an effect of posture on anticipatory anxiety

Intern. J. Neuroscience, 118:227–237, 2008Copyright C© 2008 Informa Healthcare USA, Inc.ISSN: 0020-7454 / 1543-5245 onlineDOI: 10.1080/00207450701750463

AN EFFECT OF POSTURE ON ANTICIPATORYANXIETY

DARREN M. LIPNICKIDON G. BYRNE

School of PsychologyThe Australian National UniversityCanberra, Australia

This study investigated the effects of body posture on state anxiety and psychologicalstress. Twenty normal adults performed a demanding mental arithmetic task in bothstanding and supine conditions, with subjective measures of anxiety and stressobtained before, immediately, and 10 min after the task. Participants were found toexperience anticipatory anxiety when standing, although not when supine. Themechanism underlying this effect remains to be determined, although it couldinvolve a postural difference in baroreceptor load.

Keywords anxiety, baroreceptors, posture, psychological stress, standing, supine

INTRODUCTION

Body posture has been shown to influence various aspects of psychologicalfunctioning, including arousal (Caldwell et al., 2003; Caldwell et al., 2000;Cole, 1989; Elliott et al., 2005; Vaitl & Gruppe, 1990), somatosensation(Peru et al., 2006; Shimoda & Ikuta, 2000), visual detection (Caldwell et al.,2003; Peru & Morgant, 2006; Vercruyssen & Simonton, 1994), and cognition(Lipnicki & Byrne, 2005; Schulman & Shontz, 1971). There is reason to thinkthat posture also influences state anxiety and psychological stress, with onepotential mechanism for such a relationship involving baroreceptors.

Received 9 June 2007.Address correspondence to Darren M. Lipnicki, Zentrum fur Weltraummedizin Berlin,

Arnimallee 22, 14195 Berlin, Germany. E-mail: [email protected]

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228 D. M. LIPNICKI AND D. G. BYRNE

Baroreceptors are stretch-sensitive sensory receptors in the aorta, carotidarteries, heart, and lungs. An increase in either blood pressure or local bloodvolume increases the load on baroreceptors, in turn elevating their level ofstimulation. Baroreceptors convey their stimulation level to cardiovascularcenters in the brainstem, and it is through the regulation of autonomic nervoussystem activity by these centers that baroreceptors have a crucial role in bloodpressure homeostasis (Mohrman & Heller, 2003). One situation in which thishomeostatic role is called on is a change in body posture. To illustrate, uponstanding from a recumbent position gravity draws blood away from the upperbody and into the legs; this unloads baroreceptors, which in turn typicallyinvokes compensatory elevations in vascular resistance and heart rate thatprevent blood pressure (and cerebral perfusion) from falling. In contrast tothe standing position, with a greater volume of blood in the upper bodybaroreceptors are loaded when lying down, which typically results in lessvascular resistance and a slower resting heart rate when lying down than whenstanding up (Mohrman & Heller, 2003; Smith & Ebert, 1990).

In addition to their homeostatic role baroreceptors induce effects notdirectly related to blood pressure maintenance. These include reductions inboth cortical arousal and sensitivity to painful stimuli when baroreceptors arestimulated, as shown in animals (Randich & Maixner, 1984; Vaitl & Gruppe,1991) and humans (Rau & Elbert, 2001). Further, it is possible that baroreceptorstimulation reduces anxiety (Dworkin, 1988; Dworkin et al., 1994). Cited assupport for this idea are studies in rats, in whom behaviors thought to reflectanxiety were reduced by pharmacological elevations in blood pressure; an effectabolished when baroreceptors were denervated (Dworkin et al., 1979; Szekelyet al., 1963). It has also been suggested that baroreceptor stimulation can reducepsychological stress, with human participants reporting greater stress followinga demanding mental task when a task-associated blood pressure elevation waspharmacologically blocked with a peripherally acting β-blocker (Schweizeret al., 1991).

Given the relatively high load on baroreceptors, the extra-homeostaticeffects of baroreceptor stimulation could be expected to develop when lyingdown (as compared to a more upright posture). In keeping with this notionare findings of lower cortical arousal (Cole, 1989; Vaitl & Gruppe, 1990) andreduced sensitivity to electrical stimuli (Shimoda & Ikuta, 2000) in participantswhen supine than when tilted head-up toward the vertical. Accordingly, ifbaroreceptor stimulation reduces anxiety (Dworkin, 1988; Dworkin et al., 1994)and psychological stress (Schweizer et al., 1991) then it could be expected thatthese negative affective states will be experienced less intensely when lying

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POSTURE AND ANXIETY 229

down than when in a more upright posture. Conflicting with this idea, however,reports of mental task performance tending to be rated as more stressful whensupine than when tilted head-up to 70◦ (Sloan et al., 1995), and for state anxietyto be similar across supine, seated and standing conditions (Hennig et al., 2000).The present study addressed this discrepancy through a more comprehensiveinvestigation than those previously reported. This involved obtaining measuresof state anxiety and psychological stress in standing and supine conditionsduring rest periods and in association with performing a demanding mentalarithmetic task.

MATERIALS AND METHODS

Participants

Data from 20 volunteers (16 women) aged 18 to 33 years (mean ± SD = 20.6 ±4.2) were used in the study. Participants were screened with the trait form ofthe State-Trait Anxiety Inventory (STAI) (Spielberger et al., 1983): the mean(± SD) score was 37.0 ± 7.5. An additional participant whose score exceededa relevant norm (Creamer et al., 1995) by three SDs was excluded. Two furtherparticipants were excluded after becoming faint while standing. Participantsprovided informed written consent before the study, which was approved bythe Australian National University Human Research Ethics Committee.

Subjective Measures

State anxiety was assessed with both the short-form of the STAI state scale(Marteau & Bekker, 1992) and a 0–10 scale (Benotsch et al., 2000; Lampicet al., 1996). Psychological stress was measured with a 0–10 scale (Ng et al.,2003; Pruessner et al., 2003). The 0–10 scale statements were structured likeSTAI short-form statements, for example, “How anxious you are feeling rightnow, that is, at this moment”; participants circled a number from 0 (Not atall anxious) to 10 (Extremely anxious). Mental arithmetic task difficulty wasassessed with a 0–10 scale. A ratings sheet with appropriate statements wasprovided only when required and collected immediately upon completion (theexperimenter was not present while responses were made).

Procedures

Each participant attended two sessions; these were held 2–10 days apart(mean ± SD = 4.5 ± 2.1) and began at a similar time of day. The mental

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arithmetic task was performed in one session (task) and not the other (control);session order was counterbalanced across participants. Both task and controlsessions contained standing and supine conditions, the order of which wascounterbalanced across participants (although the same for a given participantacross sessions). Mental arithmetic task instructions were to alternativelysubtract 13 and add 5 from a starting number as quickly and accurately aspossible, voicing the response after each operation (as per Schweizer et al.,1991). An error resulted in being asked to begin again from the starting number:800 in one postural condition and 850 in the other (counterbalanced across theseand not revealed until the task was to begin). The task was practiced before theexperimental procedures of a task session until correct provision of the first 6responses starting from 660.

The experimental procedures were conducted in a bare cubicle, and beganwith a participant sitting quietly for 5 min before either standing or lying supineon a mattress (with pillows for head support). The participant then rested for8 min before completing a ratings sheet, spent another 2 min resting, and theneither performed the mental arithmetic task or rested (control session) for 2 min;a second ratings sheet was completed immediately afterward. A further 10 minwere spent resting before completing a third ratings sheet. The participant wasthen seated, and these procedures (beginning with sitting quietly for 5 min)were repeated for the alternate postural condition. Participants were asked totry and stay still and quiet while resting and to remain awake with their eyesopen at all times (this was monitored over closed circuit television, with areminder to keep the eyes open needed only rarely).

Heart Rate

Heart rate was measured throughout both the experimental and control sessionswith a Finapres 2300 Blood Pressure Monitor (Ohmeda, Madison, WI). Thisenabled an effect of posture on baroreceptor activity to be demonstrated.

Analysis

State anxiety and psychological stress difference scores were produced bysubtracting control session values from corresponding task session values;this was done to control for any variation in negative affect that developedsimply as a function of time. Difference scores were analyzed using the mul-tivariate approach to repeated-measures ANOVA. Anxiety analyses (including

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Figure 1. Mean (± SEM) difference scores for state anxiety (0–10 scale; STAI short-form) andpsychological stress (0–10 scale) before (Baseline), and immediately (Task) and 10 min (Recovery)after the mental arithmetic task, in both the standing and supine conditions. Scores are task sessionvalues minus corresponding control session values.

follow-ups) were doubly multivariate, with scores from both the 0–10 scaleand STAI short-form being dependent variables.

RESULTS

State Anxiety

Anxiety difference scores (Figure 1) were influenced by rating period (F(4,16) =6.80, p < .01), being greater immediately after the mental arithmetic task thanat baseline (F(2,18) = 10.99, p = .001) and after recovery (F(2,18) = 13.87, p <

.001). A posture by period interaction (F(4,16) = 4.62, p < .05) resulted frombaseline anxiety difference scores being greater in the standing condition thanin the supine condition (F(2,18) = 3.73, p < .05). This, in turn, stems fromhigher raw baseline anxiety scores (Figure 2) in the task session than in the

Figure 2. Mean (+ SEM) raw scores for state anxiety (0–10 scale; STAI short-form) andpsychological stress (0–10 scale) at baseline in the standing and supine conditions of both thecontrol and task sessions.

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control session for the standing condition (F(2,18) = 9.92, p = .001) but not thesupine condition (p > .05). These results suggest that participants experiencedanticipatory anxiety when standing, although not when supine; a conclusionsupported by baseline anxiety scores being higher when standing than whensupine in the task session (F(2,18) = 13.12, p < .001), and no different betweenpostural conditions in the control session (p > .05).

Psychological Stress

There was a main effect of rating period for stress difference scores (Figure 1;F(2,18) = 14.42, p < .001), which were greater immediately after the mentalarithmetic task than at baseline (F(1,19) = 23.46, p < .001) and after recovery(F(1,19) = 30.38, p < .001). Stress difference scores exhibited no statisticallysignificant effects of postural condition, with neither a main effect nor a postureby period interaction (p > .05). Nevertheless, a pattern of baseline data similarto those indicating an influence of posture on anticipatory anxiety was observedfor psychological stress (see Figure 2).

Mental Arithmetic Task

A trend for more mental arithmetic task responses when standing (mean ± SD =27.0 ± 11.5) than when supine (25.0 ± 11.9; t(19) = 1.98, p < .07) is likely tobe due to the slightly (although non-significantly, p > .05) higher number oferrors made when supine (1.9 ± 1.6) than when standing (1.6 ± 1.8). This isbecause the time taken to restart the sequence following an error detracted fromthe time available for providing responses. It can thus be concluded that mentalarithmetic task performance was unaffected by postural condition. Difficultyratings did not differ between the standing (5.3 ± 2.2) and supine (5.1 ± 2.5)conditions (p > .05).

Heart Rate

Heart rate was significantly greater (p < .001) at baseline when standing(mean ± SD = 94.0 ± 14.4 bpm and 96.6 ± 15.3 bpm for the control andexperimental sessions, respectively) than when supine (72.3 ± 10.2 bpm and74.9 ± 12.0 bpm). Heart rate reactivity to the mental arithmetic task (tasksession value minus control session value) was significantly greater (p < .01)when supine (15.1 ± 13.2 bpm) than when standing (6.2 ± 14.5 bpm).

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DISCUSSION

The major finding of the current study was an influence of posture onanticipatory anxiety, which participants experienced when standing, althoughnot when lying down. Intrinsic to this result was an anxiogenic environmentalcontext (i.e., an impending mental arithmetic task). In the absence of ananxiogenic context posture does not appear to influence state anxiety underresting conditions, at least in participants who present with low state anxiety,as per both the control session data of the current study and a previous report(Hennig et al., 2000). It remains to be seen if this is also the case for participantswith higher overall levels of state anxiety.

Although never aiming to directly test the idea, the current study wasmotivated by the possibility that baroreceptor stimulation reduces anxiety(Dworkin, 1988; Dworkin et al., 1994). When lying down, an increase inblood volume load increases the extent to which baroreceptors are stimulated(Mohrman & Heller, 2003; Smith & Ebert, 1990). One manifestation ofthis is a reflex decrease in heart rate, as demonstrated in the current studythrough participants having a lower heart rate when supine than when standing.Accordingly, the absence of anticipatory anxiety in participants when supinecould stem from an anxiolytic effect of the relatively high load on baroreceptorsin that posture (Smith & Ebert, 1990). Potentially underlying any such effectis a baroreceptor-mediated reduction in cortical arousal (Dworkin, 1988) whensupine (Cole, 1989; Vaitl & Gruppe, 1990). The present finding could beinterpreted as giving credence to assumptions of relaxation procedures beingmost effective when lying down (see Garvin et al., 2001). However, ratherthan an anxiolytic effect when supine, it is possible that postural differences inanticipatory anxiety arose from an anxiety response having been facilitated, orprimed to develop (see Berntson et al., 1998), when standing.

A number of mechanisms could potentially facilitate or prime an anxietyresponse when standing, or be otherwise implicated in an influence of postureon anticipatory anxiety. Peripheral physiological activity may play an importantrole in affective states (Damasio, 2003; James, 1884; Lange, 1912). Consistentwith this, Cameron et al. (2000) suggested that peripheral autonomic effectsmay facilitate the anxiogenic properties of yohimbine. Similarly, the greaterheart rate when standing, or other postural differences in autonomic nervoussystem activity (Mohrman & Heller, 2003; Smith & Ebert, 1990), could havecontributed to participants in the current study experiencing anticipatory anxietywhen standing, although not when supine. Also, anxiety is associated withcentral noradrenergic activity (e.g., Tanaka et al., 2000), which is potentially

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greater when standing up than when lying down (see Lipnicki & Byrne, 2005).Postural differences in anticipatory anxiety may therefore have developedin association with postural differences in central noradrenergic activity.Consideration should also be given to a difference in physical comfort betweenthe standing and supine conditions, and to postural differences in muscle activityand vestibular information, as muscular relaxation decreases state anxiety(Pawlow & Jones, 2002) and the neural substrates of anxiety are linked tothose of balance control (Balaban, 2002).

It was found in the current study that posture influenced anxiety prior tothe performance of a mental arithmetic task, with a similar (although non-significant) pattern observed for psychological stress. In contrast, there was nodifference in anxiety or stress between standing and supine conditions followingcompletion of the task. Of even greater contrast is the previously reported trendfor the stress associated with mental task performance to be greater whensupine than when more upright (Sloan et al., 1995). Assuming baroreceptorloading does work to reduce anxiety (Dworkin, 1988; Dworkin et al., 1994)and stress (Schweizer et al., 1991) responses to mental tasks when supine,this may be opposed (or overridden) by other effects. Compensatory mentaleffort is required to adequately perform cognitive tasks when baseline arousalis low (Clark et al., 1987; Portas et al., 1998). In the current study posturedid not affect task performance, despite baseline arousal likely to have beenlower when supine than when standing (Cole, 1989; Vaitl & Gruppe, 1990). Itis therefore possible that negative affect associated with compensatory mentaleffort (Hockey, 1997) when supine offset any baroreceptor-mediated anxiolyticor stress-reducing effects (although a difference in mental effort expenditurebetween the supine and standing conditions was not reflected in ratings of taskdifficulty). In addition, afferent feedback or interoceptive effects associatedwith the greater heart rate reactivity to mental arithmetic when supine thanwhen standing (as also found by Rusch et al., 1981) could have promoted anegative affective response when supine.

In conclusion, it was found in the current study that participantsexperienced anticipatory anxiety when standing, although not when lying down.The mechanism underlying this effect remains to be determined, although itcould involve a postural difference in baroreceptor load. The current study addsto previous research showing differences in psychological functioning betweenthe supine and more upright postures, with affected functions including arousal(Caldwell et al., 2003; Caldwell et al., 2000; Cole, 1989; Elliott et al., 2005;Vaitl & Gruppe, 1990), somatosensation (Peru et al., 2006; Shimoda & Ikuta,2000), visual detection (Peru & Morgant, 2006; Vercruyssen & Simonton,

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1994), and cognition (Lipnicki & Byrne, 2005; Schulman & Shontz, 1971).Such effects may have relevance for research involving brain scans, as theseare typically obtained only in the supine posture.

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an effect of posture on anticipatory anxiety

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