2015 IEEE International Conference on Systems, Man, and Cybernetics

Analysis of Skip Motion as a Recovery Strategy
after an Induced Trip
Kento Mitsuoka∗ , Yasuhiro Akiyama∗ , Yoji Yamada∗ , and Shogo Okamoto∗∗ Department of Mechanical Science and Engineering

Nagoya University, Nagoya, Japan
falls is necessary to determine the timing, position, and torqueof fall avoidance assists.
Tripping is the major reason for falls during gait motion [8].
Therefore, many studies have aimed to reveal the mechanism
underlying recovery motion after triggering events. In thesestudies, tripping was induced using a flat plate or block thatwas fixed on the ground [9] [10]. Through these studies, twocommon recovery strategies were observed [11]. One strategy
was referred to as the lowering strategy and was observed when
the subjects tripped during the late swing phase. The trippedfoot was placed immediately in front of the obstacle and theother leg was subsequently lifted over the obstacle and placed
behind it. The second strategy, called the elevating strategy,
was observed when the subject tripped during the early swing
phase. The tripped leg was quickly lifted over the obstacle andthen placed behind it [12].
However, the type of tripping in these experiments waslimited. Only flat obstacles were used under conditions inwhich the toe hit the obstacle in all trials. In general, tripping
does not always occur at the toe and can also occur at theshank and ankle. Thus, it is possible that different recovery
strategies are used under different conditions. In particular, a
trip at the ankle may have more of an impact because it might
be difficult to absorb the reaction force using the ankle joint
and overcome the obstacle because the foot may get stuck on
the obstacle. However, tripping caused upon the impeded ankle
has hardly been studied thus far. One specific condition wasinvestigated by [13] where a rope transiently arrested ankle
movement, which is still one limited cause of tripping.
Thus, measurement and analysis of recovery motions aftera trip induced at the ankle under the more general conditions
are required to understand the fall avoidance strategy. Suchunderstanding will help to design assist strategy of gait assistant devices such as physical assistant robots to prevent falls
among elderly individuals.

Abstract—Falls are one of the main causes of decreased qualityof life and activities of daily living among the elderly individuals.
Because tripping is the most common cause of falls, some typesof recovery motions have been reported after tripping induced
by a flat plate or block. Common recovery strategies observed
under these conditions included elevating and lowering strategies.
Especially, in case that the swinging leg contacts an obstacle atits early phase, the elevating strategy has been considered as a
typical recovery motion. In this study, we performed a tripping
experiment using a crossbar in the early and middle of swing
phase. As a result, a new recovery strategy, which differed fromthose previously reported, was observed. In the new strategy,referred to as the skip strategy, the subjects lifted and moved
the standing leg forward after tripping. The subjects utilized thisstrategy to recover from an early phase trip. The participants had
to exert enough knee flexion and ankle plantar flexion torques ofrecovery leg with the skip strategy and enough knee extension,
hip extension, and ankle plantar flexion torques of recovery leg
with the lowering strategy. Especially for the elderly who cannot
perform large joint torques, gait assistant devices such as physicalassistant robots are required to compensate such joint torques inorder to perform these recovery motions.
Index Terms—Fall, Physical assistant robot, Recovery motion,
Skip strategy, Trip

I. I NTRODUCTION
The elderly population is rapidly increasing around theworld. Aging can cause many diseases and injuries, which
can decrease the quality of life (QOL) and activities of dailyliving (ADL) of the elderly. Falls are one of the main reasonsfor injuries. In fact, approximately 10 to 30% of the elderly
experience a fall, and 10% have severe injuries and limitations
in daily activities [1]. Falls can also lead to death or to theindividual becoming bedridden [2]. Even in cases in whichthese injuries could be avoided, the fear of falling also makes
the elderly more inactive and decreases the QOL and ADL [3].
Thus, it is important to protect the elderly from falling.
More than 100 factors have been proposed to affect therisk of falling. These factors have been classified as internal(related to physical ability) or external (related to environmental conditions) [4]. These factors can influence each other andcause a fall.
In general, the elderly has less strength of muscle andjoint torques [5] [6]. It is one of the factor of falling forthe elderly. Physical assistant robots have been developed tosupport elderly to walk better [7]. However, it is difficult toassist fall avoidance motion because of the complexity of the
motion. Thus, an investigation of the critical factors that cause
978-1-4799-8697-2/15 $31.00 © 2015 IEEE
DOI 10.1109/SMC.2015.167

II. M ETHODS
A. Subjects
Two males who were 23 and 25 years old participatedin the study. The weights of the patients were 60 and 70
kg and their heights were 165 and 170 cm, respectively.
Neither participant had a history of impaired balance, falls,
neurological disease, or visual deficits. The experiments were
performed with the approval of the Institutional Review Board
911

TABLE I
N ORMAL GAIT PARAMETERS

Subject A
Subject B

During experiment
After experimentDuring experiment
After experiment

Step length [m]Step time [s]Ratio of half gait [%]Walking speed [m/s]0.79 ± 0.02
0.78 ± 0.02
0.76 ± 0.04
0.75 ± 0.04

0.51 ± 0.02
0.52 ± 0.01
0.50 ± 0.02
0.51 ± 0.03

49.6 ± 1.90
50.6 ± 0.48
47.9 ± 1.14
47.8 ± 1.42

1.51 ± 0.05
1.50 ± 0.01
1.52 ± 0.05
1.48 ± 0.03

Notes : Values are mean ± SD

of Nagoya University. Written informed consent was provided
by each subject.

Safety harness
Load cell

B. Apparatus

Speed guide䘊
Metronome

Full body kinematic data was recorded by the motion
capture system (MAC3D System, Motion Analysis Corp.)
consisting of 10 cameras with 25 reflective markers that wereattached to the entire body. A movable foot force plate (M3D
Force plate, Tec Gihan Corp.) was used to measure the groundreaction force. A load cell (RSCC-200kg, Unipulse Corp.)
monitors applied force on the safety harness. All of the devices
were synchronized and recorded with a sample frequency of
100 Hz. Three axis force sensors (USL-08-H6, Tech Gihan
Corp.) were also attached to the obstacle to detect the timingof the trip perturbation. For safety, a harness was connected tothe shoulders and waist, and a knee protector and ankle joint
supporter were utilized.
An overview of the experimental setup is shown in Fig. 1.
Tripping was induced to a leg using a stick-shaped, aluminumcrossbar as an obstacle. The position of the obstacle wasmanipulated using a linear motor actuator (GLM25-CE, THK
Corp.). A speed guide that moved in front of the subject
along the walkway was used to control walking speed. The
walking speed was set as 1.5 m/s for all subjects using a threephase induction motor (TO-K, HITACHI Corp.). An electricalmetronome was also used to control the walking cadence. The
walking cadence was determined for each subject individually.

Obstaclewith actuated linear slider
Foot force plate0.25 mWalkway 7mFig. 1.

Experimental setup

support limb were defined as middle phase trips. To avoid theprediction of tripping, the order of the dummy and trip trials,
the perturbed limbs, and the trip perturbation phases wererandomized. In addition, half-covered glasses, which prevented
the subject from visualizing what was below, were used toprevent prediction of the obstacle position.
In total, 30 trials that included 20 dummy trials (in whichtrip perturbation did not occur), five early phase trip trials andfive middle phase trip trials were performed. After all the trials
were completed, an additional five trials were recorded underconditions in which the subjects knew that no obstacle existedon the walkway. As described later, by this method, early andmiddle phases at which the ankle contacts the obstacle wereclearly discerned.

C. Protocol
The subjects wore tight sportswear and markers were puton the suits after agreement. The subjects were then instructed
to walk continuously on the walkway to get used to walking
with the speed guide and the electrical metronome and until
the gait was stable. The position of the obstacle for each trip
phase was decided based on the gait motion of the subject after
stabilization.
After the completion of the practice trials, the trip experiment was initiated. During the experiment, the subject
occasionally tripped at the ankle while walking. Trip perturbations were applied during the early and middle swing
phases. These phases were defined based on the relative
positions of the support and swing limbs. Perturbations that
occurred behind the support limb were defined as early phase
trips. In contrast, perturbations that occurred in front of theD. Data Analysis
All data was filtered using a 6 Hz Butterworth filter. Falls
were detected based on a force that was applied to the safety
harness, with a threshold value that corresponded to 5% of the
body weight. Heel contact and toe-offs were detected when
the ground reaction force exceeded 10 N. To calculate internal joint torque, the Software for Interactive Musculoskeletal
Modeling (SIMM, Mulsculographics Inc., CA) was used.

912

TABLE II
T RIAL DETAIL

Subject A
Subject B

Early phase trip

Middle phase trip

Fall

Recover

Fall

Recover

0
0

4
4

2
1

3
4

0.2










−0.4
−0.7

1
1

30
30

20
20

0.8

Obstacle

0.5
0.2



−0.1








−0.4
−0.7

−1
−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m]

1.5

2

0.5
0.2
−0.1

1.5

1.5

1.5

Obstacle0−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m]Vertical distance [m]1.5

Vertical distance [m]

1.8

0.3

1.2
0.9
0.6

Obstacle

0.3
1.5(d) Skip strategy20−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m](e) Lowering strategy

1.5

2

1.5

2

(c) Normal gait

1.8

0.6





−1
−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m]

2

1.8

0.9



−0.4

(b) Lowering strategy

1.2



−0.7

−1
−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m](a) Skip strategy

Vertical distance [m]TotalLateral distance [m]Obstacle

0.5

−0.1

Missed data

0.8
Lateral distance [m]Lateral distance [m]0.8

Dummy trial

1.2
0.9
0.6
0.3

1.5

2

0
−2 −1.5 −1 −0.5 0 0.5 1
Traveling distance [m](f) Normal gait

Fig. 2. Approximate foot contact trajectory and recovery motions of the skip strategy, the lowering strategy, and the normal gait. (a), (b), and (c) are the footcontact trajectory with step number, and (d), (e), and (f) are the motions of each strategy with trajectory of center of mass (COM). In this figure, left leg (dottedline) was tripped by obstacle and right leg (solid line) was used as recovery leg. In each strategy, the first recovery step was step number 3 in fig. 2(a) and step
number 4 in fig. 2(b).

III. R ESULTS

phase trips. Additionally, three trials were classified as falls.
Early phase trips were induced an average of 0.29 ± 0.09 m
behind the position of the support limb. Middle phase trips
were induced an average of 0.25 ± 0.09 m in front of the
support limb. Two different recovery strategies, the loweringand skip strategies, were observed in this experiment. The
approximate motions for the recovery strategies and normal
walking are shown in Fig. 2.

A. Recovery motions
The gait timing for the two subjects is shown in Table I.
As a result of the speed guide and metronome, the gait timing
varied only slightly and did not differ significantly during theexperiment. This indicated that the existence of an obstacle did
not affect the gait motion of the subjects. Thus, when analyzing
the data, it was not necessary to modulate the gait motion of
the subjects during the experiment.
As shown in Table II, 18 trip data sets were successfullyrecorded; 8 of them were early phase and 10 were middle

The lowering strategy was used after an middle phase
trip. In the lowering strategy, the tripped leg was placed on
the ground immediately after the trip and the other leg was913

TABLE III
F IRST RECOVERY STEP PARAMETERS AFTER TRIPPING

Lowering strategy

Subject A
Subject B

First recovery step length [m]First recovery step time [s]First recovery step length [m]First recovery step time [s]∗Skip strategy

Fall

Recover

Recover

1.01∗
0.55∗
0.81 ± 0.07
0.31 ± 0.11

0.90 ± 0.10
0.58 ± 0.02
0.83 ± 0.04
0.47 ± 0.04

0.52 ± 0.04
0.38 ± 0.01
0.33 ± 0.05
0.26 ± 0.01

Only one data was recorded in this case

TABLE IVCOM MAXIMUM HORIZONTAL , VERTICAL , AND ABSOLUTE VELOCITY AFTER TRIPPING

Subject A

Subject B

Skip strategy
Lowering strategy
Normal gait
Skip strategy
Lowering strategy
Normal gait

Horizontal velocity [m/s]Vertical velocity [m/s]Absolute velocity [m/s]

0.99± 0.08
1.17 ± 0.04
1.36 ± 0.09
0.94 ± 0.09
1.11 ± 0.02
1.33 ± 0.05

0.67 ± 0.09
1.22 ± 0.10
0.24 ± 0.03
0.99 ± 0.09
1.40 ± 0.11
0.26 ± 0.05

1.42 ± 0.04
2.18 ± 0.05
1.69 ± 0.10
1.54 ± 0.02
2.23 ± 0.11
1.70 ± 0.07

TABLE VM AXIMUM KICKING AND BRAKING FORCES AFTER TRIPPING

1st recovery step

Tripping

Subject A
Subject B

Skip strategy
Lowering strategy
Skip strategy
Lowering strategy

2nd recovery step

Kicking force [N]Braking force [N]Kicking force [N]Braking force [N]28.8 ± 24.3
192 ± 30.5
102 ± 22.2
205 ± 14.7

166 ± 32.0
245 ± 30.1
149 ± 78.4
248 ± 32.7

61.5 ± 32.4
157 ± 52.4
75.8 ± 49.1
122 ± 32.7

296 ± 25.9
253 ± 54.0
203 ± 40.3
195 ± 76.2

Notes : Values are mean ± SD

subsequently lifted over the obstacle and placed behind of itas the recovery leg. Instead of the elevating strategy, which wasreported previously, the skip strategy was used after early phase
trips. In the skip strategy, the standing leg which is opposite
to tripped leg, was lifted to the front and used as the recovery
leg immediately after the trip.

both subjects. In all cases involving falls, the subjects utilized
the lowering strategy for recovery. In the case of a fall, therecovery step was not significantly different from the cases
involving recovery.

B. First recovery step

According to the center of mass (COM) trajectory, whichis shown in Fig. 2, the vertical displacement of COM waslargest in case of lowering strategy. Table IV showed recorded
COM maximum horizontal, vertical, and absolute velocity
after tripping. The maximum absolute velocity of COM after
trip was different with skip strategy and lowering strategy.
Compared with the normal gait and lowering strategy, thevelocity of COM for skip strategy was smaller than that for thenormal gait and lowering strategy. In contrast, the maximum
absolute velocity of COM of lowering strategy was larger thannormal gait.

C. COM maximum absolute velocity

The recovery step time was defined as the duration between
the time when the recovery leg was lifted and when it wasplaced on the ground after the trip. The recovery step length
was defined as the distance between the heel position of therecovery leg when lifted and the heel position of the recovery
leg when it was placed on the ground after tripping. The
recovery step time and the duration of each strategy are shownin Table III. The recovery step time and the length after
the trip were significantly different. Both were much shorterfor the skip strategy compared to the lowering strategy for914

TABLE VI
E STIMATED MAXIMUM JOINT FLEXION AND EXTENSION TORQUES OF THE HIP, KNEE , AND ANKLE OF RECOVERY LEG

Flexion torques (Dorsal flexion torque for ankle)

Subject A

Subject B

Skip strategy
Lowering strategy
Normal gait
Skip strategy
Lowering strategy
Normal gait

Extension torques (Plantar flexion torque for ankle)

Subject A

Subject B

Skip strategy
Lowering strategy
Normal gait
Skip strategy
Lowering strategy
Normal gait

Tripping to 1st recovery step

1st recovery step to 2nd recovery step

Hip [Nm]

Knee [Nm]Ankle [Nm]Hip [Nm]

Knee [Nm]Ankle [Nm]6.06± 20.9
51.4 ± 3.51
19.8 ± 26.7
21.8 ± 12.3
51.2 ± 15.7
47.8 ± 20.1

97.7 ± 15.1
32.0 ± 4.90
47.4 ± 10.3
88.7 ± 15.6
35.2 ± 6.21
17.4 ± 3.90

4.32 ± 0.78
9.55 ± 1.25
6.24 ± 2.19
5.36 ± 0.72
6.44 ± 2.31
13.4 ± 1.96

16.9 ± 23.3
32.0 ± 20.4
16.3 ± 8.97
2.37 ± 24.9
48.8 ± 39.8
4.15 ± 14.6

45.7 ± 5.91
86.7 ± 16.8
56.6± 20.3
70.2 ± 26.2
67.6 ± 34.6
33.8± 4.12

4.18 ± 0.99
3.13 ± 1.76
2.30 ± 0.88
1.16 ± 2.38
0.08 ± 6.88
0.63 ± 0.63

Tripping to 1st recovery step

1st recovery step to 2nd recovery step

Hip [Nm]

Knee [Nm]Ankle [Nm]Hip [Nm]

Knee [Nm]Ankle [Nm]167± 22.9
73.8 ± 11.6
190 ± 38.6
185 ± 16.5
82.3 ± 13.9
123 ± 30.6

26.9 ± 8.33
51.5 ± 2.34
11.6 ± 14.3
24.2 ± 2.86
42.1 ± 6.98
34.9 ± 3.59

102 ± 23.6
79.0 ± 7.86
81.6 ± 7.41
106 ± 5.84
73.3 ± 8.01
68.9 ± 5.02

195 ± 41.2
349 ± 56.2
169 ± 28.3
343 ± 43.1
425 ± 54.2
189 ± 8.40

33.8 ± 8.56
98.5 ± 9.78
14.5± 7.82
11.1 ± 25.72
24.4 ± 21.8
19.0± 10.7

150 ± 7.87
146 ± 8.72
93.2 ± 5.50
100 ± 13.7
96.0 ± 11.2
74.7 ± 2.47

Notes : Values are mean ± SD

strategy were larger than those with the lowering strategy andnormal gait. The joint torque for the lowering strategy had
two different characteristics. The knee extension torque at thetime of the first recovery step after the trip was larger for the
lowering strategy than for both the skip strategy and normal
gait. The hip extension torque at the time between the firstand second recovery steps was larger for the lowering strategy
than those for the other strategies.

D. Kicking and Braking force
The differences between the recovery strategies were also
apparent in terms of the ground reaction force in the traveling
direction. The force that pushed the body in the direction of
travel was defined as the kicking force. The force exerted in the
direction opposite to travel, which decelerated forward motion,
was defined as the braking force. The maximum kicking andbraking forces for the three phases, which are defined as thetime between the trip and the first recovery step and thetime between the first and second recovery steps, are shownin Table V. During the time between the trip and the firstrecovery step, the kicking force was larger with the lowering
strategy than with the skip strategy. This indicated that theforce, which accelerated forward motion, was larger with thelowering strategy. It also occurred at the phase between the firstand second recovery steps. The braking force for the lowering
strategy was larger during the time between the trip and the firstrecovery step than that observed for the skip strategy. These
results indicated that the subjects tried to decelerate forward
motion with a large braking force, but that a step was notenough to achieve deceleration. Therefore, the kicking force
was still large during the time between the first and second
recovery steps.

IV. D ISCUSSION
To avoid the obstacle at the middle phase of the swing
phase, we observed the lowering strategy, which is in linewith the previous studies. In contrast, for the obstacle at theearly phase of the swing phase, the participants performed
the skip strategy whereas the previous studies reported theelevating strategy as the most typical avoidance action. The
most plausible cause for this new strategy was the shape ofthe obstacle used in the experiment. Previously, the elevating
strategy was observed after early phase trips induced by a flat
plate or block. Under these conditions, the tripped leg wasable to overcome the obstacle. However, in our experiments,
it was not easy to overcome the obstacle because the crossbar
disturbed the motion of lifting tripped leg. Thus, the skip
strategy might have been utilized instead of the elevating
strategy.
The skip strategy was characterized by a hopping motion
and a decrease in the velocity of the COM after the trip,
as shown in Fig. 2. During this motion, large knee flexion
and ankle plantar flexion torques were observed. The datasuggested that the estimated knee flexion torque was morethan twice that of the normal gait (Table VI). The estimatedE. Estimated joint torques of recovery leg
Table VI showed the estimated maximum joints torque ofrecovery leg. The ankle plantar flexion torque at the time
between the first and second recovery steps was larger inthe skip and lowering strategies than in the normal gait for
both subjects. The knee and ankle plantar flexion torques at
the time of the first recovery step after the trip with the skip915

the early swing phase. With the skip strategy, the subject lifted
the stance leg forward just after tripping and used the leg asa recovery leg. We analyzed the internal joint torques needed
to perform observed recovery motions using a muscleskeletal
software, and found that during this motion, the subject had toexert larger knee and ankle plantar flexion torques by the timeof the first recovery step, compared with the other strategies.
On the other hand, to achieve lowering strategy, the subject
had to perform larger hip and knee extension torques andankle plantar flexion torque. Furthermore, we have suggested
joint torques and timings which are required to perform suchrecovery motions. It might be helpful for gait assistant devices
such as physical assistant robots when they assist such fall
avoidance motions.

ankle plantar flexion torque was almost 25% larger than thatof normal gait. It is likely that the knee flexion torque wasmainly important for the success of the skip strategy.
Furthermore, from the point of the ground reaction force,when the subject used the skip strategy, the kicking force on
the horizontal plane was always lower than that of the lowering
strategy (Table V). This indicated that the kicking force was
directed more vertically in the skip strategy compared to thelowering strategy. On the other hand, the kicking force ofthe lowering strategy was predominantly in the horizontal
direction. In other words, the COM was accelerated as a resultof the large kicking force. This also resulted in a difference in
the COM velocity between the skip and lowering strategies.
The main factor that determined which strategy was selected
was the tripping phase. The subjects selected the lowering
strategy in all trials when they tripped in the middle phase.
In contrast, the skip strategy was selected in all trials in which
the trip occurred in the early phase. In addition, falls only
occurred when the lowering strategy was utilized. This could
be explained by differences in the horizontal position of the
COM when the subject tripped. In cases of a middle phase
trip, the COM had already moved forward compared to theearly phase trip (Fig. 2). Thus, the COM velocity for theskip strategy was smaller than that of the lowering strategy.
It was also caused by the initial position of the COM when
the trip occurred. When a middle phase trip occurred, the COM
was already in a forward position compared to an early phase
trip (Fig. 2). Thus, the velocity of the COM was difficult
to decelerate when the lowering strategy was utilized. This
explains why the lowering strategy was only utilized after falls,
and not with the skip strategy.
According to these results, a middle phase trip has morerisks than a early phase trip. In addition, lager joint torques
are required to achieve these recovery strategies than normal
walking. Especially, enough knee flexion torque and ankle
plantar flexion torque at the time of first recovery step after thetrip, and ankle plantar flexion torque at the time between thefirst and second recovery steps are needed to succeed in skip
strategy. To perform lowering strategy, larger knee extension
torque at the time of the first recovery step after the trip, andhip extension torque and ankle plantar flexion torque at thetime between the first and second recovery steps are required.
However, it might be difficult for the elderly to perform thoselarger torques because of weakened strength of muscle. Thus,
gait assistant devices such as physical assistant robots might
be helpful to decrease risks of fall if they can assist suchjoint torques at suggested timings. In addition, we still needto check the possibility whether the elderly also can performthese strategies, especially skip strategy or not.

ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI Grant Number 26750121.
R EFERENCES[1] K. DeGoede, J. Ashton-Miller, and A. Schultz, “Fall-related upper body
injuries in the older adult: a review of the biomechanical issues,” Journalof biomechanics, vol. 36, no. 7, pp. 1043–1053, 2003.
[2] H. Luukinen, M. Herala, K. Koski, R. Honkanen, P. Laippala, and S.-L.
Kivelä, “Fracture risk associated with a fall according to type of fallamong the elderly,” Osteoporosis International, vol. 11, no. 7, pp. 631–
634, 2000.
[3] C. L. Arfken, H. W. Lach, S. J. Birge, and J. P. Miller, “The prevalence
and correlates of fear of falling in elderly persons living in the community.” American journal of public health, vol. 84, no. 4, pp. 565–570,
1994.
[4] M. C. Nevitt, S. R. Cummings, and E. S. Hudes, “Risk factors forinjurious falls: a prospective study,” Journal of gerontology, vol. 46,
no. 5, pp. M164–M170, 1991.
[5] S. V. Brooks and J. A. Faulkner, “Skeletal muscle weakness in old age:
underlying mechanisms.” Medicine and science in sports and exercise,vol. 26, no. 4, pp. 432–439, 1994.
[6] D. G. Thelen, A. B. Schultz, N. B. Alexander, and J. A. Ashton-Miller,“Effects of age on rapid ankle torque development,” The Journals ofGerontology Series A: Biological Sciences and Medical Sciences, vol. 51,
no. 5, pp. M226–M232, 1996.
[7] R. Bogue, “Exoskeletons and robotic prosthetics: a review of recent
developments,” Industrial Robot: An International Journal, vol. 36, no. 5,
pp. 421–427, 2009.
[8] A. Blake, K. Morgan, M. Bendall, H. Dallosso, S. Ebrahim, T. Arie,
P. Fentem, and E. Bassey, “Falls by elderly people at home: prevalence
and associated factors,” Age and ageing, vol. 17, no. 6, pp. 365–372,
1988.
[9] M. J. Pavol, T. M. Owings, K. T. Foley, and M. D. Grabiner, “Mechanisms leading to a fall from an induced trip in healthy older adults,”
The Journals of Gerontology Series A: Biological Sciences and Medical
Sciences, vol. 56, no. 7, pp. M428–M437, 2001.
[10] M. Pijnappels, M. F. Bobbert, and J. H. van Dieën, “Contribution of the
support limb in control of angular momentum after tripping,” Journal ofbiomechanics, vol. 37, no. 12, pp. 1811–1818, 2004.
[11] A. Schillings, B. Van Wezel, T. Mulder, and J. Duysens, “Muscularresponses and movement strategies during stumbling over obstacles,”Journal of Neurophysiology, vol. 83, no. 4, pp. 2093–2102, 2000.
[12] J. J. Eng, D. A. Winter, and A. E. Patla, “Strategies for recovery from atrip in early and late swing during human walking,” Experimental Brain
Research, vol. 102, no. 2, pp. 339–349, 1994.
[13] C. Smeesters, W. C. Hayes, and T. A. McMahon, “The threshold trip
duration for which recovery is no longer possible is associated withstrength and reaction time,” Journal of Biomechanics, vol. 34, no. 5, pp.
589–595, 2001.

V. C ONCLUSIONSIn this study, we measured recovery motion after ankle
tripping induced by a crossbar. As a result, a new recovery
strategy, which was named the skip strategy, was observed
whereas the elevating strategy was thought to be a major
recovery motion against the contact with an obstacle during916