The goal of this clinical trial is to evaluate the short-term (24-hour) effect of conventional transcutaneous electrical nerve stimulation (TENS) compared to Sham TENS on pain induced by resistance training in women with fibromyalgia. The main questions it aims to answer are:

1. Can conventional TENS effectively reduce pain intensity and unpleasantness during and after a resistance training session in women with fibromyalgia?
2. Is there a significant difference between the effects of conventional TENS and Sham TENS on exercise-induced pain in this population? Researchers will compare a conventional TENS group to a Sham TENS group to see if active TENS application during and after resistance training sessions leads to reduced pain levels and improved pain perception.

Participants will:

• Undergo a supervised resistance training session
• Receive either conventional TENS or ShamTENS treatment during and after exercise
• Complete pain assessments, measuring both intensity and unpleasantness, before, during, and up to 24 hours after exercise sessions

Study Design

Randomization with minimization will be performed using MinimPy software to ensure group balance, considering the following factors:

• Pain intensity, assessed using a pain scale (0-10)
• Fibromyalgia severity, assessed using the Fibromyalgia Impact Questionnaire
• Age

Participants will be informed that different types of TENS stimulation may or may not produce a sensation, and this does not impact treatment effectiveness.

Resistance Exercise Protocol The resistance training protocol involves a leg press exercise performed at a 90° starting angle. Participants will undergo a familiarization period, a warm-up, and a repetitions-to-failure (RTF) test to determine their 5-repetition maximum (RM). The Epley equation will be used to predict the 1RM. Participants will then perform two sets of 10 repetitions at 60% of their estimated 1RM, with a 1-minute rest between sets. A kinesiologist will supervise the training session.

Data Collection

Baseline Assessments:

Demographic information (age, gender identity, height, weight, medication) Physical activity level (Physical Activity Questionnaire for Adults) Fibromyalgia severity (Fibromyalgia Impact Questionnaire) Kinesiophobia (Tampa Scale of Kinesiophobia) Treatment expectations (Numerical scale from -100 to 100) Blood pressure Repetitions to Failure Test (RTF)

Participants will also complete a digital logbook via REDCap to record their pain levels at the end of the day and 24 hours post-exercise.

Inclusion Criteria:

• Be 40 years of age or older.
• Have a diagnosis of fibromyalgia.
• Experience persistent pain in the lumbo-pelvic and lower limb region with an intensity ≥ 3/10 on a numerical scale of 0 to 10 (0 = no pain, 10 = worst pain imaginable).
• Refrain from consuming caffeine and analgesics 6 hours before the experiment, and refrain from smoking cigarettes 2 hours before the experiment.

Exclusion Criteria:

• Are physically active prior to the start of the study, i.e., they meet the recommendations of 150 minutes per week of moderate to vigorous intensity activity or practice 2 times 30 minutes of muscle-strengthening exercises per week for 2 months.
• Suffer from poorly controlled cardiovascular diseases.
• Have one or more contraindications to physical activity
• Have one or more contraindications to TENS
• Have already participated in a research project involving TENS or used a TENS device in the past 10 years.

Blinding assessment: Participants' treatment expectations (before and after the first TENS application) will be used as an indicator of blinding success. The outcome assessor's blinding will be assessed by asking them to guess the group allocation.