Measurement of gait by accelerometer and walkway: a comparison study

Accelerations of the waist and lower extremities over a range of gait velocities to aid in activity monitor selection for field-based studies

This study aimed to define accelerations measured at the waist and lower extremities over a range of gait velocities to provide reference data for choosing the appropriate accelerometer for field-based human activity monitoring studies. Accelerations were measured with a custom activity monitor (± 16g) at the waist, thighs, and ankles in 11 participants over a range of gait velocities from slow walking to running speeds. The cumulative frequencies and peak accelerations were determined. Cumulative acceleration amplitudes for the waist, thighs, and ankles during gait velocities up to 4.8 m/s were within the standard commercial g-range (± 6g) in 99.8%, 99.0%, and 96.5% of the data, respectively. Conversely, peak acceleration amplitudes exceeding the limits of many commercially available activity monitors were observed at the waist, thighs, and ankles, with the highest peaks at the ankles, as expected. At the thighs, and more so at the ankles, nearly 50% of the peak accelerations would not be detected when the gait velocity exceeds a walking velocity. Activity monitor choice is application specific, and investigators should be aware that when measuring high-intensity gait velocity activities with commercial units that impose a ceiling at ± 6g, peak accelerations may not be measured.

Tri-axial accelerometer analysis techniques for evaluating functional use of the extremities

Activity monitors provide an objective mechanism for evaluating patient function. It is unclear what similarities or unique information may be yielded using different analyses. Fifteen patients scheduled to undergo shoulder arthroplasty and fifteen matched control subjects wore tri-axial accelerometer activity monitors bilaterally at the lower (wrist) and upper (biceps) arm for 3days. Measures of central tendency, variance, sample entropy, and asymmetry were calculated. A novel technique to evaluate time distribution of activity intensity was also performed. Within both groups there was a difference in central tendency and variance when comparing dominant and non-dominant limbs for both the lower (

CONTROLS

Mean Activity, P<0.001; Max Activity, P<0.001;

PATIENTS

Mean Activity, P=0.044; Max Activity, P=0.009) and upper (

CONTROLS

Mean Activity, P<0.001; Max Activity, P=0.046;

PATIENTS

Mean Activity, P=0.002; Max Activity, P=0.049) arm. Within group differences were also present for lower arm entropy in both groups (CONTROLS, P<0.001; PATIENTS P=0.041), and at the upper arm for patients (P=0.003). There were differences between groups for the asymmetry index for both the lower (P=0.033) and upper arm (P=0.005), and maximum activity level of the lower arm (P=0.05). Between group differences were present for time distribution of activity intensity, as the involved upper arm of patients was inactive for a greater time than controls (P=0.013). These results highlight unique information provided by multiple analysis methods, and include a novel approach of evaluating the distribution of time spent across variable intensity activities.

Estimation of walking energy expenditure by using support vector regression

This paper develops a new predictor of walking energy expenditure from wireless measurements of body movements using triaxial accelerometers. Reliable data were collected from repeated walking experiments in different conditions on a treadmill with simultaneous measurement of expired oxygen and carbon dioxide. Support vector regression, a powerful non-linear regression method, was used to process and model the data. This novel processing method sets this investigation apart from existing papers. Good results were achieved in the robust estimation of walking related energy expenditure from a number of variables derived from triaxial accelerometer and treadmill speed.

Gait and muscle activation changes in men with knee osteoarthritis

The aim was to examine the biomechanics of level- and stair-walking in men with knee osteoarthritis (OA) at different pre-determined gait speeds and to compare the results with those obtained from healthy control subjects. Special emphasis was placed on the estimation of joint loading. Fifty-four men with knee OA (50-69 years) and 53 healthy age- and sex-matched controls were enrolled in the study. The participants walked barefoot in the laboratory (1.2 m/s+/-5%), corridor (1.2; 1.5 and 1.7 m/s+/-5%), and climbing and coming down stairs (0.5 and 0.8 m/s+/-5%) separately. Joint loading was assessed with skin mounted accelerometers (SMAs) attached just above and below the more affected knee joint. The 3-D ground reaction forces (GRFs) and muscle activation with surface-electromyography (EMG) from vastus medialis (VM) and biceps femoris (BF) were also measured simultaneously. There were no differences in SMA variables between groups during level-walking, but maximal loading rate (LR(max)) was higher bilaterally in the controls (P<.05). Patients loaded their lower extremity more forcefully especially during stair descent at faster speed. The distinctions in muscle activation both at level- and stair ambulation in VM and BF muscles revealed that the patients used different strategies to execute the same walking tasks. It is concluded that the differences in measured SMA and GRF parameters between the knee OA patients and the controls were only minor at constant gait speeds. It is speculated that the faster speeds in the stair descent subjected the compensatory mechanisms to the maximum highlighting the differences between groups.

Gait posture estimation using wearable acceleration and gyro sensors

A method for gait analysis using wearable acceleration sensors and gyro sensors is proposed in this work. The volunteers wore sensor units that included a tri-axis acceleration sensor and three single axis gyro sensors. The angular velocity data measured by the gyro sensors were used to estimate the translational acceleration in the gait analysis. The translational acceleration was then subtracted from the acceleration sensor measurements to obtain the gravitational acceleration, giving the orientation of the lower limb segments. Segment orientation along with body measurements were used to obtain the positions of hip, knee, and ankle joints to create stick figure models of the volunteers. This method can measure the three-dimensional positions of joint centers of the hip, knee, and ankle during movement. Experiments were carried out on the normal gait of three healthy volunteers. As a result, the flexion-extension (F-E) and the adduction-abduction (A-A) joint angles of the hips and the flexion-extension (F-E) joint angles of the knees were calculated and compared with a camera motion capture system. The correlation coefficients were above 0.88 for the hip F-E, higher than 0.72 for the hip A-A, better than 0.92 for the knee F-E. A moving stick figure model of each volunteer was created to visually confirm the walking posture. Further, the knee and ankle joint trajectories in the horizontal plane showed that the left and right legs were bilaterally symmetric.

Reproducibility of Loading Measurements With Skin-Mounted Accelerometers During Walking

OBJECTIVE

To examine reproducibility of load measurements with skin-mounted accelerometers (SMAs) during walking.

DESIGN

Reliability study.

SETTING

A motion analysis laboratory.

PARTICIPANTS

Ten healthy young men.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Two triaxial accelerometers were fixed to the subjects' skin above and below the knee joint. The subjects walked barefoot at their preferred speed and at a constant speed (1.3m/s, +/-5%) in a gait laboratory and along a corridor. The same protocol was repeated over 2 days. Initial peak acceleration (IPA), peak-to-peak (PP) acceleration, and maximal and average acceleration transient rates (ATRs) were calculated. The coefficient of variation (CV) and Pearson linear correlation coefficient were calculated to measure reproducibility of SMA load measurements.

RESULTS

IPA and PP acceleration had good interday repeatability (CV <15%). The repeatability of average ATR and maximal ATR parameters was generally not acceptable. The loading variables obtained from ground reaction forces and SMA measurements during gait revealed high linear correlations, indicating that with SMA measurements it is possible to predict certain ground reaction force loading parameters.

CONCLUSIONS

SMAs are practical for use in clinical environments to collect acceleration data that may be used to estimate joint loads.

Analysis of orientation error of triaxial accelerometers on the assessment of energy expenditure

This paper investigates the effects of orientation error in the positioning of triaxial accelerometers on the assessment of energy expenditure. Four subjects walked on a treadmill at varying velocities ranging from 4km.h^-1to 5km.h^-1. During each test, a triaxial accelerometer attached to the lower back at arbitrary orientations to record body accelerations. Energy expenditure was estimated by the sum of the integrals of the absolute value of accelerometer output from all the three measurement directions. Based on theoretical analysis and experimental observations, it is concluded that small orientation errors (&lt; 3&#176;) have no distinguishable effects on the estimation of energy expenditure. We propose an efficient method to compensate for larger orientation errors. The experimental results verified the effectiveness of this proposed compensation method.

Interstride trunk acceleration variability but not step width variability can differentiate between fit and frail older adults

Variability of gait may be regarded as a sign of adaptability and thus a requirement for successful locomotion, or as a sign of impaired balance control. In this study we examined the role of step width variability (SWV) and interstride trunk acceleration variability in two groups of fit and frail old people. We investigated the association of these measures and how they differentiated the two groups. We examined 33 fit older adults (mean age 73 years, S.D. 3.3 years) and 32 frail old people (mean age 80 years, S.D. 4.0 years). Subjects performed timed walking at different speeds ranging from very slow to very fast. SWV was measured from footprints. Trunk accelerations were registered by a triaxial accelerometer and interstride trunk acceleration variability assessed by an unbiased autocorrelation procedure. All measures were normalized to a walking speed of 0.9 m/s to avoid the confounding effect of gait speed on speed dependent gait parameters. SWV demonstrated low association with the trunk variability measures, and did not differ between groups. The frail group had lower mediolateral (P=0.015), but higher vertical (P=0.015) and anteroposterior (P<0.02) trunk variability than the fit group. Trunk variability classified 80% of the subjects correctly into their respective group (sensitivity=0.75, specificity=0.85). The findings are compatible with a notion that mediolateral interstride trunk variability represents a different aspect of motor control than variability in the direction of propulsion.

Classification of basic daily movements using a triaxial accelerometer

A generic framework for the automated classification of human movements using an accelerometry monitoring system is introduced. The framework was structured around a binary decision tree in which movements were divided into classes and subclasses at different hierarchical levels. General distinctions between movements were applied in the top levels, and successively more detailed subclassifications were made in the lower levels of the tree. The structure was modular and flexible: parts of the tree could be reordered, pruned or extended, without the remainder of the tree being affected. This framework was used to develop a classifier to identify basic movements from the signals obtained from a single, waist-mounted triaxial accelerometer. The movements were first divided into activity and rest. The activities were classified as falls, walking, transition between postural orientations, or other movement. The postural orientations during rest were classified as sitting, standing or lying. In controlled laboratory studies in which 26 normal, healthy subjects carried out a set of basic movements, the sensitivity of every classification exceeded 87%, and the specificity exceeded 94%; the overall accuracy of the system, measured as the number of correct classifications across all levels of the hierarchy, was a sensitivity of 97.7% and a specificity of 98.7% over a data set of 1309 movements.

Accelerometry: providing an integrated, practical method for long-term, ambulatory monitoring of human movement

Accelerometry offers a practical and low cost method of objectively monitoring human movements, and has particular applicability to the monitoring of free-living subjects. Accelerometers have been used to monitor a range of different movements, including gait, sit-to-stand transfers, postural sway and falls. They have also been used to measure physical activity levels and to identify and classify movements performed by subjects. This paper reviews the use of accelerometer-based systems in each of these areas. The scope and applicability of such systems in unsupervised monitoring of human movement are considered. The different systems and monitoring techniques can be integrated to provide a more comprehensive system that is suitable for measuring a range of different parameters in an unsupervised monitoring context with free-living subjects. An integrated approach is described in which a single, waist-mounted accelerometry system is used to monitor a range of different parameters of human movement in an unsupervised setting.

Test-retest reliability of trunk accelerometric gait analysis

The purpose of this study was to determine the test-retest reliability of a trunk accelerometric gait analysis in healthy subjects. Accelerations were measured during walking using a triaxial accelerometer mounted on the lumbar spine of the subjects. Six men and 14 women (mean age 35.2; range 18-57) underwent the same protocol on 2 consecutive days. The raw acceleration signals from six self-selected walking speeds were transformed into a horizontal-vertical coordinate system to remove unwanted variability caused by gravity. Acceleration root mean square values, cadences, step and stride lengths were then computed and interpolated using quadratic curve fits and point estimates were calculated at a standardised walking speed of 1.35 m/s. Relative reliability was determined using two models of intraclass correlation coefficients (ICC(1,1) and ICC(3,1)) to assess any systematic shifts and absolute reliability was determined using measurement error (ME). The results of the study showed high ICC values (0.77-0.96) and ME values of 0.007-0.01 g for mean acceleration; 0.009 m for step lengths; 0.022 m for stride length and 1.644 step/min for cadences. In conclusion, the method was found to be reliable and may have a definite potential in clinical gait analysis.

The use of accelerometry to detect heel contact events for use as a sensor in FES assisted walking

Current sensors for the control of functional electrical stimulation (FES) assisted walking in hemiplegic individuals are not wholly satisfactory, as they are either not implantable or ineffectual in the detection of heel contact events. This study describes the use of an accelerometer placed on the trunk to detect heel contact events of both legs based on the examination of the anterior-posterior horizontal acceleration signal. Four subjects wore an accelerometer over their lumbar spine. Footswitches placed on the sole of one foot recorded the heel contact and heel off times for that foot. The acceleration signal was reduced to a series of pulses by studying the negative-positive changes in acceleration. It was found that there was approximately a 150 ms delay between heel contact and the negative-positive change in acceleration. This delay was consistent across different walking speeds, but was different between subjects and when hemiplegic gait was simulated. Therefore, accelerometers placed on the trunk are valid sensors for the detection of heel contact events during FES assisted walking.

Analysis and decomposition of signals obtained by thigh-fixed uni-axial accelerometry during normal walking

The use of piezo-resistive uni-axial accelerometer signals in gait analysis is complicated by the fact that the measured signal is composed of different types of acceleration. The aim of the study is to obtain insight into the signal from a tangential accelerometer attached to the thigh during walking. Six subjects walk with three different speeds. Simultaneous measurements are performed with accelerometers, footswitches and an opto-electronic system. The components of the accelerometer signal are calculated from the opto-electronic system. A clear relationship is found between the measured and calculated accelerometer signals (range RMS: 0.76-3.69 m x s(-2), range rms: 0.22-0.61). The most pronounced feature is a high positive acceleration peak (> 10 m x s(-2)) at the end of the cycle. The gravitational acceleration during one cycle is characterised by a sinusoidal shape, whereas the inertial acceleration contains higher-frequency components (up to 20 Hz). During the major part of the gait cycle, the gravitational and inertial acceleration make opposing contributions to the signal As a result, the gravitational acceleration influences the amplitudes of the measured acceleration signal, the shape and peaks of which are mainly determined by the inertial acceleration. Because the gravitational and inertial accelerations differ in frequency components, the application for gait analysis remains feasible.

A triaxial accelerometer and portable data processing unit for the assessment of daily physical activity

The present study describes the development of a triaxial accelerometer (TA) and a portable data processing unit for the assessment of daily physical activity. The TA is composed of three orthogonally mounted uniaxial piezoresistive accelerometers and can be used to register accelerations covering the amplitude and frequency ranges of human body acceleration. Interinstrument and test-retest experiments showed that the offset and the sensitivity of the TA were equal for each measurement direction and remained constant on two measurement days. Transverse sensitivity was significantly different for each measurement direction, but did not influence accelerometer output (< 3% of the sensitivity along the main axis). The data unit enables the on-line processing of accelerometer output to a reliable estimator of physical activity over eight-day periods. Preliminary evaluation of the system in 13 male subjects during standardized activities in the laboratory demonstrated a significant relationship between accelerometer output and energy expenditure due to physical activity, the standard reference for physical activity (r = 0.89). Shortcomings of the system are its low sensitivity to sedentary activities and the inability to register static exercise. The validity of the system for the assessment of normal daily physical activity and specific activities outside the laboratory should be studied in free-living subjects.