Assessing the stability of human locomotion: a review of current measures

Falling poses a major threat to the steadily growing population of the elderly in modern-day society. A major challenge in the prevention of falls is the identification of individuals who are at risk of falling owing to an unstable gait. At present, several methods are available for estimating gait stability, each with its own advantages and disadvantages. In this paper, we review the currently available measures: the maximum Lyapunov exponent (λS and λL), the maximum Floquet multiplier, variability measures, long-range correlations, extrapolated centre of mass, stabilizing and destabilizing forces, foot placement estimator, gait sensitivity norm and maximum allowable perturbation. We explain what these measures represent and how they are calculated, and we assess their validity, divided up into construct validity, predictive validity in simple models, convergent validity in experimental studies, and predictive validity in observational studies. We conclude that (i) the validity of variability measures and λS is best supported across all levels, (ii) the maximum Floquet multiplier and λL have good construct validity, but negative predictive validity in models, negative convergent validity and (for λL) negative predictive validity in observational studies, (iii) long-range correlations lack construct validity and predictive validity in models and have negative convergent validity, and (iv) measures derived from perturbation experiments have good construct validity, but data are lacking on convergent validity in experimental studies and predictive validity in observational studies. In closing, directions for future research on dynamic gait stability are discussed.

Pregnancy-related pelvic girdle pain (PPP), I: Terminology, clinical presentation, and prevalence

Pregnancy-related lumbopelvic pain has puzzled medicine for a long time. The present systematic review focuses on terminology, clinical presentation, and prevalence. Numerous terms are used, as if they indicated one and the same entity. We propose "pregnancy-related pelvic girdle pain (PPP)", and "pregnancy-related low back pain (PLBP)", present evidence that the two add up to "lumbopelvic pain", and show that they are distinct entities (although underlying mechanisms may be similar). Average pain intensity during pregnancy is 50 mm on a visual analogue scale; postpartum, pain is less. During pregnancy, serious pain occurs in about 25%, and severe disability in about 8% of patients. After pregnancy, problems are serious in about 7%. The mechanisms behind disabilities remain unclear, and constitute an important research priority. Changes in muscle activity, unusual perceptions of the leg when moving it, and altered motor coordination were observed but remain poorly understood. Published prevalence for PPP and/or PLBP varies widely. Quantitative analysis was used to explain the differences. Overall, about 45% of all pregnant women and 25% of all women postpartum suffer from PPP and/or PLBP. These values decrease by about 20% if one excludes mild complaints. Strenuous work, previous low back pain, and previous PPP and/or PLBP are risk factors, and the inclusion/exclusion of high-risk subgroups influences prevalence. Of all patients, about one-half have PPP, one-third PLBP, and one-sixth both conditions combined. Overall, the literature reveals that PPP deserves serious attention from the clinical and research communities, at all times and in all countries.

Pelvis-thorax coordination in the transverse plane during gait

The coordination between horizontal pelvic and thoracic rotations during treadmill walking was studied in ten subjects. Previous studies have considered the relative phase using the mean to characterize coordination mode, and the S.D. as an index of its stability. However, to use S.D. of relative phase as a measure for coordinative stability, the underlying oscillations have to be phase-locked at a certain value. Random fluctuations around this value can then be viewed as stochastic perturbations of a stable fixed point, resulting in a unimodal distribution of relative phase. Using methods of circular statistics this study shows that these conditions were not met in pelvis-thorax coordination. Spectral analyses revealed that, as walking velocity increased, a triphasic component emerged in the pelvic rotations, while the thoracic rotations remained harmonic across all walking velocities. These findings refute the use of standard relative phase measures to capture pelvis-thorax coordination. An alternative measure is introduced, namely the difference between the continuous Fourier phases of the component oscillations as determined for the main frequency of the thorax oscillation. With this measure, pelvis-thorax coordination was found to evolve from in-phase coordination towards antiphase coordination as walking velocity increased. This method may be used to assess reliably the properties of pelvis-thorax coordination in both healthy and pathological gait patterns in the future.

Measuring functional abilities of patients with knee problems: rationale and construction of the DynaPort knee test

We present the rationale and design of the DynaPort knee test. The test aims at measuring knee patients' functional abilities in an unobtrusive, user-friendly way. Test persons wear several belts around their trunk and legs. The belts contain accelerometers, the signals of which are stored in a recorder, embedded in one of the belts. The knee test consists of a set of 29 tasks related to activities of daily life ("test items"). Accelerometer signals are analyzed in terms of 30 "movement features" (accelerations, angles, durations, frequencies, and some dimensionless numbers). In data analysis, the beginning and end of each test item is marked by hand; otherwise, analysis is automatic. We compared 140 knee patients with 32 healthy controls and found 541 of the 29 x 30=870 test item x movement feature combinations differed significantly between the two groups. From these 541 combinations the DynaPort knee score is calculated by the weighted averages of movement features per item, then weighted averages of items per cluster (locomotion, rising and descending, transfers, lifting and moving objects), and finally the average of the clusters. In an initial study the test-retest reliability of the knee test proved high, and the test turned out to be sufficiently responsive (0.7 patients' standard deviations improvement after 24 months). However, it remains difficult to interpret the scores in more meaningful terms than merely "better" or "worse." Extensive reliability studies in the future will further assess the validity of the test and provide more insight into the meaning of the scores. The DynaPort knee test may thus become an important instrument for evaluating patients'functional abilities in knee-related clinical practice and research.

Within-subject variability in low back load in a repetitively performed, mildly constrained lifting task

STUDY DESIGN

A repeated-measures in vivo experiment.

OBJECTIVE

To describe within-subject variability of spinal compression in repetitive lifting.

SUMMARY OF BACKGROUND DATA

Epidemiology and failure mechanics suggest that peak loads may be more predictive of injury than average loads. Nevertheless, biomechanical studies usually focus on the latter.

METHODS

Ten healthy males performed 360 lifts in 1 hour of a 45-L crate, weighted with a stable 10-kg mass on 1 day and with an unstable mass (10 kg of water) on another day. The maximum compression force in each lift was estimated, using a simple inverse dynamics model and a single equivalent muscle model.

RESULTS

The individual distributions of maximum compression force were slightly skewed to the right (average skewness 0.67). Median and 95th percentile values were used to characterize the distribution. The median (50th percentile) compression ranged from 3375 to 6125 N, and from 3632 to 6298 N in the stable and unstable load conditions, respectively. The within-subjects peak (95th percentile) compression forces were from 405 to 1767 N and from 526 to 2216 N, respectively, higher than the median values. The peak values differed significantly between conditions, whereas the difference in medians did not reach significance. Only a limited trendwise (fatigue-related) variance could be demonstrated.

CONCLUSION

Peak spinal compression by far exceeds median compression in repetitive lifting and can be affected by task conditions independently from the median. Therefore, the variability of spinal loads needs to be taken into consideration when analyzing and redesigning tasks that can cause spinal injuries.

Low-dimensional control: tonus (1963)

In 1963, an article on "Tonus" (tone), written by Nikolai A. Bernstein and Yakov M. Kots, appeared in the second edition of the Bols'aja Medicinskaja Enciclopedija [Grand Medical Encyclopedia]. The paper is now published for the first time in the English language, with Mark L. Latash as translator. In accordance with then contemporary neurophysiology and neuropsychology, the paper presented "tone" as a graded phenomenon (as opposed to all-or-none), serving to prepare the segmental level for phasic contractions. Influenced by Granit and Matthews, the authors proposed that the suprasegmental level controls the threshold and the slope of the stretch reflex. In their introduction to the present edition, the editors understand this proposal in the context of low-dimensional control, that is, control in terms of one or a few variables (as opposed to central commands specifying all the details). Selected episodes from the history of low dimensional control and its logical counterpart, spinal intelligence, are used to illustrate how difficult these ideas were to accept. As so often in new scientific developments, confusion was the rule, and in this respect the paper on "Tonus" is no exception. In the epilogue, Kots gives his personal memories of the context in which the paper was written. At the time, he was working on "equitonometry" (equi-tono-metry), measuring tonic balance with gravity eliminated. Results of equitonometric research quite naturally led to the idea that suprasegmental centers control the threshold and the slope of the tonic stretch reflex. As Kots remembers, that was "no big deal".

Berstein's anti-reductionistic materialism: On the road towards a biology of activity (1965)

Bernstein's paper, "On the Road Towards a Biology of Activity," appeared the year before his death.2 With this paper, Bernstein closed several lines of argument that he had been developing from the onset of his career in the early 1920s. The paper converges on the notion of activity. In accordance with his own shifting focus heuristic (cf. Bongaardt, 1996), Bernstein challenged future researchers of movement to integrate models of the movement functions that constitute activity. He suggested that these functions are: the coordination of movement, the planning of movement, and the exploration of better, optimal ways to move. In the 1920s, Bernstein had collaborated with his friend and colleague L.S. Vygotsky at the Moscow Institute of Experimental Psychology. Vygotsky (cf. 1926/1994) was the first to place activity at the core of Soviet psychology. According to Vygotsky, reflexology and behaviorism, then dominant approaches in psychology, were fundamentally wrong; they focus on building-blocks of behavior without addressing phenomena that stand out as typically human, most importantly, consciousness. Rather than starting with building blocks, psychology should start with the daily activity of human beings in their environment and show how this activity relates to consciousness. Forty years later, in his 1965 paper, Bernstein stressed a point that mirrors Vygotsky's: Reflexes are not building blocks of movement. The general characteristics of any movement precede the specificity of such units, whether reflexes or synergies, and this primacy pertains to the actual organization of movements as well as to the study of movement. The development and relevance of the activity concept in Bernstein's work in the period from 1925 to 1965 deserves a study of its own; here, a brief historical sketch of Bernstein's activity concept is offered, along with a few theoretical considerations concerning activity's constituent functions.

Bernstein's failure to join the space race: his commentary on Tsiolkovskii's "Mechanics in Biology" (1964)

October 4, 1957, the Soviet Union launched Sputnik I into orbit from Tyuratam in Turkistan. An event "with the suddenness and surprise of a Pearl Harbor and of the impact of a Hiroshima atomic explosion" (Stoiko, 1970, p. ix). Nor would this be the only time America lost to the Russians in the space race. November 3 of the same year, Sputnik II carried the dog Laika, the first living being who traveled, and died, in space. In the USA, Senator Lyndon B. Johnson lamented: "Control of space means control of the world" (quoted from Heppenheimer, 1997, p. 126), and attempts were made to speed up Wernher von Braun's launching program (Piszkiewicz, 1995; cf. Von Braun, 1968). Alas, on December 6, when the American rocket began to lift, "it seemed as if the gates of hell had opened up. Brilliant stiletto flames shot out from the side of the rocket near the engine. The vehicle agonizingly hesitated for a moment, quivered again, and in front of our unbelieving, shocked eyes, began to topple" (Halberstam, quoted from Heppenheimer, p. 127). Thus, at the UN, "Soviet delegates asked their American counterparts if the United States might wish to receive foreign aid under Moscow's program of technical assistance to backwards nations" (from Heppenheimer, p. 128). Von Braun finally succeeded with the Explorer I on January 31, 1958, but for the Americans the agonizing wasn't over. On August 21, 1957, the Soviet Union launched an intercontinental ballistic missile (ICBM), this time carrying a dummy, but able to carry a nuclear bomb (Harford, 1997). So, the first ICBMs in the world were aimed at the USA. And then, on the morning of April 12, 1961, Yuri Gagarin shouted "Poyekhali" ("Let's go!") (quoted from Heppenheimer, p. 172), and was launched into space at 9:06 to fly "over America" 51 minutes later. Quite naturally, the Soviet authorities wanted to show that Russia had been ahead all the time, and historical heroes were in strong demand. The Russians didn't have to look far.

The efficacy of Le Bon Départ and Sensory Integration treatment for children with developmental coordination disorder: a randomized study with six single cases

OBJECTIVE

Evaluation of the efficacy of Le Bon Départ (LBD) treatment and Sensory Integration (SI) treatment on motor performance of children with developmental coordination disorder.

DESIGN

A single subject design with multiple baseline and alternating treatments. Order of treatment and length of phase were randomized. Measurements were blinded.

SETTING

Department of Occupational Therapy at the Academic Hospital Vrije Universiteit Amsterdam, The Netherlands.

SUBJECTS

Five boys and one girl with developmental coordination disorder (age: 6.0-8.1 years).

INTERVENTIONS

Baseline condition, Le Bon Départ treatment and Sensory Integration treatment.

MAIN OUTCOME MEASURES

The Movement ABC, Praxis Tests, a rhythm test and visual analogue scales. With the exception of the Praxis Tests, lower scores indicate better performance.

RESULTS

During both treatments, the performance on the Movement ABC (x = 7.21) and the scores on the visual analogue scales (x = 46.64) were significantly better than in the baseline (Movement ABC(baseline): x = 17.38; visual analogue scales(baseline): x = 68.18). After treatment 2, performance on the Praxis Tests and scores on the visual analogue scales were significantly better than after treatment 1 (Praxis Tests: 113.54 versus 104.68; visual analogue scales: 34.74 versus 58.54). All six children performed better on the Movement ABC during treatment as compared to the baseline. Le Bon Départ led to significant improvement on all dependent variables, Sensory Integration on the visual analogue scales only. The improvements after Le Bon Départ were larger than the improvements after Sensory Integration treatment. On the rhythm test this difference was significant: LBD led to an improvement of 43.01 points, while the improvement after SI was 17.59 points (p < 0.05).

CONCLUSION

Motor performance of children with developmental coordination disorder improved significantly on all dependent variables after the combination of treatments. Le Bon Départ led to more improvement than Sensory Integration. LBD appears to be a valuable treatment method for children with developmental coordination disorder.

30 years later: On the problem of the relation between structure and function in the brain from a contemporary viewpoint (1996), part II

In the first half of the present paper, which appeared in vol. 3, issue 4 issue of Motor Control, the authors elaborated on Bernstein's (1935/1967) idea of the ambiguity of the relationship between the central command and the peripheral effect. The authors presented maybe the strongest statement so far: It is because the means are variant that the results can be invariant. As in Bernstein's 1935 paper, this was taken as evidence that there is no one-to-one relationship between structure and function in the brain. The authors discussed the history of localization theories, pointing out that neither strong localizationism nor strong anti-localizationism (as in Lashley's equipotentiality) would help understand the relation between brain structure and function. In order to understand the nature of a "brain center" for a function, the authors argued, one has to understand the concept of "function" itself. The development of "function" does not imply that the organism learns to (re)act in a stereotyped fashion, but that a control matrix is established, with non-single-valued relationships, allowing the organism to (re)act differently every time, in accordance with the need and actual situation. At the end of the first part of the paper, the authors emphasized the importance of a new basic logic of neurophysiology. In that sense, there are certain parallels between neurophysiology in the 60s (and also today, the present editors would add) and physics around the turn of the century, when Maxwell, Boltzmann, Planck, and others, created a completely new framework for theoretical physics.

Bernstein's theory of movement behavior: historical development and contemporary relevance

In present-day movement science, N. A. Bernstein's formulation of the problems of motor control is often taken as the starting point. The reliance on Bernstein has not brought agreement among his followers, however. In this article, the authors pose the following question: Does the disagreement arise from the structure of his work itself or from incomplete exploitation of his thinking? By using, inter alia, Bernstein's 24 English and German articles, the authors present an analysis of the development of Bernstein's theory of movement behavior, against the backdrop of the scientific progress in the Soviet Union in Bernstein's time and the clashes between Soviet politics and science. Bernstein addressed in his early articles the measurement and biomechanical analysis of movements. His experimental data soon indicated the need for a new understanding of the organization of movements, which he formulated in terms of coordination. Because of political problems, his work was interrupted; but after being "rehabilitated" and again allowed to work, Bernstein aimed to explain how animals find and optimize the solutions to motor problems. The structure of the theory that ensued was comprehensive exactly by virtue of his repeatedly shifting focus between the different aspects of the organization of movement: More important than the answers he gave were the questions he asked. Moreover, the way he approached those questions may help scientists solve pressing problems in present-day movement science.

Deficits in the coordination of agonist and antagonist muscles in stroke patients: implications for normal motor control

Movement impairments about a single joint in stroke patients may be related to deficits in the central regulation of stretch reflex (SR) thresholds of agonist and antagonist muscles. One boundary of the SR threshold range for elbow flexor and extensor muscles was measured in hemiparetic subjects by analysing electromyographic activity during stretching of relaxed muscles at seven different velocities. For each velocity, dynamic SR thresholds were measured as angles at which electromyographic activity appeared. These data were used to determine the sensitivity of the threshold to velocity and the static SR thresholds for flexors and extensors. In contrast to relaxed muscles in healthy subjects, static flexor and extensor thresholds lay within the physiological range in 11/12 and 4/12 subjects, respectively. This implies that, in the range between the static SR threshold and one of the physiological joint limits, relaxation of the muscle was impossible. Subjects then made slow movements against different loads to determine their ranges of active movement. Maximal flexor and extensor torques were lower in hemiparetic subjects throughout the angular range. In some subjects, ranges were found in which no active torque could be produced in either extensor or both muscle groups. These ranges were related to the boundary values of SR thresholds found during passive muscle stretch. The range in which reciprocally organized agonist and antagonist muscle activity could be generated was limited in all but one subject. When attempting to produce torque from positions outside their measured range of movement, excessive muscle coactivation occurred, typically producing no or paradoxical motion in the opposite direction. Results suggest a relationship between spasticity measured at rest and the movement deficit in stroke by demonstrating a link between motor deficits and control deficits in the central regulation of individual SR thresholds.

Detecting individual change in children with mild to moderate motor impairment: the standard error of measurement of the Movement ABC

OBJECTIVE

To assess whether the Movement ABC can be used to monitor individual change in motor performance.

DESIGN

Motor-impaired children were tested three times in succession with the Movement ABC without any intervention.

SETTING

Two schools for special education and one school for children who are chronically ill.

SUBJECTS

Three girls and 20 boys aged 6-8 years.

MAIN OUTCOME MEASURES

Scores were measured per item (0 --> 5), added to cluster scores (0 --> 10 or 15), added to form the total scores (0 --> 40). Mean scores, standard errors of measurement (SEMs) and least detectable differences (LDDs) were calculated per item, per cluster and for the total scores. A repeated measures analysis of variance was performed to test for the effects of time.

RESULTS

The total scores improved significantly from the first session (mean: 15.4 points) to the second (mean: 13.3), but not from the second to the third (mean: 13.2). Average item scores ranged from 0.6 to 2.7 points with SEMs of 0.79 --> 1.54 and LDDs of 2.20 --> 4.27. Average cluster scores ranged from 3.4 to 5.3 with SEMs of 1.51 --> 1.84 and LDDs of 4.18 --> 5.11. The SEM of the total scores equalled 3.13 with an LDD of 8.68.

CONCLUSIONS

The total score of the Movement ABC is sufficiently sensitive to monitor individual change; the cluster scores have moderate sensitivity and individual items are inappropriate to monitor individual change. The significant effect of time is interpreted as an effect of learning.

Cardiopulmonary and CD4 cell changes in response to exercise training in early symptomatic HIV infection

PURPOSE

The purposes of the present study were to assess the effects of a 12-wk laboratory based aerobic exercise program on cardiopulmonary function, CD4 cell count, and physician-assessed health status among symptomatic pre-AIDS HIV-infected individuals (N = 28) and to assess the degree to which ill health was associated with exercise relapse.

METHODS

Responses to graded exercise test, physician-assessed health status, and CD4 cell counts were determined at baseline and 12-wk follow-up for participants randomly assigned to exercise or control conditions, and reasons for exercise noncompliance were recorded.

RESULTS

Approximately 61% of exercise-assigned participants complied (> 50% attendance) with the exercise program, and analyses of exercise relapse data indicated that obesity and smoking status, but not exercise-associated illness, differentiated compliant from noncompliant exercisers. Compliant exercisers significantly improved peak oxygen consumption (VO2peak; 12%), oxygen pulse (O2pulse; 13%), tidal volume (TV; 8%), ventilation (VE; 17%), and leg power (25%) to a greater degree than control participants and noncompliant exercisers (all P < 0.05). Although no group differences in health status were found, a significant interaction effect indicated that noncompliant exercisers' CD4 cells declined (18%) significantly, whereas compliant exercisers' cell counts significantly increased (13%; P < 0.05).

CONCLUSION

We conclude that although aerobic exercise can improve cardiopulmonary functioning in symptomatic HIV-infected individuals with minimal health risks, attention to factors associated with exercise adherence is warranted.

Hugo de Vries on heredity, 1889-1903. Statistics, Mendelian laws, pangenes, mutations

The essay describes the development of Hugo de Vries's thinking on heredity from the publication of his Intracellulare Pangenesis in 1889 to the publication of Die Mutations-theorie, Volume 2, in 1903. De Vries's work in the 1890s can be characterized as an attempt to defend his theory of pangenes, especially the fundamental and controversial idea that different characters have different material hereditary carriers. Hybridization experiments served his goal. Recently discovered research notes on hybridization from 1896 suggest that, though he was unaware of Mendel's work, De Vries used the laws of dominance and recessiveness, segregation, and independent assortment to explain the 75:25 ratio in the second generation. He had discovered these laws by applying insights from probability theory to his research. In Die Mutationstheorie De Vries combined central concepts of intracellular pangenesis and his mutation theory by modifying the meanings of important terms and introducing new states of pangenes. In his attempts to describe Mendelian crossings in terms of pangenes and mutations, he became entangled in a number of contradictions. Some of his remarks suggest that he was aware that the Mendelian laws and his own theories of pangenes and mutations could not be made consistent.

The transfer of scanning training effects in visual inattention after stroke: five single-case studies

The transfer effects of scanning training of right-brain-damaged stroke patients with visual inattention have been studied. Five patients were treated according to a B-C-B-D design. The first B phase lasted for 2, 4, or 6 weeks, whereas the other intervention phases had a fixed duration of 2 weeks. During all phases physical therapy was given. Additionally, occupational therapy was applied during the B phases, training on a scanning apparatus during the C phase, and training on reading tasks during the D phase. At least three times in each intervention phase, performance was measured using a computerized visual scanning test, a line bisection test, and a letter cancellation test. In addition, wheelchair navigation was assessed. For four out of the five patients a significant positive effect of visual scanning training on visual scanning behaviour was found. This effect, however, appeared to be restricted to the task which was specifically trained. Over the group as a whole no evidence could be found for any transfer of visual scanning training effects to the domain of gross motor skills.