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Kevin Terry, Christopher Stanley, and Diane Damiano

There remains a pressing need for a stability metric that can reliably identify fall susceptibility during walking, enabling more effective gait rehabilitation for reduced fall incidence. One available metric is the maximum margin of stability (MOSmax), which is calculated using the body’s center of mass (COM) position and velocity along with the location of the maximum center of pressure (COPmax). However, MOSmax has several limitations that may limit stability assessment. Specifically, the assumptions of a fixed COP and constant ground reaction force (GRF) are not applicable to gait. To address these limitations, a modified MOS equation that allows for a variable COP and is not dependent on a constant GRF is presented here. The modified MOS was significantly lower than MOSmax throughout a significant portion of single limb support for normal walking gait. This finding indicates the MOSmax metric may lack sensitivity to instability as it may still be positive when the actual MOS indicates existing or impending instability. This comparison also showed that the MOS might offer additional information about walking stability relevant to gait assessment for fall prevention and rehabilitation. However, like other stability metrics, this capability must be established with further investigations of perturbed and pathological gait.

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Li Li

Variability has long been used as an indication of stability in the application of a dynamical systems approach to human motion (i.e., greater variability has been related to a less stable system and vise versa). This paper incorporates the probability of gait transition during walking and running at a certain speed to represent the stability of human locomotion. The mathematical representation concerning the probability of gait transition change with locomotory speed was derived for increasing walking speed and decreasing running speed. Additionally, the influence of acceleration and deceleration on the stability landscapes of walking and running was discussed based on experimental data. The influence of acceleration was also used to explain the different trends of hysteresis observed by various researchers. Walk-to-run transition speed was greater than run-to-walk transition speed, with a greater magnitude of acceleration, while the trend was reversed with a lesser acceleration magnitude. The quantitative measure of the relationship between variability and stability needs to be explored in the future.

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Reint H. Geuze

The functional integrity of the bimanual neuromotor system of Parkinson's disease (PD) subjects (stage II) compared to controls (2 × n = 16) was evaluated by measures of coordination stability of tapping in in-phase. anti-phase. and 90°-phase. Recently, intentional influence was modeled as an additive attractor function on the intrinsic dynamics, resulting in predictions tested by Scholz and Kelso (1990). In this study, the intentional influence was modulated by attaching cognitive meaning to the rhythmical pattern, which was expected to enhance the stability of coordination and, if effective, might be profitable to PD patients. Half of the PD subjects significantly lacked stability. They were less stable than controls, lost coordination at lower frequencies, and needed more time to switch between phase patterns. The reduction of stability was reflected in the progression of the disease. Cognitive meaning reduced variability of the single hands but not of relative phase, and no effect on switching time was found. The results suggest a weaker coupling strength between the limbs in PD patients lacking stability.

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Renato Rodano and Roberto Squadrone

Stability and consistency is a critical aspect in joint kinetic measurements. By applying a statistical technique, called sequential estimation procedure, the aim of this work was to determine the minimum number of trials required to obtain a stable mean for peak hip, knee, and ankle moments and powers during vertical jump. Nine competitive track and field sprinters (21.7 ± 3.5 yrs, 177.6 ± 4.3 cm, 70.8 ± 3.6 kg) performed 5 series of 5 double-legged maximum-height countermovement vertical jumps. From force platform and kinematic data, moment and power output were calculated for hip, knee, and ankle joints. The sequential estimation procedure applied to these data revealed that at least a 12-trial protocol is needed to establish a true measure for all the selected parameters. The mean number of trials for each variable was greater than 8 and less than 13. When hip moments were excluded from the analysis, a 10-trial protocol could be sufficient to reach a stable mean. In conclusion. the results of this study gave statistical evidence for the need to adopt multiple-trial protocols in order to obtain a stable mean for joint kinetic data.

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Hendrik Reimann, Tyler Fettrow, and John J. Jeka

response according to a feedback law, the CNS can increase the passive stability characteristics of the system. One well-established way to do this is to increase the average step width over multiple steps. The biomechanical rationale for this lies in the asymmetry between swing foot and stance foot at

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Cameron T. Gibbons, Polemnia G. Amazeen, and Aaron D. Likens

which a skill becomes perfected over the course of practice, as reflected in increased accuracy and stability (decreased movement variability). In their classic three-stage theory of motor learning, Fitts and Posner ( 1967 ) proposed that the individual learns the fundamentals of the action (cognitive

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Anna Lee, Tanvi Bhatt, Xuan Liu, Yiru Wang, Shuaijie Wang, and Yi-Chung (Clive) Pai

individuals’ dynamic stability. Dynamic stability is simultaneously affected by the center of mass (COM) position and its velocity relative to the base of support (BOS) (ie, COM state) to prevent environmental-induced falls. 9 Specifically, slip-perturbation training is induced by a sudden forward

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Fariba Hasanbarani and Mark L. Latash

Stability of salient performance variables (PVs) is crucial for functional actions, given the unpredictable and varying external forces and intrinsic states of the body (reviewed in Bernstein, 1967 ; Latash, 2017 ). Over the last 20 years, a method to study the dynamical stability of variables

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Alice D. LaGoy, Caleb Johnson, Katelyn F. Allison, Shawn D. Flanagan, Mita T. Lovalekar, Takashi Nagai, and Chris Connaboy

load carriage on the warfighter be understood. 1 , 9 – 12 Additional load reduces the dynamic postural stability of an individual, and compromised dynamic postural stability may contribute to the increased injury risk associated with load carriage. 13 – 15 Dynamic postural stability refers to the

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Kazem Malmir, Gholam Reza Olyaei, Saeed Talebian, Ali Ashraf Jamshidi, and Majid Ashraf Ganguie

There seems to be a relationship between poor postural control and ankle injuries. 1 Postural control is provided by interaction and coordination among sensory information, central processing, and motor outputs. Postural control system provides postural orientation and postural stability. Postural