Influence of fear of falling on anticipatory postural ... - Springer Link

Eur J Appl Physiol (2011) 111:611–620 DOI 10.1007/s00421-010-1680-7

O R I G I N A L A R T I CL E

InXuence of fear of falling on anticipatory postural control of medio-lateral stability during rapid leg Xexion E. Yiou · T. Deroche · M. C. Do · T. Woodman

Accepted: 28 September 2010 / Published online: 12 October 2010 © Springer-Verlag 2010

Abstract During leg Xexion from erect posture, postural stability is organized in advance during “anticipatory postural adjustments” (APA). During these APA, inertial forces are generated that propel the centre of gravity (CoG) laterally towards stance leg side. This study examined how fear of falling (FoF) may inXuence this anticipatory postural control of medio-lateral (ML) stability. Ten young healthy participants performed a series of leg Xexions at maximal velocity from low and high surface heights (6 and 66 cm above ground, respectively). In this latter condition with increased FoF, stance foot was placed at the lateral edge of the support surface to induce maximal postural threat. Results showed that the amplitude of ML inertial forces generated during APA decreased with FoF; this decrease was compensated by an increase in APA duration so that the CoG position at time of swing foot-oV was located further towards stance leg side. With these changes in ML APA, the CoG was propelled in the same Wnal (unipodal) position above stance foot as in condition with low FoF. These results contrast with those obtained in the literature during quiet standing which showed that FoF did not have any inXuence on the ML component of postural control. It is proposed that ML APA are modiWed with increased FoF, in such a way that the risk of a sideway fall induced by the large CoG motion is attenuated.

Communicated by Dick Stegeman. E. Yiou (&) · T. Deroche · M. C. Do Laboratory CIAMS, Team RIME, UFR STAPS, University of Paris-Sud 11, 91405 Orsay Cedex, France e-mail: [email protected] T. Woodman Institute for the Psychology of Elite Performance, Bangor University, Bangor LL57 2DG, Gwynedd, UK

Keywords Anticipatory postural adjustments · Medio-lateral stability · Fear of falling · Leg Xexion · Motor control

Introduction It is well known that voluntary leg movements performed from the erect posture are preceded by dynamical phenomena in the postural body segments referred to as “anticipatory postural adjustments” (APA; see Bouisset and Do 2008; Massion 1992, for reviews). One of the main functions of these APA is to allow the maintenance of postural equilibrium during leg movement (see Bouisset and Do 2008; Massion 1992, for reviews). Indeed, the act of lifting the swing foot (e.g. during leg Xexion or stepping initiation) induces a reduction in the size of the base of support (BoS), which is then limited to the single stance foot’s contact with the ground. It follows that if the centre of gravity (CoG) remains at the same position, its vertical projection onto the ground will then lie outside the BoS, thus creating a disequilibrium torque towards the swing leg side. Based on the conditions of dynamic stability raised by Pai and Patton (1997) and Hof et al. (2005), a stable posture on the single stance leg could then be reached only when suYcient inertial forces are generated in the opposite direction (i.e. towards the stance leg side). During voluntary leg Xexion or stepping initiation, these inertial forces are invariably generated before the time of foot-oV (FO), i.e. during APA (e.g. Jian et al. 1993; Lyon and Day 1997; Mouchnino et al. 1992; Nissan and Whittle 1990; Nouillot et al. 1992; Yiou 2005). These anticipatory inertial forces thus serve to set the initial (FO) medio-lateral (ML) CoG parameters (position and velocity) that will determine the ML CoG trajectory during the forthcoming voluntary leg movement (Lyon

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and Day 1997; Mouchnino et al. 1992). During leg raising from the erect posture, these ML APA are critical to propel the CoG above the BoS in the Wnal unipodal posture, i.e. they determine the stability of the Wnal postural equilibrium. Consequently, any factor that alters ML APA might endanger postural equilibrium and, as such, increase the risk of a fall. In addition to physiological factors, e.g. those factors associated with deterioration of neural, sensory and/or musculoskeletal systems with ageing or neurological disease (e.g. Parkinson’s), recent studies have shown that psychological factors such as fear of falling (FoF) and related concepts such as low balance conWdence may induce changes in the way postural equilibrium is controlled (e.g. Adkin et al. 2000, 2002; Brown et al. 2006; Carpenter et al. 2001, 2004, 2006; Davis et al. 2009; Hauck et al. 2008). As stressed in these studies, the discrimination between these two factors (psychological and physiological) is, therefore, important in developing adapted intervention programs directed to improve the physical and/or cognitive components of balance deWcit in persons at risk of fall. One classical way of speciWcally investigating the eVects of FoF on postural control is to have young healthy participants performing static or dynamic motor tasks while standing at the edge of diVerent surface heights, i.e. “low-threat” and “high-threat” condition. Studies have conWrmed the eVectiveness of such experimental conditions using participant self-reports of perceived balance conWdence and anxiety (Carpenter et al. 2004) or commonly used physiologic cues of arousal and anxiety (Adkin et al. 2002). The postural organization of the motor task—typically, during quiet standing (QS) (e.g. Adkin et al. 2000; Brown et al. 2006; Carpenter et al. 2001, 2006; Davis et al. 2009) or correction to unexpected perturbations (e.g. Brown and Frank 1997; Carpenter et al. 2004)—is then compared between the lowand the high-threat condition. In the situation with an increased FoF (high-threat condition), it has been repeatedly reported in these studies that the central nervous system (CNS) adopts a protective strategy that shifts the CoG farther back during QS posture (i.e. further away from the forward edge of the support surface), which restricts the CoG motion, during both the static and dynamic motor tasks. With this tighter CoG control, the risk of falling towards the forward edge of the support surface was thus attenuated. Despite an increasing interest on the relationship between FoF and postural control, only one study has investigated the eVect of FoF on the APA associated with a voluntary movement (Adkin et al. 2002). In this study, young healthy participants performed a series of rise-totoes at maximal velocity on low and high surface heights. In line with the above-reported eVects of FoF on postural control, the authors reported that the amplitude (but not the

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Eur J Appl Physiol (2011) 111:611–620

duration) of the APA [as quantiWed with antero-posterior (AP) CoG and centre of pressure (CoP) measures] was reduced with FoF. The contribution to the literature notwithstanding, this study presents some limitations that should be highlighted. Of particular note, rise-to-toes is a relatively simple task that mainly involves simultaneous rotation around both ankles. It is also a symmetrical task with respect to leg requirements and, thus, mainly involves the control of AP balance. The control of ML stability is a priori much less critical than for motor tasks with asymmetrical leg requirements and involving sequential recruitments of many joint degrees of freedom, such as leg Xexion, step initiation, etc. As such, the eVects of FoF on the anticipatory postural control of ML stability could not be revealed in the Adkin et al. (2002) study. However, it is particularly important to address the problems associated with the control of ML stability because lateral falls are common in older adults and are associated with an elevated risk of hip fracture, compared with falls in other directions (e.g. Maki et al. 2000; Maki and McIlroy 2006). Control of lateral stability may thus be an important area for fall-preventative intervention in this population. Of particular interest, recent results from the literature showed that, during bipedal QS at the forward edge of the support surface, FoF had an inXuence on the AP component of postural sway but not on the ML component (Brown et al. 2006; Hauck et al. 2008). This result would suggest that the AP component of postural control is more sensitive to FoF than the ML component. However, it seems more likely that the experimental design used in these studies was not appropriate to reveal an eVect of FoF on the ML component of postural control. SpeciWcally, the direction of threat was limited to the AP plane which may have restricted the eVect of FoF to this sole plane. Also, ML sway during static postural control is known to be smaller than AP sway (e.g. Gatev et al. 1999; Winter et al. 1998). As such, it is possible that, in contrast to AP sway, ML sway is not perceived as a threat to balance in conditions of increased FoF. Consequently, it may not be subjected to tighter control in the high-threat condition as compared to the low-threat condition. If this is true, then dynamic postural tasks involving large ML CoG displacement (such as the leg Xexion from QS posture) should induce ML protective strategies similar to those described during rise-to-toes. The goal of this study is to examine the eVects of FoF on the anticipatory postural control of ML stability in young healthy participants. A series of leg Xexions from the erect posture were purposely carried out from low and high surface heights. Based on the previous results from the literature (Adkin et al. 2002), it was hypothesized that the amplitude of the ML APA would be depressed with FoF, with a consequent greater occurrence of unsuccessful trials,


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i.e. trials where participants have to rapidly replace their swing leg on the support surface to recover equilibrium.

Methods Participants The study was performed on ten young healthy participants (29.7 § 6.18 years, 71.6 § 10.68 kg, 172.4 § 7.43 cm height; 6 males, 4 females). The use of healthy young adults as participants aims to minimize the potential confounds related to the ageing process (Hauck et al. 2008). No participants reported any falls in the previous year. All participants gave written consent after having been informed as to the nature and purpose of the experiment which was approved by local ethics committees. The study conformed to the standards set by the Declaration of Helsinki. Induction of FoF FoF was modiWed through alterations to the surface height on which participants stood (Adkin et al. 2000, 2002; Brown et al. 2006; Carpenter et al. 2001, 2004, 2006; Davis et al. 2009; Hauck et al. 2008). From the low surface height (low-threat condition), participants stood upright on a large force plate (120 £ 60 cm, Bertec, Columbus, USA) placed on the ground. The distance between the ground and the surface of the force plate was 6 cm. On the high surface height (high-threat condition), participants stood upright on a 60 cm height £ 30 cm depth £ 44 cm large iron box placed on the force plate (Fig. 1a, b). In this condition, the external side of the nondominant (stance) foot was positioned at the lateral edge of the wooden box (Fig. 1b). The CoG motion during the leg raising was mainly directed towards this edge and postural threat during leg movement was thus mainly exerted towards this direction. The big toes of the feet were positioned at the front edge of the box. The distance between the heels and the backward edge of the box was about 2 cm but depended on the size of each participant’s feet. The distance between the external boundary of the swing foot and the lateral edge of the box was about 4 cm but depended on individuals’ feet width in the initial posture. In order to increase FoF for participants, no harness system was used; however, two spotters were present to prevent actual falling (Carpenter et al. 2006). Perceived conWdence, stability and FoF There is evidence that psychological measures relating to gait and balance are task-speciWc (Carpenter et al. 1999)

Fig. 1 Initial/Wnal posture and experimental set-up. a Schematic proWle of one participant in the initial and Wnal posture in the low- and high-threat conditions. b Localization of the stance and swing feet on the box (high-threat condition) in the initial and Wnal posture from above

and that there is a need to involve independent evaluation tools for conWdence, fear, anxiety, and perceived stability. As such, task-speciWc tools have been developed, which are sensitive to change in postural threat and related to concomitant change in postural control (Carpenter et al. 2006). Self-reported perceptions were assessed in the low- and high-threat conditions. ConWdence, FoF, and coping eYcacy were assessed prior to the task, whereas anxiety and perceived stability were assessed after the task. Participants rated how conWdent they felt, how fearful they felt, and how stable they felt using a 100 mm visual analogue scale (VAS) anchored by two labels: low levels of conWdence, fear, and stability on the left side and high levels of conWdence, fear, and stability on the right side. Using an incremental scale between 0% (“not at all”) and 100% (“completely”), participants were also required to estimate their coping eYcacy in their ability to avoid a fall, maintain concentration, overcome worry and reduce nervousness about balancing or falling during the experimental task. State anxiety was assessed using a French translation of a 16-item questionnaire commonly used in past research on postural anxiety (e.g. Carpenter et al. 2006). Three scales probed distinct elements of state anxiety: somatic (6 items), worry (4 items) and concentration/disruption (6 items). Participants scored each item using a nine-point Likert scale, ranging from 1 (“I did not feel this at all”) to 9 (“I felt this extremely”). Scores for each of the 16 items were summed to determine a total anxiety score.

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Initial/Wnal posture and experimental conditions In both experimental conditions, the participants initially stood barefoot in a natural upright posture, with feet shoulder-width apart, and with the two hands joined in the back. The feet positions were marked on a millimetric paper placed over the support surface (the force plate or the box) and these marks were used as a visual reference on which the participants had to position themselves under the supervision of the experimenters. These marks were also used to delimit the BoS in the initial (bipodal) and the Wnal (unipodal) posture. The same marks were used in the low- and high-threat conditions. The iron box was placed on the force plate so that the marks in the high-threat condition were vertically superimposed to those marks in the lowthreat condition. During the whole experiment, the gaze was directed towards a small white cross placed at the eye level and 2 m distant. In each experimental condition, the participants had to perform ten trials of leg Xexion. The swing leg was the preferred leg. The task was performed at a maximal velocity following an acoustic signal (reaction time situation). Participants had to maintain their Wnal (unipodal) posture for 5 s before they could replace the swing foot on the marks of the support surface. In the Wnal posture, the knee and hip of the swing leg had to be 90° Xexed, while the knee and hip of the stance leg had to be extended (Fig. 1a). Participants had their hands clasped in their back during the whole experiment. Experimenters checked that the correct initial and Wnal postures were adopted. Instructions regarding the maximal velocity, reaction time, gaze direction, and maintenance of the Wnal posture were repeatedly recalled. The low- and high-threat conditions were randomly performed among participants to avoid the eVects of practice and/or fatigue. In addition, participants rested for 5 min between the two experimental conditions. The between-trial delay was approximately 10 s. No practice trials were allowed before data recordings. Data recordings The ground-reaction forces and the moments along the three directions were obtained with the force plate. The CoG acceleration along the ML (y⬙G) axis was obtained by the ratio between the ground-reaction force and the mass of the participant (Newton’s second law). The CoG velocity (y⬘G) and displacement (yG) were obtained with a successive integration of the y⬙G trace. This method has been reported to be valid over the brief time interval investigated (e.g. Lepers and Brenière 1995; McIlroy and Maki 1999; Zettel et al. 2002a, b). The instantaneous displacements of the CoP along the ML (yP) and the antero-posterior axis (xP) were obtained with the following approximations:

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yP = Mx/Rz (Mx: moment around the antero-posterior axis; Rz: vertical ground-reaction force) and xP = My/Rz (My: moment around the ML axis). These biomechanical data were digitized at a sampling rate of 500 Hz. Experimental variables The AP and ML CoG position in the initial posture were obtained by averaging the AP and ML CoP position over a 200 ms time window recorded during the QS period. The duration of APA corresponded to the time delay between the onset variation of the y⬙G trace from the baseline (t0) and the FO time. t0 corresponded to the instant when the y⬙G trace exceeded the mean y⬙G value (§2 standard deviations) in the QS posture. This mean “background” y⬙G value was obtained by averaging the y⬙G trace (over a 200 ms time window) during the QS period that preceded the onset of the acoustic signal. The FO time was estimated with the yP trace and with pressure captors placed under the toe of the swing foot. The amplitude of ML APA corresponded to the maximum value of the y⬙G, y⬘G and yG trace during the APA time window (Yiou and Do 2010). In order to test whether the conditions of dynamic stability at the FO time were equivalent between the high- and the lowthreat condition, we also calculated the “extrapolated centre of mass position” (YcoM) at the FO time (Hof et al. 2005): YcoM = yG + y⬘G/0, where 0 = qg/l is the eigenfrequency of the body modelled as an inverted pendulum of length l, l = H £ 0.575 (H is the body height) and g is the acceleration of gravity = 9.81 m/s2. According to Hof et al. (2005), YcoM should be within the BoS unless stability could not be maintained in the Wnal (unipodal) posture. Note that the same YcoM value can be reached with diVerent combinations of yG and y⬘G values. The maximal yG position reached in the Wnal unipodal posture was used to compare the eYciency of ML APA to propel the CoG towards stance leg side. The maximal CoG velocity along the vertical axis and the reaction time were taken as indicators of the performance of the leg raising (focal performance). If one considers that the displacement of the “postural” body segments is negligible as compared to the displacement of the “focal” body segments (i.e. the segments composing the leg that Xexed), then it can be stated that the maximal vertical CoG velocity is representative of the maximal velocity of the hip Xexion (e.g. Yiou 2005). Analyses on three participants showed that these two quantities were indeed highly correlated and peaked at the same time. The reaction time corresponded to the time delay between the departure signal and the onset rise of the y⬙G trace. In a few trials, participants repositioned their swing leg on the support surface to recover balance. The percentage of unsuccessful trials was quantiWed in each condition. In order to provide insight into these trials,


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we tested the hypothesis that YcoM in these trials did not reach a suYcient value to ensure stability in the Wnal posture. In this hypothesis, the YcoM value(s) obtained in the unsuccessful trial(s) should systematically be lower than the lowest YcoM value obtained in the successful trials (where, by deWnition, YcoM reached a suYcient value to ensure stability in the Wnal posture). When more than one single trial was unsuccessful, only the highest YcoM value was considered. In addition, we visually checked oZine whether the YcoM position obtained in each trial was under the BoS limits. This was done with the stance foot’s marks on the millimetric paper placed over the support surface. Statistics Repeated measures ANOVAs were conducted on each biomechanical and psychological variable with condition (low- vs. high-threat condition) as a within-participants factor. Alpha was set at 0.05.

Results Perceived conWdence, stability and FoF As expected, surface height was explicitly perceived as a postural threat. The high-threat condition signiWcantly increased self-reported FoF (F[1,9] = 7.75, P = 0.02) and signiWcantly decreased balance conWdence (F[1,9] = 9.04, P = 0.01). The high-threat condition also decreased participants’ coping eYcacy in the ability: to avoid a fall (F[1,9] = 9.96, P = 0.02), to overcome worry (F[1,9] = 5.80, P = 0.04), and to reduce nervousness about balancing or falling during the experimental task (F[1,9] = 9.85, P = 0.01). In contrast, coping with the ability to maintain concentration was not signiWcantly inXuenced by height condition. There was also no signiWcant diVerence between low- and high-threat conditions on retrospective measures of perceived anxiety and stability.

Fig. 2 Biomechanical traces of leg Xexion (one trial in one representative participant) and presentation of the main experimental variables. y⬙G, y⬘G, yG, z⬘G: medio-lateral (ML) centre of gravity (CoG) acceleration, ML CoG velocity, ML CoG displacement and vertical CoG velocity, respectively. t-2, t-1, t0, t1, t2: onset of quiet standing (QS) posture time window, triggering of acoustic signal, onset variation of the y⬙G trace from the baseline, swing foot-oV, time of maximal performance, respectively. SW, ST: swing and stance leg side, respectively. QS, RT, APA, LF: time windows for QS posture, reaction time, ML APA and time for leg Xexion to reach maximal velocity, respectively. y⬙GAPA, y⬘GFO, yGFO, z⬘GMAX: peak of ML CoG acceleration during APA, ML CoG velocity/displacement at the foot-oV time, peak of vertical CoG velocity, respectively

and with the yP trace). The vertical CoG velocity (which was taken as an indicator of the focal performance) reached a peak value during the voluntary leg movement. InXuence of FoF on the leg Xexion-related variables

Description of the biomechanical traces The global time course of the biomechanical traces was very similar in the low- and high-threat conditions (Fig. 2). Details of the diVerences between these two conditions are reported in the below section. During the APA, the y⬙G trace reached a peak value towards the stance leg, while the yP trace reached a peak value towards the swing leg side (these two traces mirrored each other during APA). The y⬘G trace reached a peak value a few ms before the time of swing FO. The CoG was continuously displaced towards the stance leg side and reached a maximal value above the stance foot in the Wnal unipodal posture (this statement was checked with the stance foot’s mark on the support surface

Statistical analysis showed that both the initial AP and ML CoG positions were not signiWcantly diVerent between the low- and high-threat conditions (P > 0.05). Statistical analysis also showed that both the peak value of the y⬙G trace (F[1,9] = 24.72, P < 0.001) and the y⬘G value at the FO time (F[1,9] = 13.81, P = 0.01) were signiWcantly lower in the high-threat condition than in the low-threat condition (Fig. 3). In contrast, the duration of the ML APA (F[1,9] = 25.95, P < 0.001) and the yG value at the FO time (F[1,9] = 7.08, P = 0.03) were both signiWcantly higher in the high-threat condition than in the low-threat condition. Reaction time was also signiWcantly higher in the high-threat condition than in the low-threat condition (F[1,9] = 25.95, P < 0.001).

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the low-threat condition (4 § 5%). Eight participants (out of the ten) displayed one or two unsuccessful trials in the low- and/or the high-threat condition (the two other participants did not display any unsuccessful trials). In the low-threat condition, the highest YcoM value of the unsuccessful trials was lower than the lowest YcoM value of the successful trials for two participants, and it was higher for three participants. In the high-threat condition, the highest YcoM value of the unsuccessful trials was lower than the lowest YcoM value of the successful trials for two participants and it was higher for four participants. When the low- and high-threat conditions were pooled together, the highest YcoM value of the unsuccessful trials was lower than the lowest YcoM value of the successful trials for only one participant. In this participant, the YcoM was not under the BoS.

Discussion This work examined the inXuence of FoF on the anticipatory postural control of ML stability. FoF was induced by having participants standing at diVerent surface heights above the ground (6 and 66 cm) and performing a series of leg Xexions. It was hypothesized that the amplitude of the ML APA would be altered with FoF, with a consequent greater occurrence of unsuccessful trials, i.e. trials where participants have to rapidly replace their swing leg on the support surface to recover equilibrium. InXuence of FoF on the CoG position in the initial posture Fig. 3 Comparison of the spatio-temporal features of APA in the lowand high-threat condition. APAd, y⬙GAPA, y⬘GFO, yGFO: duration of medio-lateral (ML) APA, peak of ML centre of gravity (CoG) acceleration during APA, ML CoG velocity and ML CoG displacement at the foot-oV time, respectively. Positive mean value indicates displacement, velocity or acceleration towards the stance leg side. Values given are mean § 1 standard deviation (all participants together). *, **, ***Statistical diVerence with P < 0.05, P < 0.01, P < 0.001, respectively

Statistical analysis further showed that the performance of the leg Xexion, in terms of maximal vertical CoG velocity (mean value in the two experimental conditions, 0.23 § 0.05 m/s), the maximal yG position (0.11 § 0.01 m) and the YcoM value (0.12 § 0.02 cm) were not signiWcantly diVerent between the two experimental conditions (P > 0.05). Comparison of the “extrapolated centre of mass position” at the FO time between the successful and the unsuccessful trials The percentage of unsuccessful trials was not signiWcantly diVerent between the high-threat condition (10 § 9%) and

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The results of the present study showed that neither the initial ML CoG value nor the AP CoG value changed with increased FoF. This discrepancy with the literature (in regard to the initial AP CoG value) might be ascribed to the lower surface height used in the present study (60 cm) as compared to other studies in the literature (e.g. 160 cm in Adkin et al. 2002 and in Carpenter et al. 2004; 320 cm in Adkin et al. 2008). With this lower surface height, FoF might not be suYcient to induce change in the initial CoG position. Whatever the reason(s), the present results show that the FoF-induced modiWcations of ML APA cannot be ascribed to any change in the initial CoG position. This precision is important because such changes have the potential to induce modiWcations in the APA features of subsequent voluntary leg movement (e.g. Azuma et al. 2007; Mille and Mouchnino 1998; Patchay and Gahéry 2003). Finally, the reported change in ML APA cannot be ascribed to a diVerence in the focal movement velocity either, as the peak of vertical CoG velocity remained unchanged between the two experimental conditions.


FoF may elicit diVerent protective strategies for voluntary whole-body movement Taken together, the present results are in agreement with the current view from the literature that FoF modiWes the anticipatory postural control of stability during voluntary whole-body movement (Adkin et al. 2002). However, the present results diVer from the literature on several aspects. Adkin et al. (2002) showed that the organization of APA during rise-to-toes from the erect posture was modiWed when participants stood at the edge of a high surface. In this situation, the forward CoG displacement and the forward CoG velocity at the onset of the voluntary leg movement both reached lower values, while the APA duration remained unchanged. The authors interpreted this attenuation of the initial AP CoG parameters as reXecting postural strategy directed to protect individuals from falling towards the forward edge of the surface during task execution. The authors also stressed that, with these postural changes, the risk that the CoG could not be propelled above the BoS in the Wnal posture on the toes increased, thus making participants return backward to their initial posture with feet in full contact with the surface more often. Indeed, results showed that the frequency of unsuccessful trials increased with FoF, and also that the performance of the rise-to-toes (maximal AP CoG velocity) decreased. The eYciency of the postural strategy used to perform the motor task was, therefore, mitigated. The present result that both the peak of anticipatory y⬙G value and the y⬘G value at the onset of the voluntary leg movement were lower with increased FoF is in line with the study of Adkin et al. (2002). This result can be interpreted as reXecting a protective strategy directed to attenuate the ML inertial forces that tend to induce sideway fall towards the stance leg side. It thus seems that the ML dynamics induced by the leg Xexion was perceived as a postural threat to balance in the high-threat condition. This result contrasts with recent studies in the literature which reported that, during standing at the forward edge of the support surface, the ML component of postural sway was not sensitive to FoF (Brown et al. 2006; Hauck et al. 2008). This discrepancy with the literature might be ascribed to the experimental setup used in the present study which simultaneously induced lateral and forward threat (bi-directional threat), while previous studies in the literature only induced forward threat (uni-directional threat). It is thus possible that the ML threat was speciWcally responsible for the changes of ML APA. This statement remains somewhat conjectural however, since the speciWc inXuence of the AP threat on ML APA could not be quantiWed in the present study. Alternatively, the discrepancy with the literature might be ascribed to the much higher ML CoG range motion reached during leg Xexion (t11 cm) than during QS (t1 cm; Gatev et al. 1999),

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which represents a greater threat to balance. Such large CoG motion may possibly be subjected to a tighter central control in condition with FoF, independently on the direction of threat. Clearly, future studies should be undertaken to clarify these diVerent issues. In marked contrast with the study of Adkin et al. (2002), results also showed that, at the FO time, the CoG position was located further towards the direction of the postural threat (i.e. towards the stance leg side) in the high-threat condition as compared to the low-threat condition. The greater initial CoG displacement was presumably made possible with the increase in the duration of APA. Despite these changes in initial ML CoG parameters with increased FoF, the initial conditions of dynamic stability remained unchanged, as attested by the equivalent “extrapolated centre of mass” value (Hof et al. 2005) between the low- and high-threat conditions. The CoG could thus be propelled in the same Wnal ML position above the BoS as in the lowthreat condition, i.e. it reached a stable position. These results suggest that, in the condition with increased FoF, the CNS compensated for the lower initial ML CoG velocity by an increase in the ML APA duration so that the CoG could be propelled further towards stance leg side at the FO time. With this strategy, the CoG was brought nearer to its position of equilibrium at the time when the voluntary leg raising was initiated. It thus seems that the CNS more carefully ensured that the conditions of ML stability could eVectively be reached in the Wnal posture before triggering the voluntary movement. This statement adds further insight into the notion of “protective” postural strategy in conditions of increased FoF. In addition to the slowdown of APA development (as featured with the longer APA duration and the lower CoG velocity at the FO time), results showed that the time required to initiate the APA (the reaction time) was slightly but signiWcantly lengthened with increased FoF. FoF thus had a slight aversive eVect on this aspect of the focal movement performance (in contrast to the maximal vertical CoG velocity which remained unchanged). This observation of delayed reaction time at higher surface height is consistent with previous results by Gage et al. (2003). This result might be a consequence of the more careful postural strategy used to control equilibrium during lower limb movement. Similar interpretation is classically provided in the literature to explain the increase in reaction time with ageing (e.g. Schmidt 1984, for review). The protective strategy used in the present study markedly diVers from the one described during rise-to-toes (Adkin et al. 2002), where the CNS did not delay the onset of the voluntary movement in the condition with increased FoF, which probably resulted in a more unstable Wnal posture on the toes and a lower focal movement velocity. The present results thus suggest that FoF in young healthy

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participants might elicit diVerent protective strategies for voluntary whole-body movement. Protective strategy with FoF might depend on environmental constraints The question of the factors underlying the use of diVerent protective strategies in condition with increased FoF might be raised. First, it is possible that the constraint to remain stable in the Wnal posture was greater in the present study than in the study of Adkin et al. (2002). Indeed, when participants of the present study could not maintain the Wnal unipodal posture (the “unsuccessful trials”), the swing leg had to be repositioned rapidly on the support surface and without any visual guidance (since the gaze was directed forward) for balance recovering. The vertical distance covered by the swing foot from its elevated position in the unipodal posture to the support surface was approximately 50 cm, i.e. it was relatively large. Because the surface on which participants stood was relatively small in the highthreat condition (see Fig. 1), there was an actual threat that the swing foot (or part of it) could be repositioned out of the support surface, which would endanger balance. Such an environmental constraint on foot placement may have forced individuals to develop a more careful postural strategy for leg Xexion aimed at avoiding recovering balance with the swing foot. Such an additional environmental constraint was not present during rise-to-toes at the edge of the support surface. Indeed, in case the Wnal posture on the toes could not be maintained (the “unsuccessful trials”), participants had the opportunity to come back to their initial posture by simply pivoting downwards around their ankles, without any risk of unbalance due to repositioning their heels out of the support surface. The inXuence of environmental constraint on swing foot placement had previously been reported to inXuence ML APA during rapid-triggered stepping reactions over frontal obstacle following sudden force plate translation (Zettel et al. 2002a, b). The authors reported that when no environmental constraint on swing foot placement was imposed (unconstraint trials), the CNS developed a “hybrid” control of ML stability, including the development of ML APA combined with a strategy of lateral swing foot placement to recover balance. Alternatively, when lateral stepping was obstructed by lateral barriers (constraint trials), the CNS was able to upregulate the ML APA amplitude in order to avoid lateral swing foot placement. Similarly, in the present study, the CNS might have adapted the ML APA features in order to avoid the potential risks of disequilibrium associated with rapid swing foot repositioning on the support surface. One other factor that may underlie the use of diVerent protective strategies in condition with increased FoF stems from the experimental conditions used to elicit FoF.

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SpeciWcally, the diVerential of surface height between the low- and the high-threat condition (“relative” surface height) was lower in the present study (60 cm) than in the studies of the literature (e.g. 100 cm in Carpenter et al. 2004; 120 cm in Adkin et al. 2002; 240 cm in Davis et al. 2009). Consequently, it is possible that the increase in FoF and related psychological variables from the low- to the high-threat condition was also lower. In addition, the “absolute” surface height used to induce FoF was lower in the present study (66 cm) than in previous studies (e.g. 160 cm in Adkin et al. 2002 and in Carpenter et al. 2004; 320 cm in Adkin et al. 2008). As such, it is possible that FoF in the high-threat condition was also lower as argued above (see “InXuence of FoF on the CoG position in the initial posture”). Having said this, the results from the present study conWrm as a whole that the high height condition signiWcantly increased self-reported FoF and signiWcantly decreased related balance conWdence as well as the coping eYcacy in the ability to avoid a fall, to overcome worry, and to reduce nervousness about balancing or falling during the experimental task. Although the experimental manipulation was thus successful, future studies should investigate how FoF and the associated changes in ML APA evolve with increasing “absolute” and “relative” surface heights. One reason for using a relatively low surface height was to increase ecological validity. SpeciWcally, if we are to understand the mechanisms underlying the incidence of falling in the elderly, it is important that researchers adopt experimental paradigms that are realistic in terms of FoF; such ecological validity points to surface heights that are in the region of two or three steps on stairs, for example. Another related rationale for using a relatively low surface height in the high-threat condition of the present study was to allow comparison of the present results with the results of a future study with the elderly, who may not tolerate performing leg Xexion at a surface that is too high (unpublished observations). Brown et al. (2006) recently failed to reveal an eVect of age on the relationship between FoF and postural control during the maintenance of balance in QS posture. A future study will examine whether the present experimental conditions involving more dynamical task are more discriminative at diVerentiating postural control in these two populations. Can unsuccessful trials be ascribed to altered initial conditions of dynamic stability? To answer this question, the YcoM values were compared between the successful and the unsuccessful trials, in both the low- and high-threat conditions. Results showed that when the low- and high-threat conditions were pooled together, the highest YcoM value of the unsuccessful trials was indeed lower than the lowest YcoM value of the


successful trials for only one participant. In this particular participant, the YcoM value was not under the BoS which is a necessary condition for stability (Hof et al. 2005). These results suggest that, in most participants, the unsuccessful trials might not be ascribed to altered initial conditions of dynamic stability. Unsuccessful trials might then rather be ascribed to alteration of postural control mechanisms involved in the maintenance of the QS unipodal posture. This statement might explain why the percentage of unsuccessful trials was not lower in the high-threat condition as compared to the low-threat condition (it was equivalent), although the postural strategy to control equilibrium during leg Xexion was more “careful” in the former condition.

Conclusion This work investigated the inXuence of FoF on the anticipatory postural control of ML stability during rapid leg Xexion. Results showed that when FoF increased, the CoG was displaced further towards the stance leg side at the FO time and the ML CoG velocity was attenuated. These changes of initial ML CoG parameters with increased FoF could not be explained by diVerent initial conditions of dynamic stability. These results show that, with FoF, the conditions of ML stability could eVectively be reached in the Wnal posture before triggering the voluntary movement. The present results thus provide new insights into the inXuence of FoF on postural control in young healthy participants. Future research will examine whether older adults develop similar adaptive postural behaviour with increased FoF.

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