This randomized controlled trial investigates whether slow breathing techniques influence heart rate variability, exercise self-efficacy, and resistance exercise performance in women with fibromyalgia. Participants will be randomly assigned to one of three breathing conditions (slow breathing with visual pacer, slow breathing without pacer, or spontaneous breathing) before performing a biceps curl resistance exercise. The study will examine how breathing patterns interact with psychological variables (anxiety sensitivity, pain catastrophizing, pain hypervigilance, and kinesiophobia) to affect physiological and performance outcomes.

Background: Fibromyalgia is considered one of the most representative central sensitivity syndromes, where central sensitization is the predominant characteristic. Resistance and strength training have demonstrated efficacy as therapeutic strategies for fibromyalgia patients. Slow breathing has been shown to reduce anxiety, perceived stress, and pain intensity while increasing heart rate variability (HRV), a biomarker of stress reactivity. The Fear-Avoidance Model of Pain demonstrates that anxiety sensitivity, pain catastrophizing, hypervigilance, and fear of pain/movement largely determine activity levels in chronic pain patients. However, no studies have examined how slow breathing interacts with these psychological variables to enhance resistance exercise performance in fibromyalgia patients.

Study Design: Single-session, three-arm randomized controlled trial with parallel group assignment.

Experimental Procedure:

Phase 1 - Baseline Assessment:

• Semi-structured interview for demographic and clinical information
• Administration of self-report questionnaires (detailed below)
• 5-minute baseline HRV measurement (seated, eyes open, spontaneous breathing)
• Respiration rate inferred from HRV

Phase 2 - Resistance Exercise Preparation:

• Determination of optimal weight for biceps curl exercise using submaximal protocol
• Warm-up: 3 minutes walking + 10 unloaded practice repetitions
• Progressive weight testing to determine 50% 1-RM equivalent

Phase 3 - Breathing Intervention (8 minutes): Participants randomly assigned to:

• Condition A: Slow breathing with visual pacer (6 breaths/min: 4-sec inhale, 6-sec exhale) displayed continuously
• Condition B: Slow breathing without pacer (same pattern, brief training then self-paced)
• Condition C: Spontaneous breathing at participant's natural rate
• HRV continuously recorded during all breathing conditions. Respiration rate inferred from HRV.

Phase 4 - Resistance Exercise Test:

• Biceps curl exercise at 50% estimated 1-RM
• Maximum repetitions to voluntary exhaustion
• Proper form maintained (back against wall, arms close to body, full range of motion)

Phase 5 - Post-Exercise Assessment:

• Immediate measurement of dependent variables (detailed below)
• 8 min. of rest
• HRV recording 5 min.

Safety Monitoring: Continuous observation by trained evaluators; clear stopping criteria.

Inclusion Criteria:

1. Female sex
2. Age ≥ 18 years
3. Medical diagnosis of fibromyalgia confirmed by physician
4. Capacity to understand and sign informed consent form
5. Fluency in spoken and written Spanish

Exclusion Criteria:

1. Current or past severe mental illness or neurodegenerative disease
2. Current treatment for oncological pathology, degenerative disease, or terminal illness
3. Musculoskeletal injury contraindicating biceps curl exercise
4. Connective tissue disease or arthritis
5. Inability to perform biceps curl exercise due to physical limitations

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