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A triaxial accelerometer for measuring arm movements
Applied Ergonomics 33 (2002) 541–547
A triaxial accelerometer for measuring arm movements Eva Bernmarka,b,*, Christina Wiktorina,b b
a Department of Occupational Health, Stockholm County Council, Stockholm 17176, Sweden Department of Public Health Sciences, Division of Occupational health, Karolinska Institute, Stockholm, Sweden
Received 27 April 2002; received in revised form 14 May 2002; accepted 5 July 2002
Abstract A triaxial accelerometer used as an inclinometer (INC) (Logger Technology, Sweden) needed to be evaluated for field measurements of arm postures and movements. INC consists of one portable data logger and up to four sensors. Each sensor measures the inclination to the vertical line. The sampling frequency is 20 Hz and, the logger can collect data for up to 12 h. The aim of the present study was to compare INC measurements with those from an optoelectronic measuring system—the Mac Reflex system (OPT) (Qualisys AB, Sweden). For all movements with normal to high velocities INC measured the degree of arm elevation with very high precision. At very high velocities and, especially when the direction of the velocity was perpendicular to the vertical line the largest differences between INC and OPT were found. In field studies INC has proved to be very safe and useful in assessment of arm movements. INC was easy to handle and easy to wear for the researcher and the subject, respectively. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Inclinometer; Accelerometer; Arm movements
1. Introduction Epidemiological studies indicate that frequent or sustained work with elevated arms increases the risk of neck/shoulder disorders (Ari.ens et al., 2000; Bernard, 1997; Guangyan and Buckle, 1999; Van der Windt et al., 2000). However, information about the degree of arm elevation causing this increased risk is still lacking, as is information about the duration and frequencies of harmful work with elevated arms (Winkel and Westgaard, 1992). It is also unclear whether ergonomic intervention programs facilitate recovery after an episode of neck/shoulder disorder (Westgaard and Winkel, 1997). Therefore, there is a need for accurate and precise instruments that can measure arm elevation angles for whole days without disturbing the worker ( (Hansson and Mikkelsen, 1997; Akesson et al., 1997). A portable three-axis accelerometer (Logger Technol. Sweden) used as an inclinometer has ogy HB, Malmo, been developed for measuring postures and movements *Corresponding author. Tel.: + 46-8-517-731-49; fax: +46-8-33-4333. E-mail address: [email protected] (E. Bernmark).
over time. The inclinometer system (INC) can measure the inclination of a body segment in relation to the vertical line (the line of gravity), and also the direction of the inclination. It cannot measure rotation. The instrument has been tested in laboratory tests and is considered very precise (Hansson et al., 2001a). However, the INC has not been tested for measuring human movements. In most human movements and especially in the upper extremities, rotation around the long axis of the actual body segment occurs simultaneously with other movements. It is thus important to test whether the INC really can measure the degree and the direction of arm elevation movements. The aim of the present study was to compare INC measurements with those from optoelectronic measuring system of known high precision, —the Mac Reflex system (OPT) (Peterson and Palmerud, 1996; Axelsson, 1992). The following specific questions were to be answered: (1) Can the INC measure the elevation of the upper arm in relation to the vertical line? (2) Can the INC differentiate between arm flexion and arm abduction, i.e. can the INC measure the direction of the movement?
0003-6870/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 0 2 ) 0 0 0 7 2 - 8
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E. Bernmark, C. Wiktorin / Applied Ergonomics 33 (2002) 541–547
2. Material and method
‘‘Angleprogram’’(Department of Occupational Health, Stockholm, Sweden).
2.1. Subjects Six healthy, right-handed test persons, three females and three males, participated in the tests. The mean age was 43 years (ranging from 28 to 58 years) and mean height 175.8 cm (148–193 cm). 2.2. Inclinometer The INC consists of a data logger and up to four sensors (Fig. 1). The logger, based on PCMCIA flash memory cards, has a memory capacity of 20 Mb which is enough to measure up to 12 h with four sensors. The sampling frequency is 20 Hz. The data logger is small and can be used attached to a belt around the subject’s waist. The sensors, weighing only 20 g each, are taped to the skin on the relevant body segments. Each sensor measures the total acceleration (r) acting on the body segment, inclination (y) in relation to the vertical line and the direction (j) of the inclination. Thus, the position of each sensor, and thereby the body segment, is described by spherical co-ordinates (r; y; j). The sensor cannot measure rotation around the long axis of the moving body segment. Each sensor consists of three miniature accelerometers mounted perpendicular to each other in the x, y and z directions. For a threedimensional analysis, three accelerometers are needed, each measuring both static and dynamic acceleration in one direction. Static acceleration equals gravity (1 g=9.8 m/s2) i.e. the acceleration acting on the sensor in rest. When the body segment moves, the INC sensor is also influenced by dynamic acceleration. The principle for INC measurement is to record continuously how gravity is distributed over the three accelerometers. After a recording period, the data is transferred to a computer for analysis with a software program e.g. the
2.3. Mac reflex The reference system used in this study was a threedimensional optoelectronic movement analysing system, the Mac Reflex system (OPT) (Qualisys AB, Sweden). The OPT consists of infrared flashlights mounted on video cameras, video processors, and a computer with the Mac Reflex 3D software. Small reflective spherical markers are applied to the body segment to be studied, and the rectangular co-ordinates (x, y, z) for these markers are recorded by two or up to seven of the specially adapted video cameras. The sampling frequency is 50 Hz. Each marker’s change in position can then be used to calculate e.g. distance, angle, velocity and acceleration. OPT produces position data with high precision (Axelsson, 1992; Peterson and Palmerud, 1996). 2.4. Experimental procedure One INC sensor was taped caudal to the insertion of the deltoid muscle on the right upper arm. Two 20 mm OPT markers were taped on the same arm, one on the caput of humerus 3 cm caudal to the border of acromion and one on the lateral epicondyle of humerus. In this study, The OPT consisted of three video cameras. After attachment to the subject’s arm the INC sensor was calibrated by defining the vertical line and the direction of the body segment. The vertical line (y ¼ 01) was defined by having the subject sit and bend slightly to the right with a 2 Kg weight in the hand so the arm hung vertically. The direction of the right arm was defined by having the subject stand and hold the upper arm at 901 abduction. (j ¼ 901). Five test movements of the right upper arm were then recorded simultaneously by the INC and the OPT. 2.5. Test movements Three of the five tests were performed in order to investigate how well the INC measured the degree of arm elevation (y) and the direction (j) when the arm was held in different positions i.e. the sensors were only influenced by static acceleration (gravity). The other two tests were performed to investigate how well the INC can measure arm positions (y; j) when the arm is moving at different rates i.e. when the sensors are influenced by both static and dynamic accelerations.
Fig. 1. The Inclinometer system consists of a data logger with a PCMCIA flash memory card and up to four sensors.
1. The starting position was sitting with the arm hanging by the side (y ¼ 01). The arm was then slowly flexed through the whole motion range (0–
E. Bernmark, C. Wiktorin / Applied Ergonomics 33 (2002) 541–547
2. 3.
4.
5.
1801) with 5-s long pauses at about 451, 901, 1351 and 1801 of flexion. Arm abduction was performed similarly. The starting position was sitting with the shoulder and the elbow flexed to about 901 and the upper arm in maximal outward rotation. The elbow was supported on a table. The arm was then rotated inwards through the whole rotation range with four 5-s pauses at about 01 (lower arm vertical), 451, 901 and maximal internal rotation. The starting position was standing with the arm hanging by the side. Arm flexion–extension through the whole motion range was then performed with three different rates; ‘‘slow’’, 6 arm swings per minute (0.10 Hz), ‘‘high’’, 24 arm swings per minute (0.40 Hz), and ‘‘very high’’, 45 arm swings per minute (0.75 Hz). The rate was controlled by following a person making the same movements on a video and with the help of a metronome. Each movement rate was recorded for 30 s. Each subject was instructed to paint a 1 1 m2 large board for 3 min with a paintbrush. The paint was in a tin on a table beside the subject. The upper edge of the board was at a height of 1.9 m. The subjects were instructed to work in their own way and tempo. This experiment was done in a standing position.
543
direction of arm elevation (j) was defined as the angle between the reference position and the line through the two markers projected to the horizontal plane. All data was filtered with a lowpass 4th order Butterworth filter, with a cut-off frequency of 5 Hz for the INC data, and with a cut-off frequency of 10 Hz for the OPT data (Vaughan, 1982). For each subject and each 5-s position (tests 1–3), the median angles were calculated separately for the INC and the OPT recordings. The results from the two instruments were compared using Pearson’s correlation coefficient as a linear relationship could be assumed. The total acceleration that influenced the INC in the arm-swing tests (test 4) was calculated for each subject and arm-swing rate. The medians (p50), the 5th (p5) and the 95th percentiles (p95) for the total acceleration were then calculated. For each subject, the arm elevation angles (y) during the 3 min painting (test 5) were grouped into 201intervals, 0–191, 20–391 up to 160–1801. The proportion of the total measuring time spent with the upper arm in each interval was calculated separately for the INC and the OPT measurements.
3. Results 3.1. Degree of arm elevation
2.6. Data processing When measuring the degree of arm elevation (y) without influence of dynamic acceleration the correspondence between INC and OPT was almost perfect. The correlation coefficient between INC and OPT was 1.0 for both the arm flexion positions from test 1 (Fig. 2) and the arm abduction positions from test 2 (Fig. 3). Test 3 was performed to see how the degree of arm elevation (y) from the INC and OPT measurements
Arm elevation angle, Flexion
180 Inclinometer (degree)
After the recordings, the data from the INC measurements was transferred from the logger to a PC. Spherical co-ordinates (r; y; j) were used to describe the position of the sensor i.e. the upper arm. The inclination (y) of the upper arm was defined as 01 when the arm hung vertically downward. Theoretically, the inclination ranges from 01 to 1801. The direction of the inclined arm (j) was defined as 01 when the arm was directed forwards (flexion), 901 when it was directed laterally (abducted), and 1801 when it was directed backwards (extension). The INC measurements were analysed with a software program, the ‘‘Angleprogram’’(Department of Occupational Health, Stockholm, Sweden). The Angleprogram is specially designed for analysing INC data and calculates the position (y; j) of the relevant sensor (20 times per second). It also calculates the total acceleration (r). Similar calculations were performed for OPT data i.e. (r; y; j) using the Excel spreadsheet program (Microsoft). For the OPT measurements the degree of arm elevation (y) was defined as the angle between the line through the two markers and the vertical line. The 901 arm flexion position (test 1) was the reference position (j ¼ 0) for defining the direction of the arm. The
r = 1.0 135
90
45
0 0
45 90 135 Optical system (degree)
180
Fig. 2. Plots of optical system angles versus inclinometer system angles for six test subjects and five arm flexion positions, from test 1. Correlation coefficient (r) and line of identity are also presented.
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changed when the arm was rotated along it’s long axis. The mean arm elevation angles for the two measurement systems were equal (y ¼ 721) when the arm was at maximal external rotation and almost equal at maximal internal rotation (Table 1). The mean difference between INC and OPT angles for all subjects and all positions from tests 1–3 was 11 (95%CI: 2.8–+0.1) i.e. the INC values were on average 11 lower than the OPT values. For arm elevation (y) with the influence of dynamic acceleration (tests 4–5) the INC and the OPT also corresponded well. In test 4, when the subjects performed arm swings at low and high rates, the curves coincided (Fig. 4a, b). At very high rates, the curves diverged somewhat in the turning points of the movement cycles i.e. when the arm changed direction (Fig. 4c). When the movement turned from flexion to extension the INC measurements showed somewhat higher values than the OPT. In the middle of the
Arm elevation angle, Abduction
Inclinometer (degree)
180 r = 1.0
135
At the different arm flexion positions in test 1, the direction angle (j) of the arm should theoretically be 01. However, the mean direction angles (j) for the INC ranged from 631 to 721 (Table 4). At the different arm abduction positions in test 2, j should theoretically be 901. In practice, the INC direction angles (j) ranged from 721 to 991.
4. Discussion
45
0
45
90
135
180
Optical system (degree) Fig. 3. Plots of optical system angles versus inclinometer system angles for six test subjects and five arm abduction positions, test 1. Correlation coefficient (r) and line of identity are also presented.
Table 1 Mean elevation angle (y) from INC and OPT measurements, test 3 (n ¼ 6). Minimum and maximum values (range) are also presented
Max external 01 451 Internal 901 Internal Max internal
3.2. Direction of arm elevation
90
0
Test 3
movement cycles, the INC measurements showed somewhat lower values than the OPT. At this very high velocity, the INC was not able to record the extension phase i.e. when the movement changed from extension to flexion (Fig. 4c). The total acceleration acting on the INC sensor was close to 1 g during the slow arm-swings (0.10 Hz) but when the subjects were doing very rapid arm-swings (0.75 Hz) the total accelerations ranged from 0.5 to 2.0 g (Table 2). During the 3-min painting test (test 5) the proportions of time spent in each of the nine elevations intervals were almost identical for the two measuring systems (Table 3). The highest deviation in all angle intervals was only 2% of the total recording time, i.e. to 3.6 s for 3 min measurement.
INC
OPT
Arm elevation angle (y)
Arm elevation angle (y)
Median (deg)
Range (deg)
Median (deg)
Range (deg)
72
(62–85)
72
(69–74)
74 77
(63–84) (66–84)
74 78
(70–78) (72–83)
80
(70–89)
81
(74–85)
79
(70–91)
80
(72–85)
For all positions and for all movements with normalto-high velocities the INC measured the degree of arm elevation with high accuracy. For single movements performed with very high velocity the accuracy was lower. The INC could not differentiate between arm abduction and arm flexion. Even though the two systems corresponded well in measuring arm elevation, the INC gave slightly higher values at small elevation angles and slightly lower values at high elevation angles compared, to the OPT (Figs. 2 and 3). The most plausible explanation of these small differences is errors in the OPT system, rather than in the INC. During arm elevation the arm rotates inwards raising the marker on the epicondyle more than the marker caudal to the deltoideus muscle. The line through the two OPT markers then causes an overestimation of the degree of arm elevation. The total acceleration acting on the INC sensor can be divided into static and dynamic accelerations. The static acceleration is equal to gravity (g). The dynamic acceleration can be further divided into centripetal acceleration (ac) and tangential acceleration (at) (Fig. 5). Centripetal acceleration (ac) is the product of the square of angular velocity (o) and the distance from the rotation centre to the INC sensor (d). Tangential acceleration (at) is the product of this distance (d) and
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545 OPT
Arm-swing 0.10 Hz
Arm elevation (degree)
INC 150 100 50 0 0
2
4
(a)
6
8
10
Time (sec)
OPT
Arm elevation (degree)
Arm-swing 0.40 Hz
INC
150 100 50 0 0
2
4
(b)
6
8
10
Time (sec)
OPT
Arm elevation (degree)
Arm-swing 0.75 Hz
INC
150 100 50 0 0
2
4
(c)
6
8
10
Time (sec)
Fig. 4. 3 typical curves from one test subject showing 10-s arm-swings at low rate (a), high rate (b), and very high rate (c). White curves represent Inclinometer result and the black curves represent optical measurements.
Table 2 Total acceleration from arms-swing test measured with the inclinometer (test 4). Medians, 5th percentiles, and 95th percentiles for total accelerations, arm-swings test (n ¼ 6) Rate
Low High Very high
Frequency
0.10 Hz 0.40 Hz 0.75 Hz
Total acceleration (1 g=9.81 m/s2) Median
P5
P95
1.0 1.0 1.0
1.0 0.8 0.5
1.1 1.4 2.0
the angular acceleration of the arm. The centripetal and the tangential accelerations are both proportional to the distance (d) from the rotation centre to the sensor and hence increase with distance (d). It is therefore preferable to place the sensor as close as possible to the centre of motion. The arm-swing test at different rates (test 5) showed how dynamic acceleration influenced the results. In slow arm-swings the total acceleration was close to 1 g (Table 2) i.e. the INC was not influenced by dynamic accelerations and the curves from the two systems coincided (Fig. 4a). Even when the arm-swings were performed at a higher rate (Fig. 4b) the curves coincided, except at the turning points where they diverged a little. For very fast arm-swings (Fig. 4c), the curves differed more and the INC did not register the extension beyond the vertical line (Fig. 4c). At this high frequency the total acceleration from the INC ranged
from 0.5 to 2.0 g (table 2) . This shows that the INC has been influenced by dynamic accelerations partly altered the direction of the total acceleration and caused a deviation from the vertical line. The purpose of the painting test was to see how well the INC records movements at varying rates during a longer period. The reason for only recording 3 min was the long tracking time for the OPT. Despite this short data collection time, the painting test showed almost identical results for the two systems (Table 3). The test persons felt comfortable working with the INC attached to the arm and the instrument did not disturb their work techniques. In field studies the influence from dynamic acceleration is small, as movements with very high dynamic acceleration are usually infrequent. If the recording period is long enough, a few movements with very high dynamic acceleration do not influence the total results. However, if single movements with very high velocities are to be studied, the measuring error can become too large. A shortcoming of the INC system was that it could not differentiate between arm abduction and arm flexion (Table 4). For measuring the direction of inclination, there is a need for a system that can record the longitudinal rotation of the upper arm. Another limit of the system is that it measures the inclination and not the angle between two adjacent body segments. For example, the INC only measures the arm
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Table 3 Painting, 3 min Mean percentage time for upper arm elevation angle (y) spent in each 201 interval
INC OPT
0–191
20–391
40–591
60–791
80–991
100–1191
120–1391
140–1591
160–1801
17% 19%
28% 27%
16% 14%
14% 13%
18% 18%
6% 6%
2% 2%
0% 1%
0% 0%
Table 4 Mean arm direction angles (j) at three arm flexion and three arm abduction positions from tests 1 and 2. (n ¼ 6). Minimum and maximum values (range) also presented Arm direction angle (j) Theoretical value
Arm Arm Arm Arm Arm Arm
flexion 451 (test 1) Flexion 901 (test 1) Flexion 1351 (test 1) abduction 451 (test 2) abduction 901 (test 2) abduction 1351 (test 2)
0 0 0 90 90 90
INC
OPT
Mean (deg)
Range (deg)
Mean (deg)
Range (deg)
63 66 72 99 85 72
(48–86) (55–81) (63–84) (79–125) (72–106) (64–80)
3 1 6 93 90 100
(0–7) (0–3) (2–10) (87–103) (90–91) (87–108)
Fig. 5. The total acceleration acting on the inclinometer sensor can be divided into three accelerations, centripetal acceleration (ac), tangential acceleration (at) and, gravity (g). Degree of arm elevation (y), angular velocity (o) and, distance between movement centre and the inclinometer sensor (d) are also presented.
elevation relative to the vertical line and not to the trunk, because the system lacks reference system on the trunk. The INC was developed for measuring postures and movements during occupational work e.g. before and after the introduction of an ergonomic intervention program. The INC could also be used for measuring patients before and after rehabilitation. In this study, we tested upper arm movements but the INC can be attached to any (rigid) body segment. In earlier studies the INC has been attached to the head, the back and the upper arms (Hansson et al., 2001a, b). Earlier accelerometer-based instruments usually have only one accelerometer. Thus, they are sensitive in only one direction and can only be used to discriminate between two positions e.g. sitting—standing or between two dynamic activities such as standing—walking
(Guangyan and Buckle, 1999). They cannot quantify the degree of inclination. The OPT is a very precise instrument, but it has some limitations when studying human movements. Prior to an OPT measuring session the system must be calibrated. After that it is impossible to move the cameras without a new calibration. The OPT markers must be in the view of at least two cameras during the whole test which makes the OPT unsuitable for field studies. Moreover, when analysing OPT data the tracking time is long which make this method expensive. Other portable instruments that measure the degree of arm elevation are the Physiometer (Aara( s and Stranden, 1988), the Abduflex (Ericson et al., 1994), and the Intometer (Sporrong et al., 1999). The Physiometer employs fluid-based angle transducers. The sampling frequency is 10 Hz. The Abduflex system consists of mercury microswitches used to indicate the elevation in seven predetermined intervals of 151 intervals: 0–151up to 75–901. The sampling frequency is 1 Hz and maximum data collection time is about 8 h. A drawback with the Abduflex compared to the INC is that it is more clumsy and less precise. The Intometer consists of pressure transducers attached to the upper arms. It measures arm elevation relative to the vertical line. The Intometer can collect data up to 8 h at a frequency of 4 Hz. None of these instruments can record the degree of longitudinal rotation or the direction of inclination (flexion or abduction).
5. Conclusions The INC seems to be very useful for measuring inclination of the arm to the vertical line over time (up
E. Bernmark, C. Wiktorin / Applied Ergonomics 33 (2002) 541–547
to 12 h). The INC is small and easy to handle both for subject and the test leader. For all movements at normal to high velocities the INC measured the degree of arm elevation with very high precision related to the OPT measurements. The INC could not describe in detail single movements at very high velocities. The INC could not differentiate between arm abduction and arm flexion.
References Aar(as, A., Stranden, E., 1988. Measurement of postural angles during work. Ergonomics 31 (6), 935–944. ( ( Balogh, I., Moritz, U., Skerfving, S., Akesson, I., Hansson, G.-A., 1997. Quantifying workload in neck, shoulders and wrists in female dentists. Int. Arch. Occup. Environ. Health 69, 461–474. Ari.ens, G., Mechelen, W., Bongers, P., Bouter, L., Wal, G., 2000. Physical risk factors for neck pain. Scand J Work Environ Health 26 (1), 7–19. Axelsson, S., 1992. Photographic surveillance of industrial robot errors and evaluation of two camera systems. Chalmers University of . Technology, Department of Production engineering, Goteborg, Sweden (Paper in Swedish, abstract in English). Bernard, B.P. (Ed.) 1997. Musculoskeletal Disorders and Workplace Factors. United States Department of Health and Human Services, Cincinnati (OH). Ericson, M.O., Heijdenberg, J., Wallin, K., 1994. Abduflex. Portable system for field measurements of arm postures and motions.
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Technical Report, ISSN 1100-5781. 31, Royal Institute of Technology, Stockholm, Sweden. Guangyan, L., Buckle, P., 1999. Current techniques for assessing physical exposure to work-related musculoskeletal risks, with emphasis on posture-based methods. Ergonomics 42 (5), 674–695. ( Hansson, G.-A., Mikkelsen, S., 1997. Kinematic evaluation of occupational work. Adv. Occup. Med. Rehabilitation 3 (1), 57–69. ( Asterland, P., Holmer, N.-G., Skerfving, S., 2001a. Hansson, G.-A., Validity and reliability of triaxial accelerometers for inclinometry in posture analysis. Med. & Biol. Eng. & Comput. 39 (4), 405–413. ( Balogh, I., Unge Bystrom, . Hansson, G-A., J., Ohlsson, K., Nordan. der, C., Asterland, P., Sjolander, S., Rylander, L., Skerfving, S., 2001b. Questionnaire versus direct technical measurements for assessment of postures and movements of head, upper back, arms and hands. Scand J Work Environ Health 27 (1), 30–40. Peterson, B., Palmerud, G., 1996. Measurement of upper extremity orientation by video stereometry system. Med. Biol. Eng. & Comput. 34, 149–154. Sporrong, H., Sandsj.o, L., Kadefors, R., Herberts, P., 1999. Assessment of workload and arm position during different work sequences: A study with portable devices on construction workers. Appl. Ergon. 30, 495–503. Vaughan, C.L., 1982. Smoothing and differentiation of displacementtime data: An application of splines and digital filtering. Int. J. BioMed. Comput. 13, 375–386. Westgaard, R.H., Winkel, J., 1997. Ergonomic intervention research for improved musculoskeletal health: A critical review. Int. J. Ind. Ergon. 20, 463–500. Van der Windt, D., Thomas, E., Pope, D., Winter, A., Macfarlane, G., Bouter, L., Silman, A., 2000. Occup. Environ. Med. 57, 433–442. Winkel, J., Westgaard, R., 1992. Occupational and individual risk factors for shoulder-neck complaints. Part II-The scientific basis (literature review) for the guide. Int. J. Ind. Ergon. 10, 85–104.
A triaxial accelerometer for measuring arm movements Eva Bernmarka,b,*, Christina Wiktorina,b b
a Department of Occupational Health, Stockholm County Council, Stockholm 17176, Sweden Department of Public Health Sciences, Division of Occupational health, Karolinska Institute, Stockholm, Sweden
Received 27 April 2002; received in revised form 14 May 2002; accepted 5 July 2002
Abstract A triaxial accelerometer used as an inclinometer (INC) (Logger Technology, Sweden) needed to be evaluated for field measurements of arm postures and movements. INC consists of one portable data logger and up to four sensors. Each sensor measures the inclination to the vertical line. The sampling frequency is 20 Hz and, the logger can collect data for up to 12 h. The aim of the present study was to compare INC measurements with those from an optoelectronic measuring system—the Mac Reflex system (OPT) (Qualisys AB, Sweden). For all movements with normal to high velocities INC measured the degree of arm elevation with very high precision. At very high velocities and, especially when the direction of the velocity was perpendicular to the vertical line the largest differences between INC and OPT were found. In field studies INC has proved to be very safe and useful in assessment of arm movements. INC was easy to handle and easy to wear for the researcher and the subject, respectively. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Inclinometer; Accelerometer; Arm movements
1. Introduction Epidemiological studies indicate that frequent or sustained work with elevated arms increases the risk of neck/shoulder disorders (Ari.ens et al., 2000; Bernard, 1997; Guangyan and Buckle, 1999; Van der Windt et al., 2000). However, information about the degree of arm elevation causing this increased risk is still lacking, as is information about the duration and frequencies of harmful work with elevated arms (Winkel and Westgaard, 1992). It is also unclear whether ergonomic intervention programs facilitate recovery after an episode of neck/shoulder disorder (Westgaard and Winkel, 1997). Therefore, there is a need for accurate and precise instruments that can measure arm elevation angles for whole days without disturbing the worker ( (Hansson and Mikkelsen, 1997; Akesson et al., 1997). A portable three-axis accelerometer (Logger Technol. Sweden) used as an inclinometer has ogy HB, Malmo, been developed for measuring postures and movements *Corresponding author. Tel.: + 46-8-517-731-49; fax: +46-8-33-4333. E-mail address: [email protected] (E. Bernmark).
over time. The inclinometer system (INC) can measure the inclination of a body segment in relation to the vertical line (the line of gravity), and also the direction of the inclination. It cannot measure rotation. The instrument has been tested in laboratory tests and is considered very precise (Hansson et al., 2001a). However, the INC has not been tested for measuring human movements. In most human movements and especially in the upper extremities, rotation around the long axis of the actual body segment occurs simultaneously with other movements. It is thus important to test whether the INC really can measure the degree and the direction of arm elevation movements. The aim of the present study was to compare INC measurements with those from optoelectronic measuring system of known high precision, —the Mac Reflex system (OPT) (Peterson and Palmerud, 1996; Axelsson, 1992). The following specific questions were to be answered: (1) Can the INC measure the elevation of the upper arm in relation to the vertical line? (2) Can the INC differentiate between arm flexion and arm abduction, i.e. can the INC measure the direction of the movement?
0003-6870/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 0 2 ) 0 0 0 7 2 - 8
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E. Bernmark, C. Wiktorin / Applied Ergonomics 33 (2002) 541–547
2. Material and method
‘‘Angleprogram’’(Department of Occupational Health, Stockholm, Sweden).
2.1. Subjects Six healthy, right-handed test persons, three females and three males, participated in the tests. The mean age was 43 years (ranging from 28 to 58 years) and mean height 175.8 cm (148–193 cm). 2.2. Inclinometer The INC consists of a data logger and up to four sensors (Fig. 1). The logger, based on PCMCIA flash memory cards, has a memory capacity of 20 Mb which is enough to measure up to 12 h with four sensors. The sampling frequency is 20 Hz. The data logger is small and can be used attached to a belt around the subject’s waist. The sensors, weighing only 20 g each, are taped to the skin on the relevant body segments. Each sensor measures the total acceleration (r) acting on the body segment, inclination (y) in relation to the vertical line and the direction (j) of the inclination. Thus, the position of each sensor, and thereby the body segment, is described by spherical co-ordinates (r; y; j). The sensor cannot measure rotation around the long axis of the moving body segment. Each sensor consists of three miniature accelerometers mounted perpendicular to each other in the x, y and z directions. For a threedimensional analysis, three accelerometers are needed, each measuring both static and dynamic acceleration in one direction. Static acceleration equals gravity (1 g=9.8 m/s2) i.e. the acceleration acting on the sensor in rest. When the body segment moves, the INC sensor is also influenced by dynamic acceleration. The principle for INC measurement is to record continuously how gravity is distributed over the three accelerometers. After a recording period, the data is transferred to a computer for analysis with a software program e.g. the
2.3. Mac reflex The reference system used in this study was a threedimensional optoelectronic movement analysing system, the Mac Reflex system (OPT) (Qualisys AB, Sweden). The OPT consists of infrared flashlights mounted on video cameras, video processors, and a computer with the Mac Reflex 3D software. Small reflective spherical markers are applied to the body segment to be studied, and the rectangular co-ordinates (x, y, z) for these markers are recorded by two or up to seven of the specially adapted video cameras. The sampling frequency is 50 Hz. Each marker’s change in position can then be used to calculate e.g. distance, angle, velocity and acceleration. OPT produces position data with high precision (Axelsson, 1992; Peterson and Palmerud, 1996). 2.4. Experimental procedure One INC sensor was taped caudal to the insertion of the deltoid muscle on the right upper arm. Two 20 mm OPT markers were taped on the same arm, one on the caput of humerus 3 cm caudal to the border of acromion and one on the lateral epicondyle of humerus. In this study, The OPT consisted of three video cameras. After attachment to the subject’s arm the INC sensor was calibrated by defining the vertical line and the direction of the body segment. The vertical line (y ¼ 01) was defined by having the subject sit and bend slightly to the right with a 2 Kg weight in the hand so the arm hung vertically. The direction of the right arm was defined by having the subject stand and hold the upper arm at 901 abduction. (j ¼ 901). Five test movements of the right upper arm were then recorded simultaneously by the INC and the OPT. 2.5. Test movements Three of the five tests were performed in order to investigate how well the INC measured the degree of arm elevation (y) and the direction (j) when the arm was held in different positions i.e. the sensors were only influenced by static acceleration (gravity). The other two tests were performed to investigate how well the INC can measure arm positions (y; j) when the arm is moving at different rates i.e. when the sensors are influenced by both static and dynamic accelerations.
Fig. 1. The Inclinometer system consists of a data logger with a PCMCIA flash memory card and up to four sensors.
1. The starting position was sitting with the arm hanging by the side (y ¼ 01). The arm was then slowly flexed through the whole motion range (0–
E. Bernmark, C. Wiktorin / Applied Ergonomics 33 (2002) 541–547
2. 3.
4.
5.
1801) with 5-s long pauses at about 451, 901, 1351 and 1801 of flexion. Arm abduction was performed similarly. The starting position was sitting with the shoulder and the elbow flexed to about 901 and the upper arm in maximal outward rotation. The elbow was supported on a table. The arm was then rotated inwards through the whole rotation range with four 5-s pauses at about 01 (lower arm vertical), 451, 901 and maximal internal rotation. The starting position was standing with the arm hanging by the side. Arm flexion–extension through the whole motion range was then performed with three different rates; ‘‘slow’’, 6 arm swings per minute (0.10 Hz), ‘‘high’’, 24 arm swings per minute (0.40 Hz), and ‘‘very high’’, 45 arm swings per minute (0.75 Hz). The rate was controlled by following a person making the same movements on a video and with the help of a metronome. Each movement rate was recorded for 30 s. Each subject was instructed to paint a 1 1 m2 large board for 3 min with a paintbrush. The paint was in a tin on a table beside the subject. The upper edge of the board was at a height of 1.9 m. The subjects were instructed to work in their own way and tempo. This experiment was done in a standing position.
543
direction of arm elevation (j) was defined as the angle between the reference position and the line through the two markers projected to the horizontal plane. All data was filtered with a lowpass 4th order Butterworth filter, with a cut-off frequency of 5 Hz for the INC data, and with a cut-off frequency of 10 Hz for the OPT data (Vaughan, 1982). For each subject and each 5-s position (tests 1–3), the median angles were calculated separately for the INC and the OPT recordings. The results from the two instruments were compared using Pearson’s correlation coefficient as a linear relationship could be assumed. The total acceleration that influenced the INC in the arm-swing tests (test 4) was calculated for each subject and arm-swing rate. The medians (p50), the 5th (p5) and the 95th percentiles (p95) for the total acceleration were then calculated. For each subject, the arm elevation angles (y) during the 3 min painting (test 5) were grouped into 201intervals, 0–191, 20–391 up to 160–1801. The proportion of the total measuring time spent with the upper arm in each interval was calculated separately for the INC and the OPT measurements.
3. Results 3.1. Degree of arm elevation
2.6. Data processing When measuring the degree of arm elevation (y) without influence of dynamic acceleration the correspondence between INC and OPT was almost perfect. The correlation coefficient between INC and OPT was 1.0 for both the arm flexion positions from test 1 (Fig. 2) and the arm abduction positions from test 2 (Fig. 3). Test 3 was performed to see how the degree of arm elevation (y) from the INC and OPT measurements
Arm elevation angle, Flexion
180 Inclinometer (degree)
After the recordings, the data from the INC measurements was transferred from the logger to a PC. Spherical co-ordinates (r; y; j) were used to describe the position of the sensor i.e. the upper arm. The inclination (y) of the upper arm was defined as 01 when the arm hung vertically downward. Theoretically, the inclination ranges from 01 to 1801. The direction of the inclined arm (j) was defined as 01 when the arm was directed forwards (flexion), 901 when it was directed laterally (abducted), and 1801 when it was directed backwards (extension). The INC measurements were analysed with a software program, the ‘‘Angleprogram’’(Department of Occupational Health, Stockholm, Sweden). The Angleprogram is specially designed for analysing INC data and calculates the position (y; j) of the relevant sensor (20 times per second). It also calculates the total acceleration (r). Similar calculations were performed for OPT data i.e. (r; y; j) using the Excel spreadsheet program (Microsoft). For the OPT measurements the degree of arm elevation (y) was defined as the angle between the line through the two markers and the vertical line. The 901 arm flexion position (test 1) was the reference position (j ¼ 0) for defining the direction of the arm. The
r = 1.0 135
90
45
0 0
45 90 135 Optical system (degree)
180
Fig. 2. Plots of optical system angles versus inclinometer system angles for six test subjects and five arm flexion positions, from test 1. Correlation coefficient (r) and line of identity are also presented.
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changed when the arm was rotated along it’s long axis. The mean arm elevation angles for the two measurement systems were equal (y ¼ 721) when the arm was at maximal external rotation and almost equal at maximal internal rotation (Table 1). The mean difference between INC and OPT angles for all subjects and all positions from tests 1–3 was 11 (95%CI: 2.8–+0.1) i.e. the INC values were on average 11 lower than the OPT values. For arm elevation (y) with the influence of dynamic acceleration (tests 4–5) the INC and the OPT also corresponded well. In test 4, when the subjects performed arm swings at low and high rates, the curves coincided (Fig. 4a, b). At very high rates, the curves diverged somewhat in the turning points of the movement cycles i.e. when the arm changed direction (Fig. 4c). When the movement turned from flexion to extension the INC measurements showed somewhat higher values than the OPT. In the middle of the
Arm elevation angle, Abduction
Inclinometer (degree)
180 r = 1.0
135
At the different arm flexion positions in test 1, the direction angle (j) of the arm should theoretically be 01. However, the mean direction angles (j) for the INC ranged from 631 to 721 (Table 4). At the different arm abduction positions in test 2, j should theoretically be 901. In practice, the INC direction angles (j) ranged from 721 to 991.
4. Discussion
45
0
45
90
135
180
Optical system (degree) Fig. 3. Plots of optical system angles versus inclinometer system angles for six test subjects and five arm abduction positions, test 1. Correlation coefficient (r) and line of identity are also presented.
Table 1 Mean elevation angle (y) from INC and OPT measurements, test 3 (n ¼ 6). Minimum and maximum values (range) are also presented
Max external 01 451 Internal 901 Internal Max internal
3.2. Direction of arm elevation
90
0
Test 3
movement cycles, the INC measurements showed somewhat lower values than the OPT. At this very high velocity, the INC was not able to record the extension phase i.e. when the movement changed from extension to flexion (Fig. 4c). The total acceleration acting on the INC sensor was close to 1 g during the slow arm-swings (0.10 Hz) but when the subjects were doing very rapid arm-swings (0.75 Hz) the total accelerations ranged from 0.5 to 2.0 g (Table 2). During the 3-min painting test (test 5) the proportions of time spent in each of the nine elevations intervals were almost identical for the two measuring systems (Table 3). The highest deviation in all angle intervals was only 2% of the total recording time, i.e. to 3.6 s for 3 min measurement.
INC
OPT
Arm elevation angle (y)
Arm elevation angle (y)
Median (deg)
Range (deg)
Median (deg)
Range (deg)
72
(62–85)
72
(69–74)
74 77
(63–84) (66–84)
74 78
(70–78) (72–83)
80
(70–89)
81
(74–85)
79
(70–91)
80
(72–85)
For all positions and for all movements with normalto-high velocities the INC measured the degree of arm elevation with high accuracy. For single movements performed with very high velocity the accuracy was lower. The INC could not differentiate between arm abduction and arm flexion. Even though the two systems corresponded well in measuring arm elevation, the INC gave slightly higher values at small elevation angles and slightly lower values at high elevation angles compared, to the OPT (Figs. 2 and 3). The most plausible explanation of these small differences is errors in the OPT system, rather than in the INC. During arm elevation the arm rotates inwards raising the marker on the epicondyle more than the marker caudal to the deltoideus muscle. The line through the two OPT markers then causes an overestimation of the degree of arm elevation. The total acceleration acting on the INC sensor can be divided into static and dynamic accelerations. The static acceleration is equal to gravity (g). The dynamic acceleration can be further divided into centripetal acceleration (ac) and tangential acceleration (at) (Fig. 5). Centripetal acceleration (ac) is the product of the square of angular velocity (o) and the distance from the rotation centre to the INC sensor (d). Tangential acceleration (at) is the product of this distance (d) and
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545 OPT
Arm-swing 0.10 Hz
Arm elevation (degree)
INC 150 100 50 0 0
2
4
(a)
6
8
10
Time (sec)
OPT
Arm elevation (degree)
Arm-swing 0.40 Hz
INC
150 100 50 0 0
2
4
(b)
6
8
10
Time (sec)
OPT
Arm elevation (degree)
Arm-swing 0.75 Hz
INC
150 100 50 0 0
2
4
(c)
6
8
10
Time (sec)
Fig. 4. 3 typical curves from one test subject showing 10-s arm-swings at low rate (a), high rate (b), and very high rate (c). White curves represent Inclinometer result and the black curves represent optical measurements.
Table 2 Total acceleration from arms-swing test measured with the inclinometer (test 4). Medians, 5th percentiles, and 95th percentiles for total accelerations, arm-swings test (n ¼ 6) Rate
Low High Very high
Frequency
0.10 Hz 0.40 Hz 0.75 Hz
Total acceleration (1 g=9.81 m/s2) Median
P5
P95
1.0 1.0 1.0
1.0 0.8 0.5
1.1 1.4 2.0
the angular acceleration of the arm. The centripetal and the tangential accelerations are both proportional to the distance (d) from the rotation centre to the sensor and hence increase with distance (d). It is therefore preferable to place the sensor as close as possible to the centre of motion. The arm-swing test at different rates (test 5) showed how dynamic acceleration influenced the results. In slow arm-swings the total acceleration was close to 1 g (Table 2) i.e. the INC was not influenced by dynamic accelerations and the curves from the two systems coincided (Fig. 4a). Even when the arm-swings were performed at a higher rate (Fig. 4b) the curves coincided, except at the turning points where they diverged a little. For very fast arm-swings (Fig. 4c), the curves differed more and the INC did not register the extension beyond the vertical line (Fig. 4c). At this high frequency the total acceleration from the INC ranged
from 0.5 to 2.0 g (table 2) . This shows that the INC has been influenced by dynamic accelerations partly altered the direction of the total acceleration and caused a deviation from the vertical line. The purpose of the painting test was to see how well the INC records movements at varying rates during a longer period. The reason for only recording 3 min was the long tracking time for the OPT. Despite this short data collection time, the painting test showed almost identical results for the two systems (Table 3). The test persons felt comfortable working with the INC attached to the arm and the instrument did not disturb their work techniques. In field studies the influence from dynamic acceleration is small, as movements with very high dynamic acceleration are usually infrequent. If the recording period is long enough, a few movements with very high dynamic acceleration do not influence the total results. However, if single movements with very high velocities are to be studied, the measuring error can become too large. A shortcoming of the INC system was that it could not differentiate between arm abduction and arm flexion (Table 4). For measuring the direction of inclination, there is a need for a system that can record the longitudinal rotation of the upper arm. Another limit of the system is that it measures the inclination and not the angle between two adjacent body segments. For example, the INC only measures the arm
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Table 3 Painting, 3 min Mean percentage time for upper arm elevation angle (y) spent in each 201 interval
INC OPT
0–191
20–391
40–591
60–791
80–991
100–1191
120–1391
140–1591
160–1801
17% 19%
28% 27%
16% 14%
14% 13%
18% 18%
6% 6%
2% 2%
0% 1%
0% 0%
Table 4 Mean arm direction angles (j) at three arm flexion and three arm abduction positions from tests 1 and 2. (n ¼ 6). Minimum and maximum values (range) also presented Arm direction angle (j) Theoretical value
Arm Arm Arm Arm Arm Arm
flexion 451 (test 1) Flexion 901 (test 1) Flexion 1351 (test 1) abduction 451 (test 2) abduction 901 (test 2) abduction 1351 (test 2)
0 0 0 90 90 90
INC
OPT
Mean (deg)
Range (deg)
Mean (deg)
Range (deg)
63 66 72 99 85 72
(48–86) (55–81) (63–84) (79–125) (72–106) (64–80)
3 1 6 93 90 100
(0–7) (0–3) (2–10) (87–103) (90–91) (87–108)
Fig. 5. The total acceleration acting on the inclinometer sensor can be divided into three accelerations, centripetal acceleration (ac), tangential acceleration (at) and, gravity (g). Degree of arm elevation (y), angular velocity (o) and, distance between movement centre and the inclinometer sensor (d) are also presented.
elevation relative to the vertical line and not to the trunk, because the system lacks reference system on the trunk. The INC was developed for measuring postures and movements during occupational work e.g. before and after the introduction of an ergonomic intervention program. The INC could also be used for measuring patients before and after rehabilitation. In this study, we tested upper arm movements but the INC can be attached to any (rigid) body segment. In earlier studies the INC has been attached to the head, the back and the upper arms (Hansson et al., 2001a, b). Earlier accelerometer-based instruments usually have only one accelerometer. Thus, they are sensitive in only one direction and can only be used to discriminate between two positions e.g. sitting—standing or between two dynamic activities such as standing—walking
(Guangyan and Buckle, 1999). They cannot quantify the degree of inclination. The OPT is a very precise instrument, but it has some limitations when studying human movements. Prior to an OPT measuring session the system must be calibrated. After that it is impossible to move the cameras without a new calibration. The OPT markers must be in the view of at least two cameras during the whole test which makes the OPT unsuitable for field studies. Moreover, when analysing OPT data the tracking time is long which make this method expensive. Other portable instruments that measure the degree of arm elevation are the Physiometer (Aara( s and Stranden, 1988), the Abduflex (Ericson et al., 1994), and the Intometer (Sporrong et al., 1999). The Physiometer employs fluid-based angle transducers. The sampling frequency is 10 Hz. The Abduflex system consists of mercury microswitches used to indicate the elevation in seven predetermined intervals of 151 intervals: 0–151up to 75–901. The sampling frequency is 1 Hz and maximum data collection time is about 8 h. A drawback with the Abduflex compared to the INC is that it is more clumsy and less precise. The Intometer consists of pressure transducers attached to the upper arms. It measures arm elevation relative to the vertical line. The Intometer can collect data up to 8 h at a frequency of 4 Hz. None of these instruments can record the degree of longitudinal rotation or the direction of inclination (flexion or abduction).
5. Conclusions The INC seems to be very useful for measuring inclination of the arm to the vertical line over time (up
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to 12 h). The INC is small and easy to handle both for subject and the test leader. For all movements at normal to high velocities the INC measured the degree of arm elevation with very high precision related to the OPT measurements. The INC could not describe in detail single movements at very high velocities. The INC could not differentiate between arm abduction and arm flexion.
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