Increased walking variability in elderly persons with congestive heart failure.

OBJECTIVES: To determine the effects of congestive heart failure on a person's ability to walk at a steady pace while ambulating at a self-determined rate. SETTING: Beth Israel Hospital, Boston, a primary and tertiary teaching hospital, and a social activity center for elderly adults living in the community. PARTICIPANTS: Eleven elderly subjects (aged 70-93 years) with well compensated congestive heart failure (NY Heart Association class I or II), seven elderly subjects (aged 70-79 years) without congestive heart failure, and 10 healthy young adult subjects (aged 20-30 years). MEASUREMENTS: Subjects walked for 8 minutes on level ground at their own selected walking rate. Footswitches were used to measure the time between steps. Step rate (steps/minute) and step rate variability were calculated for the entire walking period, for 30 seconds during the first minute of the walk, for 30 seconds during the last minute of the walk, and for the 30-second period when each subject's step rate variability was minimal. Group means and 5% and 95% confidence intervals were computed. MAIN RESULTS: All measures of walking variability were significantly increased in the elderly subjects with congestive heart failure, intermediate in the elderly controls, and lowest in the young subjects. There was no overlap between the three groups using the minimal 30-second variability (elderly CHF vs elderly controls: P < 0.001, elderly controls vs young: P < 0.001), and no overlap between elderly subjects with and without congestive heart failure when using the overall variability. For all four measures, there was no overlap in any of the confidence intervals, and all group means were significantly different (P < 0.05).

Is walking a random walk? Evidence for long-range correlations in stride interval of human gait.

Complex fluctuations of unknown origin appear in the normal gait pattern. These fluctuations might be described as being 1) uncorrelated white noise, 2) short-range correlations, or 3) long-range correlations with power-law scaling. To test these possibilities, the stride interval of 10 healthy young men was measured as they walked for 9 min at their usual rate. From these time series, we calculated scaling indexes by using a modified random walk analysis and power spectral analysis. Both indexes indicated the presence of long-range self-similar correlations extending over hundreds of steps; the stride interval at any time depended on the stride interval at remote previous times, and this dependence decayed in a scale-free (fractallike) power-law fashion. These scaling indexes were significantly different from those obtained after random shuffling of the original time series, indicating the importance of the sequential ordering of the stride interval. We demonstrate that conventional models of gait generation fail to reproduce the observed scaling behavior and introduce a new type of central pattern generator model that successfully accounts for the experimentally observed long-range correlations.

Fractal dynamics of human gait: stability of long-range correlations in stride interval fluctuations.

Fractal dynamics were recently detected in the apparently "noisy" variations in the stride interval of human walking. Dynamical analysis of these step-to-step fluctuations revealed a self-similar pattern: fluctuations at one time scale are statistically similar to those at multiple other time scales, at least over hundreds of steps, while healthy subjects walk at their normal rate. To study the stability of this fractal property, we analyzed data obtained from healthy subjects who walked for 1 h at their usual, slow, and fast paces. The stride interval fluctuations exhibited long-range correlations with power-law decay for up to 1,000 strides at all 3 walking rates. In contrast, during metronomically paced walking, these long-range correlations disappeared; variations in the stride interval were random (uncorrelated) and nonfractal. The long-range correlations observed during spontaneous walking were not affected by removal of drifts in the time series. Thus the fractal dynamics of spontaneous stride interval are normally quite robust and intrinsic to the locomotor system. Furthermore, this fractal property of neural output may be related to the higher nervous centers responsible for the control of walking rhythm.

Electrical stimulation of the thigh muscles after reconstruction of the anterior cruciate ligament. Effects of electrically elicited contraction of the quadriceps femoris and hamstring muscles on gait and on strength of the thigh muscles.

The effects of neuromuscular electrical stimulation on the strength of the thigh muscles and on gait were examined in ten patients after reconstruction of the anterior cruciate ligament. The patients were randomly assigned to one of two treatment groups: neuromuscular electrical stimulation and volitional exercise, or volitional exercise alone. A four-week course of electrically elicited co-contraction of the thigh muscles resulted in significant attenuation of the characteristic loss of strength of the quadriceps as compared with volitional exercise. There was no significant difference between groups in any measure of performance of the hamstring muscles. In the group that received neuromuscular electrical stimulation, the values for cadence, walking velocity, stance time of the involved limb, and flexion-excursion of the knee during stance were significantly different from those of the volitional exercise group. Flexion-excursion of the knee during stance was directly and significantly correlated with strength of the quadriceps femoris muscle. Flexion of the knee during stance was qualitatively different in the involved extremity as compared with the uninvolved extremity in all patients. There is a rapid flexion of the knee at weight acceptance that is maintained throughout stance and probably reflects stabilization of the joint by muscular coactivation to compensate for weakness of the quadriceps. The patients who received neuromuscular electrical stimulation had stronger quadriceps muscles and more normal gait patterns than those in the volitional exercise group.

Combining position and acceleration measurements for joint force estimation.

The calculation of joint forces in biomechanics is usually based on the measurements of the kinematics of a given body segment, the estimation of the inertial properties of that segment and the solution of the 'inverse dynamics problem'. Such a process results in estimates of the joint forces and moments needed to sustain the monitored motion. This paper presents a new approach that combines position and acceleration measurements for the purpose of deriving high-quality joint force estimates. An experimental system that is based on an instrumented compound pendulum was designed and tested. The joint forces necessary to maintain a swinging motion of the pendulum were measured by an array of strain gauges, and were compared to the forces estimated by the integrated kinematic segment that measured the position and acceleration of the pendulum. The joint force measurements were also compared to the force estimates that were based on the calculated segmental acceleration generated by the differentiation of the segmental position alone. The results show a high degree of correlation between the forces estimated by the integrated segment and those measured by the strain gauges. The force estimates based on the position measurements alone were less accurate and noisier. The application of the integrated segment to the study of human kinetics is discussed and illustrated by the ankle and knee forces during slow walking. The results suggest that the use of accelerometers is necessary for the estimation of transients and high-frequency components of joint forces.

A video-based system for the estimation of the inertial properties of body segments.

A system for the estimation of the inertial properties of human body segments using advanced video technology and computer image processing was developed. The system is based on the photogrammetric technique, where three-dimensional information is determined from two separate two-dimensional video images. The inertial properties are calculated using an image-processing algorithm which provides volumetric information, coupled with a database of anatomical densities provided in the literature. In order to determine the accuracy of the system and its limitations, the system estimates of the inertial properties of solid bodies were compared to theoretically calculated values. The application of the system to kinesiological studies is illustrated by measuring the inertial properties of the shank of three subjects, and comparing the results to data generated using regression equations provided in the literature. Human factors, such as segment boundaries identification and color thresholds selection, were found to introduce the largest errors. A proper selection of the optical setting can reduce the errors to levels of 5% or better. On the average, the system overestimated the inertial properties of solid objects by 2.51% for mass, 1.21% for center of mass, 4.53% for transverse moments of inertia and 3.65% for longitudinal moment of inertia. The video-based estimates of the mass and center of mass of the shank were comparable to values obtained from anthropometric-based regression equations. The predictions of the transverse moment of inertia of the shank varied considerably among the methods. The findings suggest that a video-based system represents a promising technique for estimating inertial properties of human body segments for individual subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Mathematical modelling of the effect of sole elasticity distribution on pronation.

A coronal plane model of a distributed elastic sole has been proposed and analyzed with respect to the effects of different medial-lateral elasticity distribution on pronation under quasi-static conditions. The distributed model consists of an array of linear vertical line springs. Under minimum energy assumption, the behavior of the top surface of the interface under resultant force and moment loading was shown to be equivalent to that of a rigid-body mechanism under the same loading. The model was then combined with a rigid-link model of the lower limb. Expressions that describe the relationship of the interface aggregate parameters with pronation and the center of pressure were obtained. These expressions were confirmed by an experiment in which the elastic distribution in the interface was systematically varied and the pronation angle and the center of pressure measured. The model has the potential of being a useful analytical tool in the design of elastic soles in running shoes.

Footswitch system for measurement of the temporal parameters of gait.

Gait analysis relies upon accurate measurement of initial and end foot contact times. These times act as a reference point for correlating all other gait data and as a mean of distinguishing normal and pathologic gait. We have developed a simple, inexpensive footswitch system that provides accurate estimates of the start and end of stance phase for sequential steps. The estimates of the beginning and end of stance phase do not require custom footwear, extensive calibration, or precise placement of the sensor within the shoe. The system is based on a commercially available transducer and can be readily reproduced for use in a laboratory setting for less than $50. We describe this system, as well as its validation. To assess the accuracy of this footswitch system, we compared footswitch based estimates of initial and end foot contact times with those obtained using a force platform as 10 people took 30 steps (10 each at slow, normal and fast walking rates) across a force platform. Both estimates coincided within +/- 10 ms (mean: 0 +/- 3 ms; N = 300) for the start of stance phase and within +/- 22 ms (mean: -1 +/- 8 ms; N = 300) for the end of stance phase. For stance duration, the differences ranged from -24 to 28 ms (mean: 1 +/- 10 ms; N = 300). In combination, these measures can be used to estimate stance duration to within 3% of force plate determined values for steps with stance durations ranging from 446 to 1594 ms. Estimates of swing and stride duration also are within 5% of force plate determined values. This system should therefore prove to be a useful tool for augmenting laboratory based investigations of gait.

A quantitative comparison of a position measurement system and accelerometry.

An electro-mechanical system to characterize the dynamic performance of a position measurement system was constructed. The system produced pure sine wave kinematics over the frequency range of 1-11 Hz. Synchronous measurements of the position (using infra-red light emitting diodes) and the acceleration were taken at discrete frequencies. The position signal was filtered and differentiated twice to obtain an estimated acceleration. The acceleration estimate was compared to the acceleration reading from the accelerometer, and both were compared to the theoretical acceleration. The comparison was based on extracting average features of the signal like amplitude, offset and noise. The results show that the accelerometer measurements matched the theoretical amplitude to within 1-3% over most of the range but showed significant offset drift. The acceleration estimates based on the position measurements were highly dependent on the filtering scheme, showed no significant offset but had higher levels of noise. The experimental measurements and the mathematical analysis quantitatively compared the dynamic performance of the position measurement system and the accelerometer. Such a system could be used to optimize the performance of position measurement devices, by comparing different filtering and/or differentiating schemes.

Theoretical considerations in laser hair removal.

A systematic and logical approach for laser hair removal demands an understanding of its biologic and physical bases. This article presents an overview of hair anatomy and physiology followed by a mathematically nonrigorous review of tissue optics and thermal responses to laser irradiation. The reader is provided with a step by step approach to laser hair removal.

1989 Volvo Award in biomechanics. Mechanical recruitment of low-back muscles. Theoretical predictions and experimental validation.

A biomechanical model for studying lumbar muscle load sharing for a class of physical tasks that involve gravitational loading (holding weights) of the upper body in an erect posture is presented. The model assumes that the lumbar muscles balance the externally applied flexion and lateral bending moments. The concept of a 'loading plane' whose axes are the two bending moments is introduced. Any point in the plane can be viewed as a 'loading-point' describing a combination of bending moments that are applied to the body. The study of lumbar-muscle load sharing revealed loading conditions that required activation or deactivation of a particular muscle. The loading plane thus could be divided into regions of activity and inactivity for each muscle, separated by a 'switching curve.' The concept of 'switching curves' proved very useful for examining previously described physiologic assumptions on the loading conditions of particular muscle groups, and for grouping the 22 muscles described in the model into ten functional units. Electromyographic validation studies were conducted and showed a high degree of correlation between the model predictions and actual measurements for the contralateral (with respect to the load) muscles and to a lesser degree of correlation for the ipsilateral muscles.

Limitations of quasi-static estimation of human joint loading during locomotion.

The forces and moments at the ankle, knee and hip joints of the human lower limbs are divided into static and inertial components. They are calculated for various activities ranging from slow walking to running. The relative roles of these two components in the 'total' joint loads are studied, and the limitations of using a quasi-static analysis approach for joint load approximation are discussed. The results indicate that the static loads only reflect the gravitational and external reactions between the body and the environment, whereas the inertial loads provide dynamic information on each body segment involved. The effect of the inertial forces and moments becomes more important as the speed of locomotion increases; where the more proximal joints in the human lower extremity are concerned; and where the shear components of the force and moment are of interest. On the other hand, it seems that most of the joint moments in the lower extremity during walking and even running could reasonably be approximated by static components.

The kinematometer--an integrated kinematic sensor for kinesiological measurements.

The calculation of human joint forces and moments during locomotion is usually based on the solution of the "inverse dynamics problem." A new approach, called the Integrated Kinematic Sensor (IKS) approach, is proposed. It combines measurements of position, linear acceleration and angular velocity, coupled with six degrees of freedom analysis of rigid body motion, for the purpose of deriving high quality link kinematics and joint loads (force and moment) estimates. The IKS approach is tested on an instrumented compound pendulum to simulate the swing of a lower limb segment. The results show a high degree of correlation between the loads estimated by the IKS and those directly measured by the instrumented joint. The approach is illustrated by studying the kinematic and dynamic variables of the human shank segment during normal walking. The results agree with the basic patterns reported in the literature, while adding new information on transients during heel strike and toe off.

The study of kinematic transients in locomotion using the integrated kinematic sensor.

A system based on the integrated kinematic sensor (IKS) was used to study the three-dimensional (3-D) kinematics of human lower limb during walking and running. The linear displacement, angular velocity, and linear acceleration of the foot, shank, and thigh segments were directly measured using three IKS's. The results clearly showed the heel strike impact in both walking and running, illustrating the high frequency components that exist in those activities. This paper illustrates the limitations of standard position measurements to capture transients associated with phase transitions, not only in acceleration estimates, but also in the determination of segmental angular velocities. An error analysis was conducted to determine the relative contribution of the accelerometer and the angular rate sensor to the determination of the segmental center of mass (COM) acceleration. The results suggest that in practical kinesiological applications, adding either an accelerometer or an angular rate sensor can remarkably increase the accuracy of segmental COM acceleration estimates.

Testing of a biomechanical model of the lumbar muscle force distribution using quasi-static loading exercises.

The study of lumbar muscle force distribution in response to externally applied loads is based on the introduction of biomechanical models of the lumbar region. The evaluation of such models requires the execution of loading exercises while monitoring the EMG activity of certain lumbar muscles. This work uses muscle activity maps as the major design tool of such exercises, provided that the subject is constrained to an upright erect posture. The maps describe the predicted muscle force for a given combination of externally applied bending moments. A series of shoulder adduction exercises were designed and the EMG signals of eight lumbar muscles were measured while subjects performed the exercises. The results show good agreement between the model predictions and the EMG measurements, especially when the load and the muscle were contralateral to one another.

Contribution of measurement system artifacts to systolic spikes.

A clinical study was conducted to determine if the systolic spike often observed in an invasive blood pressure display originates in the measurement system. The clinical system consisted of a catheter, stopcocks, 4 ft of tubing, an amplifier (containing a 12-Hz low-pass filter), and a continuous flush device. A second transducer was connected directly to the patient's catheter, in parallel with the clinical system. Its output was conditioned by an amplifier modified to exclude the low-pass filter. Simultaneous measurements were taken from both transducers, recorded, and later analyzed. A comparison of the traces in the time domain, as well as an analysis of the signals in the frequency domain, suggests that the systolic spike did not originate in the pressure tubing or in the amplifier system.

The effects of external bending moments on lumbar muscle force distribution.

A detailed biomechanical model of the low-back musculature that predicts muscle-force distribution in response to external loading is presented. The paper shows that the class of loading tasks that involve an erect posture and an arbitrary load placed on the upper limbs can be described as a loading plane whose axes are the flexion and lateral bending moments. Under these conditions, the individual muscle forces are described as a three-dimensional surface defined by the loading plane and termed the muscle activity surface (MAS). The MAS and the loading plane intersect along the switching curve which separates the load combinations that activate the muscle from those that do not. The paper suggests the existence of a recruitment order of low back muscles in response to external loads and presents a comprehensive framework for examining earlier studies that used EMG measurements to validate physiological and mechanical predictions.

Algorithm to identify components of arterial blood pressure signals during use of an intra-aortic balloon pump.

Existing bedside cardiovascular monitors often inaccurately measure arterial blood pressure during intra-aortic balloon pump (IABP) assist. We have developed an algorithm that correctly identifies features of arterial pressure waveforms in the presence of IABP. The algorithm is adaptive, functions in real-time, and uses information from the electrocardiographic (ECG) and arterial blood pressure signals to extract features and numeric values from the arterial blood pressure waveform. In its current form, it requires reliable ECG beat detection and was not intended to operate under conditions of extremely poor balloon timing. The algorithm was evaluated by an expert (P.F-C.) on a limited data set, which consisted of 12 1-minute epochs of data recorded from 6 intensive care unit patients. A criterion for selection of patients was that the ECG beat detector could detect ECG beats correctly from the waveforms. The overall sensitivity and positive predictivity for beat detection were 94.04% and 100%, respectively. For feature identification, the overall sensitivity was greater than 89%, positive predictivity was 100%, and the false-positive rate was 0%. The performance measures may be biased by the criteria for patient selection. This approach to identifying waveform features during IABP improves the accuracy of measurements. The utility of using 2 sources of information to improve measurement accuracy has been demonstrated and should be applicable to other physiologic signal-processing applications.