### INTRODUCTION

### MATERIALS AND METHODS

### Participants

*d*effect sizes ranging from 0.82 to 3.66 for the effect of autonomic dysfunction on the time constant (τ) of the on- transient HR response (Almas et al., 2017; Regensteiner et al., 1998; Silva et al., 2017). Considering the smallest effect size of 0.82 (Almas et al., 2017), the required number of participants to achieve a power of 0.95 and an alpha of 0.05 was estimated to be nine in each group. Thus, 10 women with a clinical diagnosis of FM according to the American College of Rheumatology criteria (Wolfe et al., 1990; Wolfe et al., 2010) and 10 age-matched apparently healthy CON women participated in this study. The exclusion criteria adopted for both groups were: (a) any cardiovascular abnormalities (arrhythmias and heart failure), (b) articular or bone injury, (c) tobacco usage, (d) use of chronotropic and antihypertensive drugs that could affect autonomic modulation, and (e) pregnancy. The FM patients were taking antidepressant (60%), analgesic (50%), anticonvulsant (40%), anti-inflammatory (10%), and muscle relaxant (10%) drugs. Participants signed an informed consent statement previously approved by the local ethics committee (number protocol approved 5.091.561). This study followed the recommendations established in the Declaration of Helsinki.

### Procedures

### Maximal graded exercise test

### Constant-load submaximal exercise

### On-transient HR response

_{(}

_{t}_{)}is the HR response, HR

_{0}is the HR average of the last 30 sec before the beginning of exercise (i.e., baseline HR value), A

_{on}is the amplitude of the HR response,

*t*is time, and τ is the time constant (i.e., time to reach 63% of the final HR) (Fig. 1). The goodness of fit was measured by the standard error of estimate (SEE) and the coefficient of determination (R

^{2}).

### HR and HR variability

_{1}), (b) standard deviation of the continuous long-term RRi variability (SD

_{2}), and (c) the SD

_{1}/SD

_{2}ratio. The SD

_{1}was used to represent the parasympathetic modulation on sinus node, and the SD

_{2}to represent both parasympathetic and sympathetic components (Shaffer and Ginsberg, 2017). The SD

_{1}/SD

_{2}ratio was used to represent sympathetic and parasympathetic balance (Shaffer and Ginsberg, 2017).

### Statistical analyses

_{on}, HR

_{0}, SEE, and R

^{2}), and absolute and relative mean HR were all compared between FM and CON groups using the Student

*t*-test. The homogeneity of variances was confirmed via Levene test. The mean HR and HRV indices were compared using two-way analysis of variance (group: CON and FM vs. time: 5–10 and 10–15 min). The SDNN, RMSSD, SD

_{1}, and SD

_{2}indices were logarithmically (Ln) transformed to meet the assumptions of parametric testing (i.e., Ln-SDNN, Ln-RMSSD, Ln-SD

_{1}, Ln-SD

_{2}, respectively). The sphericity of the variances was confirmed via Mauchly’s test. The significance was accepted when

*P*≤0.05. All analyses were performed using Jamovi free software (ver. 2.3).

### RESULTS

*P*>0.05). However, the FM patients had lower absolute and relative peak HR than the CON women during the maximal graded exercise test (Table 1).

_{on}, and HR

_{0}(all

*P*>0.05). In addition, the SEE values for τ and A

_{on}, expressed in absolute and relative values, were similar between the FM and CON groups (

*P*>0.05). The R

^{2}of the fitted function was also similar between the FM and CON groups (

*P*> 0.05).

_{1}, Ln-SD

_{2}, and SD

_{1}/SD

_{2}(all

*P*>0.05). In addition, there was no difference between FM and CON groups for mean HR relative to peak HR (FM: 78%±9% vs. CON: 73%±7%,

*P*=0.19) or relative to maximum predicted HR (FM: 65%±8% vs. CON: 68%±8%,

*P*= 0.53).