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PubMed1996

The endarterectomy-produced common carotid artery step: a harbinger of early emboli and late restenosis.

PURPOSE: This study analyzes the role of the carotid endarterectomy (CEA)-produced common carotid step or shelf on early and late CEA outcome. A simple method of reconstruction that covers the exposed media and intima of the step and smooths out the flow channel is presented. METHODS: Since 1984 intraoperative geometric measurements were made on 1019 CEA. Duplex scans were made after surgery on 968 (95%); 234 (23%) were monitored for at least 5 years. Of the first 920 CEA, 261 (28%) had a step > or = 2 mm. Of the last 99 operations, the 27 (27%) with a step > or = 2 mm had inversion plication reconstruction of the step. RESULTS: Three patients had early post-CEA neurologic events attributed to a common carotid step, two had transient ischemic attacks, and one had a mild stroke. All three had a CEA-produced step > or = 2 mm (1.2% of 261, p = 0.03 when compared with no unexplained early neurologic events in the 659 without a step). Duplex scans were normal in all three except for a common carotid artery step. Six patients had seven reoperations for common carotid artery restenosis at 23 to 104 months (mean 73 months) after CEA. All had a step > or = 2 mm at the origin CEA (2.7% of 261, p = 0.001 when compared with no reoperations for common carotid artery stenosis in the 659 CEA without a step). All but one patient had > 75% restenosis. Four were asymptomatic. All seven with restenosis were within 2 mm of the original common carotid end point step. Four restenoses were concentric, two eccentric, and one ulcerated. Six of the seven arteries were originally reconstructed with a greater saphenous vein patch that extended 3 to 6 mm proximal to the step. Early postoperative duplex scans of the 27 recent CEA with step reconstruction demonstrated a tortuous but smooth common carotid artery flow channel. CONCLUSION: The CEA-produced common carotid artery step is a potential source of both early postoperative emboli and late restenosis. The incidence is approximately 4% in patients with a CEA step > or = 2 mm. CEA vein patch reconstruction does not prevent restenosis in the region of the step but may lower the incidence. Although not proven, reconstruction techniques directed at covering the exposed step media and intima and smoothing the flow channel may favorably alter outcome.

Carotid Artery, Common↗
PubMed1987

Tactile discrimination of shape: responses of rapidly adapting mechanoreceptive afferents to a step stroked across the monkey fingerpad.

Responses of rapidly adapting Meissner corpuscle mechanoreceptive afferent fibers (RAs) to steps of varying shape stroked across the distal fingerpad were recorded from anesthetized monkeys. A series of flat plates were used, each having an increase in thickness (a step) in the middle so that one-half of the plate was thicker than the other. The cross-sectional shape of the step approximated that of a half-cycle sinusoid, 0.5 mm high. The width (half-cycle wavelength) of the sinusoidal step was varied from 0 to 3.13 mm, producing a series of step shapes that differed in steepness and curvature. These steps could be broadly categorized into 2 groups, "steep" and "gradual." Each step was stroked back and forth under constant compressional force, using a servocontrolled mechanical stimulator. The RA's response to a step provided a spatial pattern of action potentials in which the occurrence of each impulse corresponded to a position of the step on the skin. This response consisted of a single "burst" of impulses to the sinusoidal portion of the step. Changes in stroke direction, step shape, or velocity of stroking primarily affected the RA discharge rate during the burst, and, less consistently, the spatial width of the burst. For a given step shape and stroke velocity, the discharge rate was greater for strokes from the low to the high side of the step than for strokes from the high to the low side. Discharge rate was greater for steep than for gradual steps and, for a given step, it increased with stroke velocity. All the major features of the responses were interpreted as being due predominantly to the sensitivity of the RA to vertical velocity at the most sensitive spot on its receptive field, together with a sensitivity to the rate of change in skin curvature at that spot. RA discharge rate distinguished not only the gross differences between steep and gradual steps, but also some of the finer differences in sharpness among steep steps. From a comparison with the human capacity for tactile discrimination of the steps, it was concluded that RAs, through their discharge rates, provide primarily "intensive" information about the sharpness of shapes.

Adaptation, Physiological↗
PubMed2004

Changes in pathology instruction and student performance on the United States Medical Licensing Examination Step 1, 1995-2000: a nationwide 6-year longitudinal study.

A multi-institutional collaborative longitudinal study was conducted to assess the effect of curricular change (overall curricular format and detailed pathology instruction) and school policy regarding the Step 1 requirement on the United States Medical Licensure Examination (USMLE) Step 1 Total and Step 1 Pathology scores over a period of 6 years (1995 to 2000). Detailed descriptions of instruction (overall curricular format and the format and number of hours of instruction in various areas of pathology) and the school policy of USMLE Step 1 requirement for classes entering in 1993 to 1998 were matched with Step 1 Total scores and Step 1 Pathology scores for 48,166 students from 73 schools who took the corresponding June 1995 to 2000 examination. Hierarchical linear modeling was used for analysis while controlling for students' MCAT-bpv (Medical College Admission Test, composite of biological and physical sciences and verbal reasoning) scores and undergraduate grade point average (GPA). Results indicated that the GPA and MCAT-bpv scores of students significantly impacted their Step 1 Total score and Step 1 Pathology score for each year. The mean MCAT-bpv scores of entering classes showed a steady increase from 1993 to 1998. About this same time, Step 1 Total scores were on the rise (especially for classes entering 1994 to 1996, after which they stabilized). During the same time period (1993 to 1998), there was a substantial move toward integrated instruction of Pathology. The mean Step 1 scores of schools with integrated instruction were slightly higher than those of schools with nonintegrated instruction, but the difference was not significant. Analyses of variance were run to examine the effect of change of curriculum on Step 1 Total and Step 1 Pathology scores. Our analysis does not appear to show a cause-and-effect relationship between changes in curriculum toward integrated instruction and the increasing Step 1 scores during the study period. The MCAT-bpv scores showed an upward pattern during the study period, and given that they are strong predictors of Step 1 scores, it appears that the rising MCAT-bpv scores are a major contributory factor to the upward trend in the scores during the study period. The only clearly significant result at the school level is that a requirement to pass Step 1 before moving on to clinical rotations is positively related to Step 1 Total and Step 1 Pathology scores.

Analysis of Variance↗
PubMed1997

Kinetics of calcium steps underlying calcium oscillations in melanotrope cells of Xenopus laevis.

Melanotrope cells of Xenopus laevis display intracellular calcium oscillations which are generated at the plasma membrane and travel as a wave through the cytoplasm into the nucleus. An oscillation involves discrete increases in intracellular Ca2+ ('steps'), followed by a relatively smooth return to the basal Ca2+ level. The aim of our investigation was to determine what role these steps play in shaping the Ca2+ signal in melanotrope cells, by conducting a high resolution spatio-temporal analysis of the kinetics of the Ca2+ steps. To this end Fura-red loaded cells were analysed by confocal laser scanning microscopy using the line scanning method to achieve 6 ms time resolution. Furthermore, the kinetics of the steps were analysed in 3 different intracellular areas, to see if there are spatial differences in Ca2+ signalling kinetics. The results showed that each calcium oscillation is built up by 3-4 steps that were generated very quickly and had approximately the same size. Following each Ca2+ step, there was a slow removal of calcium before the next step boosted the overall level of Ca2+. Since the Ca2+ steps were most pronounced directly beneath the plasma membrane, they appear to be generated in this region. The speed of the Ca2+ wave near the membrane exceeded 40 microns/s, indicating an active mechanism for wave propagation. In deeper regions of the cell, the wave speed was much slower (about 8 microns/s) and the size of each step was smaller, indicating that regulation occurs within a narrower range of [Ca2+]i. Inside the nucleus, however, the calcium wave accelerated again (23 microns/s). Treatment with TRH evoked a high amplitude Ca2+ transient and increased the number of Ca2+ steps to 5 or 6. Each step had approximately the same size as the steps of the pretreatment Ca2+ oscillations. Caffeine treatment, which increased the frequency of the oscillations, had no effect on the number or the size of the Ca2+ steps, but it reduced the time needed for each step to reach its maximum height. We suggest a possible 'building block' function for the Ca2+ steps, whereby a cell generates more steps to achieve a high oscillation amplitude or accelerates the speed of the steps to increase the frequency of oscillations. Both phenomena may play a crucial role in the encoding of information transduced from an extracellular input to the intracellular target.

Animals↗
PubMed1983

Rate-limiting step: a quantitative definition. Application to steady-state enzymic reactions.

The generality of the concept of a rate-limiting step in enzymic reactions recently has been questioned [Northrop, D. B. (1981) Biochemistry 20, 4056-4061] because, in simulated isotopic experiments, alterations of the step identified as rate limiting by current definitions do not consistently affect Vmax in the expected manner. In this paper a definition for a rate-limiting step is posed that eliminates such inconsistencies while the thrust of the original concept is retained. Thus, for any steady-state process involving a linear reaction sequence the rate-limiting step is taken as the "most sensitive" step, or the step which, if perturbed, causes the largest change in overall velocity, v. In both V and V/K enzymic systems the most sensitive step is identified by the relative magnitude of the sensitivity function, SFj, for the various forward steps. If forward steps are identified by kj, SFj is equal to delta(1/v)/[delta(1/kj)/(1/kj)], when the equilibrium constant for the step involving kj is maintained constant. The corresponding sensitivity index, SIj, is a normalized function of SFj (the normalizing factor is v) such that the sum of the values for SIj is equal to 1. In addition, there is an exact relationship between the sensitivity index for the isotopic step and the fraction of the intrinsic isotopic effect that is expressed in the overall rate of the reaction (when the intrinsic effect is taken as the fractional difference in reciprocal rate constant produced by the isotope). A procedure is described for approximating the sensitivity function for the various steps in a reaction sequence on the basis of the Gibbs energy profile for that reaction and thus identifying the most sensitive step. This approach also is used to consider the general question of whether a rate-limiting step should be specified for a multistep enzymic reaction. Identifying the rate-limiting step as the most sensitive step in a reaction sequence means that no aspect of the concept of minimal rate should be automatically considered as a property of a rate-limiting step.

Enzymes↗
PubMed2006

Physiological changes in gentian axillary buds during two-step preculturing with sucrose that conferred high levels of tolerance to desiccation and cryopreservation.

BACKGROUND AND AIMS: Induction of dehydration tolerance is a key to achieving high survival rates in cryopreservation of plant specimens. It has been reported previously that two-step preculturing with sucrose effectively increased desiccation tolerance in axillary buds of gentian (Gentiana scabra), which allow the buds to survive cryopreservation. This study is aimed at characterizing each step of this preculturing and to elucidate physiological changes induced during this preculturing. METHODS: In standard two-step preculture, excised gentian axillary buds were incubated for 11 d on MS medium with 0.1 m sucrose at 25 degrees C (first step: mild osmotic stress was given) and the subsequent incubation on MS medium with 0.4 m and 0.7 m sucrose for 1 d each (second step). The levels of abscisic acid (ABA), proline and soluble sugars in gentian buds during the preculture were determined. Effects of various combinations of two-step preculturing and of exogenous ABA and proline were studied. KEY RESULTS: During the first preculture step, there was a transient increase in ABA content peaking on day 4, which declined to a background level at the end of the first and second step preculturing. Proline level increased steadily during the first preculture step and increased further in the second preculture step. Incubating buds with medium containing proline, instead of the two-step preculturing, did not allow them to survive desiccation. Incubating buds with ABA instead of 0.1 m sucrose-preculturing effectively increased desiccation tolerance only when it was followed by the second preculture step. Fluridone, an ABA synthesis inhibitor included in the two-step preculture medium, reduced desiccation tolerance of the buds. The normal first-step preculture increased the levels of soluble sugars 2.4-fold, especially sucrose and raffinose. Buds treated with the second preculture step had greatly increased sucrose levels. CONCLUSIONS: These observations lead to the hypothesis that the first preculture step involves ABA-mediated cellular changes and the second step induces loading of sucrose in the gentian buds.

Abscisic Acid↗
PubMed2005

Attention demanding tasks during treadmill walking reduce step width variability in young adults.

BACKGROUND: The variability of step time and step width is associated with falls by older adults. Further, step time is significantly influenced when performing attention demanding tasks while walking. Without exception, step time variability has been reported to increase in normal and pathologically aging older adults. Because of the role of step width in managing frontal plane dynamic stability, documenting the influence of attention-demanding tasks on step width variability may provide insight to events that can disturb dynamic stability during locomotion and increase fall risk. Preliminary evidence suggests performance of an attention demanding task significantly decreases step width variability of young adults walking on a treadmill. The purpose of the present study was to confirm or refute this finding by characterizing the extent and direction of the effects of a widely used attention demanding task (Stroop test) on the step width variability of young adults walking on a motorized treadmill. METHODS: Fifteen healthy young adults walked on a motorized treadmill at a self-selected velocity for 10 minutes under two conditions; without performing an attention demanding task and while performing the Stroop test. Step width of continuous and consecutive steps during the collection was derived from the data recorded using a motion capture system. Step width variability was computed as the standard deviation of all recorded steps. RESULTS: Step width decreased four percent during performance of the Stroop test but the effect was not significant (p = 0.10). In contrast, the 16 percent decrease in step width variability during the Stroop test condition was significant (p = 0.029). CONCLUSION: The results support those of our previous work in which a different attention demanding task also decreased step width variability of young subjects while walking on a treadmill. The decreased step width variability observed while performing an attention demanding task during treadmill walking may reflect a voluntary gait adaptation toward a more conservative gait pattern emphasizing frontal plane control of the trunk. Extension of the experimental paradigm to older adults and mechanistic approaches to link step width variability to dynamic stability, and falls, in a cause-effect manner are necessary.

Journal Article↗
PubMed1987

Tactile discrimination of shape: responses of slowly adapting mechanoreceptor afferents to a step stroked across the monkey fingerpad.

The representation of shape in the responses of slowly adapting mechanoreceptive afferent fibers (SAs) in monkeys was investigated. A series of flat plates was used, each having an increase in thickness (a step) in the middle so that one-half of the plate was thicker than the other. The cross-sectional shape of the step approximated that of a half-cycle of a sinusoid. The height of the step was fixed at 0.5 mm, while its width (half-cycle wavelength) was varied from 0 to 3.13 mm, resulting in step shapes that varied in steepness and curvature. The steps fell into 2 categories, characterized as "steep" and "gradual." A servocontrolled mechanical stimulator stroked each step across the distal fingerpad from the high to the low side of the step and back, while maintaining the contact force at 20 gm wt. Evoked action potentials in single SAs innervating the fingerpads of anesthetized monkeys were recorded. Each SA's response to a step provided a spatial response profile (discharge rate as a function of step position) that reflected the distribution of curvature across the step shape. All the major features of the SA response could be consistently explained as being due to the sensitivity of the SA to the amount and rate of change in skin curvature. The response profile was altered by changes in stroke direction, step shape, and stroke velocity. Differences in stroke direction (back and forth) were indicated by differences in pattern of response: a "burst-pause-burst" for strokes from high to low, and a "pause-burst-pause" for strokes from low to high; a greater discharge rate in response to the step for low to high strokes, and for some SAs, the reduction or absence of basal discharge in one of the directions. The discharge rate during the burst for either direction of stroking was greater for steep than for gradual steps, and increased, for a given step shape, with increases in stroke velocity. Regardless of differences in stroke velocity, steep steps were distinguished from gradual steps by having narrower burst widths for low-to-high strokes and narrower pause widths for high-to-low strokes. The same stimuli were delivered to the human fingerpad, and the capacities of humans to discriminate between the steps were measured. It was concluded that the spatial features of SA responses, representing the widths of regions of active and inactive SA populations, as well as the intensive feature of discharge rate, accounted for the gross sensory discriminations of shape.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed2004

Variability of step kinematics in young and older adults.

Fall-related injuries are the most common and serious medical problems facing older adults. Recent studies of older adults have focused on the variability of step kinematics and the relationship to falling. The accuracy of step variability estimates is proportional to the number of steps that are collected. The use of an instrumented treadmill allows simultaneous collection of spatial and temporal step kinematics for a large number of continuous steps. The current study was conducted to determine the influence of age, walking velocity and handrail use on the variability of step kinematics using a treadmill protocol. Eighteen young adults (average age: 27.7 +/- 3.3 years) and 12 healthy older adults (average age: 73.4 +/- 2.3 years) were recruited from the community. Temporal and spatial gait parameters were quantified using custom designed software from measurements collected during treadmill walking. The primary independent variables were the variability of step length, step width, and step time. Step width variability of older adults was significantly larger than that of young adults. Walking velocity did not influence step kinematic variability. Handrail usage influenced the variability of step length and step width, but not of step time. The present results, and those of previous studies, point to a consistent relationship between age and step width variability. Since step width variability has been implicated in falls, further research is warranted.

Adult↗
PubMed1999

Effects of age on balance assessment using voluntary and involuntary step tasks.

BACKGROUND: Taking a step while standing and modifying a step while walking are two strategies often used to maintain balance when balance disturbances are encountered during activities of daily living. This study investigated whether performance on an involuntary step task, which is assumed to be a surrogate for fall recovery abilities, was comparable to performance on a voluntary step task. METHODS: The performance of a voluntary and an involuntary step task was measured in healthy young adult (mean age 21 years) and healthy elderly adult (mean age 68 years) female subjects. Subjects stepped as fast as possible in the direction of a minimally destabilizing lateral waist pull (voluntary step task), or they responded naturally to a large destabilizing lateral waist pull (involuntary step task). The effects of age, task, and their interaction on the primary outcome variables of step foot liftoff time, landing time, step length, and step height were examined. RESULTS: In the voluntary step task, the older adults, compared to the young, required significantly more time to lift their foot (Young: 307 msec; Elderly: 424 msec). In the involuntary step task, the elderly were as quick as the young in lifting their foot (Young: 322 msec; Elderly: 335 msec). The young lifted their foot at about the same time for the two tasks. The elders, on the other hand, lifted their foot significantly earlier in the involuntary step task, compared to the voluntary step task (Vol: 424 msec; Invol: 335 msec). CONCLUSIONS: A voluntary step task underestimates the ability of healthy elderly adults to respond quickly when large destabilizing balance disturbances are encountered.

Accidental Falls↗
PubMed1991

The two power limits conditioning step frequency in human running.

1. At high running speeds, the step frequency becomes lower than the apparent natural frequency of the body's bouncing system. This is due to a relative increase of the vertical component of the muscular push and requires a greater power to maintain the motion of the centre of gravity, Wext. However, the reduction of the step frequency leads to a decrease of the power to accelerate the limbs relatively to the centre of gravity, Wint, and, possibly, of the total power Wtot = Wext + Wint. 2. In this study we measured Wext using a force platform, Wint by motion picture analysis, and calculated Wtot during human running at six given speeds (from 5 to 21 km h-1) maintained with different step frequencies dictated by a metronome. The power was calculated by dividing the positive work done at each step by the duration of the step (step-average power) and by the duration of the positive work phase (push-average power). 3. Also in running, as in walking, a change of the step frequency at a given speed has opposite effects on Wext, which decreases with increasing step frequency, and Wint, which increases with frequency; in addition, a step frequency exists at which Wtot reaches a minimum. However, the frequency for a minimum of Wtot decreases with speed in running, whereas it increases with speed in walking. This is true for both the step-average and the push-average powers. 4. The frequency minimizing the step-average power equals the freely chosen step frequency at about 13 km h-1: it is higher at lower speeds and lower at higher speeds. The frequency minimizing the push-average power approaches the freely chosen step frequency at high speeds (around 22 km h-1 for our subjects). 5. It is concluded that the increase of the vertical push does reduce the step-average power, but that a limit is set by the increase of the push-average power. Between 13 and 22 km h-1 the freely chosen step frequency is intermediate between a frequency minimizing the step-average power, eventually limited by the maximum oxygen intake (aerobic power), and a frequency minimizing the push-average power, set free by the muscle immediately during contraction (anaerobic power). The first need prevails at the lower speed, the second at the higher speed.

Biomechanical Phenomena↗
PubMed2000

Could different directions of infant stepping be controlled by the same locomotor central pattern generator?

This study examined the idea of whether the same central pattern generator (CPG) for locomotion can control different directions of walking in humans. Fifty-two infants, aged 2-11 mo, were tested. Infants were supported to walk on a treadmill at a variety of speeds. If forward stepping was elicited, stepping in the other directions (primarily sideways and backward) was attempted. The orientation of the infant on the treadmill belt determined the direction of stepping. In some infants, we also attempted to obtain a smooth transition from one direction to another by gradually changing the orientation of the infant during a stepping sequence. Limb segment motion and surface electromyography from the muscles of the lower limb were recorded. Most infants who showed sustained forward walking also could walk in all other directions. Thirty-three of 34 infants tested could step sideways. The success of eliciting backward stepping was 69%. Most of the infants who did not meet our backward stepping criteria did, however, make stepping movements. The different directions of stepping had similar responses to changes in treadmill speed. The relationship between stance and swing phase durations and cycle duration were the same regardless of the direction of stepping across a range of speeds. Some differences were noted in the muscle activation patterns during different directions of walking. For example, the hamstrings were much more active during the swing phase of backward walking compared with forward walking. The quadriceps was more active in the trailing leg during sideways walking. In some infants, we were able to elicit stepping along a continuum of directions. We found no discrete differences in either the electromyographic patterns or the temporal parameters of stepping as the direction of stepping was gradually changed. The results support the idea that the same locomotor CPG controls different directions of stepping in human infants. The fact that most infants were able to step in all directions, the similarity in the response to speed changes, and the absence of any discrete changes as the direction of stepping was changed gradually are all consistent with this hypothesis.

Brain Stem↗
PubMed1997

Optimal walking in terms of variability in step length.

The optimal condition in speed, step rate, and step length of human walking has been reported in terms of temporal consistency, energy cost, and attentional demand. No study, however, has been conducted on the optimal condition in terms of spatial variability of walking. This study examined whether there is an optimal walking speed with minimum intrasubject variability in step length and step width during free walk (experiment 1) and whether there is an optimal step rate with minimum step length variability during walking with imposed step rates (experiment 2). Wearing shoes with ink-applied felt squares attached to the heels, healthy students walked on a flat walkway (0.6 x 16 m) at five different speeds with a freely chosen step rate in experiment 1 and walked at three different speeds with five different step rates in experiment 2. Free walk was found to have the fewest variable errors (VEs) in step length approximately at preferred walking speed. Variable error in step width increased linearly with an increase in walking speed. Under imposed step rates, VEs in step length were the fewest when walking with step rates close to those in free walk. Our everyday walking is performed most frequently at preferred speed and/or with freely chosen step rate, thereby optimizing the consistency of gait performance. Intrasubject variability in step length may be a useful measure for evaluation of walking.

Adult↗
PubMed2000

The Dopamine/D1 receptor mediates the phosphorylation and inactivation of the protein tyrosine phosphatase STEP via a PKA-dependent pathway.

The striatal-enriched protein tyrosine phosphatase (STEP) family is expressed within dopaminoceptive neurons of the CNS and is particularly enriched within the basal ganglia and related structures. Alternative splicing produces several isoforms that are found in a number of subcellular compartments, including postsynaptic densities of medium spiny neurons. The variants include STEP(61), a membrane-associated protein, and STEP(46), a cytosolic protein. The C terminals of these two isoforms are identical, whereas the N-terminal domain of STEP(61) contains a novel 172 amino acid sequence that includes several structural motifs not present in STEP(46). Amino acid sequencing revealed a number of potential phosphorylation sites in both STEP isoforms. Therefore, we investigated the role of phosphorylation in regulating STEP activity. Both STEP(61) and STEP(46) are phosphorylated on seryl residues by a cAMP-dependent protein kinase (PKA)-mediated pathway in striatal homogenates. The specific residues phosphorylated in STEP(61) were identified by site-directed mutagenesis and tryptic phosphopeptide mapping as Ser160 and Ser221, whereas the major site of phosphorylation in STEP(46) was shown to be Ser49. Ser160 is located within the unique N terminal of STEP(61). Ser221 and Ser49 are equivalent residues present in STEP(61) and STEP(46), respectively, and are located at the center of the kinase-interacting motif that has been implicated in protein-protein interactions. Phosphorylation at this site decreases the activity of STEP in vitro by reducing its affinity for its substrate. In vivo studies using striatal slices demonstrated that the neurotransmitter dopamine leads to the phosphorylation of STEP via activation of D1 receptors and PKA.

Adenosine Triphosphate↗
PubMed2001

Perturbed step initiation in cerebellar subjects: 2. Modification of anticipatory postural adjustments.

Although ataxias of stance and gait are frequent manifestations of cerebellar disease, the number of human studies examining stance or gait in cerebellar subjects is limited. In the present study, we examined whether anticipatory postural adjustments were impaired in cerebellar subjects during perturbed and unperturbed step initiation. The first aim was to show possible abnormalities in timing, force and kinematic parameters of anticipatory postural adjustments in unperturbed stepping in cerebellar subjects. Second, we examined the ability of cerebellar subjects to modify anticipatory postural adjustments associated with step initiation in response to a backward translation. Finally, we asked whether cerebellar subjects (and controls) make use of predictive knowledge of perturbation amplitude in perturbed stepping. Only few abnormalities of anticipatory postural adjustments were found in cerebellar subjects compared to controls. Both in the unperturbed and perturbed step conditions, force production as well as step length and step velocity were reduced in cerebellar subjects compared to controls, suggesting compensatory slowing. Cerebellar subjects also appeared to be less able to use predictive information of perturbation amplitude to scale anticipatory postural adjustments than control subjects. Nevertheless, in unperturbed steps, temporal parameters of anticipatory postural adjustments were preserved in cerebellar subjects. When subjects voluntarily initiated a step in response to the surface translation, both control and cerebellar subjects adapted by executing the anticipatory postural adjustments for step more rapidly. Furthermore, both control and cerebellar subjects were able to use online information regarding perturbation amplitude to scale parameters of step initiation in perturbed stepping. Overall, our findings suggest that the cerebellum is neither critical for the basic motor program underlying unperturbed step initiation nor for many adaptive changes occurring during perturbed step initiation. Like its role in predictive scaling of automatic postural responses to external perturbations, the cerebellum appears to be important for predictive adaptation of anticipatory postural adjustments during step initiation.

Adolescent↗
PubMed1999

Biomechanical influences on balance recovery by stepping.

Stepping represents a common means for balance recovery after a perturbation to upright posture. Yet little is known regarding the biomechanical factors which determine whether a step succeeds in preventing a fall. In the present study, we developed a simple pendulum-spring model of balance recovery by stepping, and used this to assess how step length and step contact time influence the effort (leg contact force) and feasibility of balance recovery by stepping. We then compared model predictions of step characteristics which minimize leg contact force to experimentally observed values over a range of perturbation strengths. At all perturbation levels, experimentally observed step execution times were higher than optimal, and step lengths were smaller than optimal. However, the predicted increase in leg contact force associated with these deviations was substantial only for large perturbations. Furthermore, increases in the strength of the perturbation caused subjects to take larger, quicker steps, which reduced their predicted leg contact force. We interpret these data to reflect young subjects' desire to minimize recovery effort, subject to neuromuscular constraints on step execution time and step length. Finally, our model predicts that successful balance recovery by stepping is governed by a coupling between step length, step execution time, and leg strength, so that the feasibility of balance recovery decreases unless declines in one capacity are offset by enhancements in the others. This suggests that one's risk for falls may be affected more by small but diffuse neuromuscular impairments than by larger impairment in a single motor capacity.

Adult↗
PubMed1999

Physiological responses and perceptions of exertion in a step aerobics session.

The aims of this study were to establish the cardiovascular and metabolic demands of a university step aerobics session entitled 'Uni-Step' performed at three step heights, and to evaluate the use of heart rate and ratings of perceived exertion for the estimation of exercise intensity during this mode. Ten female participants in step aerobics (mean VO2max = 47.7, s = 6.8 ml.kg-1.min-1) performed a 40-min Uni-Step routine on steps of height 6, 8 and 10 inches (15.2, 20.3 and 25.4 cm). Oxygen uptake, heart rate and ratings of perceived exertion were recorded throughout each test. Maximum oxygen uptake (VO2max) and maximum heart rate were measured using a continuous treadmill protocol. The mean intensities were 45.6%, 51.6% and 56.2% VO2max for the 6-, 8- and 10-inch steps respectively. The mean percent heart rate reserves were 57.2%, 63.6% and 70.1% at these three heights respectively. Correlations indicated a weak relationship between %VO2max and ratings of perceived exertion for the 6- and 8-inch steps (r = 0.61 and 0.66 respectively) but a stronger one for the 10-inch step (r = 0.79). Uni-Step performed on the two highest steps was of a sufficient relative intensity to improve or maintain the cardiovascular fitness of participants in this study. The lowest step may be useful for participants of lower fitness. Heart rate overestimated the metabolic cost of Uni-Step at all three step heights and therefore caution is advised if used to predict intensity. Low correlations between %VO2max and ratings of perceived exertion at the two lower step heights indicate that ratings of perceived exertion may have limited utility in prescribing training intensity.

Adult↗
PubMed1996

Impairment of posturo-kinetic co-ordination during initiation of forward oriented stepping movements in parkinsonian patients.

In order to differentiate between a specific impairment affecting gait initiation and a non-specific deficit in the postural adjustment which occurs prior to any forward oriented stepping movement, 3 forward oriented movements (FOMs), performed by a group of parkinsonian patients and a group of healthy age-matched subjects, were compared in the present study. These FOMs all consisted of initiating 1 step, but differed in their respective planning characteristics. The first consisted of initiating normal walking. The second consisted of initiating a single step, while the third was a visually guided task, consisting of placing the foot just behind a mark on the ground. In all 3 FOMs, the postural phase, i.e., the time elapsing between the initial shift of the center of pressure (CP) and the onset of the first step, was significantly longer in the patients than in the healthy subjects, whereas the duration of the subsequent movement phase, i.e., that of the first step, was within the same range in both groups. The horizontal reaction forces that led to a forward center of gravity (CG) acceleration during the postural phase were markedly reduced in the patients in all 3 FOMs, and the maximal velocity of the iliac crest marker, which corresponds approximately to that of the CG, decreased significantly in the patients. In addition, the length of the first step was significantly shorter in the patients than in the healthy subjects, in all 3 FOMs. The EMG pattern differed significantly between the patients and the healthy subjects; the amplitudes of the early tibialis anterior (TA) and vastus lateralis (VL) activations often decreased and were unilateral rather than bilateral. In addition, the gastrocnemius medialis (GM) burst associated with foot lift-off at the end of the postural phase was either absent or greatly reduced, thus suggesting that the co-ordination between the preparatory postural adjustment of the whole body and the actual stepping movement was impaired. The present results suggested that the lengthening of the postural phase is a common deficit in all FOM tasks in parkinsonian patients and is due to the impaired production of the requisite propulsive forces providing the forward acceleration of the CG. Consequently, a shortening of the first step length occurs. However, the step length is reduced less in the FOM tasks which provide some information about the goal of the first step (single step, visually guided step) than in a normal walking task, during which such information is missing. This suggests that although the stepping movement can be improved with the aid of any sensory cue about the end of the step in patients with Parkinson's disease, the postural phase will always be prolonged whichever FOM task they perform.

Aged↗