- Email: [email protected]
Comparison between an accelerometer and a three-dimensional motion analysis system for the detection of movement
Physiotherapy 98 (2012) 256–259
Comparison between an accelerometer and a three-dimensional motion analysis system for the detection of movement P.Y.M. Chung, G.Y.F. Ng ∗ Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China
Abstract Objective To examine the reliability and concurrent validity of an accelerometer compared with a three-dimensional (3D) motion analysis system for measuring the motor reaction time of the lower limbs, to validate a simple method for objective clinical assessment of movement onset. Design Timing of the onset of knee extension movement in response to an audio signal was measured simultaneously with an accelerometer and a 3D VICON motion analysis system. Participants Twelve able-bodied subjects with a mean age of 26 (standard deviation 2.3) years. Results Good reliability was found for both instruments, although the intraclass correlation coefficient (ICC) was higher for the accelerometer (ICC3,1 = 0.739; P < 0.001). The mean motor reaction time measured by the accelerometer and the VICON system was 205.0 and 196.9 mseconds, respectively. Good agreement was found between the paired measurements (mean ICC = 0.774), and the average 95% limits of agreement were −56.4 to 72.5 mseconds. Conclusion In conclusion, the accelerometer had good reliability for the measurement of movement onset time. Agreement between the measurements from the accelerometer and the VICON system was high. However, the limits of agreement covered a wide range, so absolute timings for movement onset derived from these methods should not be used interchangeably. © 2011 Published by Elsevier Ltd on behalf of Chartered Society of Physiotherapy. Keywords: Accelerometer; Movement onset; Reaction time
Introduction Reaction time is an important determining factor in many sporting events. Most reports on reaction time have focused on the upper limbs [1–5], possibly because upper limb reaction time can be measured easily using a keyboard or a thumb switch. However, these methods are not applicable to the lower limbs, and it has been reported that motor control differs between the upper and lower limbs [6]. Christou and Rodriguez [6] examined the transfer of motor performance between ipsilateral upper and lower limbs by measuring the force and time accuracy of goal-directed isometric contractions of the muscles. They found that all components of motor learning cannot be transferred between the upper and lower limbs. As such, it is necessary to measure the lower limb
∗
Corresponding author. Tel.: +852 27666721; fax: +852 23308656. E-mail address: [email protected] (G.Y.F. Ng).
response to stimuli in order to investigate the effect of sports training on motor performance involving the lower limbs. Three-dimensional (3D) motion analysis technology has been widely used in gait studies and is regarded as the gold standard for the analysis of movement [7–10]. Most 3D motion analysis systems use cameras to capture the trajectories of reflective markers attached to body parts during movements. Spatiotemporal kinematics parameters of the movements can be acquired after data processing, and can be used to measure the onset timing of body movements. However, due to the complex set-up and sophisticated equipment involved, this technology is not easily applied in the clinical setting. Accelerometers are relatively simple instruments that measure movements involving acceleration. Most accelerometers contain a piezo-electric element and a seismic mass embedded in a sensor case; when the piezo-electric element is compressed or sheared by the seismic mass during acceleration, it generates voltage signals proportional to the
0031-9406/$ – see front matter © 2011 Published by Elsevier Ltd on behalf of Chartered Society of Physiotherapy. doi:10.1016/j.physio.2011.06.003
P.Y.M. Chung, G.Y.F. Ng / Physiotherapy 98 (2012) 256–259
force. Accelerometers have been used for the analysis of gait [11], to monitor physical activities [12,13], to estimate the body centre of mass [10], and to measure muscle activities with perturbation in postural testing [14]. Good test–retest reliability has been reported for accelerometer recordings [intraclass correlation coefficient (ICC) 0.7 to 0.97] [15–18]. The portability and relatively low cost of an accelerometer make it suitable for movement detection and biomechanical analysis for both upper and lower limbs in the clinical setting. However, to date, the validity of accelerometers in the measurement of reaction time has not been determined. Therefore, this study aimed to determine the concurrent validity for an accelerometer and a 3D motion analysis system in measuring motor reaction times of the knee.
Methods Twelve able-bodied subjects with a mean age of 26 [standard deviation (SD) 2.3] years volunteered to participate in this study. Their mean height was 164 (SD 9.4) cm and mean weight was 58 (SD 9.4) kg. The right leg was the dominant leg for all subjects. This study was approved by the Human Subjects Ethics Sub-committee of the Hong Kong Polytechnic University, and subjects gave their written informed consent prior to testing. The onset time of lower limb movement was measured using a uni-axial, piezo-electric accelerometer (Model 8772A10, Kistler Instrument Corp., Amherst, NY, USA) and an eight-camera VICON motion analysis system (v-370, Vicon Motion Systems, Oxford, UK). The eight cameras were located 2 m above the floor in a laboratory measuring 4 × 10 m. Subjects sat in the centre of the room with the sagittal plane aligned with the Y-axis of the VICON system, and hip and knee flexed at 90◦ . The accelerometer was attached to the tibial tuberosity of the right leg, and a spherical, reflective marker was placed over the accelerometer. Subjects were asked to extend their right knee as fast as possible on hearing a ‘beep’ triggered by the operator. The audio signal occurred at any time from 1 to 9 seconds after a ‘start’ verbal cue given by the operator. The audio signal was output to the VICON system via an analogueto-digital converter, and displayed graphically as a voltage signal in order to synchronise the VICON system and the accelerometer. The accelerometer signals were sent to an analogue-to-digital converter at a sampling rate of 1000 Hz, and captured with LabVIEW software (National Instrument, Austin, TX, USA). The sampling frequency of the VICON cameras was set at 250 Hz, and VICON Nexus 1.4 (Vicon Motion Systems, Oxford, UK) was used to reconstruct the sagittal trajectories of the reflective markers. Both sets of data were exported to Microsoft Excel (Microsoft Corp., Redmond, WA, USA) for data processing. As the accelerometer was uni-axial and force was registered along the axis of movement, the anteroposterior trajectory of the reflective marker was used for comparison. Movement
257
Table 1 Mean movement onset times measured by the accelerometer and the VICON system, and the mean differences between the paired measurements for each trial. Subject
Mean (SD) values of the five measurements (mseconds)
Accelerometer
VICON
1 2 3 4 5 6 7 8 9 10 11 12
179.2 (31.6) 249.0 (37.1) 186.6 (18.7) 196.4 (22.7) 272.4 (29.4) 253.4 (20.5) 156.4 (22.2) 244.8 (23.5) 160.2 (34.4) 215.6 (27.6) 118.8 (21.6) 225.6 (16.2)
194.6 (46.9) 228.4 (39.9) 189.0 (18.7) 159.8 (24.5) 224.8 (41.7) 201.8 (21.0) 140.6 (14.9) 238.6 (34.2) 159.8 (34.1) 229.8 (28.9) 153.2 (21.4) 243.6 (19.8)
Mean difference (95% CI) of paired measurements for each trial (mseconds) −15.4 (−80.7 to 49.9) 20.6 (0.3 to 40.9) −2.4 (−9.1 to 4.3) 36.6 (21.9 to 51.3) 47.6 (18 to 77.2) 53.6 (7.4 to 99.8) 15.8 (−37.3 to 68.9) 6.2 (−18.2 to 30.6) 0.4 (−70.2 to 71) −14.2 (−43 to 14.6) −34.4 (−25.4 to 43.4) −18.0 (−6.2 to −29.8)
SD, standard deviation; CI, confidence interval.
onset was defined as any deviation in the anteroposterior direction lasting for ≥5 mseconds with a difference of at least 2 SD from the resting signal (captured 50 mseconds before the audio signal for both sets of data). Data analysis Each subject was tested five times and all trials were used to determine the reliability of the two instruments (ICC3,1 ). ICC2,1 and limits of agreement were calculated for each trial to assess the level of agreement between the paired measurements [19,20]. Results Table 1 shows the mean measurements for each subject. The ICC values for the accelerometer and the VICON system were 0.739 (P < 0.001) and 0.542 (P < 0.001), respectively. The mean movement onset time measured with the accelerometer and the VICON system was 205 (SD 50.8) and 197 (SD 44.8) mseconds, respectively. Agreement between the paired measurements was good (ICC2,1 0.729 to 0.822) (Table 2). The mean difference between the paired measurements was 8 mseconds, and the average limits of agreement were −56.4 to 72.5 mseconds (Table 2). Fig. 1 is an example of a Bland and Altman plot which examines if there is any bias in the two measurements, and shows the difference between two measurements (accelerometer and VICON system) plotted against the mean values for the two instruments. Discussion Both the accelerometer and the VICON system demonstrated good reliability for the measurement of movement
258
P.Y.M. Chung, G.Y.F. Ng / Physiotherapy 98 (2012) 256–259
Table 2 Intraclass correlation coefficient (ICC) values and limits of agreement for the five trials.
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
ICC values
Mean (SD) difference between the paired measurements (mseconds)
Lower limit of agreement (mseconds)
Upper limit of agreement (mseconds)
0.75* 0.73* 0.76* 0.82* 0.82*
4.2 (32.9) 6.2 (34.0) 10.0 (34.2) 8.5 (33.1) 11.3 (30.1)
−60.3 −60.5 −57.0 −56.4 −47.7
68.7 72.9 77.0 73.4 70.4
SD, standard deviation. * P < 0.005.
values, and more of the differences were found at the positive end (Table 1). This suggests that there may be bias between the two instruments in that the movement onset time measured by the accelerometer is slower than that measured by the VICON system. This could be due to different mechanisms for the detection of movement. The accelerometer detects force and translates it to movement, whereas the VICON system captures the movement of the reflective markers per se. Signals from the accelerometer are generated by deformation of the piezo-electric crystal inside the sensor when it is compressed or sheared by the seismic mass during movement. Therefore, a certain degree of force is needed to deform the piezo-electric element before an electric charge can be built up, and time is needed for the voltage to reach a level that can be detected by the equipment. These factors may have led to the delay in reaction time for accelerometer measurements. Conclusion Fig. 1. Example of Bland and Altman plot.
onset, and the ICC for the accelerometer was higher than that for the VICON system. The difference in sampling frequency between the two instruments may account for the discrepancy in reliability, as it has been reported that the measurement error of motion analysis would increase with a decreasing sampling frequency and an increasing movement speed [21]. Therefore, an accelerometer with a higher sampling rate would provide more reliable measurements than a 3D motion analysis system with a lower sampling frequency. Although the level of agreement between the accelerometer and the VICON system was good, the Bland and Altman plot showed that the limits of agreement were quite large, with 95% of the accelerometer measurements falling within the range of 56.4 mseconds earlier or 72.5 mseconds later than the VICON measurements. As the mean reaction time of two measurements was 200.9 mseconds, the range of agreement would equate to 64% of the mean movement onset time; this is considered to be clinically significant. Therefore, the data obtained from the two instruments should not be used interchangeably. Although the Bland and Altman plots revealed no obvious bias, the mean differences for all five trials were positive
In conclusion, the accelerometer showed good reliability for the measurement of motor reaction times. However, the reaction times detected by the accelerometer were slower than those detected using the VICON system, so the absolute values of movement onset timing measured by the two instruments should not be used interchangeably. As accelerometers have the advantage of being relatively simple technology for the detection of movement, their use in the clinical setting is warranted. Acknowledgements The authors thank Mr Cheung Yat-man for his technical advice, and Dr. Raymond Chung of the Hong Kong Polytechnic University for technical help and statistical advice in this study. Ethical approval: Human Subjects Ethics Sub-committee of the Hong Kong Polytechnic University (Project ID: HSEARS20110509001). Funding: University Research Grant from the Hong Kong Polytechnic University. Conflict of interest: None declared.
P.Y.M. Chung, G.Y.F. Ng / Physiotherapy 98 (2012) 256–259
References [1] Dogan G. Multiple-choice reaction and visual perception in female and male elite athletes. J Sports Med Phys Fitness 2009;49:91–6. [2] Fontani, Lodi L, Felici A, Migliorini S, Corradeschi F. Attention in athletes of high and low experience engaged in different open skill sports. Percept Mot Skills 2006;102:791–805. [3] Nakamoto H, Mori S. Sport-specific decision-making in a Go/Nogo reaction task: difference among nonathletes and baseball and basketball players. Percept Mot Skills 2008;106:163–70. [4] Zwierko T, Osinski W, Lubinski W, Czepita D, Florkiewicz B. Speed of visual sensorimotor processes and conductivity of visual pathway in volleyball players. J Hum Kinet 2010;23:21–7. [5] Dane S, Hazar F, Tan Ü. Correlation between eye–hand reaction time and power of various muscles in badminton players. Int J Neurosci 2008;118:349–54. [6] Christou EA, Rodriguez TM. Time but not force is transferred between ipsilateral upper and lower limbs. J Motor Behav 2008;40:186–9. [7] L’Hermette M, Savatier X, Baudry L, Tourny-Chollet C, Dujardin F. A new portable device for assessing locomotor performance. Int J Sports Med 2008;29:322–6. [8] McGinley JL, Baker R, Wolfe R, Morris ME. The reliability of threedimensional kinematic gait measurements: a systematic review. Gait Posture 2009;29:360–9. [9] Pomeroy VM, Evans E, Richards JD. Agreement between an electrogoniometer and motion analysis system measuring angular velocity of the knee during walking after stroke. Physiotherapy 2006;92:159–65. [10] Lee HK, Cho SP, You JH, Lee KY. The concurrent validity of the body center of mass in accelerometric measurement. In: Proceedings of the 29th Annual International Conference of the IEEE, EMBS, vol. 2007. 2007. p. 23–6.
259
[11] Lord S, Rochester L, Baker K, Nieuwboer A. Concurrent validity of accelerometry to measure gait in Parkinsons disease. Gait Posture 2008;27:357–9. [12] Harrington DM, Welk GJ, Donnelly AE. Validation of MET estimates and step measurement using the ActivPAL physical activity logger. J Sports Sci 2011;29:627–33. [13] Davis MG, Fox KR, Hillsdon M, Sharp DJ, Coulson JC, Thompson JL. Objectively measured physical activity in a diverse sample of older urban UK adults. Med Sci Sports Exerc 2011;43:647–54. [14] Ng ECT, Chui MPY, Siu AYK, Yam VWN, Ng GY. Ankle positioning and knee perturbation affect temporal recruitment of the vasti muscles in people with patellofemoral pain. Physiotherapy 2010;97:65–70. [15] Henriksen M, Lund H, Moe-Nilssen R, Bliddal H, Danneskio-Samsoe B. Test–retest reliability of trunk accelerometric gait analysis. Gait Posture 2004;19:288–97. [16] Moe-Nilssen R. Test–retest reliability of trunk accelerometry during standing and walking. Arch Phys Med Rehabil 1998;79:1377–85. [17] Van Hees VT, Slootmaker SM, Groot GD, Van Mechelen W, Van Lummel RC. Reproducibility of triaxial seismic accelerometer (DynaPort). Med Sci Sports Exerc 2009;41:810–7. [18] Armstrong WJ, McGregor SJ, Yaggie JA, Bailey JJ, Johnson SM, Goin AM, et al. Reliability of mechanomyography and triaxial accelerometry in the assessment of balance. J Electromyogr Kinesiol 2010;20:726–31. [19] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1: 307–10. [20] Portney LG, Walkins MP. Foundation of clinical research: application to practice. 3rd edn. Upper Saddle River: Pearson/Prentice Hall; 2009. [21] Polk JD, Psutka SP, Demes B. Sampling frequencies and measurement error for linear and temporal gait parameters in primate locomotion. J Hum Evol 2005;49:665–79.
Available online at www.sciencedirect.com
Comparison between an accelerometer and a three-dimensional motion analysis system for the detection of movement P.Y.M. Chung, G.Y.F. Ng ∗ Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China
Abstract Objective To examine the reliability and concurrent validity of an accelerometer compared with a three-dimensional (3D) motion analysis system for measuring the motor reaction time of the lower limbs, to validate a simple method for objective clinical assessment of movement onset. Design Timing of the onset of knee extension movement in response to an audio signal was measured simultaneously with an accelerometer and a 3D VICON motion analysis system. Participants Twelve able-bodied subjects with a mean age of 26 (standard deviation 2.3) years. Results Good reliability was found for both instruments, although the intraclass correlation coefficient (ICC) was higher for the accelerometer (ICC3,1 = 0.739; P < 0.001). The mean motor reaction time measured by the accelerometer and the VICON system was 205.0 and 196.9 mseconds, respectively. Good agreement was found between the paired measurements (mean ICC = 0.774), and the average 95% limits of agreement were −56.4 to 72.5 mseconds. Conclusion In conclusion, the accelerometer had good reliability for the measurement of movement onset time. Agreement between the measurements from the accelerometer and the VICON system was high. However, the limits of agreement covered a wide range, so absolute timings for movement onset derived from these methods should not be used interchangeably. © 2011 Published by Elsevier Ltd on behalf of Chartered Society of Physiotherapy. Keywords: Accelerometer; Movement onset; Reaction time
Introduction Reaction time is an important determining factor in many sporting events. Most reports on reaction time have focused on the upper limbs [1–5], possibly because upper limb reaction time can be measured easily using a keyboard or a thumb switch. However, these methods are not applicable to the lower limbs, and it has been reported that motor control differs between the upper and lower limbs [6]. Christou and Rodriguez [6] examined the transfer of motor performance between ipsilateral upper and lower limbs by measuring the force and time accuracy of goal-directed isometric contractions of the muscles. They found that all components of motor learning cannot be transferred between the upper and lower limbs. As such, it is necessary to measure the lower limb
∗
Corresponding author. Tel.: +852 27666721; fax: +852 23308656. E-mail address: [email protected] (G.Y.F. Ng).
response to stimuli in order to investigate the effect of sports training on motor performance involving the lower limbs. Three-dimensional (3D) motion analysis technology has been widely used in gait studies and is regarded as the gold standard for the analysis of movement [7–10]. Most 3D motion analysis systems use cameras to capture the trajectories of reflective markers attached to body parts during movements. Spatiotemporal kinematics parameters of the movements can be acquired after data processing, and can be used to measure the onset timing of body movements. However, due to the complex set-up and sophisticated equipment involved, this technology is not easily applied in the clinical setting. Accelerometers are relatively simple instruments that measure movements involving acceleration. Most accelerometers contain a piezo-electric element and a seismic mass embedded in a sensor case; when the piezo-electric element is compressed or sheared by the seismic mass during acceleration, it generates voltage signals proportional to the
0031-9406/$ – see front matter © 2011 Published by Elsevier Ltd on behalf of Chartered Society of Physiotherapy. doi:10.1016/j.physio.2011.06.003
P.Y.M. Chung, G.Y.F. Ng / Physiotherapy 98 (2012) 256–259
force. Accelerometers have been used for the analysis of gait [11], to monitor physical activities [12,13], to estimate the body centre of mass [10], and to measure muscle activities with perturbation in postural testing [14]. Good test–retest reliability has been reported for accelerometer recordings [intraclass correlation coefficient (ICC) 0.7 to 0.97] [15–18]. The portability and relatively low cost of an accelerometer make it suitable for movement detection and biomechanical analysis for both upper and lower limbs in the clinical setting. However, to date, the validity of accelerometers in the measurement of reaction time has not been determined. Therefore, this study aimed to determine the concurrent validity for an accelerometer and a 3D motion analysis system in measuring motor reaction times of the knee.
Methods Twelve able-bodied subjects with a mean age of 26 [standard deviation (SD) 2.3] years volunteered to participate in this study. Their mean height was 164 (SD 9.4) cm and mean weight was 58 (SD 9.4) kg. The right leg was the dominant leg for all subjects. This study was approved by the Human Subjects Ethics Sub-committee of the Hong Kong Polytechnic University, and subjects gave their written informed consent prior to testing. The onset time of lower limb movement was measured using a uni-axial, piezo-electric accelerometer (Model 8772A10, Kistler Instrument Corp., Amherst, NY, USA) and an eight-camera VICON motion analysis system (v-370, Vicon Motion Systems, Oxford, UK). The eight cameras were located 2 m above the floor in a laboratory measuring 4 × 10 m. Subjects sat in the centre of the room with the sagittal plane aligned with the Y-axis of the VICON system, and hip and knee flexed at 90◦ . The accelerometer was attached to the tibial tuberosity of the right leg, and a spherical, reflective marker was placed over the accelerometer. Subjects were asked to extend their right knee as fast as possible on hearing a ‘beep’ triggered by the operator. The audio signal occurred at any time from 1 to 9 seconds after a ‘start’ verbal cue given by the operator. The audio signal was output to the VICON system via an analogueto-digital converter, and displayed graphically as a voltage signal in order to synchronise the VICON system and the accelerometer. The accelerometer signals were sent to an analogue-to-digital converter at a sampling rate of 1000 Hz, and captured with LabVIEW software (National Instrument, Austin, TX, USA). The sampling frequency of the VICON cameras was set at 250 Hz, and VICON Nexus 1.4 (Vicon Motion Systems, Oxford, UK) was used to reconstruct the sagittal trajectories of the reflective markers. Both sets of data were exported to Microsoft Excel (Microsoft Corp., Redmond, WA, USA) for data processing. As the accelerometer was uni-axial and force was registered along the axis of movement, the anteroposterior trajectory of the reflective marker was used for comparison. Movement
257
Table 1 Mean movement onset times measured by the accelerometer and the VICON system, and the mean differences between the paired measurements for each trial. Subject
Mean (SD) values of the five measurements (mseconds)
Accelerometer
VICON
1 2 3 4 5 6 7 8 9 10 11 12
179.2 (31.6) 249.0 (37.1) 186.6 (18.7) 196.4 (22.7) 272.4 (29.4) 253.4 (20.5) 156.4 (22.2) 244.8 (23.5) 160.2 (34.4) 215.6 (27.6) 118.8 (21.6) 225.6 (16.2)
194.6 (46.9) 228.4 (39.9) 189.0 (18.7) 159.8 (24.5) 224.8 (41.7) 201.8 (21.0) 140.6 (14.9) 238.6 (34.2) 159.8 (34.1) 229.8 (28.9) 153.2 (21.4) 243.6 (19.8)
Mean difference (95% CI) of paired measurements for each trial (mseconds) −15.4 (−80.7 to 49.9) 20.6 (0.3 to 40.9) −2.4 (−9.1 to 4.3) 36.6 (21.9 to 51.3) 47.6 (18 to 77.2) 53.6 (7.4 to 99.8) 15.8 (−37.3 to 68.9) 6.2 (−18.2 to 30.6) 0.4 (−70.2 to 71) −14.2 (−43 to 14.6) −34.4 (−25.4 to 43.4) −18.0 (−6.2 to −29.8)
SD, standard deviation; CI, confidence interval.
onset was defined as any deviation in the anteroposterior direction lasting for ≥5 mseconds with a difference of at least 2 SD from the resting signal (captured 50 mseconds before the audio signal for both sets of data). Data analysis Each subject was tested five times and all trials were used to determine the reliability of the two instruments (ICC3,1 ). ICC2,1 and limits of agreement were calculated for each trial to assess the level of agreement between the paired measurements [19,20]. Results Table 1 shows the mean measurements for each subject. The ICC values for the accelerometer and the VICON system were 0.739 (P < 0.001) and 0.542 (P < 0.001), respectively. The mean movement onset time measured with the accelerometer and the VICON system was 205 (SD 50.8) and 197 (SD 44.8) mseconds, respectively. Agreement between the paired measurements was good (ICC2,1 0.729 to 0.822) (Table 2). The mean difference between the paired measurements was 8 mseconds, and the average limits of agreement were −56.4 to 72.5 mseconds (Table 2). Fig. 1 is an example of a Bland and Altman plot which examines if there is any bias in the two measurements, and shows the difference between two measurements (accelerometer and VICON system) plotted against the mean values for the two instruments. Discussion Both the accelerometer and the VICON system demonstrated good reliability for the measurement of movement
258
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Table 2 Intraclass correlation coefficient (ICC) values and limits of agreement for the five trials.
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
ICC values
Mean (SD) difference between the paired measurements (mseconds)
Lower limit of agreement (mseconds)
Upper limit of agreement (mseconds)
0.75* 0.73* 0.76* 0.82* 0.82*
4.2 (32.9) 6.2 (34.0) 10.0 (34.2) 8.5 (33.1) 11.3 (30.1)
−60.3 −60.5 −57.0 −56.4 −47.7
68.7 72.9 77.0 73.4 70.4
SD, standard deviation. * P < 0.005.
values, and more of the differences were found at the positive end (Table 1). This suggests that there may be bias between the two instruments in that the movement onset time measured by the accelerometer is slower than that measured by the VICON system. This could be due to different mechanisms for the detection of movement. The accelerometer detects force and translates it to movement, whereas the VICON system captures the movement of the reflective markers per se. Signals from the accelerometer are generated by deformation of the piezo-electric crystal inside the sensor when it is compressed or sheared by the seismic mass during movement. Therefore, a certain degree of force is needed to deform the piezo-electric element before an electric charge can be built up, and time is needed for the voltage to reach a level that can be detected by the equipment. These factors may have led to the delay in reaction time for accelerometer measurements. Conclusion Fig. 1. Example of Bland and Altman plot.
onset, and the ICC for the accelerometer was higher than that for the VICON system. The difference in sampling frequency between the two instruments may account for the discrepancy in reliability, as it has been reported that the measurement error of motion analysis would increase with a decreasing sampling frequency and an increasing movement speed [21]. Therefore, an accelerometer with a higher sampling rate would provide more reliable measurements than a 3D motion analysis system with a lower sampling frequency. Although the level of agreement between the accelerometer and the VICON system was good, the Bland and Altman plot showed that the limits of agreement were quite large, with 95% of the accelerometer measurements falling within the range of 56.4 mseconds earlier or 72.5 mseconds later than the VICON measurements. As the mean reaction time of two measurements was 200.9 mseconds, the range of agreement would equate to 64% of the mean movement onset time; this is considered to be clinically significant. Therefore, the data obtained from the two instruments should not be used interchangeably. Although the Bland and Altman plots revealed no obvious bias, the mean differences for all five trials were positive
In conclusion, the accelerometer showed good reliability for the measurement of motor reaction times. However, the reaction times detected by the accelerometer were slower than those detected using the VICON system, so the absolute values of movement onset timing measured by the two instruments should not be used interchangeably. As accelerometers have the advantage of being relatively simple technology for the detection of movement, their use in the clinical setting is warranted. Acknowledgements The authors thank Mr Cheung Yat-man for his technical advice, and Dr. Raymond Chung of the Hong Kong Polytechnic University for technical help and statistical advice in this study. Ethical approval: Human Subjects Ethics Sub-committee of the Hong Kong Polytechnic University (Project ID: HSEARS20110509001). Funding: University Research Grant from the Hong Kong Polytechnic University. Conflict of interest: None declared.
P.Y.M. Chung, G.Y.F. Ng / Physiotherapy 98 (2012) 256–259
References [1] Dogan G. Multiple-choice reaction and visual perception in female and male elite athletes. J Sports Med Phys Fitness 2009;49:91–6. [2] Fontani, Lodi L, Felici A, Migliorini S, Corradeschi F. Attention in athletes of high and low experience engaged in different open skill sports. Percept Mot Skills 2006;102:791–805. [3] Nakamoto H, Mori S. Sport-specific decision-making in a Go/Nogo reaction task: difference among nonathletes and baseball and basketball players. Percept Mot Skills 2008;106:163–70. [4] Zwierko T, Osinski W, Lubinski W, Czepita D, Florkiewicz B. Speed of visual sensorimotor processes and conductivity of visual pathway in volleyball players. J Hum Kinet 2010;23:21–7. [5] Dane S, Hazar F, Tan Ü. Correlation between eye–hand reaction time and power of various muscles in badminton players. Int J Neurosci 2008;118:349–54. [6] Christou EA, Rodriguez TM. Time but not force is transferred between ipsilateral upper and lower limbs. J Motor Behav 2008;40:186–9. [7] L’Hermette M, Savatier X, Baudry L, Tourny-Chollet C, Dujardin F. A new portable device for assessing locomotor performance. Int J Sports Med 2008;29:322–6. [8] McGinley JL, Baker R, Wolfe R, Morris ME. The reliability of threedimensional kinematic gait measurements: a systematic review. Gait Posture 2009;29:360–9. [9] Pomeroy VM, Evans E, Richards JD. Agreement between an electrogoniometer and motion analysis system measuring angular velocity of the knee during walking after stroke. Physiotherapy 2006;92:159–65. [10] Lee HK, Cho SP, You JH, Lee KY. The concurrent validity of the body center of mass in accelerometric measurement. In: Proceedings of the 29th Annual International Conference of the IEEE, EMBS, vol. 2007. 2007. p. 23–6.
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