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symptoms. We expected to find reduced autonomic flexibility (lower HF-HRV and BRS) in association with MUSC. In addition, for the first time, we report on the association between BRS and anxiety symptoms in youths [ 18 ; for HF-HRV and depression symptoms see 19–21 ]. The participants were 921 10to 13-year-old Dutch preadolescents (53.1% girls; 11.5 8 0.5 years) from the population cohort TRacking Adolescents’ Individual Lives Survey [21–23] . Written informed consent was obtained from the parents; children participated voluntarily. The study was approved by the medical ethics committee. Internalizing symptoms (i.e. anxiety, depression and MUSC, including abdominal pain, headaches, nausea and stomachache) were assessed by the Child Behavior Checklist (CBCL, parent report) and Youth Self-Report (YSR, childreport), using DSM-IV scales [1, 24, 25] . Each informant may provide a specific aspect of the construct being measured, as agreement between their judgements is generally low (r = 0.21–0.34, p ! 0.001; [25, 26] ). Pearson’s correlations between the 3 respective CBCL and YSR scales were moderate (r = 0.30–0.60, p ! 0.001). Internalizing problems were largely in the nonclinical range (97.5%). Externalizing problem scale scores across the DSM-IV-based attention deficit/hyperactivity disorder, oppositional defiant disorder and conduct disorder scales were calculated and used as a covariate. Continuous noninvasive HR and systolic blood pressure (BP) measurements of 4 min in the supine position were conducted during spontaneous breathing [23] . HF-HRV (0.15–0.40 Hz, ln [ms 2 ]) and BRS (0.07–0.14 Hz, ln [ms/mm Hg], coherence 0.3) were calculated using power spectral analysis [22, 27–29] . A lower BRS points towards reduced responsiveness of the autonomic nervous system to regulate short-term BP changes through HR variations, reflecting low vagal and/or high sympathetic influences on the heart (r HF-HRV-BRS = 0.65, p ! 0.001, [16] ). We additionally calculated BRS2 with 0.5 coherence, as generally applied (n = 522; [22, 27] ). The respiration rate was not accounted for since the breathing rates of youngsters commonly fall within the HF range [30] . We have shown a satisfactory short-term test-retest reliability of our autonomic nervous system measurements [27] . Linear regression analyses were conducted separately for CBCLand YSR-based ratings, with HF-HRV and BRS as separate outcome variables, and all internalizing subscales entered simultaneously as predictor variables, adjusted for confounders ( table 1 ). The analyses were repeated for BRS2 to investigate concurrent validity. The tests were 2-tailed using p ! 0.05. Table 1 shows that in boys and girls MUSC were significantly associated with lower supine HF-HRV (YSR) and BRS (YSR, CBCL boys only). The results on BRS2 were similar, with a negative association between MUSC and BRS2 (YSR: B = –0.12, p ! 0.05; CBCL boys only: B = –0.17, p ! 0.05, n = 239), supporting concurrent validity. Depression symptoms (YSR) were signifiMedically unexplained somatic complaints (MUSC; e.g. headaches, abdominal pain) are common in youths and may lead to increased medical consumption, poorer academic attainment and future somatic ill health [1–3] . MUSC are strongly associated with both current and future anxiety and depression symptoms [3, 4] , which all are assumed to partly reflect the independent expression of underlying distress through bodily versus psychological symptoms, respectively [5] . Dysfunction of the autonomic nervous system as one of the major physiological stress systems has been suggested as an important mechanism in MUSC [6–10] . However, the findings on the association between MUSC and cardiac autonomic measures are still inconsistent [11–14] . The use of small-sized samples may have contributed to this. Presently, we lack large-sized population studies using a dimensional approach regarding MUSC, especially in youths, hence pointing to early autonomic risks [2, 15] . Indicators of cardiac autonomic inflexibility or dysfunction, such as reduced high-frequency heart rate variability (HF-HRV; indicating vagal activity) and, particularly, baroreflex sensitivity (BRS; indicating the flexible regulation of autonomic balance between the vagal and sympathetic nervous system) have rarely been investigated [6, 8, 9, 16] . Moreover, research into patterns of physiological functioning that discriminate between MUSC and anxiety and depression symptoms is sparse [2, 4] . The potentially confounding effects of co-occurring internalizing symptoms, which may have served to inflate effect sizes, mask effects or even influence the direction of effects, have often been neglected [17] . We investigated the relationship of dimensionally measured MUSC with HF-HRV and BRS in a large preadolescent population cohort, taking into account co-occurring internalizing Received: April 14, 2010 Accepted: July 4, 2010 Published online: November 18, 2010

psychotherapy and psychosomatics

Reduced Cardiac Autonomic Flexibility Associated with Medically Unexplained Somatic Complaints in the Context of Internalizing Symptoms in a Preadolescent Population Sample: The TRAILS Study