IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 3, NO. 4, OCTOBER 2018

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Longitudinal Rollover Strategy as Effective
Intervention to Reduce Wrist Injuries During
Forward Fall
Saeed Abdolshah , Member, IEEE, Nader Rajaei , Member, IEEE, Yasuhiro Akiyama , Member, IEEE,
Yoji Yamada, Member, IEEE, and Shogo Okamoto , Member, IEEE

Abstract—Strategies for reducing fall-related injuries have beenassessed through biomechanical studies and employed in robotics.
A better understanding of the mechanism of such fall arresting
methods can help fall prevention programs to reduce injuries andhelp robots to prevent damage to themselves. Rollover about thelongitudinal axis is an effective strategy to reduce the impact force
experienced by the hand and consequently reduce the associatedinjuries; however, neither biomechanical researchers nor roboticists have studied this useful fall arresting strategy. This study was
designed to investigate the impact force experienced by the human
hand during a forward fall using the longitudinal rollover strategy.
We designed a series of fall experiments in which the subjects were
instructed to arrest a forward fall using longitudinal rollover andbimanual strategies. The experimental results showed that during
a forward fall, longitudinal rollover considerably reduces the impact force and the risk of injuries. The kinematic data of motion
were measured and presented. A comparison of our method withvarious forward fall arresting methods reported so far in the existing literature showed that our proposed fall arresting strategy
is one of the most efficient techniques. Our study results are expected to provide robotics researchers with useful data to design
algorithms to reduce robot damage during falls.
Index Terms—Human detection and tracking, humanoid robots,
impact force, longitudinal rollover strategy during a forward fall.

I. INTRODUCTION
UMANOID robots are being increasingly used in environments designed for humans. Thus, motion planningfor unknown [1] and nonoptimal environments such as uneventerrains [2] has attracted the attention of researchers. Environmental factors such as tripping over obstacles or collision with
objects may endanger the balance of robots, which can lead toa fall. Such incidents can damage the robots severely.

H

Manuscript received February 24, 2018; accepted July 13, 2018. Date of publication August 9, 2018; date of current version August 17, 2018. This letter
was recommended for publication by Associate Editor H. Van der Kooij andEditor Y. Yokokohji upon evaluation of the reviewers’ comments. This work
was supported in part by JSPS KAKENHI under Grant 26750121, and in partby METI “Strategic international standardization promotion project: International standardization of human tolerance against fall injuries.” (Corresponding
author: Saeed Abdolshah.)
The authors are with the Department of Mechanical Systems Engineering,
Nagoya University, Nagoya 464-8603, Japan (e-mail:,saeed.abdolshah@mech.
nagoya-u.ac.jp; nader.rajaei@mae.nagoya-u.ac.jp; akiyama-yasuhiro@mech.
nagoya-u.ac.jp; yoji.yamada@mae.nagoya-u.ac.jp; okamoto-shogo@mech.
nagoya-u.ac.jp).
Digital Object Identifier 10.1109/LRA.2018.2864646

To reduce the risk of humanoid robot damage, various methods have been introduced, such as multiple contact point algorithms [3], [4], fall direction changes [5], [6], an active shockreduction motion technique [7], and optimization methods [8]–[11]. Design approaches have proposed mounting some hardcomponents [12] or impact-absorbing spots on colliding bodysegments [13], [14]. Recently, human-based methods have also
been employed to reduce the risk of robot damage. The patternof human motion during a backward fall has been applied to ahumanoid robot model [15]. The results are promising for ensuring a safe backward fall while reducing the impact velocity
considerably.
The idea of providing an active compliance mechanism for
robots has also been inspired by human motion [16], [17]. Thismechanism is based on the concept of unlocking the joints ofrobots and using the arms to function in a manner similar to
those of a human to maximize shock absorption. In this technique, the scenarios both before and after ground collision are
considered by the algorithm, and the results show that the proposed method works efficiently. Human-inspired fall algorithms
have also been applied to humanoid soccer robots [18] to predict the fall in the shortest time, recover the robot motion, anduse joints to dissipate the energy generated at impact. To apply such algorithms to real robots, the human motion should
be studied and quantified. Thus, further investigation on humanfall arresting strategies is necessary.
A wide range of studies have been conducted to explore thebiomechanical factors affecting the impact forces and the strategies to reduce fall-related injuries among humans. During aforward fall, an impact force profile is imposed on the hand,including a primary high magnitude peak (fmax1) occurring
shortly after the contact followed by a secondary lower peak(fmax2). Fall experiments may endanger subject safety becauseof the high risk of injuries. Thus, the impact force during afall from a standing position is predicted by a mathematicalmodel [19]. The simulation results show that a human fall witha locked elbow structure imposes the highest peak impact force
and should be prevented.
Biomechanical studies have focused on finding interventions
to reduce the impact force. Compliant flooring and protective
devices are considered effective interventions to reduce the impact force and consequently the level of injuries [20], [21]. The

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4188

body configuration can considerably affect the impact force experienced by the hands [22]. Thus, the natural reaction of thehuman during a falling motion [23] plays an important role inpreventing injuries. A typical fall arresting strategy includes theuse of the upper extremities to break the fall using two hands
and thereby protect the head [24]. Some interventions have been
proposed to reduce the impact force based on a bimanual forward fall.
A falling person has only 500 ms to change the body configuration prior to impact [25]. A reduction in the initial elbow angle
prior to the impact has been identified as a practical strategy for
decreasing the peak impact force and the risk of injuries. Thisfinding has been supported by both performing experiments andusing a biomechanical model [26]. The impact force is also
considerably affected by the impact velocity. Thus, decreasing
the relative velocity between the upper extremities and impact
surface attenuates the peak impact force [27].
In addition, elbow flexion after impact produces a lagged andreduced peak impact force [28]. This strategy is more efficientthan shoulder joint flexion [29]. Experimental results indicatethat an extra shear force is experienced by the shoulder joint
during a forward fall with an externally rotated forearm posture and a locked elbow; however, an internally rotated forearm
configuration reduces the risk of injuries [30].
The utilization of martial arts and judo techniques to reduce injuries during falls has been studied. Such techniques
are mainly based on sideways and backward falls and have produced a 30% reduction in the hip peak impact force [31]. The
squat response can also reduce the hip peak impact energy by
up to 43% [32]. Forward fall arresting strategies using martial
arts include a body rollover to reduce the risk of hip and shoulder injuries. According to such techniques, to break a forward
fall, the body should be rolled over the scapula of the ipsilateralshoulder. This technique is highly risky for the hip and shoulder
and additionally endangers the neck and head [33].
Fall studies have received considerable attention from sport
injury researchers. A few studies have investigated the risk of fallinjuries in sports such as inline skating and introduced strategies[34] such as the use of wrist guards to reduce fall-related injuries[35], [36]. Rollover about the longitudinal axis is an effective
strategy to reduce injuries during a forward fall in inline skating[37]; however, to the best of the authors’ knowledge, previous
studies have not investigated this strategy.
In this study, we aimed to compare the longitudinal rolloverand bimanual strategies. To determine whether fall arresting
strategies affect the impact force, the following null hypothesis was tested: the application of the bimanual and longitudinalrollover strategies would not considerably change the peak impact force experienced by the hand (i.e., the bare hand and thehand equipped with a wrist guard). To test the hypothesis, someforward fall experiments were performed and the subjects were
instructed to arrest the forward fall using the bimanual and longitudinal rollover strategies. A comparison of the impact forces
revealed that the application of the rollover strategy considerably reduced the impact force and the risk of injuries. Moreover,electromyography (EMG) data associated with the peak impact
force showed the reduced muscle activities using the rollover

IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 3, NO. 4, OCTOBER 2018technique. Kinematic data were obtained for the longitudinalrollover strategy. This study not only explores an efficient fallarresting strategy to reduce fall-related injuries but also provides
useful kinematic data that can be used to manage the fall motionof robots.
II. MATERIALS AND METHODS
A. Participants
Six healthy young male subjects participated in this study.
Their ages ranged between 20 and 38 years (mean ± standard
deviation [SD] = 25.5 ± 5.5). Their average heights and weights
were 173.1 ± 5.9 cm and 68.3 ± 11.5 kg, respectively. None of
the subjects had any records of major medical or neurological illnesses or movement disorders. All of them were right handed,and one of the subjects was a professional inline skater. The
subjects were trained to arrest a forward fall using the bimanualand longitudinal rollover strategies. All the subjects provided
written consent before the experiments. This study was conducted with the approval of the Institutional Review Board of
Nagoya University, Japan. All the experiments were performedin accordance with the approved guidelines.
B. Experimental Protocol
The subjects were instructed to kneel on a soft surface while
maintaining an upright posture. They were instructed to extend their hands forward while keeping the back straight. The
subjects were free to choose their preferred elbow angle. They
were asked not to bend the upper torso backward or forward.
Two force plates (M3D Force Plate, Tec Gihan Corp., Japan)
were mounted in front of the subjects to measure the impact
force experienced by each hand. The force plates were covered
with soft foam paddings to prevent injuries. The stiffness of thepaddings was 62.5 kN/m, which was measured using the Discovery Hybrid Rheometer (DHR-2-NA, TA Instruments, US).
The muscular activity was measured by attaching three wireless surface EMG devices (Noraxon USA Inc., Scottsdale, AZ)
on each set of biceps and triceps muscles. To obtain the elbow angles during the fall motion, a goniometer (Noraxon USA
Inc., Scottsdale, AZ) was attached to each elbow. A motion
capture system (MAC3D System, Motion Analysis Corp.) with
10 cameras and 25 reflective markers was used to measure thetorso rotation and impact velocity during the fall motion. All
the devices were synchronized, and the data were recorded at afrequency of 1 kHz. The full-wave EMG data were rectified andfiltered using a low-pass filter with a cut-off frequency of 50 Hz.
The EMG data were normalized relative to the average reference EMG values obtained from the maximum contractions ofeach muscle.
Two types of experiments were performed:
Experiment 1 – Bimanual strategy: The subjects were asked
to rotate their bodies in the sagittal plane about the flexionextension axis of the knees. They were instructed to arrest theirfall by making the two hands land on the force plates with amotion similar to a pendulum free-fall motion. The subjects
were trained to land on both their hands simultaneously. The

ABDOLSHAH et al.: LONGITUDINAL ROLLOVER STRATEGY AS EFFECTIVE INTERVENTION TO REDUCE WRIST INJURIES

Fig. 1. Forward fall arresting motion using (a) bimanual and (b) rollover
strategies. The bimanual fall arresting strategy involves body rotation in thesagittal plane and landing with both hands; however, in the longitudinal rollover
strategy, the body rotates not only in the sagittal plane, but also about thelongitudinal axis. The landing occurs only on the right hand.

participants were free to choose the position of their upper extremity. This experiment was performed with bare hands andhands equipped with wrist guards (Fig. 1(a)).
Experiment 2 – Longitudinal rollover strategy: The subjects
were instructed to rotate their bodies in the sagittal plane about
the flexion-extension axis of the knees while gradually rotating
their bodies about the longitudinal axis so that only the righthand made contact with the ground. The fall was broken by theright hand while the body continued to rotate and landed on theright side. To avoid any kind of injuries, a mattress was placedon the right side of the experimental setup. This experiment was
performed with both bare hands and hands equipped with wrist
guards (Fig. 1(b)). In this experiment, ground contact occurred
first with the hands and then with the lower extremity.
The wrist guards were provided by Denso Sports Inc., Japan.
Each subject performed twelve trials—six trials with wrist
guards and the rest with bare hands. Initially, they were instructed to use the bimanual fall breaking technique, which is avery common reaction among people during a fall. The rollovertraining and the associated trials were carried out after the bimanual experiments. If the rollover training were carried out
at the beginning, the training might have affected the bimanual fall arresting trials. For each fall arresting strategy, training
was continued until the subjects could perform the strategy correctly. It is noteworthy that the number of training trials wasdifferent depending on the subjects and strategies. For example,

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Fig. 2. Impact force experienced by hands during forward fall motion usingbimanual and longitudinal rollover strategies with (a) bare hand and (b) handequipped with wrist guard. All the impact force profiles are associated withtwo peaks. The primary peak has a higher magnitude and frequency, which isfollowed by the secondary one.

performing the bimanual arresting strategy is normally easierthan performing the other strategy. Moreover, all the experiments were recorded and checked by experts to verify the accuracy of motion.
To examine the null hypothesis, an independent sample t-test
was performed. Here, the mean values of the peak impact forces
experienced by the hand during the fall motion were compared
between the bimanual and longitudinal rollover strategies. Statistical analysis was carried out using SPSS software (version
23; IBM Corporation, Armonk, NY). A statistically significant
value was considered to be p < 0.05. The mean of the peak
impact forces was summarized based on the mean value ± SD.
The average left and right hand impact forces were reported.
III. RESULTS
Fig. 2 shows a few samples of average hand impact forces
associated with the bimanual strategy and the right hand impact
force associated with the rollover strategy. The experiments
were performed with (a) bare hands and (b) hands equippedwith wrist guards. For all the impact force profiles, two peaks
can be identified; the primary one has a higher magnitude andfrequency, which is followed by a secondary peak. In addition
to the two peaks, a trivial peak can be detected shortly after thecontact (9 ± 3 ms) in the bimanual fall arresting strategy.

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Fig. 3. Elbow angle during forward fall arresting using bimanual and rollover
strategies. The graph shows the data from 0.05 s prior to the hand–groundcontact.

All the subjects exhibited a considerably higher magnitude ofpeak impact forces using the bimanual strategy: 788 ± 141 and699 ± 135 N for the peak bimanual impact force with a barehand and a hand equipped with a wrist guard, respectively. The
application of the rollover strategy decreased the peak impact
force by an average of 59% (p < 0.002) (331 ± 85 and 278± 81 N for the peak impact force using the rollover arresting
strategy with a bare hand and a hand equipped with a wrist guard,
respectively). It can be inferred that the impact force experienced
by the hand for the bimanual strategy was approximately 117.6%(bare hand) and 104.3% (hand equipped with a wrist guard) of
the body weight; however, the impact force reduced to 49.4%(bare hand) and 41.5% (hand equipped with a wrist guard) of
the body mass for the rollover strategy.
The application of the rollover strategy affected the second
peak by an average of 51% (p < 0.0005) (246 ± 52 and 217 ±45 N for the peak rollover impact force with a bare hand anda hand equipped with a wrist guard, respectively). The peak
impact force was achieved faster in the rollover technique (15 ±4 ms) than in the bimanual fall technique (23 ± 5 ms), andthe total contact time reduced to approximately 200 ms in therollover technique.
Our experimental results replicated the previous findings thatthe wrist guards and compliant floorings attenuated the peak
impact force experienced by the hand. The effect of the wrist
guards on the first peak was higher than that on the second one,which was measured as 497 ± 89 and 455 ± 87 N for the barehand and the hand equipped with the wrist guard, respectively,
for the bimanual arresting strategy. The second peak appeared at
76 ± 6 ms. The time variation between the two hands contacting
the ground was 6 ± 3 ms. This delay was negligible and lessthan that in previous studies [22].
Fig. 3 illustrates the variation in the elbow angle for a few exemplary trials. The impact occurred when the time was 0 s. During the bimanual forward fall, the subjects arrested the fall withan initial elbow angle of 163° ± 6°. The force experienced by thehand caused the elbow to flex up to 131° ± 17° and after breaking
the fall, the elbow extended. The elbow extension started after
the second peak, and the elbow angle ended up at approximately
152° ± 11°. For the rollover arresting strategy, an initial slightflexion occurred, and then, the elbow returned to the nearly fully
extended position. The flexion during rollover arresting was anIEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 3, NO. 4, OCTOBER 2018

Fig. 4. Torso rotation about longitudinal axis during forward fall using rollover
fall arresting strategy.

average of only 8° followed by an extension. The extension
expedited the torso rotation about the longitudinal axis.
A few samples of the torso rotation in global coordinate system is shown in Fig. 4. The graph was obtained using the results
obtained from the markers on the pelvis, shoulder, STRN, C7,
and clavicle. From the initial body posture, the subjects rotatedtheir torsos by an angle of 71° ± 12° about the longitudinal
axis at the moment of contact. The torso rotation was quickened
using their right hand, and the subject fell on the mattress on theright side.
The hand impact velocity was strongly associated with theimpact force experienced by the hand. In our experiments, theabsolute vertical hand impact velocity was estimated using themarkers on the hand. The hand impact velocity associated withthe bimanual strategy was 3.2 ± 0.4 m/s; however, the application of the rollover strategy reduced the impact velocity to
1.8 ± 0.3 m/s. Such results show the superiority of the rollover
strategy over the bimanual arresting strategy to reduce the riskof injuries.
The EMG data were calculated as an average normalized
value over a period of 50 ms, which covered the peak impact
force. The triceps and biceps EMG values were 0.65 ± 0.25 and0.62 ± 0.21, respectively, for the bimanual arresting strategy;
however, the muscle activities reduced during the application of
the rollover arresting strategy, with EMG values of 0.42 ± 0.18
and 0.40 ± 0.22 for the triceps and biceps muscles, respectively.
IV. DISCUSSION
In this study, we sought to assess whether a longitudinalrollover fall arresting strategy could reduce the impact force experienced by the hand and the risk of injuries. Although wrist
fracture strongly depends on bone strength, the magnitude of
the induced impact force considerably affects the possibility ofa fracture occurrence. Thus, we assumed that the force experienced by the hand was strongly associated with the risk of wrist
injuries.
Our results provided evidence that altering the fall arresting
strategy to longitudinal rollover reduced the magnitude of the
impact force experienced by the hand. Thus, the null hypothesis
was rejected. The most efficient strategy reported so far to reduce the hand peak impact force claimed only a 27% reduction[22]; however, our proposed arresting strategy reduced the im-

ABDOLSHAH et al.: LONGITUDINAL ROLLOVER STRATEGY AS EFFECTIVE INTERVENTION TO REDUCE WRIST INJURIES

pact force by 59%. Although our discussion of fracture risk was
based on the peak impact force, one may assume that the energy
absorbed by the hand was strongly proportional to the risk of
injuries. From our study results, it can be inferred that the corresponding wrist energy absorption during the rollover strategy
diminished because of an earlier peak with a smaller magnitude.
One advantage of our study was the use of human subjects,
whose motion data were obtained through a series of safe experiments in which the depicted motion was similar to real humanfall motion. In the bimanual arresting strategy, we found the
body configuration to be unnatural and difficult to control andinstruct at impact. Thus, the subjects were allowed to choose
their preferred landing posture in the bimanual technique.
The subjects were instructed to try to make contact with theground with both the hands simultaneously. We did not observea major difference between the impact forces experienced by
the left and right hands during the bimanual forward fall. Thisfinding reflected the fact that the bimanual fall arresting strategy
was performed properly. In the rollover strategy, all the subjects
were instructed to arrest the fall with their right hand. The reason
for this instruction was the fact that right-handed subjects havea better capability to arrest a fall using their dominant hand.
Based on the data obtained from the motion capture system,no major difference was found between the initial postures ofthe subjects in the rollover and bimanual strategies. Thus, it
can be inferred that although the subjects were aware of thestrategy they had to apply to arrest the fall, the initial posture andconsequently the results were not affected by their awareness.
The recorded fall motion of the professional skater and theother subjects were observed carefully, and no special differences were found between them. Although, it can be concluded
that the training and the arresting strategies were conducted
appropriately, more investigation is necessary to assure the accuracy of subjects’ fall breaking motion and the initial posture.
In addition to the two peaks, we observed a trivial early peakin bimanual fall arresting. This peak can be interpreted to becaused by the effect of elbow flexion after the contact and is notconsidered to be an independent peak in previous studies [28].
Moreover, in our bimanual experiments, the first peak appeared
later than that in similar studies [19]. The most likely explanation
for this delay is the effect of using compliant flooring and theelbow flexion at impact.
The use of human subjects restricted the experiments to asafe range of impact forces. The use of a padded force plate isa common strategy to prevent injuries during fall experiments[22]. Although the impact force was considerably lower than thatduring an actual fall, this does not impair the outcome of thisstudy. In our experiment, the maximum force experienced by
the hands occurred during bimanual fall arresting with the barehands (788 ± 141 N), which was below the reported measuresof the wrist fracture threshold.
Previous studies showed that in forward falls, knee impact
might occur considerably earlier than hand impact [38]. Thus,
similar to previous studies [19], we assumed that during theforward fall, the knees contact the ground before the hands. Thisorder of contact makes the velocity of the feet and shins to be
zero and does not affect the upper extremity velocity. Moreover,previous research studies performed fall experiments from the4191

kneeling position, claiming that the working mechanism of afall arresting technique is similar to the mechanism of falls fromkneeling and standing positions [31]. Therefore, we asked thesubjects to keep their shins fixed on the ground.
To check the muscle activation, the EMG data associated withthe peak impact force were calculated. Although the muscle
activation rate may not change as quickly among the elderly asthat among young people [39], muscle weakness is one of themost important reasons for falls. This weakness may restrain theproper arresting strategy for a forward fall. The rollover strategy,
which requires lower muscle activity, can help the elderly inmore easily arresting a fall.
Although our study considered only one type of wrist guard,
our assessment showed that the application of the guard lowered
the magnitude of the peak impact force for both the bimanualand rollover fall arresting strategies. This reduction effect was
lower on the second peak than on the first peak. Wrist guards
can play a major role by not only reducing the impact force but
also constraining the wrist motion. Further experiments using
various wrist guards are needed to confirm the effect of such preventive guards on the impact force during the rollover arresting
strategy.
V. CONCLUSION
Only few studies have previously explored the effect of fallarresting strategies on the impact force experienced by the handduring a forward fall. The present study is the first one to quantify the effect of a longitudinal rollover fall arresting strategy
on the impact force experienced by the hand and to compare
the results with those of the bimanual strategy. To achieve thestudy objective, human subjects were hired to perform the experiments. The subjects were trained to arrest a fall motion using the bimanual and longitudinal rollover strategies througha set of forward fall experiments. The bimanual strategy is
the most common technique used by humans to arrest forward
falls [23].
Our results revealed the advantages of the longitudinalrollover strategy. This strategy reduces the peak impact force
experienced by the hands by 59%. To the best of the authors’knowledge, this is one of the most efficient fall arresting strategies reported so far to reduce the peak impact force experienced
by the hand. Although changes in the experimental conditions
may vary the results to some extent, our findings are obtainedusing a standard and reasonable experimental condition that isvery similar to an actual fall motion.
We also sought to determine the upper extremity posture during the forward fall and presented the results for the elbow angle
and torso rotation to realize the longitudinal rollover arresting
strategy. From a biomechanical point of view, such data can beused in fall prevention programs to reduce the risk of injuries.
The use of assistive devices has widely increased in the last
few years [40]. Such devices can be equipped with special algorithms to reduce fall injuries. Our study results can provide
useful data for these devices. In addition, the findings of thisstudy are noteworthy because the obtained information can beused to enable humanoid robots to demonstrate human-like fallmotion and hence reduce damage.

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It is unclear as to what extent the elderly and robots can makeuse of the longitudinal rollover strategy successfully. Previous
studies [18] have illustrated that robots can successfully copewith human-inspired techniques, and hence, our study results
can be used to design longitudinal rollover algorithms for humanoid robots.
In conclusion, we have shown that altering the fall arresting
strategy can considerably affect the risk of injuries. We limited
our study to the bimanual and rollover strategies. Further investigations are necessary to identify other fall arresting strategies
and compare their results with those of the bimanual and rollover
strategies.
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