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Jeffrey M. Haddad, Jeff L. Gagnon, Christopher J. Hasson, Richard E.A. Van Emmerik and Joseph Hamill

Postural stability has traditionally been examined through spatial measures of the center of mass (CoM) or center of pressure (CoP), where larger amounts of CoM or CoP movements are considered signs of postural instability. However, for stabilization, the postural control system may utilize additional information about the CoM or CoP such as velocity, acceleration, and the temporal margin to a stability boundary. Postural time-to-contact (TtC) is a variable that can take into account this additional information about the CoM or CoP. Postural TtC is the time it would take the CoM or CoP, given its instantaneous trajectory, to contact a stability boundary. This is essentially the time the system has to reverse any perturbation before stance is threatened. Although this measure shows promise in assessing postural stability, the TtC values derived between studies are highly ambiguous due to major differences in how they are calculated. In this study, various methodologies used to assess postural TtC were compared during quiet stance and induced-sway conditions. The effects of the different methodologies on TtC values will be assessed, and issues regarding the interpretation of TtC data will also be discussed.

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Daniel Maykranz, Sten Grimmer and Andre Seyfarth

The work-loop method is frequently used to determine the mechanical work performed by a system, for instance, when analyzing muscles or describing the work balance at the joint level. While for these examples usually only one-dimensional movements are investigated, for two- or three-dimensional movements, such as leg function during walking and running, the work-loop has to be adapted. In this paper, we present an analytical derivation that extends the work-loop method to two-dimensional sagittal plane movements. Three effects contribute to the mechanical work of the leg: (1) forces directed along the leg axis, (2) forces acting perpendicular to the leg axis, and (3) a shift of the center of pressure (COP) during stance. These three contributors to the mechanical work performed can be interpreted as three general tasks of the leg. To demonstrate the new work-loop method, we analyzed experimental data on hopping, running and walking. The results indicate that the proposed new generalized work-loop concept is suitable for describing the overall mechanical work performed on the COM during stance with energy consistent net work balances. Depending on the type of gait, specific contributions of each work term were found that characterize leg function during locomotion.

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James W. Youdas, Erica F. Loder, Jody L. Moldenhauer, Christine R. Paulsen and John H. Hollman

Context:

Hip-abductor weakness is associated with many lower extremity injuries. A simple procedure to assess hip-abductor performance is necessary in patient populations.

Objective:

To describe the change in pelvic-on-femoral position of the stance limb before and after 45 seconds of resisted sidestepping.

Design:

Cross-sectional comparative.

Setting:

Laboratory.

Participants:

24 healthy women (24.6 ± 3.5 years) and 14 healthy men (24.5 ± 3.0 years).

Main Outcome Measures:

Pelvic-on-femoral position in degrees in single-leg stance before and after 45 seconds of resisted sidestepping.

Results:

The difference between the baseline and postexercise measurements for both men and women was significant (P < .05). The effect of the resisted-sidestepping exercise on the hip abductors was not statistically different between men and women.

Conclusions:

Forty-five seconds of resisted sidestepping using an elastic band produced a change in pelvic-on-femoral position in healthy adults. This test might be useful to detect impaired performance in hip abductors of patients with injury elsewhere in the musculoskeletal system.

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Young-Hoo Kwon, Lonn Hutcheson, Jeffrey B. Casebolt, Joong-Hyun Ryu and Kunal Singhal

The purpose of this study was to investigate the effects of transversely sloped ballasted walking surface on gait and rearfoot motion (RFM) parameters. Motion analysis was performed with 20 healthy participants (15 male and 5 female) walking in six surface-slope conditions: two surfaces (solid and ballasted) by three slopes (0, 5, and 10 degrees). The gait parameters (walking velocity, step length, step rate, step width, stance time, and toe-out angle) showed significant surface effect (p = .004) and surface-slope interaction (p = .017). The RFM motion parameters (peak everted/inverted position, eversion/inversion velocity, and acceleration) revealed significant surface (p = .004) and slope (p = .024) effects. The ballasted conditions showed more cautious gait patterns with lower walk velocity, step length, and step rate and longer stance time. In the RFM parameters, the slope effect was more notable in the solid conditions due to the gait adaptations in the ballasted conditions. Ballast conditions showed reduced inversion and increased eversion and RFM range. The RFM data were comparable to other typical walking conditions but smaller than those from running.

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Ferdous Wahid, Rezaul Begg, Noel Lythgo, Chris J. Hass, Saman Halgamuge and David C. Ackland

Normalization of gait data is performed to reduce the effects of intersubject variations due to physical characteristics. This study reports a multiple regression normalization approach for spatiotemporal gait data that takes into account intersubject variations in self-selected walking speed and physical properties including age, height, body mass, and sex. Spatiotemporal gait data including stride length, cadence, stance time, double support time, and stride time were obtained from healthy subjects including 782 children, 71 adults, 29 elderly subjects, and 28 elderly Parkinson’s disease (PD) patients. Data were normalized using standard dimensionless equations, a detrending method, and a multiple regression approach. After normalization using dimensionless equations and the detrending method, weak to moderate correlations between walking speed, physical properties, and spatiotemporal gait features were observed (0.01 < |r| < 0.88), whereas normalization using the multiple regression method reduced these correlations to weak values (|r| < 0.29). Data normalization using dimensionless equations and detrending resulted in significant differences in stride length and double support time of PD patients; however the multiple regression approach revealed significant differences in these features as well as in cadence, stance time, and stride time. The proposed multiple regression normalization may be useful in machine learning, gait classification, and clinical evaluation of pathological gait patterns.

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Semyon Slobounov, Tao Wu and Mark Hallett

Human upright posture is a product of a complex dynamic system that relies on integration of input from multimodal sensory sources. Extensive research has explored the role of visual, vestibular, and somatosensory systems in the control of upright posture. However, the role of higher cognitive function in a participant’s assessment of postural stability has been less studied. In previous research, we showed specific neural activation patterns in EEG associated with recognition of unstable postures in young healthy participants. Similar EEG patterns have been recently observed in regulation of posture equilibrium in dynamic stances. This article evaluates participants’ postural stability in dynamic stances and neural activation patterns underlying visual recognition of unstable postures using event-related functional MRI (fMRI). Our results show that the “stable” participants were successful in recognition of unstable postures of a computer-animated body model and experienced egocentric motion. Successful recognition of unstable postures in these participants induces activation of distinct areas of the brain including bilateral parietal cortex, anterior cingulate cortex, and bilateral cerebellum. In addition, significant activation is observed in basal ganglia (caudate nucleus and putamen) but only during perception of animated postures. Our findings suggest the existence of modality-specific distributed activation of brain areas responsible for detection of postural instability.

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Gurtej S. Grewal, Rachel Baisch, Jacqueline Lee-Eng, Stephaine Wu, Beth Jarrett, Neil Humble and Bijan Najafi

Context:

Improvements in postural stability in figure skaters can play a significant role in performance, as well as reducing fall risk.

Objective:

To explore the effect of custom foot insoles on postural stability in advanced figure skaters.

Design:

Exploratory study.

Setting:

Out of laboratory.

Participants:

Nine advanced figure skaters were recruited and 7 completed the study (age 38 ± 18.5 y, body-mass index 25 ± 3.6 kg/m2).

Intervention:

Custom foot insoles.

Main Outcome Measures:

Primary outcome of changes in postural stability (PS) quantified by center-of-mass sway with secondary outcomes of ankleand hip-joint sway and joint range of motion. Sway measurements were assessed using body-worn sensors while participants wore skates on ice. PS was assessed in single-leg stance, as well as during gliding on the dominant foot.

Results:

A significant improvement in static PS was observed after 6-wk use of custom insoles. Center-of-mass sway reduced significantly on average by 48.44% (P = .023), and ankle-joint sway reduced by 45.7% (P = .05) during single-leg-stance balance measurements. During the gliding maneuver nonsignificant changes were observed for both ankle- and knee-joint range of motion.

Conclusion:

The results of this study suggest proof of concept toward benefits of custom insoles in improving postural stability in advanced figure skaters. To generalize the findings, randomized controlled trials with larger sample sizes are warranted.

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Thomas A. Stoffregen, Karen Adolph, Esther Thelen, Kathleen M. Gorday and Yang-Yi Sheng

This study was undertaken to determine whether young children, after only a few weeks standing experience, could respond adaptively to the dynamical constraints imposed by different support surfaces. The spontaneous postural motions of young children (13-14 months old) were observed as they stood on surfaces that differed in length, friction, and rigidity. There were no externally imposed perturbations to stance. Children's postural control was remarkably adaptive: There were few falls on any of the surfaces. Moreover, the children showed surface-specific utilization of manual postural control (holding onto wooden poles), suggesting that manual control is an adaptive strategy for postural control. Finally, kinematic analysis suggested that, in some instances, children were able to employ independent control of the hips, contrary to previous models which had suggested that hip motions could not be controlled before the age of 3 years. Small, slow hip movements useful in controlling spontaneous sway (unperturbed stance) may serve as a basis for the development of larger, faster hip movements that are associated with imposed perturbations.

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Kristof Kipp, Tyler N. Brown, Scott G. McLean and Riann M. Palmieri-Smith

The purpose of this study was to examine the combined impact of experience and decision making on frontal plane knee joint biomechanics during a cutting maneuver. Kinematic and kinetic data were collected from 12 recreationally active and 18 NCAA Division I female athletes during execution of anticipated and unanticipated single-leg land-and-cut maneuvers. Knee joint abduction angles and external knee joint abduction torques were calculated and discrete peak stance-phase variables were extracted. Angle and torque time-series data were also submitted to separate functional data analyses. Variables derived from the functional data analyses indicated that decision making influenced knee abduction angle and torque time series in the recreational group only. Specifically, these variables pointed to greater knee abduction at the end of stance as well as a greater, albeit delayed peak in knee abduction torque at the beginning of landing in the recreational athletes during the unanticipated condition. In addition, the recreational athletes displayed greater discrete peak knee abduction angles than the Division I athletes regardless of condition. Discrete peak knee abduction torque did not differ between groups or conditions.

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Kazuhiro Ishimura and Shinji Sakurai

This study investigates the potential asymmetries between inside and outside legs in determinants of curved running speed. To test these asymmetries, a deterministic model of curved running speed was constructed based on components of step length and frequency, including the distances and times of different step phases, takeoff speed and angle, velocities in different directions, and relative height of the runner’s center of gravity. Eighteen athletes sprinted 60 m on the curved path of a 400-m track; trials were recorded using a motion-capture system. The variables were calculated following the deterministic model. The average speeds were identical between the 2 sides; however, the step length and frequency were asymmetric. In straight sprinting, there is a trade-off relationship between the step length and frequency; however, such a trade-off relationship was not observed in each step of curved sprinting in this study. Asymmetric vertical velocity at takeoff resulted in an asymmetric flight distance and time. The runners changed the running direction significantly during the outside foot stance because of the asymmetric centripetal force. Moreover, the outside leg had a larger tangential force and shorter stance time. These asymmetries between legs indicated the outside leg plays an important role in curved sprinting.