Publications by authors named "Yi-Chung Pai"

61 Publications

Gait Slip-Induced Fall-Type Assessment Based on Regular Gait Characteristics in Older Adults.

J Appl Biomech 2022 Apr 28:1-7. Epub 2022 Apr 28.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL,USA.

Older adults could experience split falls or feet-forward falls following an unexpected slip in gait due to different neuromuscular vulnerabilities, and different intervention strategies would be required for each type of faller. Thus, this study aimed to investigate the key factors affecting the fall types based on regular gait pattern. A total of 105 healthy older adults who experienced a laboratory-induced slip and fall were included. Their natural walking trial immediately prior to the novel slip trial was analyzed. To identify the factors related to fall type, gait characteristics and demographic factors were determined using univariate logistic regression, and then stepwise logistic regression was conducted to assess the slip-induced fall type based on these factors. The best fall-type prediction model involves gait speed and recovery foot angular velocity, which could predict 70.5% of feet-forward falls and 86.9% of split falls. Body mass index was also a crucial fall-type prediction with an overall prediction accuracy of 70.5%. Along with gait parameters, 84.1% of feet-forward falls and 78.7% of split falls could be predicted. The findings in this study revealed the determinators related to fall types, which enhances our knowledge of the mechanism associated to slip-induced fall and would be helpful for the development of tailored interventions for slip-induced fall prevention.
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http://dx.doi.org/10.1123/jab.2021-0337DOI Listing
April 2022

Can a single session of treadmill-based slip training reduce daily life falls in community-dwelling older adults? A randomized controlled trial.

Aging Clin Exp Res 2022 Jul 2;34(7):1593-1602. Epub 2022 Mar 2.

Department of Physical Therapy, College of Applied Health and Sciences, University of Illinois at Chicago, Chicago, IL, 60612, USA.

Background: Task-specific training with single-session overground slip simulation has shown to reduce real-life falls in older adults.

Aims: The purpose of this study was to determine if fall-resisting behavior acquired from a single-session treadmill-based gait slip training could be retained to reduce older adults' falls in everyday living over a 6-month follow-up period.

Methods: 143 community-dwelling older adults (≥ 65 years old) were randomly assigned to either the treadmill-based gait slip training group (N = 73), in which participants were exposed to 40 unpredictable treadmill slips, or the control group (N = 70), in which participants walked on a treadmill at their comfortable speed. Participants reported their falls from the preceding year (through self-report history) and over the following 6 months (through fall diaries and monitored with phone calls).

Results: There was no main effect of time (retrospective vs. prospective fall) and training (treadmill training vs. control) on fall reduction (p > 0.05 for both). The survival distributions of event of all-cause falls or slip falls were comparable between groups (p > 0.05 for both).

Discussion: Unlike overground slip training where a single training session could significantly reduce everyday falls in a 6-month follow-up period, the results indicated that one treadmill-based gait slip training session by itself was unable to produce similar effects.

Conclusion: Further modification of the training protocol by increasing training dosage (e.g., number of sessions or perturbation intensity) may be necessary to enhance transfer to daily living. This study (NCT02126488) was registered on April 30, 2014.
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http://dx.doi.org/10.1007/s40520-022-02090-3DOI Listing
July 2022

Kinematic synergies in over-ground slip recovery outcomes: Distinct strategies or a single strategy?

Gait Posture 2022 06 17;95:270-276. Epub 2021 Feb 17.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL, 60612, United States. Electronic address:

Background: After experiencing an unexpected slip perturbation, individuals' behavioral performance can be classified into three categories: recovery, feet-forward fall, and split fall. Researchers are uncertain whether these differences in slip outcomes are due to distinct strategies or part of a single strategy.

Research Question: Whether older adults with different behavioral outcomes during their novel slip have different kinematic synergies?

Methods: The kinematic synergies were extracted from segment angles in 87 participants using principal component analysis (PCA). The first two principal components (PC1 and PC2) in pre-slip, early-reactive, and late-reactive phases were compared across different slip outcomes.

Results: Results showed that the kinematic synergies in pre-slip and early-reactive phases are highly consistent among the three outcomes (recovery, split fall, and feet-forward fall). For the late-reactive phase, both split falls and feet-forward falls showed different kinematics synergies from recoveries.

Significance: Our findings indicated that a single strategy might be used for different slip outcomes in the pre-slip and early-reactive phases, while distinct strategies were used by fallers compared to recovered individuals. Specifically, larger trunk flexion in pre-slip phase, larger knee flexion and plantar flexion of the slipping limb in both early-reactive and late-reactive phase, and larger knee extension of the recovery limb in late-reactive phase would lower the fall risk. This study would help to assess the vulnerabilities in control strategy, according to which individualized treatment could be provided to reduce predisposition to specific types of falls.
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http://dx.doi.org/10.1016/j.gaitpost.2021.01.025DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8368075PMC
June 2022

The retention of fall-resisting behavior derived from treadmill slip-perturbation training in community-dwelling older adults.

Geroscience 2021 04 25;43(2):913-926. Epub 2020 Sep 25.

Department of Physical Therapy, University of Illinois at Chicago, 1919 W. Taylor Street, Fourth Floor, Chicago, IL, 60612, USA.

The purpose of this study was to determine whether and to what extent the immediate generalization of treadmill slip-perturbation training could be retained over 6 months to resist overground slip-induced falls. Four protocols (Tc: treadmill control; Tt: treadmill slip-perturbation training; Oc: overground control; Ot: overground slip-perturbation training) from two randomized controlled trials were compared in which two training protocols were executed with single-session repeated slip-perturbation training on the treadmill or overground context, while two control protocols were executed without repeated training. A total of 152 community-dwelling older adults (≥ 65 years) who were trained by one of the four protocols and tested by an overground slip in the initial session attended a retest session 6 months later. Falls were detected by a load cell. Data collected from motion analysis system and force plates were used to calculate stability. Tt group had no significant change in fall incidence from initial post-training test to retest. Tt group had significantly lower fall incidence (p < 0.05) and higher reactive stability (p < 0.05) than Tc group in retest. Tt group had significantly higher fall incidence (p < 0.05) and lower reactive stability (p < 0.01) than Ot group. The generalization of a single session of treadmill slip-perturbation training to overground slip resulted in inferior outcomes compared with overground slip-perturbation training (absolute retention), although the training generalization could be retained over 6 months (relative retention). Thus, treadmill slip-perturbation training could be more convenient to use if future dose-response studies indicate better or equal efficacy to overground slip-perturbation training.
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http://dx.doi.org/10.1007/s11357-020-00270-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8110680PMC
April 2021

Can Treadmill Slip-Perturbation Training Reduce Longer-Term Fall Risk Upon Overground Slip Exposure?

J Appl Biomech 2020 Aug 25:1-9. Epub 2020 Aug 25.

University of Illinois at Chicago.

The purpose was to examine and compare the longer-term generalization between 2 different practice dosages for a single-session treadmill slip-perturbation training when reexposed to an overground slip 6 months later. A total of 45 older adults were conveniently assigned to either 24 or 40 slip-like treadmill perturbation trials or a third control group. Overground slips were given immediately after initial training, and at 6 months after initial training in order to examine immediate and longer-term effects. The performance (center of mass stability and vertical limb support) and fall percentage from the laboratory-induced overground slips (at initial posttraining and at 6 mo) were measured and compared between groups. Both treadmill slip-perturbation groups showed immediate generalization at the initial posttraining test and longer-term generalization at the 6-month retest. The higher-practice-dosage group performed significantly better than the control group (P < .05), with no difference between the lower-practice-dosage and the control groups at the 6-month retest (P > .05). A single session of treadmill slip-perturbation training showed a positive effect for reducing older adults' fall risk for laboratory-induced overground slips. A higher-practice dosage of treadmill slip perturbations could be more beneficial for further reducing fall risk.
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http://dx.doi.org/10.1123/jab.2019-0211DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8344091PMC
August 2020

Which Are the Key Kinematic and Kinetic Components to Distinguish Recovery Strategies for Overground Slips Among Community-Dwelling Older Adults?

J Appl Biomech 2020 Jun 3:1-11. Epub 2020 Jun 3.

University of Illinois at Chicago.

Slip outcomes are categorized as either a backward loss of balance (LOB) or a no loss of balance (no-LOB) in which an individual does not take a backward step to regain their stability. LOB includes falls and nonfalls, while no-LOB includes skate overs and walkovers. Researchers are uncertain about which factors determine slip outcomes and at which critical instants they do so. The purpose of the study was to investigate factors affecting slip outcomes in proactive and early reactive phases by analyzing 136 slip trials from 68 participants (age: 72.2 [5.3] y, female: 22). Segment angles and average joint moments in the sagittal plane of the slipping limb were compared for different slip outcomes. The results showed that knee flexor, hip extensor, and plantar flexor moments were significantly larger for no-LOB than for LOB in the midproactive phase, leading to smaller shank-ground and foot-ground angles at the slip onset, based on forward dynamics. In the early reactive phase, the hip extensor and plantar flexor moments were larger for no-LOB than for LOB, and all segment angles were smaller for no-LOB. Our findings indicate that the shank angle and knee moment were the major determinants of slip outcomes in both proactive and reactive phases.
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http://dx.doi.org/10.1123/jab.2019-0285DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8344101PMC
June 2020

Is There an Optimal Recovery Step Landing Zone Against Slip-Induced Backward Falls During Walking?

Ann Biomed Eng 2020 Jun 12;48(6):1768-1778. Epub 2020 Mar 12.

Department of Physical Therapy, University of Illinois at Chicago, 1919, W Taylor St, (M/C 898), Chicago, IL, 60612, USA.

Recovery stepping in response to forward slips has the potential to not only rebuild the base of support to prevent backward falling, but also provide extra limb support to prevent downward falling. Hence, recovery stepping is often necessary for fall prevention following an unexpected slip. However, less is known about whether recovery foot placement could affect the likelihood of recovery following a slip. The purpose of this study was to determine whether there is an optimal recovery landing zone within which older adults have a higher likelihood of recovery. 195 participants experienced a novel, unannounced forward slip while walking on a 7-m walkway. The center of mass (COM) stability (computed from its position and velocity), vertical limb support (computed from change in hip kinematics), and recovery limb joint moments (computed from joint kinematics and ground reaction force) in the sagittal plane were analyzed. The results showed that a longer distance between recovery foot landing position and the projected COM position at recovery foot touchdown (relative recovery step placement) was conducive to stability improvement but adverse to limb support enhancement, and vice versa for a shorter distance. Relative recovery step placement could predict the recovery likelihood with an accuracy of 67.3%, and the recovery rate was greater than 50% when the distance between recovery foot and COM is less than 0.3 × foot length. This study also found more posterior stepping could be attributed to insufficient ankle plantar flexor and hip flexor moments in the pre-swing phase, while more anterior stepping was induced by insufficient hip and knee extensor moments in the following swing phase.
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http://dx.doi.org/10.1007/s10439-020-02482-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8314641PMC
June 2020

The Role of Recovery Lower Limb Segments in Post-Slip Determination of Falls Due to Instability or Limb Collapse.

Ann Biomed Eng 2020 Jan 23;48(1):192-202. Epub 2019 Jul 23.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL, 60612, USA.

Slip-related falls can be induced by instability or limb collapse, but the key factors that determine these two fall causations remain unknown. The purpose of this study was to investigate the factors that contribute towards instability-induced and limb-collapse-induced slip-related falls by investigating 114 novel slip trials. The segment angles and moments of the recovery limb after slip-onset from pre-left-touchdown (pre-LTD) to post-left-touchdown (post-LTD) were calculated, and logistic regression was used to detect which variable contributed most to instability-induced and limb-collapse-induced falls. The results showed that recovery from instability was determined by the angle of the thigh at LTD (87.7%), while recovery from limb collapse was determined by the angle of the shank at post-LTD (90.4%). Correspondingly, instability-induced falls were successfully predicted (81.5%) based on the initial thigh angle at pre-LTD and the following peak thigh moment, while limb-collapse-induced falls were successfully predicted (85.5%) based on the initial shank angle at LTD and the following peak shank moment. According to our findings, taking a shorter recovery step and/or increasing the counterclockwise moment of the thigh after pre-LTD would help individuals resist instability-induced falls, while taking a larger recovery step and/or increasing the clockwise moment of the shank post-LTD would help resist limb-collapse-induced falls. The findings of this study are crucial for future clinical applications, because individually tailored reactive balance training could be provided to reduce vulnerability to specific types of falls and improve recovery rates post-slip exposure.
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http://dx.doi.org/10.1007/s10439-019-02327-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6930351PMC
January 2020

Treadmill-gait slip training in community-dwelling older adults: mechanisms of immediate adaptation for a progressive ascending-mixed-intensity protocol.

Exp Brain Res 2019 Sep 8;237(9):2305-2317. Epub 2019 Jul 8.

Department of Physical Therapy, College of Applied Health and Sciences, University of Illinois at Chicago, 1919 West Taylor Street (M/C 898), Chicago, IL, 60612, USA.

The study purpose was to investigate whether older adults could improve their stability against a backward loss of balance (BLOB) after receiving repeated treadmill slips during walking and to see how such adaptive changes would be affected by practice dosage (combination of slip intensity and the number of slips at each intensity). Twenty-five healthy community-dwelling older adults received forty treadmill slips given over eleven blocks at five intensities (P1-P1-P2-P3-P4-P5-P4-P5-P5-P3-P1, larger number indicating higher intensity). Center of mass (COM) stability was calculated as the shortest distance of the instantaneous COM position and velocity relative to the base of support (BOS) from a theoretical threshold for BLOB (larger stability value indicated a better stability against BLOB). Stability, step length, and trunk angle were measured before and after slip onset to reflect proactive and reactive control, respectively. The first slips at each intensity block (i.e., P1, P3, P4, and P5) were compared with the first slips in the last blocks at those intensities to examine main effects of training dosage (intensity and repetition). Improvements in proactive and reactive stability were more pronounced for receiving more slips at larger intensities than fewer slips at smaller intensities. Older adults only demonstrated partial positive scaling effects to proactively, not reactively, establish a more stable initial COM state. The improved proactive stability was associated with an anterior shift of COM position relative to the BOS, resulting from a shorter pre-slip step length. The improved reactive stability was associated with an anterior shift of COM position, resulting from a larger compensatory step length and a faster COM velocity relative to the BOS. Our findings indicated that treadmill-gait slip perturbations elicited similar proactive and reactive control to that from over-ground slip perturbations, but greater slip intensity and repetition might yield more immediate adaptive improvements.
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http://dx.doi.org/10.1007/s00221-019-05582-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8376032PMC
September 2019

Limb Collapse or Instability? Assessment on Cause of Falls.

Ann Biomed Eng 2019 Mar 7;47(3):767-777. Epub 2019 Jan 7.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL, USA.

What causes an older adult to fall? Could the same factor lead to a recurring fall? The purposes of this study sought to address these questions by developing a causal-based assessment method for detection of the initial biomechanical cause of fall, and investigating the causation of 97 falls (out of 195 community dwelling older adults who participated in this study) based on this method. The unrecoverable limb collapse, or unrecoverable instability, along with its point of no return was defined, and the assessment method was established. Both the novel and the second slips of 97 participants who experienced laboratory induced slip related falls were assessed. The results showed that these older adults had more limb collapse (59.8%) initiated falls than instability (40.2%; and 32.0% of which from anteroposterior instability while only 8.2% from mediolateral instability) initiated falls. Interestingly, the majority (86.4%) of those 22 repeated fallers fell twice because of the same cause. These findings shed light on the vulnerability and the causation of recurring falls, which is one of the most challenging healthcare issues that an active but aging population is facing.
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http://dx.doi.org/10.1007/s10439-018-02195-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6382554PMC
March 2019

Can treadmill-slip perturbation training reduce immediate risk of over-ground-slip induced fall among community-dwelling older adults?

J Biomech 2019 02 14;84:58-66. Epub 2018 Dec 14.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL, USA. Electronic address:

The purpose of this study was to determine any potential falls-resistance benefits that might arise from treadmill-slip-perturbation training. One hundred sixty-six healthy community-dwelling older adults were randomly assigned to either the treadmill-slip-training group (Tt) or the treadmill-control group (Tc). Tt received 40 slip-like perturbations during treadmill walking. Tc received unperturbed treadmill walking for 30 min. Following their treadmill session, both groups were exposed to a novel slip during over-ground walking. Their responses to this novel slip were also compared to previously collected data from participants who received either over-ground-slip training (Ot) with 24 slips or over-ground walking (Oc) with no training before experiencing their novel over-ground slip. Fall rates and both proactive (pre-slip) and reactive (post-slip) stability were assessed and compared for the novel over-ground slip in groups Tt, Tc, and Oc, as well as for the 24th slip in Ot. Results showed Tt had fewer falls than Tc (9.6% versus 43.8%, p < 0.001) but more falls than Ot (9.6% versus 0%, p < 0.001). Tt also had greater proactive and reactive stability than Tc (Tt > Tc, p < 0.01), however, Tt's stabilities were lower than those of Ot (p < 0.01). There was no difference in fall-rate or reactive stability between Tc and Oc, though treadmill walking did improve the proactive stability control of the latter. While the treadmill-slip-training protocol could immediately reduce the numbers of falls from a novel laboratory-reproduced slip, such improvements were far less than that from the motor adaptation to the over-ground-slip-training protocol.
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http://dx.doi.org/10.1016/j.jbiomech.2018.12.017DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6361674PMC
February 2019

Can higher training practice dosage with treadmill slip-perturbation necessarily reduce risk of falls following overground slip?

Gait Posture 2018 03 20;61:387-392. Epub 2018 Mar 20.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL, United States. Electronic address:

Background: Perturbation training is an emerging paradigm to reduce idiopathic falls (without clinical signs or symptoms) in older adults. While a higher threat dosage (intensity) in motor learning often directly relates to greater adaptation, retention, and generalization, little is known whether increasing the practice dosage (repetition) of slip-perturbation training would necessarily improve its outcomes.

Research Question: Can higher practice dosage of treadmill slip-perturbation training lead to greater generalization to an overground slip immediately after the training?

Methods: Forty-five community-dwelling older adults (73.5 ± 5.6 years old) participated in the present study. They were conveniently assigned to three groups with equivalent treadmill walking duration: treadmill slip-perturbation training group with 40 practice dosage, 24 practice dosage, and zero practice dosage (without slip-perturbation). Later on during overground walking, all of them were exposed to the same generalization test (a novel slip on a walkway). Their recovery outcomes (fall, or no fall; balance loss, or no balance loss) and center of mass stability were compared.

Results: Higher practice dosage did not show significantly less incidence of fall, balance loss, or greater stability in comparison to lower practice dosage (p > .05). The present study showed that there was no evidence of dose-response relationship when the practice dosage was set above the 24 trials of practice dosage in treadmill slip-perturbation training.

Significance: Contrary to our hypothesis, increased practice dosage (40-slips) in treadmill slip-perturbation training from the commonly used threshold (24-slips) did not necessarily benefit immediate generalization from treadmill to overground walking among community-dwelling older adults.
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http://dx.doi.org/10.1016/j.gaitpost.2018.01.037DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8312761PMC
March 2018

Gait Speed and Dynamic Stability Decline Accelerates Only in Late Life: A Cross-sectional Study in Community-Dwelling Older Adults.

J Geriatr Phys Ther 2019 Apr/Jun;42(2):73-80

Department of Physical Therapy, University of Illinois at Chicago.

Background And Purpose: Incidence of falls increases with age whereas gait speed declines. The purposes of this study were to examine (1) whether gait speed and center-of-mass (COM) velocity declined steadily across ages in a linear fashion among community-dwelling older adults, and (2) whether such decline corresponded to the similar decline in dynamic stability, which is governed by the control of their COM position and COM velocity relative to base of support (BOS).

Methods: A total of 184 community-dwelling older adults (≥65 years) participated in the cross-sectional study. The participants were categorized into 5 age groups (65-69, 70-74, 75-79, 80-84, and 85+ years) and were asked to walk on the 7-m walkway at their preferred walking speed. Their speed, gait pattern, relative COM position, and relative COM velocity were measured.

Results: Very close relationship was confirmed between a clinical gait speed measurement and the COM velocity (R = 0.875, P < .05), which enabled us to use the 2 terms interchangeably. Gait speed decline was not noticeable from 65 to 84 years of age (P > .05), but it accelerated after 85 years of age. This decline was most likely influenced by a reduction in both step length (P < .05) and cadence (P < .05). Similarly, dynamic stability against backward loss of balance changed little between 65 and 84 years of age (P > .05). Yet, it declined significantly after 85 years of age (P < .05), primarily affected by the reduction in the COM velocity relative to the BOS, whereby the COM position relative to the BOS remained constant during their walking.

Conclusion: Expected steady decline in gait speed and in the control of gait stability cannot be confirmed. Rather, we found that both declined precipitously only after 85 years of age, when the risk of falls is likely to increase correspondingly.
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http://dx.doi.org/10.1519/JPT.0000000000000171DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6690591PMC
March 2020

The recovery response to a novel unannounced laboratory-induced slip: The "first trial effect" in older adults.

Clin Biomech (Bristol, Avon) 2017 Oct 17;48:9-14. Epub 2017 Jun 17.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, United States. Electronic address:

Background: After a single slip, older adults rapidly make adaptive changes to avoid or eliminate further backward loss of balance or a fall. This rapid adaptation has been termed the "single trial effect". The purpose of this study was to explore the relationship between the motor errors subjects experienced upon a novel slip and the selection and execution of corrective response by which they modified their ongoing gait pattern and turned it into a protective step.

Methods: A forward slip was induced in the laboratory among 145 community-living older (≥65year old) adults who were protected by an overhead full body harness system. An eight-camera motion analysis system recorded subjects' kinematics, which was used to compute their instability (motor error), recovery step placement (response selection), and stability gain (motor correction).

Findings: A linear relationship was found between the stability errors at recovery foot liftoff and the distance between the recovery foot and slipping foot at the time of its touchdown, reflecting an appropriate selection of response that was proportionate to the motor error. A linear relationship was also found between this step modification and resulting stability gain, indicating that greater step modification resulted in greater stability gain. This learning behavior was surprisingly consistent regardless whether the outcome was a recovery or a fall.

Interpretations: These results suggest that fallers and non-fallers all have an intact motor learning foundation that has enabled them to rapidly improve their stability in subsequent exposures.
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http://dx.doi.org/10.1016/j.clinbiomech.2017.06.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5600159PMC
October 2017

Can Recovery Foot Placement Affect Older Adults' Slip-Fall Severity?

Ann Biomed Eng 2017 08 4;45(8):1941-1948. Epub 2017 May 4.

Department of Physical Therapy, University of Illinois at Chicago, 1919 West Taylor St., Room 426 (M/C 898), Chicago, IL, 60612, USA.

Following a slip occurred in the overground walking, a fall can be classified into two exclusive categories: feet-forward fall or split fall. The purposes of this study were to investigate whether the placement of the recovery foot would determine the slip types, the likelihood of fall, and the severity associated with each fall. The fall severity was estimated based on the impact velocity of body segments or trunk orientation upon fall arrest. One hundred ninety-five participants experienced a novel, unannounced slip while walking on a 7-m walkway. Kinematics of a full-body marker set was collected by a motion capture system which was synchronized with the force plates and loadcell. The results showed that the recovery foot landing position relative to the projected center of mass position at the recovery foot touchdown determined the slip type by 90.8%. Feet-forward slips led to significantly lower rate of falls than did split slips (47.6 vs. 67.8%, p < 0.01). Yet, feet-forward falls were much more dangerous because they were associated with significantly greater estimated maximum hip impact velocity (p < 0.001) and trunk backward leaning angle (p < 0.001) in comparison to split falls.
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http://dx.doi.org/10.1007/s10439-017-1834-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5529239PMC
August 2017

Retention of the "first-trial effect" in gait-slip among community-living older adults.

Geroscience 2017 02 7;39(1):93-102. Epub 2017 Feb 7.

Department of Physical Therapy (MC 898), University of Illinois at Chicago, 1919 W. Taylor Street, Fourth Floor, Chicago, IL, 60612, USA.

"First-trial effect" characterizes the rapid adaptive behavior that changes the performance outcome (from fall to non-fall) after merely a single exposure to postural disturbance. The purpose of this study was to investigate how long the first-trial effect could last. Seventy-five (≥ 65 years) community-dwelling older adults, who were protected by an overhead full body harness system, were retested for a single slip 6-12 months after their initial exposure to a single gait-slip. Subjects' body kinematics that was used to compute their proactive (feedforward) and reactive (feedback) control of stability was recorded by an eight-camera motion analysis system. We found the laboratory falls of subjects on their retest slip were significantly lower than that on the novel initial slip, and the reactive stability of these subjects was also significantly improved. However, the proactive stability of subjects remains unchanged between their initial slip and retest slip. The fall rates and stability control had no difference among the 6-, 9-, and 12-month retest groups, which indicated a maximum retention on 12 months after a single slip in the laboratory. These results highlighted the importance of the "first-trial effect" and suggested that perturbation training is effective for fall prevention, with lower trial doses for a long period (up to 1 year). Therefore, single slip training might benefit those older adults who could not tolerate larger doses in reality.
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http://dx.doi.org/10.1007/s11357-017-9963-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5352700PMC
February 2017

Neuromuscular responses differ between slip-induced falls and recoveries in older adults.

J Neurophysiol 2017 02 2;117(2):509-522. Epub 2016 Nov 2.

W. H. Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, Georgia; and.

How does the robust control of walking and balance break down during a fall? Here, as a first step in identifying the neuromuscular determinants of falls, we tested the hypothesis that falls and recoveries are characterized by differences in neuromuscular responses. Using muscle synergy analysis, conventional onset latencies, and peak activity, we identified differences in muscle coordination between older adults who fell and those who recovered from a laboratory-induced slip. We found that subjects who fell recruited fewer muscle synergies than those who recovered, suggesting a smaller motor repertoire. During slip trials, compared with subjects who recovered, subjects who fell had delayed knee flexor and extensor onset times in the leading/slip leg, as well as different muscle synergy structure involving those muscles. Therefore, the ability to coordinate muscle activity around the knee in a timely manner may be critical to avoiding falls from slips. Unique to subjects who fell during slip trials were greater bilateral (interlimb) muscle activation and the recruitment of a muscle synergy with excessive coactivation. These differences in muscle coordination between subjects who fell and those who recovered could not be explained by differences in gait-related variables at slip onset (i.e., initial motion state) or variations in slip difficulty, suggesting that differences in muscle coordination may reflect differences in neural control of movement rather than biomechanical constraints imposed by perturbation or initial walking mechanics. These results are the first step in determining the causation of falls from the perspective of muscle coordination. They suggest that there may be a neuromuscular basis for falls that could provide new insights into treatment and prevention. Further research comparing the muscle coordination and mechanics of falls and recoveries within subjects is necessary to establish the neuromuscular causation of falls.

New & Noteworthy: A central question relevant to the prevention of falls is: How does the robust control of walking and balance break down during a fall? Previous work has focused on muscle coordination during successful balance recoveries or the kinematics and kinetics of falls. Here, for the first time, we identified differences in the spatial and temporal coordination of muscles among older adults who fell and those who recovered from an unexpected slip.
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http://dx.doi.org/10.1152/jn.00699.2016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5288485PMC
February 2017

Generalization of treadmill perturbation to overground slip during gait: Effect of different perturbation distances on slip recovery.

J Biomech 2016 Jan 22;49(2):149-54. Epub 2015 Nov 22.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, USA. Electronic address:

Treadmill-perturbation training (TM-training) may improve a person׳s fall-resistance, whereby adjusting slip distance can be a simple way to manipulate training intensity. The purpose of this study was to determine the effects of different slip distances in TM-training (12-cm vs. 18-cm) on its generalization to the recovery from a novel "free" slip during overground walking. Generalization here means the ability to apply learned skill from TM-training to slip recovery during overground walking. Thirty-six young adults in the TM_12 or the TM_18 group underwent either a 12-cm or an 18-cm slip during the treadmill walking for seven times, or in the control group were not exposed to any perturbation. Their responses were also contrasted with previously reported results from overground-perturbation training (OG-training) in which participants received either a 12-cm or an 18-cm slip during level walking with the same number of repetitions. Everyone was then exposed to the same generalization test during a novel "free" slip in overground walking. Their proactive and reactive control of stability was measured and compared. TM-training displayed a significant training effect in comparison to the control group (p<0.05), while most of the improvements were found in the reactive control of stability and were much-limited in comparison to that of OG-training. Also unlike OG-training, no significant differences were found between the results obtained from the TM_12 and the TM_18 groups (p>0.05). These results underscore the further needs to investigate the potential of the treadmill as a convenient instrument that can effectively deliver perturbation training.
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http://dx.doi.org/10.1016/j.jbiomech.2015.11.021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4793378PMC
January 2016

Intensity and generalization of treadmill slip training: High or low, progressive increase or decrease?

J Biomech 2016 Jan 26;49(2):135-40. Epub 2015 Jun 26.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, USA. Electronic address:

The purpose of this study was to investigate the efficacy of different treadmill slip training protocols on the transfer of reactive and proactive control of center of mass stability to a novel, over-ground slip. Four training protocols were investigated: high-intensity (HI; acceleration of all treadmill slips=12m/s(2)), low-intensity (LO; acceleration of all treadmill slips=6m/s(2)), progressively increasing intensity (INCR; acceleration of treadmill slips increasing from 6m/s(2) to 12m/s(2) over the course of training), and progressively decreasing intensity (DECR; acceleration of treadmill slips decreasing from 12m/s(2) to 6m/s(2) over the course of training). From a pool of 36 young subjects, nine were randomly assigned to each training protocol (HI, LO, INCR, and DECR). In each protocol, subjects underwent a series of 24 treadmill slips before they experienced a novel slip during over-ground walking. Measures from these subjects were compared across groups and to data from control subjects (CTRL, n=9) who had experienced a novel over-ground slip without treadmill training as part of a previous experiment. The results showed that treadmill slip training improved balance control on over-ground slip and had a larger effect on subjects׳ reactive control of stability (44.3%) than on proactive control (27.1%) in comparison with the CTRL group. HI yielded stronger generalization than LO, while INCR was only marginally better than DECR. Finally, the group means of stability displayed a clear ascending order from CTRL, LO, DECR, INCR, to HI.
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http://dx.doi.org/10.1016/j.jbiomech.2015.06.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4691437PMC
January 2016

Can sacral marker approximate center of mass during gait and slip-fall recovery among community-dwelling older adults?

J Biomech 2014 Dec 30;47(16):3807-12. Epub 2014 Oct 30.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, USA. Electronic address:

Falls are prevalent in older adults. Dynamic stability of body center of mass (COM) is critical for maintaining balance. A simple yet accurate tool to evaluate COM kinematics is essential to examine the COM stability. The purpose of this study was to determine the extent to which the COM position derived from body segmental analysis can be approximated by a single (sacral) marker during unperturbed (regular walking) and perturbed (gait-slip) gait. One hundred eighty seven older adults experienced an unexpected slip after approximately 10 regular walking trials. Two trials, the slip trial and the preceding regular walking trial, monitored with a motion capture system and force plates, were included in the present study. The COM positions were calculated by using the segmental analysis method wherein, the COM of all body segments was calculated to further estimate the body COM position. These body COM positions were then compared with those of the sacral marker placed at the second sacral vertebra for both trials. Results revealed that the COM positions were highly correlated with those of the sacrum׳s over the time intervals investigated for both walking (coefficient of correlation R>0.97) and slip (R>0.90) trials. There were detectable kinematic difference between the COM and the sacral for both trials. Our results indicated that the sacral marker can be used as a simple approximation of body COM for regular walking, and to somewhat a lesser extent, upon a slip. The benefits from the simplicity appear to overweigh the limitations in accuracy.
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http://dx.doi.org/10.1016/j.jbiomech.2014.10.027DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4469384PMC
December 2014

Can stability really predict an impending slip-related fall among older adults?

J Biomech 2014 Dec 14;47(16):3876-81. Epub 2014 Oct 14.

Department of Physical Therapy, University of Illinois at Chicago, 1919 West Taylor St., Room 426 (M/C 898) Chicago, IL 60612, USA. Electronic address:

The primary purpose of this study was to systematically evaluate and compare the predictive power of falls for a battery of stability indices, obtained during normal walking among community-dwelling older adults. One hundred and eighty seven community-dwelling older adults participated in the study. After walking regularly for 20 strides on a walkway, participants were subjected to an unannounced slip during gait under the protection of a safety harness. Full body kinematics and kinetics were monitored during walking using a motion capture system synchronized with force plates. Stability variables, including feasible-stability-region measurement, margin of stability, the maximum Floquet multiplier, the Lyapunov exponents (short- and long-term), and the variability of gait parameters (including the step length, step width, and step time), were calculated for each subject. Sensitivity of predicting slip outcome (fall vs. recovery) was examined for each stability variable using logistic regression. Results showed that the feasible-stability-region measurement predicted fall incidence among these subjects with the highest sensitivity (68.4%). Except for the step width (with an sensitivity of 60.2%), no other stability variables could differentiate fallers from those who did not fall for the sample included in this study. The findings from the present study could provide guidance to identify individuals at increased risk of falling using the feasible-stability-region measurement or variability of the step width.
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http://dx.doi.org/10.1016/j.jbiomech.2014.10.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4469383PMC
December 2014

Association between anthropometric factors and falls in community-dwelling older adults during a simulated slip while walking.

J Am Geriatr Soc 2014 Sep;62(9):1808-10

Institute of Physical Education, Taiyuan University of Technology, Taiyuan, China.

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http://dx.doi.org/10.1111/jgs.13016DOI Listing
September 2014

Adaptive control of center of mass (global) motion and its joint (local) origin in gait.

J Biomech 2014 Aug 11;47(11):2797-800. Epub 2014 Jun 11.

Department of Physical Therapy, University of Illinois at Chicago, 1919 West Taylor Street, Room 426 (M/C 898), Chicago, IL 60612, USA. Electronic address:

Dynamic gait stability can be quantified by the relationship of the motion state (i.e. the position and velocity) between the body center of mass (COM) and its base of support (BOS). Humans learn how to adaptively control stability by regulating the absolute COM motion state (i.e. its position and velocity) and/or by controlling the BOS (through stepping) in a predictable manner, or by doing both simultaneously following an external perturbation that disrupts their regular relationship. Post repeated-slip perturbation training, for instance, older adults learned to forward shift their COM position while walking with a reduced step length, hence reduced their likelihood of slip-induced falls. How and to what extent each individual joint influences such adaptive alterations is mostly unknown. A three-dimensional individualized human kinematic model was established. Based on the human model, sensitivity analysis was used to systematically quantify the influence of each lower limb joint on the COM position relative to the BOS and the step length during gait. It was found that the leading foot had the greatest effect on regulating the COM position relative to the BOS; and both hips bear the most influence on the step length. These findings could guide cost-effective but efficient fall-reduction training paradigm among older population.
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http://dx.doi.org/10.1016/j.jbiomech.2014.06.001DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4184911PMC
August 2014

Perturbation training can reduce community-dwelling older adults' annual fall risk: a randomized controlled trial.

J Gerontol A Biol Sci Med Sci 2014 Dec 24;69(12):1586-94. Epub 2014 Jun 24.

Department of Biomedical and Health Information Sciences, University of Illinois at Chicago.

Background: Previous studies indicated that a single session of repeated-slip exposure can reduce over 40% of laboratory-induced falls among older adults. The purpose of this study was to determine to what degree such perturbation training translated to the reduction of older adults' annual falls risk in their everyday living.

Methods: Two hundred and twelve community-dwelling older adults (≥65 years old) were randomly assigned to either the training group (N = 109), who then were exposed to 24 unannounced repeated slips, or the control group (N = 103), who merely experienced one slip during the same walking in the same protective laboratory environment. We recorded their falls in the preceding year (through self-reported history) and during the next 12 months (through falls diary and monitored with phone calls).

Results: With this single session of repeated-slip exposure, training cut older adults' annual risk of falls by 50% (from 34% to 15%, p < .05). Those who experienced merely a single slip were 2.3 times more likely to fall during the same 12-month follow-up period (p < .05) than those who experienced the 24 repeated slips. Such training effect was especially prominent among those who had history of falls.

Conclusion: A single session of repeated-slip exposure could improve community-dwelling older adults' resilience to postural disturbances and, hence, significantly reduce their annual risk of falls.
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http://dx.doi.org/10.1093/gerona/glu087DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4296119PMC
December 2014

Reduced intensity in gait-slip training can still improve stability.

J Biomech 2014 Jul 26;47(10):2330-8. Epub 2014 Apr 26.

Department of Physical Therapy, University of Illinois, Chicago, IL 60612, USA. Electronic address:

Perturbation training with "free" slips (i.e., with long slip distance) has been able to successfully improve stability and to reduce the incidence of falls among older adults. Yet, it is unclear whether a highly constrained training with reduced slip distance (and hence training intensity) can achieve similar effects. The purpose of this study was to investigate whether short-distance slips could also improve the control of stability, and whether such improvements could be generalized to a novel, "free" slip. Thirty-six young subjects were randomly assigned to either one of the two training groups, which underwent seven training trials with constrained slips of either 12-cm or 18-cm in distance before encountering a novel, "free" slip (up to 150 cm) in the test trial; or the control group, which only experienced the same test trial of a novel, "free" slip. The results showed that while both training groups were able to significantly improve their control of stability in training; the 18-cm group had significantly better reactive control of stability than the 12-cm group. During the "free" slip, such advantage enabled the 18-cm group to exhibit significantly less balance loss incidence than 12-cm group (58.3 vs. 83.3%) and the controls (100%). These differences could be fully accounted for when we assume that the central nervous system directly controls slip velocity or slip distance during adaptation, whereby the level of similarity between training trials and the test trial governs the degree of generalization. The findings that low intensity training may still improve stability warrant further investigations among older adults.
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http://dx.doi.org/10.1016/j.jbiomech.2014.04.021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4098761PMC
July 2014

Learning from laboratory-induced falling: long-term motor retention among older adults.

Age (Dordr) 2014 Jun 26;36(3):9640. Epub 2014 Mar 26.

Department of Physical Therapy (MC 898), University of Illinois at Chicago, 1919 W. Taylor Street, Fourth Floor, Chicago, IL, 60612, USA,

Falls in older adults are a major health and societal problem. It is thus imperative to develop highly effective training paradigms to reduce the likelihood of falls. Perturbation training is one such emerging paradigm known to induce shorter term fall reduction in healthy young as well as older adults. Its longer term benefits are not fully understood, however. The purpose of this study was to determine whether and to what degree older adults could retain their fall-resisting skills acquired from a single perturbation training session. Seventy-three community-dwelling older adults (≥65 years) received identical single-session perturbation training consisting of 24 slips. This was delivered through unannounced unlocking (and mixed with relocking) of low-friction movable sections of the walkway. A single retest was subsequently scheduled based on a three-stage sequential, pre-post-retest design. Outcome measurements, taken upon the first (novel) and the 24th (final) slips of the initial session and the retest slip, included fall-or-no-fall and stability (quantified by the shortest distance from relative motion state of the center-of-mass and the base-of-support to the limits of stability) at instants prior to (proactive) and after (reactive) the onset of the slip. The training boosted subjects' resilience against laboratory-induced falls demonstrated by a significant reduction from 42.5 % falls on the first slip to 0 % on the 24th slip. Rate of falls which occurred during the laboratory retest remained low in 6-month (0 %), 9-month (8.7 %), and 12-month retest (11.5 %), with no significant difference between the three time intervals. Such reduction of laboratory-induced falls and its retention were attributable to the significant training-induced improvement in the proactive and reactive control of stability. This unique pre-post-retest design enabled us to provide scientific basis for the feasibility of a single session of perturbation training to "inoculate" older adults and to reduce their annual risk of falls in everyday living.
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http://dx.doi.org/10.1007/s11357-014-9640-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4082608PMC
June 2014

Generalization of treadmill-slip training to prevent a fall following a sudden (novel) slip in over-ground walking.

J Biomech 2013 Jan 8;46(1):63-9. Epub 2012 Nov 8.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, USA.

The purposes of the study were to determine (1) whether treadmill-slip training could reduce the likelihood of falls during a novel slip in over-ground walking, and (2) to what extent such (indirect) training would be comparable to (direct) over-ground-slip training. A treadmill-slip training group (Group A, n=17) initially experienced repeated perturbations on treadmill intended to simulate forward-slip in over-ground walking. Perturbation continued and its intensity reduced when necessary to ensure subjects' successful adaptation (i.e., when they could land their trailing foot ahead of the slipping foot in at least 3 of 5 consecutive trials). They then experienced a novel slip during over-ground walking. Another 17 young adults in Group B experienced an identical novel slip that served as the controls. They then underwent more slip trials during over-ground walking. Their 16th slip trial was analyzed to represent the over-ground-slip training effect. Eight subjects (47%) in Group A fell upon their first treadmill slip, while all adapted successfully after a minimum of 15 slip trials. Upon the novel slip during over-ground walking, none of them fell in comparison to four subjects (23.5%) fell in Group B upon the same trial (p<0.05). Group A's control of stability, both proactive and reactive, was significantly better than that of Group B's on their first over-ground slip, while the level of improvement in the control of stability derived from indirect treadmill training was not as strong as that from direct over-ground-slip training, as demonstrated in Group B's 16th slip trial (p<0.001). These results clearly demonstrated the feasibility of fall reduction through treadmill-slip training.
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http://dx.doi.org/10.1016/j.jbiomech.2012.10.002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3532900PMC
January 2013

Effect of externally cued training on dynamic stability control during the sit-to-stand task in people with Parkinson disease.

Phys Ther 2013 Apr 8;93(4):492-503. Epub 2012 Nov 8.

Department of Physical Therapy, College of Applied Health Science, University of Illinois at Chicago, Chicago, Illinois 60612, USA.

Background: Previous studies have shown that people with Parkinson disease (PD) have difficulty performing the sit-to-stand task because of mobility and stability-related impairments. Despite its importance, literature on the quantification of dynamic stability control in people with PD during this task is limited.

Objective: The study objective was to examine differences in dynamic stability control between people with PD and people who were healthy and the extent to which externally cued training could improve such control during the sit-to-stand task in people with PD.

Design: This was a quasi-experimental controlled trial.

Methods: The performance of 21 people with PD was compared with that of 12 older adults who dwelled in the community. People with PD were randomly assigned to 2 groups: a group that did not receive training and a group that received audiovisually cued training (3 times per week for 4 weeks) for speeding up performance on the sit-to-stand task. Outcome measures recorded at baseline and after 4 weeks included center-of-mass position, center-of-mass velocity, and stability against either backward or forward balance loss (backward or forward stability) at seat-off and movement termination.

Results: Compared with people who were healthy, people with PD had greater backward stability resulting from a more anterior center-of-mass position at seat-off. This feature, combined with decreased forward stability at movement termination, increased their risk of forward balance loss at movement termination. After training, people with PD achieved greater backward stability through increased forward center-of-mass velocity at seat-off and reduced the likelihood of forward balance loss at movement termination through a posterior shift in the center-of-mass position.

Limitations: The study applied stability limits derived from adults who were healthy to people with PD, and the suggested impact on the risk of balance loss and falling is based on these theoretical stability limits.

Conclusions: For people with PD, postural stability against backward balance loss at task initiation was increased at the expense of possible forward balance loss at task termination. Task-specific training with preparatory audiovisual cues resulted in improved overall dynamic stability against both forward and backward balance loss.
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http://dx.doi.org/10.2522/ptj.20100423DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3613339PMC
April 2013

Adaptive control reduces trip-induced forward gait instability among young adults.

J Biomech 2012 Apr 28;45(7):1169-75. Epub 2012 Feb 28.

Department of Physical Therapy, University of Illinois at Chicago, Chicago, IL 60612, USA.

A vital functional plasticity of humans is their ability to adapt to threats to posture stability. The purpose of this study was to investigate adaptation to repeated trips in walking. Sixteen young adults were recruited and exposed to the sudden (electronic-mechanical) release of an obstacle, 11-cm in height, in the path of over ground walking during the mid-to-late left swing phase. Although none of the subjects fell on the first of eight unannounced, consecutive trips, all of them had to rely on compensatory step with a step length significantly longer than their regular to reduce their instability. In the subsequent trials, they were able to rapidly make adaptive adjustments in the control of their center-of-mass (COM) stability both proactively and reactively (i.e., before and after hitting or crossing the obstacle), such that the need for taking compensatory step was substantially diminished. The proactive adaptations included a reduced forward COM velocity that lessened forward instability in mid-to-late stance and an elevated toe clearance that reduced the likelihood of obstacle contact. The reactive adjustments were characterized by improved trunk control (by reducing its forward rotation) and limb support (by increasing hip height), and reduced forward instability (by both the posterior COM shift and the reduction in its forward velocity). These findings suggest that young adults can adapt appropriately to repeated trip perturbations and to reduce trip-induced excessive instability in both proactive and reactive manners.
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http://dx.doi.org/10.1016/j.jbiomech.2012.02.001DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3378399PMC
April 2012
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