECG of the subject were measured. The results demonstrated that the acceleration peaks of these key body segments in the chest-back direction had a highly significant positive correlation with the impact level. But their correlation in the head-foot direction was lower than that in the chest-back direction except that of the head. The acceleration peaks of these key body segments in the chest-back direction had a negative correlation with the supine angle. But the acceleration peaks of ilium in the head-foot direction had a positive correlation with the supine angle, and that of the chest almost bore no correlation. There were nonlinear relations between acceleration peak and supine angle at the head and the shoulder respectively, and the acceleration peaks of the head and shoulder in the head-foot direction reached minimum at about 40◦ supine angle. It is concluded that the acceleration responses of the key body segments demonstrate different properties between the chest-back direction and the head-foot direction. It is recommended that the angle of the seat back be adjusted about 40◦ before the spacecraft landing in order to prevent potential head injuries. © 2007 Elsevier Masson SAS. All rights reserved. Keywords: Landing impact; Dynamic responses; Impact acceleration; Human safety
1. Introduction When a manned spacecraft descends to an Earth landing by parachute, the crewmen are exposed to the ground impact forces because of an abrupt deceleration. Our previous studies have demonstrated that human body can tolerate certain levels of landing impact forces, but the high-level impact forces may cause disadvantageous influence on the human body, and even threaten the lives of crew in case of emergencies occurring during a spacecraft landing, such as the failure of the braking rockets ignition [3]. In order to study human responses to impact forces in certain body orientations likely to occur during the landing of Apollo command model, Brown et al. [1] simulated 24 body positions and conducted 288 experiments * Corresponding author.
E-mail address: [email protected] (B. Liu). 1270-9638/$ – see front matter © 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2007.08.004
in which subjects were exposed to the acceleration peak from 5.5 to 30.7g and the onset from 300 to 2500 g/sec. It was observed that impact forces produced effects to the nervous, cardio-respiratory and musculoskeletal systems; and that man can endure certain predicted Apollo landing impact forces in different body orientations without significant incapacitation or undue pain. Weis et al. [9] exposed 20 different volunteer subjects in 75 experiments to six different impact configurations in seven different body orientations by means of a vertical drop tower decelerating with a water inertia-piston and cylinder brake. The drop velocity at brake entry ranged from 4.28 to 8.47 m/s, the peak from 13.5 to 26.6g, the onset from 386 to 1380 g/sec, and the duration from 56 to 75 ms. Neither the subjective reports nor clinical findings indicated that the tolerance end point had been reached in Weis’ series. Stapp et al. [4,5] summarized the human tolerance to the deceleration and reported a series of impact experimental results by using the
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rocket sled in which 58 human volunteer subjects in 146 experiments were exposed to 16 body positions, 7 configurations of the onset (1000, 1500 and 2000 g/sec), magnitude (10, 15, 20 and 25g) and duration from 60 to 130 ms. The results indicated that all the body positions and impact configurations were within voluntary tolerance limits except the forward facing 45◦ reclining position at 25.4g measured on the sled with the onset of 1000 g/sec and the duration of 60 ms, in which compression of soft tissues around 6th, 7th, 8th thoracic vertebrae caused pain and stiffness for 60 days. Head fixation, automation retraction of harness before impact, and energy attenuation to keep the impact force magnitude below 20g and pulse duration less than 60 ms were recommended. Wang Yulan, Cheng Zilong et al. [7,8] conducted many experiments on men, dummies and animals by means of a vertical drop tower, and presented the human tolerance limitations to deceleration. Zhuang Xiangchang et al. [10] summarized the application of the mechanical model in the analysis of human responses to impact forces. In a word, we have known that human responses have relation to the peak g, onset and duration as well as orientations [6]. However, the previous researches had been in general limited to the pathological and physiological effects of the impact. This study was initiated to observe the acceleration responses of the key body segments to the landing impact. This information would furnish guidelines to be used in setting human body position in the spacecraft. 2. Methods 2.1. Test program A total of 45 human impact tests were conducted on 5 young male subjects. All subjects were chosen from male volunteers who met the requirements of a thorough medical evaluation. Selected subjects ranged in age from 19 to 21 years, in height from 166 to 173 cm, and in weight from 59 to 65 kg. There were 4 impact levels for the test of supine angle 20◦ . The acceleration peak ranged from 4 to 10g and the duration from 50 to 80 ms (measured on the dropping platform). The impact level was increased by increment of 2g if the test subject felt little or no effect. The occurrence of adverse reaction, based on an evaluation of the subjective, clinical and physiological responses of the subject by a medical monitor, determined next level of the impact forces tested. Each subject was exposed to 4 impacts in 4 days and to only one impact on the same day lest there would be any accumulated injury effect due to the repetitious impact forces. 5 body positions were selected at the supine angles from 20 to 60◦ . At each body position, the impact peak of 10g and duration of 50 ms (measured on the dropping platform) kept constant. The supine angle was increased by increment of 10◦ if the test subject felt little or no effect. Each subject was exposed to 5 impacts in 5 days and to only one impact on the same day lest there would be any accumulated injury effect due to the repetitious impact forces.
Fig. 1. Acceleration curve on the platform.
2.2. Test equipment The laboratory test equipment used in these experiments was the ISME Vertical Deceleration Tower. It is a guided free-fall device with a controlled deceleration produced by a plunger which displaces water from a cylinder. The entry velocity is controlled by the drop height. The peak of acceleration from 4 to 10g is produced by the rising height of the dropping platform from 1.2 to 1.8 m respectively with the corresponding entry velocities from 4.90 to 6.00 m/s. The deceleration pattern is controlled by the plunger shape. The deceleration pattern (Fig. 1) is readily reproduced. A space seat was installed on the dropping platform of the tower (Fig. 2(a)). The foot-end of the seat was connected to a rigid support frame by a hinge, and the head of the seat was fixed with a metal pole. The angle between the seat back and horizontal plane could be adjusted from 20 to 60◦ (increment of 10◦ ) by increasing the height of the head supporting. The interior profile of the seat was molded closely to the exterior surface of the seat cushion used in these tests. The interior profile of the seat cushion was molded closely to the test subject’s body contour. Before used in these experiments, the seat cushions had been tested to determine their mechanical properties [2]. The seat cushion was made of dacron web and polyurethane foam. The restraint system used is shown in Fig. 2(b). It consists of the chest and leg complex, all of straps 5 cm width dacron webbing and 4000 kg test strength. The chest complex consists of two torso straps and two shoulder belts fastened with a safety belt latch anterior to the sternum. The leg complex consists of a continuous belt on each side which starts at the seat hitch, passes through the knee and connects with the other side seat hitch. 2.3. Instrumentation and measurement Acceleration instrumentation consisted of strain gage accelerometers: one accelerometer (EG&G 3145-20, weight 0.013 kg, measurement range: 20g, frequency responses 0–1050 Hz) mounted in the direction of linear motion on the platform of the impact tower; another same model accelerometer was mounted to the seat lateral frame (approximately to the plane of man–seat system gravity center) in the direction of lin-
B. Liu et al. / Aerospace Science and Technology 12 (2008) 331–336
(a)
333
(b)
Fig. 2. Installation of seat and the body position, the restraint system and the fixation of the accelerometer on the subject’s head, shoulder, chest and ilium.
ear motion; four accelerometer clusters of two oriented in the subjects chest-back and head-foot directions (EG&G 3145-20, weight 0.046 kg, measurement range: 20g, frequency responses 0–1050 Hz) were secured by strap fasteners over the subject’s head, shoulder, chest and ilium respectively (Fig. 2(b)). The acceleration signals picked up by the accelerometers were amplified and filtered by a voltage amplifier and a low-pass filter with the cut-off frequency of 500 Hz (model A4100), and they were recorded on the magnetic tape by a SONY-1021 magnetic recorder. A DATA-6000 signal analysis instrument processed, displayed and printed these signals. At the same time, the electrocardiogram of the test subject was recorded by a remote ECG-5403 instrument, which was used to monitor the physiological responses pre-run, impact and 5-minute post-impact. In the time domain we made statistical analysis on the peaks of acceleration at the key segments including mean, standard deviation and linear or nonlinear regression analyses. 2.4. Subject protocol The subject was examined and instrumented one hour before the test. After instrumentation checkout in a medical room, the subject put on the “SZ” space-suit (weight 9.8 kg including helmet) and walked to the platform of the impact tower, and then was positioned and strapped into the seat, and briefed about the expected impact by the medical monitor. A 40-second
countdown was begun. ECG data were continuously collected from 40 seconds pre-impact to at least 5 minutes post-impact. The restraints were loosened from the subject immediately after the impact. He was asked about the test and observed for physical signs of the impact. After leaving the impact tower, the subject walked to the checkout room for a medical examination. The examination consisted of checking the vital signs, deep tendon reflexes, ocular reflexes and fundi, heart, lungs, abdomen and gross evaluation of the musculoskeletal system. The subject submitted a written description of his reaction immediately and 24 hours following the impact test. 3. Results 3.1. Dynamic responses at different impact levels The typical acceleration-time curves measured at the head, shoulder, chest and ilium are shown in Fig. 3. Their patterns are similar to damped concussive waves. It can be seen that there are dynamic overshoots at the subject’s head and chest. The results of statistic analyses of acceleration peaks at the head, shoulder, chest and ilium under 4–10g landing impacts are shown in Table 1. It shows that human acceleration responses at different segments vary with the level of landing impact, and there is a certain individual variation. In order to observe the changes in these key segments responses with
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(a)
(b)
(c)
(d)
Fig. 3. Variations of acceleration with time at the subject’s head (a), shoulder (b), chest (c) and ilium (d). (Note: ax denotes acceleration in the chest-back direction; az denotes acceleration in the head-foot direction; aseat denotes acceleration at the seat in the impact direction.) Table 1 Peak acceleration of human body responses at different g levels of impact Position platform seat head z head x shoulder z shoulder x chest z chest x ilium z ilium x
Peak acceleration (mean ± s, n = 5) unit: g g-level 1
g-level 2
g-level 3
g-level 4
4.27 ± 0.12 4.27 ± 0.12 6.38 ± 1.62 8.05 ± 1.68 3.93 ± 1.02 4.84 ± 1.72 6.30 ± 1.87 8.40 ± 1.51 5.48 ± 3.62 8.14 ± 1.44
5.80 ± 0.06 5.43 ± 0.05 9.34 ± 2.93 10.12 ± 2.98 5.92 ± 1.38 5.84 ± 2.31 8.86 ± 3.53 11.19 ± 2.42 5.66 ± 4.69 9.50 ± 1.04
7.43 ± 0.23 7.59 ± 0.29 13.57 ± 4.81 13.74 ± 4.00 8.65 ± 5.43 7.99 ± 2.27 8.45 ± 2.59 13.93 ± 1.56 5.89 ± 2.24 12.20 ± 2.04
9.32 ± 0.17 9.36 ± 0.11 18.07 ± 3.29 18.89 ± 1.85 6.75 ± 1.82 11.92 ± 0.93 10.39 ± 3.97 17.67 ± 2.16 6.00 ± 1.12 15.21 ± 1.17
Table 2 Regression equation between acceleration and impact level Body segment
Direction
Equation
head
Z X Z X Z X Z X
Y Y Y Y Y Y Y Y
shoulder chest ilium
= −3.18548 + 2.25553u (4 u 10) = −1.44832 + 2.12414u (4 u 10) = 2.13763 + 0.62666u (4 u 10) = −1.32584 + 1.34704u (4 u 10) = 4.59038 + 0.59239u (4 u 10) = 1.52128 + 1.69247u (4 u 10) = 5.15827 + 0.09019u (4 u 10) = 1.99133 + 1.39181u (4 u 10)
Correlation coefficient 0.83 0.87 0.39 0.83 0.37 0.87 0.06 0.91
Note: Y , response acceleration (g); u, input acceleration (g). z denotes headfoot direction; x denotes chest-back direction.
Note: z denotes head-foot direction; x denotes chest-back direction.
3.2. Dynamic responses in different body positions the impact level, we conducted the linear regression analyses shown in Table 2. The results show that the peaks at the head, shoulder, chest and ilium in the chest-back direction have a highly significant correlation with the impact level, and increase with the rise of the impact level. But the increasing rate is faster at the head, followed by the chest, ilium and shoulder. In the head-foot direction, the peaks at the head have a higher correlation and increase with the impact level, but the peaks at the shoulder and chest have a lower correlation, and the peaks at the ilium have almost no correlation.
The results of statistic analyses of acceleration peaks at the head, shoulder, chest and ilium under 10g at different supine angles from 20 to 60◦ are shown in Table 3. It shows that human acceleration responses at different segments vary with the supine angle, and there is a certain individual variation. In order to observe the changes in the key segments responses with the supine angles, we conducted the linear and nonlinear regression analyses shown in Table 4. It can be seen that the peaks at the head, chest and ilium in the chest-back direction have a higher correlation, and decrease with the rise of the supine an-
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Table 3 Peak acceleration of human body responses in different body positions Position platform seat head z head x shoulder z shoulder x chest z chest x ilium z ilium x
Peak acceleration (mean ± s, n = 5) unit: g 20◦
30◦
40◦
50◦
60◦
9.32 ± 0.17 9.36 ± 0.11 18.07 ± 3.29 18.89 ± 1.85 6.75 ± 1.82 11.92 ± 0.93 10.39 ± 3.97 17.67 ± 2.16 6.00 ± 1.12 15.21 ± 1.17
9.67 ± 0.20 9.69 ± 0.30 11.81 ± 3.00 16.67 ± 2.17 6.29 ± 0.95 13.43 ± 0.78 15.42 ± 3.61 15.21 ± 1.86 8.11 ± 3.03 13.88 ± 1.45
9.35 ± 0.35 9.55 ± 0.41 10.36 ± 2.44 12.91 ± 0.81 4.93 ± 0.56 12.87 ± 2.49 15.38 ± 3.52 12.07 ± 1.30 12.70 ± 3.76 12.72 ± 2.89
9.61 ± 0.62 10.00 ± 1.02 15.32 ± 3.92 10.91 ± 1.78 8.55 ± 1.78 10.71 ± 3.92 12.60 ± 2.37 10.05 ± 1.86 13.83 ± 1.97 7.79 ± 1.01
9.50 ± 0.10 10.29 ± 0.56 16.66 ± 4.26 7.58 ± 1.18 8.83 ± 1.75 9.24 ± 2.73 12.13 ± 1.53 7.21 ± 1.99 13.04 ± 1.21 6.19 ± 2.11
Note: z denotes head-foot direction; x denotes chest-back direction. Table 4 Regression equation between acceleration and supine angle Body segment
Direction
Equation
head
Z X Z X Z X Z X
Y Y Y Y Y Y Y Y
shoulder chest ilium
Correlation coefficient
= 0.0158α 2 − 1.2559α + 35.76 (20◦ α 60◦ ) = 24.7396 − 0.28378α (20◦ α 60◦ ) = 0.0048α 2 − 0.3291α + 11.402 (20◦ α 60◦ ) = 14.86 − 0.08066α (20◦ α 60◦ ) = 12.9232 + 0.00652α (20◦ α 60◦ ) = 22.8624 − 0.26056α (20◦ α 60◦ ) = 2.8156 + 0.19794α (20◦ α 60◦ ) = 20.8104 − 0.24132α (20◦ α 60◦ )
0.88 0.94 0.79 0.43 0.03 0.91 0.74 0.87
Note: Y , response acceleration (g); α, supine angle (◦ ). z denotes head-foot direction; x denotes chest-back direction.
gle. But the peaks at the shoulder in the chest-back direction have a lower correlation although they decrease with the rise of the supine angle. There are nonlinear relations between acceleration peak and supine angle at the head and the shoulder respectively (Fig. 4), and the acceleration peaks of the head and shoulder in the head-foot direction reached minimum at about 40◦ supine angle. The peaks at the ilium in the head-foot direction increase with the rise of the supine angle, but the peaks at the chest in the head-foot direction have almost no correlation with the supine angle. 3.3. Physiological reactions The subjects complained that they had a feeling of shock at the head and ilium after exposed to the peak from 4 to 10g duration from 50 to 80 ms at 20◦ supine angle and the peak 10g duration 50 ms at the supine angles from 20 to 60◦ , and that they were nervous before the test. The subject’s heart rate speedup was observed from recordings in ECG of the subject during the landing impact because of nervousness. As soon as the platform of impact tower rose, the heart rate of the subject began to speed up. At the moment of landing, the heart rate speeded up again. The maximum heart rate reached 113 per minute. After the impact the heart rate slowed down and recovered one minute later. The ECG of the subject also showed that decreases in the amplitude of R and T wave existed simultaneously with the increase of heart rate during the impact. But the changes could recover within one minute after the impact. Except these changes in ECG of the subject there were no other abnormalities.
4. Discussion When the human body is exposed to the landing impact in supine position, it will make dynamic responses to the impact in both the chest-back and head-foot directions. There are dynamic overshoots at the head and chest, which reflects the viscoelastic behavior of the human body. According to the results of the experiments, human dynamic responses increase with the rise of impact level, but the increase rates are not the same at different parts of the human body. The reason may be related to the body segment’s mechanical properties. With the increase of the angle of body position, human responses demonstrate different properties between the chest-back direction and the head-foot direction. These characteristics may be determined by the biomechanical properties of the human spine. The tolerance of the human body to the overload along the chest-back direction is higher than that along other directions. So astronauts are usually restrained in the seat at about 20◦ supine angle during the spacecraft launch. But what is the suitable body position before the landing in the stage of the spacecraft reentry? The greater a local acceleration is, the worse the damage. So we hope that the acceleration responses of human body, especially at the head, can be minimized during a landing. This experiment indicated that acceleration response at the head reached minimum at about 40◦ body position. Therefore, it may be advantageous that the angle of the seat back is adjusted about 40◦ before the spacecraft landing. Why do the acceleration responses at the head and shoulder reach the minimum at about 40◦ supine angle? We think it may
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(a)
(b)
Fig. 4. Nonlinear regression between acceleration and supine angle. (a) At head in the head-foot direction. (b) At shoulder in the head-foot direction.
be related to the muscle tension near the head-neck or upper limbs. As we know, at different levels of external forces, muscle tissues conduct static or dynamic contraction with different strengths to resist the impact load. When the body position angle is about 40◦ , the muscle near the head-neck or upper limbs may be in the best tension state. In this condition, the stiffness of the head-neck and the shoulder may increase. Therefore, the acceleration responses at the head and shoulder reach the minimum. In addition, we also found that it is very useful that the subjects contract the skeletal muscles actively before and during the landing impact. 5. Summary 45 human impact tests were accomplished. The results indicated that a set of impact conditions was tolerable. The acceleration responses of key body segments were measured. The peaks of the responses of these segments at different impact levels and supine angles were statistically analyzed. The results showed that the acceleration responses of the key body segments demonstrated different properties between the chestback direction and the head-foot direction. It is recommended that the angle of the seat back be adjusted about 40◦ before the spacecraft landing in order to prevent potential head injuries.
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