Generalization of motor adaptation to repeated-slip perturbation across tasks.

Abstract

Similar adaptations improve both proactive and reactive control of center-of-mass (COM) stability and limb support against gravity during different daily tasks (e.g., sit-to-stand and walking) as a consequence of perturbation training for resisting falls. Yet it is unclear whether--or to what extent--such similarities actually promote inter-task generalization. The purpose of this study was therefore to determine whether young adults could indeed transfer their adaptive control, acquired from sit-to-stand-slip, to improve their likelihood of a recovery from an unannounced novel slip in walking. Subjects underwent either repeated slips during sit-to-stand before experiencing an unannounced, novel slip during walking (training group, n=20), or they received no prior training before the same gait-slip (control group, n=23). The subjects demonstrated training-induced generalization of their improved proactive control of stability in post-training (unperturbed) gait pattern that was more stable against backward balance loss than was that of their own pre-training pattern as well the gait pattern of the subjects in the control group. Upon the unannounced novel gait-slip, the training group showed significantly lower incidence of both falls and balance loss than that shown by the control, resulting from the improvements in the reactive control of limb support and slip velocity, which directly influenced the control of their COM stability. Such transfer could occur when the subjects' central nervous system recalibrates the non-task-specific, generalized representation of stability limits during the initial training to guide both their feed-forward adjustments and their feedback responses. The findings of the inter-task generalization suggests that behavioral changes induced via the perturbation training paradigm have the potential to prevent falls across the spectrum of cyclic and non-cyclic activities.

Full text

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1



GENERALIZATION OF MOTOR ADAPTATION TO REPEATED-SLIP
PERTURBATION ACROSS TASKS


Ting-Yun Wang,a,b Tanvi Bhatt,a Feng Yang,a Yi-Chung Paia,b
aDepartment of Physical Therapy & bDepartment of Kinesiology and Nutrition
University of Illinois at Chicago, Chicago, IL 60612



Running title: Inter-task generalization



Corresponding author: Yi-Chung (Clive) Pai, Ph.D.
Department of Physical Therapy (MC 898)
University of Illinois at Chicago
1919 W. Taylor Street, Fourth Floor, Chicago, Illinois 60612
Tel: +1-312-9961507; Fax: +1-312-9964583
Email: cpai@uic.edu

*Manuscript
Click here to view linked References

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ABSTRACT
1
Similar adaptations improve both proactive and reactive control of center-of-mass (COM) 2
stability and limb support against gravity during different daily tasks (e.g., sit-to-stand and 3
walking) as a consequence of perturbation training for resisting falls. Yet it is unclear 4
whether – or to what extent – such similarities actually promote inter-task generalization. 5
The purpose of this study was therefore to determine whether young adults could indeed 6
transfer their adaptive control, acquired from sit-to-stand-slip, to improve their likelihood of a 7
recovery from an unannounced novel slip in walking. Subjects underwent either repeated 8
slips during sit-to-stand before experiencing an unannounced, novel slip during walking 9
(training group, N=20), or they received no prior training before the same gait-slip (control 10
group, N=23). The subjects demonstrated training-induced generalization of their improved 11
proactive control of stability in post-training (unperturbed) gait pattern that was more stable 12
against backward balance loss than was that of their own pre-training pattern as well the gait 13
pattern of the subjects in the control group. Upon the unannounced novel gait-slip, the 14
training group showed significantly lower incidence of both falls and balance loss than that 15
shown by the control, resulting from the improvements in the reactive control of limb support 16
and slip velocity, which directly influenced the control of their COM stability. Such transfer 17
could occur when the subjects’ central nervous system recalibrates the non-task-specific, 18
generalized representation of stability limits during the initial training to guide both their 19
feed-forward adjustments and their feedback responses. The findings of the inter-task 20
generalization suggests that behavioral changes induced via the perturbation training 21
paradigm have the potential to prevent falls across the spectrum of cyclic and non-cyclic 22
activities. 23

24
Keywords: perturbation training, sit-to-stand, gait, stability, limb support, fall.25

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A vital functional plasticity of the central nervous system (CNS) is its ability to take motor 1
adaptations obtained from one situation and apply it appropriately to different ―contexts.‖
2
Such context can mean different effectors (e.g., interlimb generalization) (Bhatt and Pai, 3
2008a; Morton et al., 2001; Sainburg and Wang, 2002), different environmental constraints 4
(e.g., moveable-platform-to-slippery-floor generalization) (Bhatt and Pai, 2009), or different 5
task objectives (i.e., inter-task generalization) (Abeele and Bock, 2003; Conditt et al., 1997; 6
Lam and Dietz, 2004; Morton and Bastian, 2004; Seidler, 2004). The latter, namely the inter-7
task generalization, conventionally defined by the improvement of performance in one task 8
resulting from adaptive skills acquired from training in a different task (Schmidt and Lee, 9
2005), is especially important to clinical interventions that can focus only on a limited small 10
subset of daily activities. 11
12
Previous findings have illustrated the CNS’s ability to generalize its learned experience and
13
response to similar perturbations occurring in untrained tasks (Lam and Dietz, 2004; Morton 14
and Bastian, 2004; Seidler, 2004). As a prerequisite to generalization, motor adaptation 15
requires a recalibration of motor control to meet novel and changing sensory demands 16
(Bastian, 2008). During this process, the CNS builds and updates its corresponding 17
representation (i.e., a neural representation of the relation between motor commands and 18
movements) where the altered task variables are transformed into intrinsic (e.g., individual 19
joint angle) or extrinsic (e.g., endpoint motion) variables, to more accurately predict the 20
actual outcome using feed-forward mechanisms (Imamizu et al., 1998; Wolpert and 21
Ghahramani, 2000). Few studies have suggested that the CNS is able to code and generalize 22
motor adaptation to changing task objectives by utilizing such an internal representation 23
resulting from motor practice (Conditt et al., 1997; Shadmehr and Mussa-Ivaldi, 1994). 24
When an acquired internal representation is more generalized and not specific to certain 25

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effectors, environments, or tasks, a greater degree of motor transfer is likely to be measurable 1
(Morton et al., 2001). 2
3
In the context of posture and locomotor control, successful proactive adaptation (i.e., action 4
before onset of perturbation) would include a combination of change within the feed-forward 5
mechanism and its influence on feedback loops. Adaptation can also occur reactively (after 6
onset of perturbation) within ―feedback-error based‖ mechanisms (Atkeson, 1989), as is
7
apparent from the attenuation of both muscle activation and degree of postural sway with 8
repeated support-surface translations (Horak et al., 1989; Nashner, 1976). An appropriate 9
feed-forward mechanism can produce movements that accurately match the predicted sensory 10
consequences and involve little need for real-time feedback adjustment in error correction. 11
12
Similarly, both young and older adults were able to adapt to prevent falls and balance loss 13
after repeated exposure to slips induced during sit-to-stand and in walking (Bhatt et al., 14
2006a; Pai et al., 2010; Pavol and Pai, 2002; Pavol et al., 2004a). For both tasks, such 15
adaptive control was achieved by improving proactive and reactive control of horizontal 16
stability and vertical limb support against gravity (through both feed-forward and feedback 17
mechanisms) (Pai et al., 2010). It is unclear, however, whether such similarities actually 18
promote inter-task generalization. 19
20
The purpose of this study was therefore to determine whether young adults could transfer 21
their adaptive control acquired from a single-session of repeated-slip exposure during sit-to-22
stand to increase the likelihood of recovery from a novel slip in walking. We hypothesized 23
that these subjects were able to generalize motor adaptation to yield better proactive and 24
reactive control of gait stability and limb support (resulting in their reduced incidence of falls 25

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and balance loss in novel gait-slip) than we found in the control group, who received no such 1
training. 2
3
4
EXPERIMENTAL PROCEDURES 5
Subjects.
Forty-three young adults (26 women; 17 men; age 26±5 years; height 6
168±9 cm; mass 64±11 kg) participated in either the training or control group (training: 7
N=20; control: N=23). Although 46 subjects were recruited initially, only these 43 subjects 8
who completed the protocol and had a full data set were included for analyses. The 9
remaining three subjects were excluded because of incomplete training or missing data (see 10
Perturbation training during sit-to-stand). All subjects gave informed consent and were 11
given full and careful explanation of the purposes and procedures in the study. The study 12
was approved by the Institutional Review Board. None of the subjects had histories of 13
neurological, musculoskeletal, or other systemic disorders that would have affected their 14
postural control. All were right-leg dominant (as determined by self-report of a preference to 15
kick a ball with the right leg). 16
17
Experimental setup.
The experimental setup was similar to that of our previous 18
studies (Bhatt et al., 2005; Pavol et al., 2002) (Fig. 1). The stool with adjustable seat level 19
(50 ~ 62 cm) was bolted to a transducer (MC3A-6-250, AMTI, Newton, MA) and was 20
supported by a specially-built platform. A slip was induced by a side-by-side pair of low- 21
friction movable platforms (65 × 30 × 0.6 cm, coefficient of friction < 0.05), each of which 22
was mounted on a frame with two rows of linear bearings. The frame was bolted onto two 23
force plates (OR6-7-1000, AMTI, Newton, MA) to measure the ground reaction force. The 24
platforms were free to slide 150 cm and 90 cm forward for the right and left, respectively, 25

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when unlocked by a computer-controlled release mechanism. During sit-to-stand, the slip 1
was induced shortly after seat-off by simultaneously unlocking both platforms when the stool 2
supported less than 10% body weight and the forward velocity of the person’s body center-3
of-mass (COM) exceeded 20 cm/s, to standardize the training conditions. Platform release 4
data were computed in real-time from force plates and the transducer beneath the stool, using 5
a computer program written in LabView (National Instruments Inc., Austin, TX). During 6
walking, the slip was induced by computer-controlled unlocking of the moveable platforms at 7
touchdown of the slipping foot. The subjects wore a full-body safety harness and their own 8
athletic shoes. The length of the rope that connected the harness to the overhead low-friction 9
trolley was adjusted so the body parts above the ankles could not touch the ground. A load 10
cell measured the force exerted on the harness. 11
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Perturbation training during sit-to-stand.
The training group first underwent a 13
regular walking protocol the same as the controls for baseline measure before any slip or their 14
formal training (Fig. 2). After a 5-minute rest break, with one foot placed on each platform, 15
they sat on the stool in a standard position with trunk straight, ankles dorsiflexed at 10°, 16
knees flexed at 100°, and elbows flexed at 90° (Pavol and Pai, 2002). They were asked to 17
stand up ―as fast as possible‖ upon an auditory cue, without using their arms, and to maintain
18
the standing position. They only knew that a slip would occur later on. After 5 nonslip (i.e.,
19
regular sit-to-stand) trials, a slip was induced without warning. 20
21
After that first slip, the subjects were informed that they ―may or may not‖ experience more 22
slips during the subsequent trials. They were also told that they should ―try not to fall‖ upon
23
a slip and remain standing still. The training protocol included a block of 5 consecutive slip 24
trials (including the first slip). This was followed by a block of 3 nonslip trials; another block 25

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of 5 consecutive slip trials; the second block of 3 nonslip trials, and a mixed block of slip and 1
nonslip trials (consisted of 4 slip and 3 nonslip trials interspersed: see Fig. 2b). The initial 2
block of repeated slips was for the CNS to have the opportunity to establish a prediction-3
error-based baseline, whereby the second block of slips was to strengthen the adaptive effects 4
and reduce the washout effect (Bhatt et al., 2006b; Pavol and Pai, 2002). The mixed block 5
given at the end was to introduce uncertainty, and thus the CNS had to take into account for 6
different conditions in order to optimize the performance (Izawa et al., 2008). 7
8
Because successful adaptation is a prerequisite for generalization just as it is for retention, all 9
subjects were expected to have successful recovery (no falls) from slips in the mixed block. 10
Two subjects failed to meet this criterion. Previous findings have established that subjects 11
can adapt to sit-to-stand-slips in 5 slips (Pavol and Pai, 2002) and that those persons who did 12
not adapt well will have poor retention (Bhatt et al., 2006b). It is conceivable these two 13
subjects might have benefited from additional practice to acquire proper motor adaptation, 14
but that is beyond the scope of the current design. Because of the small sample size, on the 15
other hand, it is impossible to investigate any transfer effect that remained with them. These 16
two subjects, therefore, were excluded from further testing and analysis. One subject in the 17
training group with a misplaced hip marker was also excluded from data analysis. 18
19
The control and the generalization test in gait-slip.
The control group was instructed 20
to walk a block of 8 trials at their self-selected pace and told that they might experience a slip 21
later on. They were told to try not to fall and continue walking in the case of a slip. On the 22
9th trial, a slip was induced on the right side without warning (Fig. 2a). After the sit-to-stand- 23
slip, the subjects in the training took a 5~10 min rest break. They were then asked to perform 24
unperturbed walking trials, and were told that they might experience a slip later on without 25

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warning. They should ―try not to fall,‖ and continue walking in the case of a slip. A slip was 1
induced on the right side on the 6th trial (Fig. 2b). 2
3
COM state stability and limb support. The data of 28 light-reflective markers 4
placed on bilateral upper and lower extremities, torso, and platforms, was recorded at 120 Hz 5
using an 8-camera motion capture system (Motion Analysis Corporation, Santa Rosa, CA). 6
Marker displacement data was lowpass filtered at marker-specific cut-off frequencies (range 7
4.5~9 Hz) using fourth-order Butterworth filters. Force plate data, harness-load cell data, and 8
trigger-release onset signals were collected at 600 Hz and synchronized with motion data at 9
the time of data collection. 10
11
The COM position and velocity in the sagittal plane were calculated using a 13-segment rigid 12
body model (de Leva, 1996). The COM position (XCOM/BOS) was defined as the absolute 13
COM position in anteroposterior direction relative to the rear of base-of-support (BOS). The 14
COM velocity (VCOM/BOS) was calculated from differentiation of COM position and 15
normalized to g bh

, where g was the acceleration due to gravity and bh was height of the 16
subject (McMahon, 1984). 17
18
Stability was quantified by measuring the shortest distance between the instantaneous COM 19
motion state (i.e., the COM position and velocity relative to the BOS) and the predicted 20
feasible stability region (FSR) limits for backward balance loss under slip conditions (Pai and 21
Iqbal, 1999; Yang et al., 2007, 2008). Greater values indicate greater stability against 22
backward balance loss and falls (Bhatt et al., 2005; Espy et al., 2010; Pai et al., 2003). Limb 23
support was measured by the instantaneous vertical height of the midpoint between a 24
person’s two hips and normalized by subjects’ height (Pai et al., 2006; Pavol and Pai, 2007).
25

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A lower hip height indicates poorer limb support against gravity, which is the primary 1
causative factor for falls (Yang et al., 2009). 2
3
In both sit-to-stand and gait, several variables were obtained at pre- and postslip instants to 4
document changes in proactive and reactive control, respectively (i.e. actions that occur 5
respectively before and after the perturbation). During sit-to-stand-slip, proactive adaptation 6
was examined by measuring preslip stability, the horizontal COM velocity at seat-off, and hip 7
height, which were obtained at the instant seat-off (20 ms before onset of slip perturbation). 8
Seat-off was defined as the time at which the vertical force on the stool dropped below 10% 9
body weight. Reactive adaptation was examined by measuring postslip stability and hip 10
height at 300 ms after slip onset, an instance determining outcomes of fall vs. recoveries at up 11
to 70% accuracy (Pai et al., 2010). During walking, proactive adaptation was examined by 12
measuring preslip stability, gait speed (i.e., the absolute COM velocity in anteroposterior 13
direction), and hip height at touchdown of the slipping limb (i.e., the right limb, RTD), which 14
occurred 30 ms before slip onset. Reactive adaptation during gait was examined by 15
measuring postslip stability and hip height at recovery (left) foot liftoff (LLO) and 16
touchdown (LTD). The instance of the recovery-foot touchdown was close to 300 ms after 17
slip onset, the time instant chosen for examining reactive changes during sit-to-stand-slip. 18
The instance of 300 ms after slip onset was chosen for the sit-to-stand-slip instead of the 19
recovery foot touchdown because some subjects did not take a recovery step to regain their 20
balance during reslips in the sit-to-stand-slip. 21
22
Kinematic measures.
The following variables were analyzed to increase our 23
understanding of the factors contributing to the adaptive changes in stability control (Bhatt et 24
al., 2006a): step length, foot angle, and the slip (BOS) velocity. Step length affected the 25

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COM location at RTD and was measured as the distance between heel markers of the leading 1
foot and the contralateral foot (normalized to subjects’ body height). The foot angle at RTD 2
influenced subsequent braking impulses and was measured as the angle between foot segment 3
(line joining the heel and fifth metatarsal) and the horizontal. The BOS velocity directly 4
affected the relative velocity between the COM and BOS; it was obtained from the velocity 5
of the sliding platform marker. During the sit-to-stand-slip, the greater of the two maximum 6
velocities attained by the markers on the two sliding platforms represented the peak BOS 7
velocity. During gait-slip, the BOS velocity was obtained from the velocity of the right 8
sliding platform marker at LLO and LTD after slip onset. 9
10
Outcome of the slip trials. For sit-to-stand-slip, falls occurred when the midpoint 11
between the hips descended to within 5% body height of its initial seated height. Outcomes 12
were classified as recoveries if the mean force on the safety harness did not exceed 4.5% of 13
body weight over any one-second period (Pavol and Pai, 2002). Backward balance loss 14
occurred when the subject took a backward recovery step. Forward balance loss occurred 15
when the subject took a forward recovery step. The remainder of the trials (i.e., subjects 16
recovered but did not step to regain balance) were classified as ―no loss of balance.‖ 17
18
For gait-slip, the outcome of slip was classified as a fall if the peak load cell force during the 19
slip trial exceeded 30% body weight (Yang and Pai, 2011). A full recovery occurred when 20
the average load cell force on the harness did not exceed 4.5% of body weight over any 1-21
second period after slip onset (Yang and Pai, 2011). Backward loss of balance occurred 22
when subjects place their contralateral limb posterior to the slipping heel upon landing. The 23
trials during which subjects landed their contralateral limb anterior to the slipping heel were 24
classified as ―no loss of balance‖ (Bhatt et al., 2006a). To guarantee the rigor of the study, 25

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the subjects would have been considered as ―harness-assisted‖ and excluded from further
1
analysis if their load cell force exceeded the 4.5% criteria in both activities. 2
3
Statistics.
To examine training effects during sit-to-stand, slip outcomes, and 4
adaptive changes in pre- and postslip stability, the horizontal COM velocity at seat-off, peak 5
BOS velocity, and hip height of the first (S1) and last (S14) slips were compared by 6
Wilcoxon signed rank test and paired-t tests respectively. To further evaluate feed-forward 7
adjustments in COM state stability, a one-way repeated-measure ANOVA with post-hoc 8
paired-t test with Bonferroni corrections was conducted to compare stability at seat-off 9
among the following trials: S1; S5 (the last slip trial of the first slip block); N1 (the first 10
nonslip trial of the first nonslip block); and S14. To determine whether the training group 11
and the controls behaved the same way at the baseline, seven variables taken at RTD of the 12
baseline measures were compared: the COM position and velocity, stability, gait speed, hip 13
height, step length, and foot angle. To examine the generalization of the training effect on 14
post-training unperturbed gait, the same seven variables were compared to those from the 15
pre-training with paired t-tests, and to those from the control with t-tests (Fig. 2). 16
17
To examine the generalization of the training effect on responses to a novel, unannounced 18
gait-slip (Fig. 2), the outcomes of the training group were compared to those of the controls 19
by Chi-square test. A 2-by-2 mixed model ANOVA, with group (training and control) as 20
between factor and event (LLO and LTD) as the repeated factor, was conducted to compare 21
the difference in postslip stability, BOS velocity, and hip height. Post-hoc independent t-tests 22
and paired t-tests were performed to compare between-group and within-group differences 23
followed by significant main effect or interaction. All analyses were performed using SPSS 24
(V.15.0, SPSS Inc, IL). 25

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1

2
RESULTS 3
Adaptation to sit-to-stand-slip.
All subjects experienced backward balance loss 4
when the slip was first induced during sit-to-stand. Thirty-five percent of the subjects in the 5
training group fell on the first slip trial (S1) during sit-to-stand. They were able to adapt 6
rapidly and thus to reduce falls to 0% on the 3rd slip (S3) and thereafter (P<0.01). This was 7
initially achieved by overcompensation, that all subjects experience a forward balance loss 8
when the slip stopped abruptly (N1 in Fig. 3). However, they were able to significantly 9
reduce the incidence of backward balance loss to 25% (P<0.05) and forward balance to 0% 10
on the last slip (Fig. 3).
11

12
The decrease in the incidence of falls and balance loss resulted from improvements in 13
stability and limb support. Both pre- and postslip stability significantly increased from the 14
first (S1) to the last slip (S14) (P<0.001 and P<0.01, respectively: see Fig. 4a). The 15
horizontal COM velocity at seat-off also increased significantly, from S1 to S14 (P<0.001). 16
The improvement in pre- and postslip stability was paralleled by decreased peak BOS 17
velocity after slip onset (P<0.001) (Fig. 4b). Notably, preslip stability against backward 18
balance loss significantly increased, from S1 to S5 and to N1 (P<0.001), and also increased 19
the risk for forward balance loss (Fig. 3). The re-adjustments came immediately thereafter 20
(i.e., preslip stability significantly decreased from N1 to S14, P<0.01) such that the COM 21
state was located within the shaded area – an area that has been predicted to be stable under 22
both slip and nonslip conditions (Fig. 5). In the end, preslip stability on S14 was still greater 23
than that of S1 (P<0.001). In contrast to the stability findings, only postslip hip height 24
showed significant increase from the first to the last slip (P<0.05) (Fig. 4c). 25

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1
Regular walking and unperturbed gait.
There were no significant between-group 2
differences in the baseline measurements [the COM position (P=0.786), the COM velocity 3
(P=0.573), stability (P=0.540), gait speed (P=0.633), hip height (P=0.965), step length 4
(P=0.631), and foot angle (P=0.607)]. The subjects in the training group demonstrated 5
noticeable generalization evidenced by increased stability at RTD and changes of gait pattern 6
during unperturbed gait. More forward COM position (P<0.01), greater stability (P<0.001), 7
shorter step (P<0.05), smaller foot angle (i.e., landing more flat-footed) (P<0.001) were 8
found in post-training unperturbed gait in comparison to those in pre-training regular walking 9
(Fig. 6a, 6c, 6e, and 6f), yet no difference was found in COM velocity (P=0.086) and hip 10
height (P=0.069) during post-training unperturbed gait (Fig. 6b and 6d). However, there was 11
no difference in the gait speed between the pre-training regular walking and post-training 12
unperturbed gait (P=0.091). The generalization effect was also evidenced by more forward 13
COM position (P<0.05), greater stability (P<0.01), and smaller foot angle (P<0.01) at RTD 14
(Fig. 6a, 6c, and 6f) demonstrated on post-training unperturbed gait of the training group in 15
comparison to those on regular walking of the control group. No difference was found in the 16
gait speed on post-training unperturbed gait of the training group in comparison to that 17
recorded during regular walking of the control group (P=0.306). 18
19
Generalization in gait-slip.
The training group demonstrated a measurable 20
generalization effect in responding to a novel gait-slip, as evidenced by better slip outcomes 21
and improved reactive control of stability and limb support. The training group showed a 22
significant decrease in both the incidence of falls and backward balance loss after 23
perturbation training during sit-to-stand. Significant improvement was found in the incidence 24
of backward balance loss; only 75% of the subjects in the training group lost their balance in 25

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gait-slip as compared to 100% balance loss in the control group (P<0.05). The five people 1
from the training group did exceptionally well by not even losing their balance in gait-slip. 2
While 26% of the subjects in the control group fell during gait-slip, none of the subjects in 3
the training group fell (P<0.05). 4

5
The postslip stability had a significant group-by-event interaction (F(1,41)=4.828, P<0.05), 6
and a significant group main effect (F(1,41)=19.159, P<0.001). The training group 7
demonstrated greater stability at both LLO (P<0.001) and LTD (P<0.01) as compared to the 8
control group (Fig. 7a). Postslip stability remained constant from LLO to LTD for the 9
training group (P=0.247), but decreased significantly from LLO to LTD for the control group 10
(P<0.001) (Fig. 7a). The BOS velocity demonstrated significant group and event main 11
effects (F(1,41)=13.977, P<0.01 and F(1,41)=23.603, P<0.001, respectively) but no group-12
by-event interaction (F(1,41)=0.350, P=0.557). The training group had lower BOS velocity 13
than did the control group at both LLO (P<0.001) and LTD (P<0.01) (Fig. 7b). The postslip 14
hip height demonstrated significant group-by-event interaction (F(1,41)=10.12, P<0.01). The 15
training group’s postslip hip height was still not different from that of the control at LLO 16
(P=0.232), but it became significantly higher later at LTD (P<0.05) (Fig. 7c).
17
18
Elimination of those five exceptional subjects did not affect preslip stability; thus the 19
remaining subjects still demonstrated greater preslip stability as compared to the control 20
subjects (P<0.05). Their elimination would have affected postslip stability (Fig. 7a) and BOS 21
velocity (Fig. 7b), but not limb support (Fig. 7c). Both training and control groups would 22
have showed a significant decrease in stability and an increase in BOS velocity from LLO to 23
LTD (all P<0.05); however, the training group still had greater stability at both LLO and 24
LTD (P<0.001 and P<0.05, (respectively) and lower BOS velocity at LLO (P<0.01) as 25

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compared to the control (Fig. 7a and 7b). Finally, none of the outcomes was eliminated due 1
to harness-assisted. 2
3
4
DISCUSSION
5
The findings demonstrated that young adults were able to transfer the acquired motor 6
adaptation from sit-to-stand-slip to gait-slip; this was made evident by their lower incidence 7
of balance loss and falls than that measured in the control group on the gait-slip. It is clear 8
that this finding is a consequence of training-induced improvements in both proactive and 9
reactive control of stability and in reactive control of limb support against gravity. 10

11
Adaptation and generalization in proactive control. Our previous work has shown 12
that 3 to 5 slips were sufficient (Pavol and Pai, 2002) to achieve the objective of perturbation 13
cancellation, an essential process of motor adaptation. Depending on the type of task, initial 14
acquisition may take as few as 1-3 trials (Bhatt et al., 2006a; Bunday et al., 2006; Lang and 15
Bastian, 1999; Morton and Bastian, 2004; Pavol and Pai, 2002), yet it can require trials 16
numbering in the hundreds (Conditt et al., 1997; Shadmehr and Mussa-Ivaldi, 1994). Motor 17
adaptations to artificial force fields and prism glasses do not occur in everyday living. Thus, 18
the adaptation process and subsequent learning takes longer, as motor commands are 19
relatively new (Baraduc and Wolpert, 2002; Conditt et al., 1997; Shadmehr and Mussa-Ivaldi, 20
1994). In contrast, the process of adaptation to familiar but potentially life-threatening 21
perturbations is usually very swift; in some instances, it may require only 1-3 trials (Bunday 22
et al., 2006). Such a rapid rate of adaptation in locomotion may result from factors such as 23
past real-life experience, large initial task errors, and a penalty-driven mechanism (i.e., a 24
potential of fall and subsequent deadly injury) (Bunday et al., 2006). 25