Effect of arm swing on single-step balance recovery

Human Movement Science 38 (2014) 173–184

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Effect of arm swing on single-step balance recovery Kuangyou B. Cheng ⇑, Yi-Chang Huang, Shih-Yu Kuo Institute of Physical Education, Health, and Leisure Studies, National Cheng Kung University, Tainan, Taiwan

a r t i c l e

i n f o

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PsycINFO classification: 2330 4010 Keywords: Balance strategy Forward fall Arm movement Coordination Tether-release method

a b s t r a c t Balance recovery techniques are useful not only in preventing falls but also in many sports activities. The step strategy plays an important role especially under intense perturbations. However, relatively little is known about the effect of arm swing on stepping balance recovery although considerable arm motions have been observed. The purpose of this study was to examine how the arms influence kinematic and kinetic characteristics in single-step balance recovery. Twelve young male adults were released from three forward-lean angles and asked to regain balance by taking a single step under arm swing (AS) and arm constrained (AC) conditions. It was found that unconstrained arms had initial forward motion and later upward motion causing increased moment of inertia of the body, which decreased falling angular velocity and allowed more time for stepping. The lengthened total balance time included weight transfer and stepping time, although duration increase in the latter was significant only at the largest lean angle. In contrast, step length, step velocity, and vertical ground reaction forces on the stepping foot were unaffected by arm swing. Future studies are required to investigate optimal movement strategies for the arms to coordinate with other body segments in balance recovery and injury reduction. Ó 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Institute of Physical Education, Health, and Leisure Studies, National Cheng Kung University, Tainan 701, Taiwan. Tel.: +886 6 2757575x81813; fax: +886 6 2766427. E-mail address: [email protected] (K.B. Cheng). http://dx.doi.org/10.1016/j.humov.2014.08.011 0167-9457/Ó 2014 Elsevier B.V. All rights reserved.

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1. Introduction Maintaining balance during standing and walking is critical for healthy daily living. Since the cost of medical treatment after falls is usually expensive, falls prevention rather than injury treatment should be the top priority. More thorough understanding of biomechanical characteristics of balance recovery and more efficient/effective balance movements are certainly necessary. Applying balance recovery techniques can be observed in many sports including gymnastics, dancing, and martial arts. Excellent balance ability is required in these sports activities to counteract excessive body linear/angular momentum. It can be achieved by muscle activation and joint coordination, and this ability can be improved through training (Misiaszek, 2003). In addition, strategies for standing balance recovery have been identified and classified as: (1) ankle strategy, (2) hip strategy, (3) step strategy, and (4) load–unload strategy (Duncan, Studenski, Chandler, Bloomfeld, & LaPointe, 1990; Winter, 1995). Based on the equations of motion of an inverted pendulum, three mechanisms for standing balance were also revealed: (1) change of the center of pressure (COP) position with respect to ground projection of the center of mass (COM), (2) counter-rotation of body segments about the COM, and (3) use of external forces other than the ground reaction force (Hof, 2007). Although minor perturbations can usually be recovered by the ankle and hip strategies, the step strategy is often called upon as a natural and preferred reaction especially under violent disruption of balance (Duncan et al., 1990). In the step strategy, the central nervous system sends signals to indicate the need to re-position the base of support by stepping forward, backward or laterally until a new base of support which stabilizes body posture is achieved. Many experimental paradigms have been utilized to simulate loss of balance in order to investigate stepping or other balance recovery strategies. For instance, these methods include having unexpected obstacles or floor translation during walking (Maki, McIlroy, & Fernie, 2003; Pijnappels, Bobbert, & van Dieën, 2004) or a sudden pushing/pulling force during upright stance (Rogers, Hedman, Johnson, Cain, & Hanke, 2001). For producing repeatable balance perturbations, the tether-release method has often been used (Hsiao-Wecksler, 2008) to investigate parameters such as step time, step lengths, joint torques, and joint angles. Balance ability between different age and/or gender groups was often assessed by simulating forward falls with this method (Thelen, Wojcik, Schultz, Ashton-Miller, & Alexander, 1997; Wojcik, Thelen, Schultz, Ashton-Miller, & Alexander, 1999). Vigorous arms movements have been observed during balance recovery (Dietz, Fouad, & Bastiaanse, 2001; Marigold, Bethune, & Patla, 2003; Roos, McGuigan, Kerwin, & Trewartha, 2008). Muscle activation latency was found to be around 80 ms for arms, and 35–40 ms for feet. Arm movements have been considered to represent two strategies in balance recovery: the protective and preventive strategies (Roos et al., 2008). Old adults generally apply the protective strategy to grasp something for support and avoid direct impact on the floor. However, young adults are more likely to utilize the preventive strategy, for instance, to shift the body center of mass (COM) opposite to the direction of the fall (Allum, Carpenter, Honegger, Adkin, & Bloem, 2002; Misiaszek, 2003). The arms can also produce joint torque to counteract the angular momentum generated by loss of balance (Pijnappels, Bobbert, & van Dieën, 2005; Roos et al., 2008). Furthermore, in human walking although arm movements are mainly passive, there is an active contribution of muscles rather than purely stretch reflexes (Meyns, Bruijn, & Duysens, 2013). The functional role of arm swing in postponing momentum transfer from the arms to the trunk was identified in tripping movements (Pijnappels, Kingma, Wezenberg, Reurink, & van Dieën, 2010). In addition, although arm swing would have a negative effect on balance at initial perturbation, the recovery actions of the arms still contribute to the overall gait stability (Bruijn, Meijer, Beek, & van Dieën, 2010). However, detailed investigation on how arm movements affect standing balance recovery using the stepping strategy has not been found. Previous studies on regaining balance have focused on lower body extremities or recovery from small perturbations. Although considerable arm motions have been observed during stepping balance recovery, the effects of arms remain unclear and further investigation is necessary. For example, it is unknown whether movement time, step length, and/or ground reaction forces during balance recovery are affected by the use of arms. Thus, the purpose of this study was to examine how the arms

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influence kinematic and kinetic characteristics in single-step balance recovery under different forward lean conditions. 2. Methods 2.1. Participants Twelve healthy participants (age: 21–26 years; height: 1.67–1.78 m; mass: 54.4–78.7 kg) were recruited in this study. The means and standard deviations of the age, height and mass were 23.85 ± 1.87 years, 1.72 ± 0.04 m, and 66.57 ± 9.79 kg, respectively. To exclude possible influence from gender difference, only males were recruited. None of them participated in sports activities requiring extensive balance training (such as in gymnastics, dancing, etc.) or suffered from lower/upper extremity injuries in the past 6 months. All the experimental procedures were approved by National Cheng Kung University Research Ethics Committee, Tainan, Taiwan. Prior to the experiments each participant was given an information sheet outlining experimental procedures and the associated risks and benefits of participation. Informed consent form was obtained from each participant. 2.2. Experimental setup and protocol The experimental setup was similar to that of a previous study (Wojcik, Thelen, Schultz, AshtonMiller, & Alexander, 2001). A tether-release system with a cable attached to the back of the participants supported forward-lean postures. Cable length was adjusted to allow participants to lean forward while holding a straight body posture (Fig. 1). Although cable tension was not measured, the release system enabled stable initial postures. After a preliminary testing session of the experimenters, initial body lean angles were set at 12.5°, 15°, and 17.5° measured from the vertical line. These angles were chosen because the requested movement was to take a single forward step to regain balance. At smaller lean angles balance could be recovered without taking a step, and larger angles discouraged the desire for continuing this experiment. The initial lean angles were insured by a laser line pointer (KML-3000, LASIC ELECTRO-OPTICS CO., Taiwan) emitting a red laser line which passed through the heel and shoulder (acromion process) from the participant’s right side and projected onto an enlarged protractor on the left side. Participants were instructed to recover balance by taking a single step after the start of simulated falls which were induced by releasing the cable without any

Fig. 1. Initial forward-lean postures maintained using a tether-release system. With the support system participants leaned forward while holding a straight body posture. Initial body lean angles were set at 12.5°, 15°, and 17.5° measured from the vertical line.

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pre-notice. They were also instructed to react naturally without delayed or hurried stepping. The preferred stepping leg happened to be the right leg for all the participants in this study. Participants attempted balance recovery under arm constrained (AC) and arm swing (AS) conditions. In AC condition arm motions were constrained by folding the arms on the front of the chest using elastic bandages, and balance recovery was achieved only by moving the trunk and lower limbs for taking a forward step. In AS condition, although initially the arms were positioned as in AC condition, participants could move their arms to assist balance recovery. To acquire natural responsive movements, no instruction on how to move the arms was given in either condition. Under each lean angle/arm swing condition, participants repeated balance recovery movements for five times. A pre-testing session of 10 min allowed the participants to practice movements under all different kinds of conditions. To reduce the effect of learning, different lean angle/arm swing trials were carried out in random order. Kinematic data were collected by Visualeyez™ (VZ4000, Phoenix Technologies Inc., Canada) motion capture system with the sampling rate at 100 Hz. Markers were secured at the ankles, lateral knees, greater trochanters, acromion processes, lateral humeral epicondyles, wrists, and the sacrum. Ground reaction forces (GRF) were recorded by two force plates (BP400600-2000, Advanced Mechanical Technology Inc., Massachusetts, USA) at the sampling rate of 1000 Hz. The first force plate was under both heels before tether release and the second one was located in front for landing the stepping leg. Preliminary trials on the rigid floor and adjustment of initial standing position under each condition ensured natural stepping rather than constrained or targeted stepping on the force plate. 2.3. Data analyses Weight transfer time (TT) was defined to be from movement initiation of the shoulder marker to movement of the right ankle marker, which is equivalent to the duration between the start of forward fall to the start of taking a step. The start of marker movement was defined as its position started to differ from the average position plus three standard deviations in either forward or vertical direction during the initial preparation period (Kim, Kim, & Im, 2011). Weight transfer time was not found by examining GRF because in the present study initially both feet were on the same force plate together with noticeable minor oscillations in the recorded force data prior to cable release, which increased difficulty in determining movement onset timing. The oscillations were consistent across different conditions and thus were not indicative of anticipation at different lean angles. Total balance time (TBT) was weight transfer time TT plus step time (ST, defined from the end of TT to landing on the second force plate). The step length L was defined as the horizontal distance between the ankle marker of the stepping foot at the start and at landing, and consequently the stepping velocity was L/ST. Step lengths were normalized by body height (BH) for each participant. Kinematic data were analyzed by VZAnalyzer™ (V3.50, Phoenix Technologies Inc., Canada) for joint angle calculation. Arm movements were analyzed by similar definitions of joint motion and segment orientation in baseball pitching literature (Fleisig et al., 1996; Stodden, Fleisig, McLean, & Andrews, 2005). Kinematic data and GRF data were filtered by a Butterworth 4th-order zero lag low-pass filter with a cutoff frequency of 7 and 50 Hz, respectively. Whole body COM position was estimated from the mass and COM position of each body segment, which were obtained from total body mass and joint marker positions with relative mass and COM location parameters (de Leva, 1996; Zatsiorsky, Seluyanov, & Chugunova, 1990). The force data were normalized by body weight for each participant. The mean loading rate was defined as the time rate of increase of the impact force to its first peak when stepping on second force plate. Statistical comparisons were done using two-way analyses of variance with repeated measures to determine the effects of arm movement condition, lean angle, and interaction. For any significant main effect/interaction, post hoc tests were performed using paired t-tests with Bonferroni correction (Carbonneau & Smeesters, 2014). The significance level was set at p < .05. 3. Results All participants were able to regain balance from the three initial lean angles by taking a single step. As was reported previously (Corbeil, Bloem, van Meel, & Maki, 2013), considerable between-subject

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variations of arm movements were found in the present study. Arm movements were classified according to the position and movement trajectory of the elbow marker. They were found to be generally symmetrical with three patterns: upward, lateral, and circular (similar to the butterfly stroke in swimming) movements with either flexed or extended arms. At first only the first two patterns were defined. The arm movement was upward if the right elbow was at a higher position than the shoulder at landing the step. Other than the upward arm motions, the arms in general moved laterally with much less anterior/posterior displacements. Thus, arm movements were lateral if the elbow was below the shoulder at landing. Since one participant occasionally moved his arms backward–upward–forward (but using the original definition this pattern was improperly classified as an upward motion), the circular pattern was also identified. Statistical analyses were performed for all the participants (Tables 1 and 2). However, since the majority of them (7 out of 12) consistently moved the arms upward and the focus of this study was on the effect of using arms rather than examining various arm movements, only the upward movement pattern was presented in the figures below. The example movement patterns of one participant at 15° lean angle revealed clear distinction between AC and AS conditions especially in movement duration (Figs. 2a and 2b & Table 1). Similar to previous findings of insignificant medial–lateral COM displacements (Carty, Mills, & Barrett, 2011), in this study the stepping foot landed almost directly in front (0.3 ± 0.6 cm to the right of its original position), indicating negligible lateral displacement although lateral stepping was not restricted. Since the observed arm movements were generally symmetric and had virtually no effect on balance in the mediolateral direction, only sagittal plane projections of joint angles/angular Table 1 Comparison of the transfer time, step time, total balance time, step length, step velocity, and maximum anterior/vertical COM displacement (between the initial position and the farthest/highest position within total balance time) under AS (arm swing) and AC (arm constrained) conditions at different lean angles. Step lengths and COM positions were normalized by body height for each participant. The symbol (*) and superscripts (alphabets) denote significant difference between arm movement conditions and between lean angles, respectively. AS

AC

Transfer time (ms) 12.5° 15° 17.5°

164.15 ± 15.67⁄ 157.40 ± 37.56⁄ 156.91 ± 38.64⁄

107.24 ± 31.41⁄ 105.23 ± 27.60⁄ 102.43 ± 22.70⁄

Step time (ms) 12.5° 15° 17.5°

399.99 ± 83.86 391.67 ± 82.86 376.56 ± 56.19⁄

361.43 ± 70.60 365.58 ± 62.68 334.38 ± 31.87⁄

Total balance time (ms) 12.5° 15° 17.5°

564.14 ± 83.67⁄ 549.08 ± 79.15⁄ 533.456 ± 62.25⁄

468.67 ± 61.16⁄a 470.81 ± 62.60⁄b 436.81 ± 36.08⁄ab

Step length (%BH) 12.5° 15° 17.5°

58.27 ± 3.63cd 59.47 ± 3.22c 60.29 ± 3.20d

57.74 ± 3.25ef 59.07 ± 3.47e 59.84 ± 4.21f

Step velocity (m/s) 12.5° 15° 17.5°

2.61 ± 0.60 2.72 ± 0.59 2.80 ± 0.41

2.84 ± 0.57 2.85 ± 0.53 3.10 ± 0.36

Maximum anterior COM displacement (%BH) 12.5° 15° 17.5°

6.60 ± 0.41 6.56 ± 0.37 6.54 ± 0.38

6.47 ± 0.45 6.42 ± 0.45 6.40 ± 0.49

Maximum vertical COM displacement (%BH) 12.5° 15° 17.5°

2.16 ± 0.45⁄ 2.27 ± 0.40⁄ 2.25 ± 0.41⁄

1.26 ± 0.31⁄ 1.22 ± 0.28⁄ 1.20 ± 0.26⁄

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K.B. Cheng et al. / Human Movement Science 38 (2014) 173–184 Table 2 Peak vertical ground reaction forces (GRF) and mean loading rate under AS (arm swing) and AC (arm constrained) conditions at three lean angles. The data were normalized by body weight for each participant. No significant differences were found between any lean angle/arm movement conditions. AS

AC

Peak GRF value (BW) 12.5° 15° 17.5°

1.50 ± 0.27 1.51 ± 0.33 1.56 ± 0.30

1.52 ± 0.33 1.48 ± 0.26 1.57 ± 0.32

Loading rate (BW/s) 12.5° 15° 17.5°

45.13 ± 17.21 46.85 ± 25.34 49.14 ± 18.02

50.78 ± 27.25 47.60 ± 22.82 51.26 ± 18.27

Fig. 2a. Sagittal joint/segment angles and angular velocities under AC condition at 15° lean angle. Data presented were averaged from a participant’s five trials under this condition. The trunk angle was defined as the angle between the horizontal line and trunk segment (the line joining the hip and shoulder joints). The knee angle was 180° when the leg was fully straight, and was decreased at knee flexion. The ankle angle of the stepping leg increased at plantarflexion and became the largest prior to landing.

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Fig. 2b. Sagittal joint/segment angles and angular velocities under AS condition at 15° lean angle. Data presented were averaged from the same (as in Fig. 2a) participant’s five trials under this condition. The trunk, knee, and ankle angles were defined as in AC condition, and the shoulder angle was defined as the angle between trunk segment and upper arm (the line joining the shoulder and elbow joints).

velocities were presented (Figs. 2a and 2b). When arm swing was allowed, considerable shoulder flexion preceded other joint motions. In this example movement pattern, regaining balance with arm motion lengthened both weight transfer time (TT) and step time (ST). Maximum COM height was raised by about 0.03 m due to the elevated arms. Although arm swing did not change step length significantly (Table 1), forward/upward arm movement and lengthened step length (by about 0.07 m) moved the COM more forward at landing the step in this example movement (Fig. 3). Statistical comparisons in the kinematics included weight transfer time (TT), step time (ST), total balance time (TBT), step length (L), step velocity (V), and maximum anterior/vertical COM displacement (between the initial position and the farthest/highest position within total balance time). Weight transfer time was longer (F(1, 11) = 46.038; p < .001) when arm movements were allowed, and significant differences were found at all three initial lean angles (Table 1). Step time was also longer (F(1, 11) = 5.455; p = .039) in AS than in AC, but significance difference was found only at 17.5° (Table 1). However, there was no significant difference in TT or ST between any of the three lean angles (Table 1). The total balance time (TBT) was longer in AS than in AC (F(1, 11)=45.010; p < .001) with significant difference at all the initial lean angles. Difference in TBT was also found between lean angles

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Fig. 3. Whole body COM position under AC (top) and AS (bottom) condition at 15° lean angle. The data were averaged from the five trials used in Figs. 2a and 2b. The x and z directions are anterior–posterior and vertical directions, respectively. In this example movement pattern between movement initiation to landing, arm swing increased maximum COM displacement more in the vertical than in the anterior position.

(F(2, 22)=3.471; p = .049) with significantly shorter total time at 17.5° than at the other two angles under only AC condition. No significant difference in step length (L) between AS and AC was found. However, L changed significantly at different lean angles (F(2, 22) = 8.092; p = .002). More specifically, L increased when the lean angle increased from 12.5° to the two steeper angles under both AS and AC conditions. Step lengths at 15° and 17.5° lean angles, however, were not significantly different. Mean stepping velocities ranged from 2.61 to 3.10 m/s with no significant differences between lean angles or arm movement conditions. Although the anterior displacements of the COM were not significantly affected by arm swing or initial lean postures, arm swing resulted in greater COM displacement in the vertical direction (F(1, 11) = 49.825; p < .001) with significant differences at all three initial lean angles (Table 1). Furthermore, no significant interaction (arm swing  lean angle condition) was found in any of the above kinematical variables. Kinetic comparison involved ground reaction forces (GRF) and mean loading rate. Some participants were found to employ forefoot and rear-foot landing interchangeably. No significant differences were found between any lean angle/arm movement conditions (Table 2). 4. Discussion Although arm movements have frequently been observed in coping with loss of balance, previous studies mainly focused on how lower extremities influenced balance recovery, and made comparisons

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across different age/gender groups or intensity levels of balance perturbation. Detailed investigation on how arm motions might affect stepping balance recovery has not been found. Since taking a step is a common strategy for maintaining balance, the purpose of the present study was to investigate how the arms influence movement characteristics during single-step balance recovery. In comparing balance recovery with and without arm swing, the weight transfer time (TT) was found to be significantly longer in AS than in AC at all three lean angles. Although the step time (ST) was significantly longer in AS than in AC only at 17.5° lean angle, the total balance time (TBT) was still longer in AS than in AC under all three angle conditions. This means that without arm motion, the participants had shorter preparation time for initiating a step and performed stepping movements more hastily. Longer TT in AS condition was probably due to somewhat forward arm movement upon cable release although it was usually followed by obvious upward arm motion (Figs. 2a and 2b). The initial forward arm motion delayed forward moving of the rest of the body (based on the conservation of momentum) and consequently postponed the timing for step initiation. Although with arm swing the ST was longer only at the largest lean angle, greater average values at other lean angles and significantly longer TBT at all the lean angles could be explained theoretically using an inverted pendulum. With mass m on top of a massless stick, the angular acceleration of an inverted pendulum under gravity is inversely proportional to its length. Numerical integration reveals that longer pendulum length increases the time to travel the same angular displacement. Similarly, Roos et al. (2008) indicated that arm elevation in balance recovery could raise body COM and provide more time for placing the stepping leg. Pijnappels et al. (2010) also pointed out that raised arms increased the moment of inertia of the whole body, which decreased angular velocity of the body’s forward fall. In the current study the arms had initial forward motion and mostly upward movements afterwards, which caused only slightly forward but mainly upward COM displacement. This meant that applying momentum conservation (by initial forward arm motion) and increasing the moment of inertia (by lengthening the inverted pendulum) to decrease the angular velocity of falling forward rather than using counterweight for balance was the main strategy observed. Although longer total balance time in AS condition was mainly due to lengthened weight transfer time (TT), other factors could also influence its duration. For instance, the fear of less protection due to constrained arm motion in AC condition might have resulted in unloading the stepping leg prior to cable release and consequently short TT. However, analyses of COP positions have excluded this possibility. It is also possible that longer TT in AS condition was because the participants felt safer when they could move their arms freely and initiate stepping at a later time. Whether this hypothesis is true will require investigation in future studies. Moreover, since the inertial effects from single body segments to whole body dynamics of lifting movements have shown to be small (Lindbeck & Arborelius, 1991), in the present study the inertial effects of segment movements were not examined in detail. Although most of the participants had seemingly greater peak GRF and greater average loading rate in AC than in AS, the differences were not significant. Arm swing might have no influence on GRF and loading rate, but greater variations in the results due to some participants’ using rear-foot and forefoot landing strategies interchangeably might be the reason for this insignificance. It is also likely that at certain lean angles one landing strategy is more preferred than the other. Since increased landing impact has been considered a cause for running injury (Hamill, Russell, Gruber, & Miller, 2011), whether arm motions could alleviate foot impact and reduce lower limb injuries requires future investigation. The present kinematic data included time/length/velocity parameters associated with taking a forward step. Weight transfer time in AS condition was about 157–164 ms, which seemed to be shorter than 265 and 258 ms in young adults at cable loads of 15% and 20% body weight, respectively (Thelen et al., 1997). Nonetheless, the current results were more comparable with 147 and 157 ms in young and middle age adults, respectively, who recovered balance from maximum forward leaning (Carbonneau & Smeesters, 2014). In the study of Thelen et al. (1997) cable loads of 15% and 20% body weight were equivalent to 13.5° and 17.3° lean angles, respectively (with detailed cable load to lean angle conversion in their study). In Carbonneau and Smeesters’s study (2014) maximum forward lean angles were 26.0° and 17.8° for young and middle age adults, respectively. Similar to regaining balance from forward leaning by walking (Do, Breniere, & Brenguier, 1982), TT in the present study was independent of the magnitude of forward inclination. However, it has also been revealed that TT decreased

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with increasing lean angle (Thelen et al., 1997). Since the decrease in TT was found to border on statistical significance (p = .053) (Thelen et al., 1997), the current results of generally shorter average TT at larger lean angles (but without statistical significance) appear to be comparable with both previous studies. As was reported previously for young males (Thelen et al., 1997; Wojcik et al., 1999), step time (ST) in the present study did not vary with increasing lean angle. The average step length L (around 57–60% BH) in this study was comparable with the results of Wojcik et al. (1999) in which the condition of 20% cable load (equivalent to the lean angle of 17.3°) resulted in the L of around 50% BH for young male participants. Although L was expected to increase with increasing lean angle (due to enlarged base of support at landing and lengthened moment arm for counteracting the moment causing a forward fall) as in previous studies (Thelen et al., 1997; Wojcik et al., 1999), in this study L increased significantly only when the lean angle increased from 12.5° to the other angles. This minor disagreement with previous findings could be explained by regaining balance from a relatively ‘‘comfortable’’ range of forward inclination in this study rather than assessing the participants’ maximum lean angles (Thelen et al., 1997; Wojcik et al., 1999). The present GRF data were compared to those in previous studies. Peak vertical GRF in the present study were comparable with those under the pre-fatigue condition of young participants (Mademli, Arampatzis, & Karamanidis, 2008). Rather than comparing GRF on the stepping foot, in a previous study lower limb joint torques were measured and knee and hip torques (especially in the young male group) were found to increase with increasing forward lean magnitude (Wojcik et al., 2001). The same trend in GRF was not observed in the present results. This is probably because in this study participants’ landing strategies were not restricted to a single one. Since some participants employed forefoot and rear-foot landing interchangeably, substantial variations in peak GRF were resulted. Although all the participants regained balance successfully with a single step, the high success rate might be a result of young age and moderate initial lean angles. In tripping recovery during walking, delayed response reactions which caused insufficient time for correct positioning the recovery leg and unsuccessful balance recovery were observed in older adults (Roos et al., 2008). It can therefore be argued that, under severe balance perturbations or for those with less muscle strength and longer reaction time (as in the elderly people), arm motions which allow extra preparatory movement time may play an important role in balance recovery. There were certain limitations in this study. The major ones were that results obtained from healthy young males might not be applicable to other groups of people, and the lean angles chosen could not represent all kinds of balance perturbations. Since equivocal results on the effect of perturbation uncertainty have been reported (Gilles, Wing, & Kirker, 1999), it is also unclear whether predictable direction and amplitude of the balance perturbations could be used to investigate responses to unpredictable disturbances in daily living. Moreover, recovering balance in just a single step rather than several steps might also affect the results. Although the instructions of limiting the number of steps did not affect young adults’ responses (Cyr & Smeesters, 2007), a minor effect was identified in older adults (Cyr & Smeesters, 2009).

5. Conclusions This study revealed that arm movements could allow more time for executing taking a step in balance recovery. The lengthened total balance time included weight transfer time and stepping time, although duration increase in the latter was significant only under the largest lean angle condition. Delayed response reactions could cause insufficient time for correct positioning the recovery leg and unsuccessful balance recovery in older adults during tripping recovery (Roos et al., 2008). It could therefore be argued that under severe balance perturbations or for those with less muscle strength and longer reaction time, arm motions which allow extra preparation and movement time may play an important role in balance recovery. In contrast, step length, step velocity, and vertical GRF on the stepping foot were unaffected by arm swing. Since various arm swing patterns were observed, future studies are required to investigate the optimal movement strategy for the arms to coordinate with other body segments in balance recovery and in injury reduction.

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Acknowledgements The authors are grateful to the financial support from National Science Council, Taiwan (NSC 101-2410-H-006-107-MY2), and to all the participants in this study for their time and cooperation.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.humov.2014.08.011.

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