Ergonomics
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
A three-dimensional ultrasonic system for posture measurement HONGWEI HSIAO & W. MONROE KEYSERLING To cite this article: HONGWEI HSIAO & W. MONROE KEYSERLING (1990) A threedimensional ultrasonic system for posture measurement, Ergonomics, 33:9, 1089-1114, DOI: 10.1080/00140139008925316 To link to this article: http://dx.doi.org/10.1080/00140139008925316
Published online: 24 Oct 2007.
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Citing articles: 17 View citing articles
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Date: 11 November 2015, At: 08:23
ERGONOMICS, 1990, VOL. 33, No.9, 1089-1114
A three-dimensional ultrasonic system for posture measurement HONGWEI HSIAO and W. MONROE KEYSERLING Center for Ergonomics, Department of Industrial and Operations Engineering, The University of Michigan, 1205 Beal Avenue, Ann Arbor, MI 48109-2117, USA Keywords: Three-dimensional; Posture; Measurement instrument; Ultrasonic;
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Joint angle; Behaviour. A time-efficient, cost-effective, and accurate system has been developed for measuring static three-dimensional joint coordinates in the laboratory. This system uses a personal computer interface to determine the distance between transmitters positioned at body joints and receivers positioned near the subject by measuring the travel time of ultrasound. Distance data are then converted to spatial coordinates and joint angles. The system can determine the location of 14 joints at one time. An experiment using three distances and fiveorientations between a transmitter and a receiver was performed to investigate the significance of measurement errors for the new system. The results showed that the standard deviation of the distance measurement was about 0·2 em for single orientation conditions and was about I em for all conditions tested. A second experiment using 11 transmitters and four receivers was performed to investigate the significance of measurement errors when determining three-dimensional coordinates. The results showed no significant difference between actual and measured coordinates. The system was then used to study the posture of a subject's upper extremity. Eight postures representing a variety of typical reaching tasks were examined. The results showed that the system was suitable for three-dimensional posture measurement.
I. Introduction Awkward body posture is a principal risk factor associated with musculoskeletal injuries and disorders during occupational activities (Chaffin et al. 1977, Ayoub et al. 1978, NIOSH 1981, Chaffin and Andersson 1984, Westgaard and Aaras 1984, Hagberg 1984, Keyserling et al. 1988). In order to provide a three-dimensional description of body posture, it is necessary iO measure the location (or coordinates) of major joints and the angles between adjacent body segments. Several approaches for measuring three-dimensional posture have been previously developed, including goniometry, photogrammetry, optoelectric analysis, video analysis, optomechanic analysis, and sonic analysis. These methods are discussed below. 1.1. Goniomelry
In this family of systems, goniometers are attached at the joints of interest to measure body angles. Several types of goniometric systems have been developed for the measurement of limb posture. Since Karpovich et al. (1960) first published their electrogoniometer study ofjoint motion during walking, the device has been a valuable tool in the study of motion and posture. However, the measurement capability of their device was limited to a single plane. In 1971, Lamoreux described a three-dimensional exoskeleton goniometer which could simultaneously record motion at three joints. Further, Hannah et al. (1979) developed a light-weight three-dimentional goniometer for recording the motion of three joints. In a study of gait, Isacson et al. (1986) modified 0014-0139/90 $3-00 © 1990 Taylor & Francis Ltd.
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the light-weight goniometer for improved attachment to the body and an improved computer interface. Several other types of electrogoniometer systems have been extensively described by Chao (1978). In general, goniometric methods involve a simple concept (i.e.,direct measurement of joint angles) and no reference to an external coordinate system. However, there are potential and real changes in postural behaviour due to the attachment of devices which can change a subject's normal motion pattern (Chao 1978). In addition, the coordinates of the measured joints are unknown. This could be a limitation in some engineering studies. Finally, mounting, aligning, and calibrating the instruments are tedious tasks which require extra care to minimize error. 1.2. Photogrammetry In photogrammetric systems, either light-reflective markers or light-emitting diodes are attached to the body to identify joint coordinates or angular data through data reduction procedures. Several photogrammetric systems for recording threedimensional aspects of postures/movements have been developed. The non-stereophotogrammetric methods used two or three cameras to record motion or posture, with an othogonal arrangement among the crossing optical axes. This technique has been used in recording the motion of arm activities (Drillis 1959), obtaining anthropometric data for an escape capsule design (Chaffee 1961),examining human posture while performing manual tasks (Kilpatrick 1970),and studying the link system of the human torso (Snyder et al. 1972). The stereophotogrammetric method was first suggested by Zeller (1953) as a technique for biomechanical applications. The method used two still cameras with parallel optical axes to obtain three-dimensional measurements. Similar methods were also used to measure body contours, surface area, and volume (Hertzberg et al. 1957, Weissman 1968), and to measure body segment movement while an operator performed pedal depression tasks (Bullock and Harley 1972).Stereophotogrammetric methods have also been used for close-range measurement such as mapping of human knee-bones (Ghosh 1983) and the anthropometric study of hands (Ghosh and Poirier 1987). In general, photogrammetric methods have the advantage that body reference markers are smaller and lower in mass than exoskeleton-goniometric systems; thus, interference with motion is minimal. The problems associated with photogramrnetric methods are that reference points or markers may be obscured (shadowed) from the camera by other body segments, and that the methods usually involve a tremendous amount of time-consuming data-decoding procedures. In addition, the extensive calculations required for the systems may produce cumulative errors (Chao 1978). 1.3. Optoelectric analysis Optoelectric systems use the same principles as photogrammetric systems to locate the joint positions. A computer-controlled optical detector is used in place of the regular camera, and an optoelectic sensing unit is used instead of film. The Selspot system is an example. It is comprised of one or more optical detectors, a set of infrared light emitting diodes with one or more control units, and a computer. Light emitting diodes (LED) arc attached at joints and other points of interest. Two or more optical detectors are used to measure postures or movements in three dimensions based on the principles of stereophotogrammetry. The computer substantially reduces data reduction and
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analysis time (Selcom Selective Electronic Inc. 1984). The advantages of optoelectric systems include the automatic processing of data and preparation of stick diagrams for visual presentation of joint motion. The disadvantages are that the data accuracy depends on environmental lighting conditions, and that the basic system is expensive (Chao 1978). 1.4. Video analysis Video systems share the same basic principles as photogrammetric systems but were developed to permit posture/motion analysis at a fast sampling rate. Either reflective markers or rapidly pulsed light emitting diodes are placed on the body. Example applications are the VICON System (Oxford Medilog Inc. 1984), and Expert Vision Motion Analysis System (Motion Analysis Corporation 1986). The VICON system consists of up to seven TV cameras, a coordinate generator, a computer, several retroreflective markers, and various instruments. The cameras are used to capture the movement of passive markers attached to the parts of the body where motion measurement is desired. Bright points appearing in the TV picture are identified by short pulses in the video signal. Each TV camera is connected to a marker detector circuit which converts these pulses into digital timing signals. A coordinate generator then computes the three-dimensional position of the markers (Oxford Medilog Tnc. 1984). The Expert Vision Motion Analysis System consists of a video camera, a video recorder, a video monitor, a video processor, a personal computer and a plotter. Video data are directly processed, or recorded on a video tape for later processing. The video processor converts images of body markers to digital outlines, which are further processed to determine postural coordinates. With one camera, the system can be used for two-dimensional studies. The system can measure three-dimensional postures by using two video cameras and the appropriate data reduction software (Motion Analysis Corporation 1987). In general, the advantages of videographic systems include the ability to handle data in real time and the easy attachment of markers. A problem with these systems is that markers can be blocked or obscured as with the photogrammetric methods (Chao 1978). Cross-point-conflict is another limitation. This occurs when a landmark's position is lost due to the inability ofthe system to track more than one landmark when two are in close proximity (Towle 1986). In most systems, lighting conditions also need to be controlled with no point-light sources in the background. In addition, videographic systems usually are expensive ($25 000 minimum) in relation to the other systems described previously (Chaffin and Anderson 1984). 1.5. OplOmechanic analysis In optomechanic systems, the joint locations are determined according to the same principles as in the stereophotographic or video systems. Optical mechanical units such as mirror scanners rathern than video cameras or electric image detectors are utilized to detect target signals. The CODA-3 Movement Monitoring System is an example of this category. It uses three in-line rotating mirror scanners to detect up to 12 reflective, target mounted, landmarks. The three-dimensional coordinates of all landmarks are computed in real time through an IBM PC or compatible computer (Towle 1986). The advantages of the system are its real time analysis and accuracy of measurement (Movement Techniques Limited 1987). The problem of landmark obscuration is
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similar to that of video systems. Other possible limitations would be cost and potential difficulties in distinguishing targets that located at the same horizontal or vertical lines. 1.6. Sonic analysis Relatively few applications of sonic techniques have been used for three-dimensional posture/motion measurements. A system was developed by Goldman and Nadler (1956) to measure displacements, velocities, and accelerations of a body segment. It consisted of a small cylindrical emitter that was strapped to the test subject, an oscillator, three microphones, and various instruments. Ultrasonic waves generated by the oscillators and broadcast by the emitter were picked up by the microphones. At each microphone, the frequency received varied from the emitted frequency according to the Doppler effect. As a result, received frequency was directly proportional to velocity. An application of this system for hand motion was reported by Kattan and Nadler (1969). Although this method was a useful technique for motion study, it was not immediately suitable for measurement of joint coordinates or joint angles (Fleischer and Lange 1983). In addition, the fact that the three microphones need to be situated along three mutually perpendicular axes was a limitation due to the possibility that the signal could be blocked or obscured by body segments. Another sonic approach was reported by Fleischer and Lange (1983), Fleischer and Becker (1986) and Berners (1986) for hand motion studies. Three receivers and up to three transmitters were used. The time delay from transmission to reception determined the distance from the transmitter to each receiver. The distances were converted to three-dimensional coordinates. The advantages of the systems were low cost and ease of set-up. However, the systems' capacity was three transmitters, thus limiting its use to a single joint. In addition, the transmitters needed to face in specific directions, which limited the ability to measure certain types of motion. 1.7. Summary Most of the systems described are relatively expensive and/or require considerable time to perform an analysis. In addition, shadowing (or obscuring of a point of interest) is a general problem. A body joint might be blocked from view by the detectors in certain postures. This limits the general applicability of these systems.
2. Objective The primary objective of this investigation was to develop and evaluate a cost-effective, time-efficient, and accurate system for measuring three-dimensional static body postures in a laboratory setting. 3.
Methodology and equipment
3.1. General principle Ultrasound has been used to measure distance in a variety of products and applications, such as automatic focusing cameras and sonar. In these applications, distance is measured in only one dimension. The ultrasonic measurement system described below is three-dimensional. It measures the travel time of ultrasound to determine the distance between transmitters placed on a subject's body and receivers placed around the subject. A personal computer interface is then used to determine the three-dimensional coordinates of the transmitters. The transmitters, which are attached to the joints of the body, generate a 42 KHz ultrasonic wave. The receivers,
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which interface with an IBM PC computer through amplifiers and a multiplexer, detect the signal. For three-dimensional analysis, three receivers are used to transform the raw distance measurements to spatial coordinates (figure I). The coordinates of a transmitter are computed with respect to a reference system in the laboratory. The equations of the relationships among transmitters and receivers are as follows: (X _X 1)2+(y_ YI)2+(Z-ZI)2=Vi
(1.1)
(X -X 2)2 +(Y- y2)2 +(Z -Z2)2 =V~
(1.2)
(X -X 3)2 +(Y- y3)2 +(Z -Z3)2 =V~
(1.3)
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where (X, Y, Z) are the unknown coordinates of the transmitter; (X I' YI,ZI) are the known coordinates of Receiver 1; (X 2' Y2 , Z 2) are the known coordinates of Receiver 2; (X 3' Y3, Z 3) are the known coordinates of Receiver 3; and
VI' V 2 , and V 3 are the measured distances from Receivers 1, 2, and 3 to the transmitter.
Since the equations are simultaneous and quadratic, two sets of coordinates may be obtained: a 'true' point and a 'false' point. The solutions for these equations are presented in Appendix I. To eliminate the 'false' answer, two alternatives are available. One alternative involves controlling experimental procedures; the other alternative involves redundant measurements. The experimental control procedure involves setting all the points to be measured on the same side ofthe plane defined by receivers I, 2, and 3. A simple test is used to eliminate the answer (i.e., point) which is located on the 'wrong' side of the plane. The redundant procedure is to use a fourth receiver. The fourth receiver can be set at any position with known coordinates X 4' Y4 , and Z4. The actual distance between this fourth receiver and the transmitter is measured V 4. The distances from the two 'candidate' points to Receiver 4 can be calculated and compared to V 4. The distance which matches V 4 corresponds to the true point. As discussed in the Introduction, shadowing (the condition where a body point is obscured from detectors [receivers] by other body segments) is a general problem for z
Receiver 2 (X2. Y2.Z2)
Figure I.
Geometric relationships among receivers and a transmitter.
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posture analysis systems which might result in missing data or measurement error. To overcome the problem, the ultrasonic system uses more than three receivers to detect signals to reduce the likelihood that a transmitter is obscured from the necessary receivers. The system can be connected to up to eight receivers. Any combination of three unobscured receivers can be used to generate a set of coordinates for a transmitter position. Because this may result in redundant data, two alternatives can be used to process redundant joint locations. The first alternative involves sorting-and-averaging procedures. Since the coordinates obtained through each combination of three receivers may be slightly deviated due to measurement error, the data are sorted in greatest to smallest order. The middle one-third points in the data set are averaged and used as the final coordinates. For example, if 20 sets of coordinates for a joint are obtained using 8 receivers, the 8th to 14th ranked values of the sorted coordinates will be averaged as the final coordinates. The other alternative involves pre-selection procedures. With this method, the experimentor selects certain receivers for a joint based on the relative location of the joint and receivers. In this case, three unobscured receivers would be chosen to obtain the coordinates. In addition to the above approaches, the ultrasonic system also allows the user to relocate receiver positions to eliminate the shadowing problem. It only takes a few minutes to calibrate receiver positions, using a set of three known points (pre-set points) and a calibration program. This technique is useful if specific postures are to be measured and no redundant receivers are used. 3.2. Components of the system The system consists of the following components: a personal computer (IBM PC or compatible), an ultrasonic measurement adaptor board for the PC, two transmitter multiplexers and a receiver multiplexer. This set-up enables the system to use up to 14 transmitters and eight receivers (figure 2). The 14 transmitters can be mounted on 14 body joints; the eight receivers can be set at any position around the transmitters. As described in the last section, only three receivers are usually needed to obtain the spatial coordinates of a transmitter. The extra receivers are provided to overcome shadow
Transmil1el' I
Receiver Multiplex.er TransmiUtt Multiplexer
Figure 2. The ultrasonic measurement system.
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3.3. Required software Two programs were developed to drive the ultrasonic measurement system. The measurement program, written in assembler language, controls the ultrasonic hardware. The data reduction program, written in PASCAL, reduces the distance data to spatial coordinates. The measurement program allows the experimentalist to select the number of transmitters and receivers to be used. The measured data can either be processed immediately through the data reduction program or be stored on a floppy disk for processing at a later time. Measurement and data processing procedures are shown in figure 4.
4. The evaluation experiments Three experiments were performed to evaluate the measurement system. They were a sensitivity and calibration experiment (Experiment I), an accuracy/precision experiment (Experiment 2), and a human postural behaviour experiment (Experiment 3). The procedures and the results are presented below. 4.1. Experiment 1: sensitivity and calibration experiment 4.1.1. Objectives: The objectives of this experiment were; (I) to test the sensitivity of the system to varying distances between the transmitter and receiver and the relative orientations between the transmitter and receiver; and (2) to calibrate the system using known coordinates of transmitters and receivers.
4.1.2. Method: A 3 x 5 x 3 full factorial analysis of variance with three replications was used to evaluate the contributions to measurement variance due to the distance
Data reduction program
Measurement program
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Table 4. Measured versus actual coordinates (ern),
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T-square
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1·14 0·95 0·49 2-20 1·17 2·02 0·11 0·13 0·04 0·94
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4.3.2. Method: The experiment measured joint surface mark locations of the right upper extremity of a subject, using eight postures (four directions of humerus and two different elbow angles) with six replications for each posture. 4.3.3. Independent variables: Humerus direction: Four directions (downward, forward, upward, and rightward) were used in the experiment. The subject was instructed to position the humerus in each direction. Elbow angle: Two elbow angles, 90 0 and 1800 (fully extended), were tested.
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4.3.4. Dependent variables: The dependent variables were the observed shoulder horizontal angle, shoulder vertical angle, and elbow angle. 4.3.5. Procedure: Six transmitters and eight receivers were used for the study. The six transmitters were mounted on the surfaces of a subject's right upper extremity, corresponding to the anterior side of Lesser Tubercle and the posterior side of the Greater Tubercle (shoulder), the Lateral Epicondyle and the Medial Epicondyle (elbow), and the Radial Styloid Process and the Ulnar Styloid Process (wrist)(figure 7). The midpoint of a line passing through the anterior side of the lesser tubercle and the posterior side of the greater tubercle was assumed to be the shoulder joint centre. The midpoint of a line passing through the medial and lateral epicondyles was assumed to be the elbow joint centre; and the midpoint of a line passing through the styloid processes of the radius and ulna was assumed to be the wrist joint centre. The eight receivers were set around the subject as shown in figure 3. The subject performed the postures while seated. She was asked to position her right humerus in four directions: downward, forward, upward, and rightward with two different elbow angles, 90 0 and 180 (full extension). The hand orientation was kept semi-prone for all the postures. Posture measurements were made with regard to three-dimensional coordinates of surface marks at the shoulder, elbow, and wrist. Joint angles were then calculated. Each posture was performed six times. The 48 observations (4 humerus positions x 2 elbow angles x 6 replications) were performed in random order. 0
4.3.6. Definitions: The joint angles are calculated on the assumption that the neutral posture of the upper extremities occurs when the arms hang down, parallel to the trunk. The joint angles and corresponding postures for the right side of the body are
3
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2. Bodyor humerus J.Greater tubm:1e 4. Lesser tubercle 5. Shoulder jointcenttt (RisbI_._vicw)
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ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
A three-dimensional ultrasonic system for posture measurement HONGWEI HSIAO & W. MONROE KEYSERLING To cite this article: HONGWEI HSIAO & W. MONROE KEYSERLING (1990) A threedimensional ultrasonic system for posture measurement, Ergonomics, 33:9, 1089-1114, DOI: 10.1080/00140139008925316 To link to this article: http://dx.doi.org/10.1080/00140139008925316
Published online: 24 Oct 2007.
Submit your article to this journal
Article views: 33
View related articles
Citing articles: 17 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=terg20 Download by: [Monash University Library]
Date: 11 November 2015, At: 08:23
ERGONOMICS, 1990, VOL. 33, No.9, 1089-1114
A three-dimensional ultrasonic system for posture measurement HONGWEI HSIAO and W. MONROE KEYSERLING Center for Ergonomics, Department of Industrial and Operations Engineering, The University of Michigan, 1205 Beal Avenue, Ann Arbor, MI 48109-2117, USA Keywords: Three-dimensional; Posture; Measurement instrument; Ultrasonic;
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Joint angle; Behaviour. A time-efficient, cost-effective, and accurate system has been developed for measuring static three-dimensional joint coordinates in the laboratory. This system uses a personal computer interface to determine the distance between transmitters positioned at body joints and receivers positioned near the subject by measuring the travel time of ultrasound. Distance data are then converted to spatial coordinates and joint angles. The system can determine the location of 14 joints at one time. An experiment using three distances and fiveorientations between a transmitter and a receiver was performed to investigate the significance of measurement errors for the new system. The results showed that the standard deviation of the distance measurement was about 0·2 em for single orientation conditions and was about I em for all conditions tested. A second experiment using 11 transmitters and four receivers was performed to investigate the significance of measurement errors when determining three-dimensional coordinates. The results showed no significant difference between actual and measured coordinates. The system was then used to study the posture of a subject's upper extremity. Eight postures representing a variety of typical reaching tasks were examined. The results showed that the system was suitable for three-dimensional posture measurement.
I. Introduction Awkward body posture is a principal risk factor associated with musculoskeletal injuries and disorders during occupational activities (Chaffin et al. 1977, Ayoub et al. 1978, NIOSH 1981, Chaffin and Andersson 1984, Westgaard and Aaras 1984, Hagberg 1984, Keyserling et al. 1988). In order to provide a three-dimensional description of body posture, it is necessary iO measure the location (or coordinates) of major joints and the angles between adjacent body segments. Several approaches for measuring three-dimensional posture have been previously developed, including goniometry, photogrammetry, optoelectric analysis, video analysis, optomechanic analysis, and sonic analysis. These methods are discussed below. 1.1. Goniomelry
In this family of systems, goniometers are attached at the joints of interest to measure body angles. Several types of goniometric systems have been developed for the measurement of limb posture. Since Karpovich et al. (1960) first published their electrogoniometer study ofjoint motion during walking, the device has been a valuable tool in the study of motion and posture. However, the measurement capability of their device was limited to a single plane. In 1971, Lamoreux described a three-dimensional exoskeleton goniometer which could simultaneously record motion at three joints. Further, Hannah et al. (1979) developed a light-weight three-dimentional goniometer for recording the motion of three joints. In a study of gait, Isacson et al. (1986) modified 0014-0139/90 $3-00 © 1990 Taylor & Francis Ltd.
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Downloaded by [Monash University Library] at 08:23 11 November 2015
the light-weight goniometer for improved attachment to the body and an improved computer interface. Several other types of electrogoniometer systems have been extensively described by Chao (1978). In general, goniometric methods involve a simple concept (i.e.,direct measurement of joint angles) and no reference to an external coordinate system. However, there are potential and real changes in postural behaviour due to the attachment of devices which can change a subject's normal motion pattern (Chao 1978). In addition, the coordinates of the measured joints are unknown. This could be a limitation in some engineering studies. Finally, mounting, aligning, and calibrating the instruments are tedious tasks which require extra care to minimize error. 1.2. Photogrammetry In photogrammetric systems, either light-reflective markers or light-emitting diodes are attached to the body to identify joint coordinates or angular data through data reduction procedures. Several photogrammetric systems for recording threedimensional aspects of postures/movements have been developed. The non-stereophotogrammetric methods used two or three cameras to record motion or posture, with an othogonal arrangement among the crossing optical axes. This technique has been used in recording the motion of arm activities (Drillis 1959), obtaining anthropometric data for an escape capsule design (Chaffee 1961),examining human posture while performing manual tasks (Kilpatrick 1970),and studying the link system of the human torso (Snyder et al. 1972). The stereophotogrammetric method was first suggested by Zeller (1953) as a technique for biomechanical applications. The method used two still cameras with parallel optical axes to obtain three-dimensional measurements. Similar methods were also used to measure body contours, surface area, and volume (Hertzberg et al. 1957, Weissman 1968), and to measure body segment movement while an operator performed pedal depression tasks (Bullock and Harley 1972).Stereophotogrammetric methods have also been used for close-range measurement such as mapping of human knee-bones (Ghosh 1983) and the anthropometric study of hands (Ghosh and Poirier 1987). In general, photogrammetric methods have the advantage that body reference markers are smaller and lower in mass than exoskeleton-goniometric systems; thus, interference with motion is minimal. The problems associated with photogramrnetric methods are that reference points or markers may be obscured (shadowed) from the camera by other body segments, and that the methods usually involve a tremendous amount of time-consuming data-decoding procedures. In addition, the extensive calculations required for the systems may produce cumulative errors (Chao 1978). 1.3. Optoelectric analysis Optoelectric systems use the same principles as photogrammetric systems to locate the joint positions. A computer-controlled optical detector is used in place of the regular camera, and an optoelectic sensing unit is used instead of film. The Selspot system is an example. It is comprised of one or more optical detectors, a set of infrared light emitting diodes with one or more control units, and a computer. Light emitting diodes (LED) arc attached at joints and other points of interest. Two or more optical detectors are used to measure postures or movements in three dimensions based on the principles of stereophotogrammetry. The computer substantially reduces data reduction and
Three-dimensional ultrasonic system for posture measurement
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analysis time (Selcom Selective Electronic Inc. 1984). The advantages of optoelectric systems include the automatic processing of data and preparation of stick diagrams for visual presentation of joint motion. The disadvantages are that the data accuracy depends on environmental lighting conditions, and that the basic system is expensive (Chao 1978). 1.4. Video analysis Video systems share the same basic principles as photogrammetric systems but were developed to permit posture/motion analysis at a fast sampling rate. Either reflective markers or rapidly pulsed light emitting diodes are placed on the body. Example applications are the VICON System (Oxford Medilog Inc. 1984), and Expert Vision Motion Analysis System (Motion Analysis Corporation 1986). The VICON system consists of up to seven TV cameras, a coordinate generator, a computer, several retroreflective markers, and various instruments. The cameras are used to capture the movement of passive markers attached to the parts of the body where motion measurement is desired. Bright points appearing in the TV picture are identified by short pulses in the video signal. Each TV camera is connected to a marker detector circuit which converts these pulses into digital timing signals. A coordinate generator then computes the three-dimensional position of the markers (Oxford Medilog Tnc. 1984). The Expert Vision Motion Analysis System consists of a video camera, a video recorder, a video monitor, a video processor, a personal computer and a plotter. Video data are directly processed, or recorded on a video tape for later processing. The video processor converts images of body markers to digital outlines, which are further processed to determine postural coordinates. With one camera, the system can be used for two-dimensional studies. The system can measure three-dimensional postures by using two video cameras and the appropriate data reduction software (Motion Analysis Corporation 1987). In general, the advantages of videographic systems include the ability to handle data in real time and the easy attachment of markers. A problem with these systems is that markers can be blocked or obscured as with the photogrammetric methods (Chao 1978). Cross-point-conflict is another limitation. This occurs when a landmark's position is lost due to the inability ofthe system to track more than one landmark when two are in close proximity (Towle 1986). In most systems, lighting conditions also need to be controlled with no point-light sources in the background. In addition, videographic systems usually are expensive ($25 000 minimum) in relation to the other systems described previously (Chaffin and Anderson 1984). 1.5. OplOmechanic analysis In optomechanic systems, the joint locations are determined according to the same principles as in the stereophotographic or video systems. Optical mechanical units such as mirror scanners rathern than video cameras or electric image detectors are utilized to detect target signals. The CODA-3 Movement Monitoring System is an example of this category. It uses three in-line rotating mirror scanners to detect up to 12 reflective, target mounted, landmarks. The three-dimensional coordinates of all landmarks are computed in real time through an IBM PC or compatible computer (Towle 1986). The advantages of the system are its real time analysis and accuracy of measurement (Movement Techniques Limited 1987). The problem of landmark obscuration is
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similar to that of video systems. Other possible limitations would be cost and potential difficulties in distinguishing targets that located at the same horizontal or vertical lines. 1.6. Sonic analysis Relatively few applications of sonic techniques have been used for three-dimensional posture/motion measurements. A system was developed by Goldman and Nadler (1956) to measure displacements, velocities, and accelerations of a body segment. It consisted of a small cylindrical emitter that was strapped to the test subject, an oscillator, three microphones, and various instruments. Ultrasonic waves generated by the oscillators and broadcast by the emitter were picked up by the microphones. At each microphone, the frequency received varied from the emitted frequency according to the Doppler effect. As a result, received frequency was directly proportional to velocity. An application of this system for hand motion was reported by Kattan and Nadler (1969). Although this method was a useful technique for motion study, it was not immediately suitable for measurement of joint coordinates or joint angles (Fleischer and Lange 1983). In addition, the fact that the three microphones need to be situated along three mutually perpendicular axes was a limitation due to the possibility that the signal could be blocked or obscured by body segments. Another sonic approach was reported by Fleischer and Lange (1983), Fleischer and Becker (1986) and Berners (1986) for hand motion studies. Three receivers and up to three transmitters were used. The time delay from transmission to reception determined the distance from the transmitter to each receiver. The distances were converted to three-dimensional coordinates. The advantages of the systems were low cost and ease of set-up. However, the systems' capacity was three transmitters, thus limiting its use to a single joint. In addition, the transmitters needed to face in specific directions, which limited the ability to measure certain types of motion. 1.7. Summary Most of the systems described are relatively expensive and/or require considerable time to perform an analysis. In addition, shadowing (or obscuring of a point of interest) is a general problem. A body joint might be blocked from view by the detectors in certain postures. This limits the general applicability of these systems.
2. Objective The primary objective of this investigation was to develop and evaluate a cost-effective, time-efficient, and accurate system for measuring three-dimensional static body postures in a laboratory setting. 3.
Methodology and equipment
3.1. General principle Ultrasound has been used to measure distance in a variety of products and applications, such as automatic focusing cameras and sonar. In these applications, distance is measured in only one dimension. The ultrasonic measurement system described below is three-dimensional. It measures the travel time of ultrasound to determine the distance between transmitters placed on a subject's body and receivers placed around the subject. A personal computer interface is then used to determine the three-dimensional coordinates of the transmitters. The transmitters, which are attached to the joints of the body, generate a 42 KHz ultrasonic wave. The receivers,
Three-dimensional ultrasonic system for posture measurement
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which interface with an IBM PC computer through amplifiers and a multiplexer, detect the signal. For three-dimensional analysis, three receivers are used to transform the raw distance measurements to spatial coordinates (figure I). The coordinates of a transmitter are computed with respect to a reference system in the laboratory. The equations of the relationships among transmitters and receivers are as follows: (X _X 1)2+(y_ YI)2+(Z-ZI)2=Vi
(1.1)
(X -X 2)2 +(Y- y2)2 +(Z -Z2)2 =V~
(1.2)
(X -X 3)2 +(Y- y3)2 +(Z -Z3)2 =V~
(1.3)
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where (X, Y, Z) are the unknown coordinates of the transmitter; (X I' YI,ZI) are the known coordinates of Receiver 1; (X 2' Y2 , Z 2) are the known coordinates of Receiver 2; (X 3' Y3, Z 3) are the known coordinates of Receiver 3; and
VI' V 2 , and V 3 are the measured distances from Receivers 1, 2, and 3 to the transmitter.
Since the equations are simultaneous and quadratic, two sets of coordinates may be obtained: a 'true' point and a 'false' point. The solutions for these equations are presented in Appendix I. To eliminate the 'false' answer, two alternatives are available. One alternative involves controlling experimental procedures; the other alternative involves redundant measurements. The experimental control procedure involves setting all the points to be measured on the same side ofthe plane defined by receivers I, 2, and 3. A simple test is used to eliminate the answer (i.e., point) which is located on the 'wrong' side of the plane. The redundant procedure is to use a fourth receiver. The fourth receiver can be set at any position with known coordinates X 4' Y4 , and Z4. The actual distance between this fourth receiver and the transmitter is measured V 4. The distances from the two 'candidate' points to Receiver 4 can be calculated and compared to V 4. The distance which matches V 4 corresponds to the true point. As discussed in the Introduction, shadowing (the condition where a body point is obscured from detectors [receivers] by other body segments) is a general problem for z
Receiver 2 (X2. Y2.Z2)
Figure I.
Geometric relationships among receivers and a transmitter.
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H. Hsiao and W M. Keyserling
posture analysis systems which might result in missing data or measurement error. To overcome the problem, the ultrasonic system uses more than three receivers to detect signals to reduce the likelihood that a transmitter is obscured from the necessary receivers. The system can be connected to up to eight receivers. Any combination of three unobscured receivers can be used to generate a set of coordinates for a transmitter position. Because this may result in redundant data, two alternatives can be used to process redundant joint locations. The first alternative involves sorting-and-averaging procedures. Since the coordinates obtained through each combination of three receivers may be slightly deviated due to measurement error, the data are sorted in greatest to smallest order. The middle one-third points in the data set are averaged and used as the final coordinates. For example, if 20 sets of coordinates for a joint are obtained using 8 receivers, the 8th to 14th ranked values of the sorted coordinates will be averaged as the final coordinates. The other alternative involves pre-selection procedures. With this method, the experimentor selects certain receivers for a joint based on the relative location of the joint and receivers. In this case, three unobscured receivers would be chosen to obtain the coordinates. In addition to the above approaches, the ultrasonic system also allows the user to relocate receiver positions to eliminate the shadowing problem. It only takes a few minutes to calibrate receiver positions, using a set of three known points (pre-set points) and a calibration program. This technique is useful if specific postures are to be measured and no redundant receivers are used. 3.2. Components of the system The system consists of the following components: a personal computer (IBM PC or compatible), an ultrasonic measurement adaptor board for the PC, two transmitter multiplexers and a receiver multiplexer. This set-up enables the system to use up to 14 transmitters and eight receivers (figure 2). The 14 transmitters can be mounted on 14 body joints; the eight receivers can be set at any position around the transmitters. As described in the last section, only three receivers are usually needed to obtain the spatial coordinates of a transmitter. The extra receivers are provided to overcome shadow
Transmil1el' I
Receiver Multiplex.er TransmiUtt Multiplexer
Figure 2. The ultrasonic measurement system.
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H. Hsiao and W M. Keyserling
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3.3. Required software Two programs were developed to drive the ultrasonic measurement system. The measurement program, written in assembler language, controls the ultrasonic hardware. The data reduction program, written in PASCAL, reduces the distance data to spatial coordinates. The measurement program allows the experimentalist to select the number of transmitters and receivers to be used. The measured data can either be processed immediately through the data reduction program or be stored on a floppy disk for processing at a later time. Measurement and data processing procedures are shown in figure 4.
4. The evaluation experiments Three experiments were performed to evaluate the measurement system. They were a sensitivity and calibration experiment (Experiment I), an accuracy/precision experiment (Experiment 2), and a human postural behaviour experiment (Experiment 3). The procedures and the results are presented below. 4.1. Experiment 1: sensitivity and calibration experiment 4.1.1. Objectives: The objectives of this experiment were; (I) to test the sensitivity of the system to varying distances between the transmitter and receiver and the relative orientations between the transmitter and receiver; and (2) to calibrate the system using known coordinates of transmitters and receivers.
4.1.2. Method: A 3 x 5 x 3 full factorial analysis of variance with three replications was used to evaluate the contributions to measurement variance due to the distance
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T-square
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1·14 0·95 0·49 2-20 1·17 2·02 0·11 0·13 0·04 0·94
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4.3.2. Method: The experiment measured joint surface mark locations of the right upper extremity of a subject, using eight postures (four directions of humerus and two different elbow angles) with six replications for each posture. 4.3.3. Independent variables: Humerus direction: Four directions (downward, forward, upward, and rightward) were used in the experiment. The subject was instructed to position the humerus in each direction. Elbow angle: Two elbow angles, 90 0 and 1800 (fully extended), were tested.
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4.3.4. Dependent variables: The dependent variables were the observed shoulder horizontal angle, shoulder vertical angle, and elbow angle. 4.3.5. Procedure: Six transmitters and eight receivers were used for the study. The six transmitters were mounted on the surfaces of a subject's right upper extremity, corresponding to the anterior side of Lesser Tubercle and the posterior side of the Greater Tubercle (shoulder), the Lateral Epicondyle and the Medial Epicondyle (elbow), and the Radial Styloid Process and the Ulnar Styloid Process (wrist)(figure 7). The midpoint of a line passing through the anterior side of the lesser tubercle and the posterior side of the greater tubercle was assumed to be the shoulder joint centre. The midpoint of a line passing through the medial and lateral epicondyles was assumed to be the elbow joint centre; and the midpoint of a line passing through the styloid processes of the radius and ulna was assumed to be the wrist joint centre. The eight receivers were set around the subject as shown in figure 3. The subject performed the postures while seated. She was asked to position her right humerus in four directions: downward, forward, upward, and rightward with two different elbow angles, 90 0 and 180 (full extension). The hand orientation was kept semi-prone for all the postures. Posture measurements were made with regard to three-dimensional coordinates of surface marks at the shoulder, elbow, and wrist. Joint angles were then calculated. Each posture was performed six times. The 48 observations (4 humerus positions x 2 elbow angles x 6 replications) were performed in random order. 0
4.3.6. Definitions: The joint angles are calculated on the assumption that the neutral posture of the upper extremities occurs when the arms hang down, parallel to the trunk. The joint angles and corresponding postures for the right side of the body are
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