Variability and Specificity in Reactive Stabilization Movements to Diverse Slip Perturbations
Phase: NA
Conditions: Slip Perturbations
Sponsor: University of Nebraska

Variability and Speciﬁcity in Reactive Stabilization Movements to Diverse Slip Perturbations 
NCT03755336 
04-20-2021  
 
 

Study Protocol 
For all gait collections, participants will arrive at the Biomechanics Research Building on the 
University of Nebraska at Omaha campus. All participants will be consented and screened prior to 
gait testing. During the consent process, participants will be asked if they can be contacted again 
for future research studies. During the screening, participants will undergo a short medical history 
and a physical activity questionnaire. participants will be asked to report their previous falls before 
the intervention. Participants will also be asked to be contacted through a phone call 6-months 
post intervention to record their fall history within the 6 months period. 
Participants will be asked to put on a form ﬁtting suit for data collection.  Body weight, height and 
extremity anthropometry will be collected (length of arm and leg segments). Retroreﬂective 
markers will be placed on the legs, arms, torso and head to obtain whole- body kinematic data. A 
ceiling-mounted safety harness will also be ﬁtted to each participant. Prior to performing any 
walking trials, the participants will be allowed to "sit" in the harness to experience how it feels when 
it catches them. 
On the initial visit, participants will perform informed consent, then be asked to provide 
demographic information (age, gender), height and body mass measurements, and a medical 
questionnaire to assess exclusion criteria. Questionnaires to assess for physical activity, and 
previous falls will be administered. Upper and lower body strength will be measured using a hand 
held dynamometer during the ﬁrst and last visits. On each of the three visits, participants will 
perform a gait variability assessment followed by a diverse slip perturbation assessment. 
Functional design and validation of a wearable apparatus for administering lifelike slip 
perturbations: To investigate slips under diverse slipping conditions, a Wearable Apparatus for 
Slipping Perturbations (WASP) has been developed and validated by students working in the 
Biomechanics Research Building under supervision of the PI. Currently, the WASP can be worn over 
standard athletic shoes and triggered remotely by the experimenter. Before the WASP is triggered, 
the participant walks with a high friction interface between the rubber external sole and laboratory 
ﬂoor. When the experimenter wirelessly triggers the WASP, the external sole releases from the 
participant’s shoe, transitioning available friction to a low friction interface between two layers of 
PTFE ﬁlm within 10 milliseconds.  Friction is reduced from a dynamic coefficient of approximately 
m=0.55 (athletic shoes on a basketball court) to m=0.10 (akin to shoes on ice). 
Gait variability assessment procedures: Before recording data, retroreﬂective markers will be 
placed on speciﬁc anatomical locations. These locations are deﬁned to create a full- body 
dynamics model. Following familiarization, subjects will walk on a treadmill for 5 minutes at a 
prescribed pace of 1.3 meters per second while whole body kinematics are collected with a 17-
camera motion capture system. Both linear and nonlinear measures of gait variability will be 
calculated to determine possible relationships between these measures and other outcome 
variables such as learning rate for reactive movements and fall rate. 
Diverse slip perturbation assessment procedures: subjects will outﬁtted with WASP and a safety 
harness connected to an overhead rail system and walk overground back and forth across a large 
motion capture area. Previous studies show that walking speed is highly correlated with some 

outcome measures, speciﬁcally variations in whole body angular momentum, gait variability 
measures, and loss of balance following a slip. 
Therefore, investigators will control gait speed across all subjects to 1.3 m/s, a typically 
comfortable speed for older adults. Participants will be informed that a slip may occur, and that if 
slipped, they should attempt to maintain balance and continue walking. Participants will then be 
slipped without warning at a randomly chosen time after 1 to 3 minutes of walking. Slips will be 
administered to both feet simultaneously during early, middle or late stance in a randomized order. 
After each slip, participants will rest seated for 5 minutes. During this rest period, motion capture 
data and force data from the harness will be downloaded, and the WASP will be outﬁtted on the 
participant for the next trial. Participants will complete four slips in each of early, middle, and late 
stance on each visit. Measures of falls, slip severity, and reactive stabilization movements will be 
calculated. Since falls will be prevented from a harness system in this study, a trial will be classiﬁed 
as a fall if the load cell in the harness system measures a peak force in excess of 30% of the 
participant's body weight. 
 
Statistical Analysis Plan 
For each outcome variable (e.g., slip severity, reactive stabilization movements, fall rates), 
normality assumptions will be assessed using Shapiro–Wilk tests and visual inspection of 
histograms and Q-Q plots. Descriptive statistics (means ± SD or medians with interquartile ranges) 
will be reported based on these assessments. 
 To determine differences between age groups (younger vs. older adults) on continuous outcomes 
(e.g., slip severity measures, reactive stabilization kinematics), linear mixed- effects models will be 
employed, with participant included as a random factor to account for repeated measurements 
within individuals. Post-hoc comparisons, when necessary, will be adjusted using Tukey or Holm–
Bonferroni corrections. 
For binary outcomes (fall/no fall, deﬁned by harness load exceeding 30% of body weight), 
generalized linear mixed-effects models (logistic regression) will be used, again with participant 
included as a random effect and age group as the primary ﬁxed factor. 
All statistical analyses will be performed using R statistical software, with signiﬁcance determined 
at α = 0.05. Any data transformations or handling of missing data will be clearly documented and 
justiﬁed.