The purpose of this study is to examine the acute and chronic effects of high frequency electrical current transfer (frequently called "TECAR") on pain and functional movement in individuals with a musculoskeletal injury or pain. The participants will be assigned into an experimental or a control group and outcome measures will be measured prior to, after, 24 and 48 hours following a single intervention session (Acute effects) as well as 3 and 6 months after the intervention (chronic effects).

The purpose of this study is to examine the acute and chronic effects of high-frequency electrical current transfer (TECAR) on pain and functional movement in individuals with musculoskeletal injuries. The project is conducted in two phases to distinguish between immediate and long-term therapeutic outcomes. In the first phase, the acute effects of a single 20-minute TECAR session (Capacitive mode, 0.5 MHz, 40-60% intensity) are compared to transcutaneous electrical stimulation (TENS, asymmetric biphasic mode, 80Hz) and a control group in participants with acute hamstring strain or acute low back pain, defined as episodes occurring for less than half the days in the past six months with a visual analogue pain intensity > 6/10. Outcome measures are obtained at baseline, immediately post-intervention, and at 24 and 48 hours. In the chronic phase, participants with hamstring injuries or chronic low back pain receive 10 sessions (3 times per week) while outcome measurements are obtained before, immediately after, and at 3 and 6 months follow-up. Statistical analysis involves a Mixed-Model Analysis of Variance (ANOVA) to evaluate group-by-time interactions for outcomes including the Visual Analogue Scale (VAS), Oswestry Disability Index (ODI), and flexibility measures. For significant results, Tukey's post-hoc tests are utilized for pairwise comparisons. Additionally, non-parametric Kruskal-Wallis and Friedman tests are used to analyze percentage changes in clinical scores. Treatment effect sizes are quantified using partial eta squared with the level of significance set at p < 0.05.

Individuals with Muscle Injury

Inclusion Criteria:

• Age over 18 years.
• Athletes: amateur soccer players or sprinters with regular, systematic training.
• Suspected Grade I or II hamstring strain.
• Cessation of training due to injury.
• No surgical intervention in the previous year (for initial assessment as potential participants).

Exclusion Criteria:

• History of injury to the same muscle (ipsilateral side) within the last 6 months.
• Other injury to the posterior aspect of the thigh.
• Other injuries or chronic pain in the trunk or lower limbs.
• Use of non-steroidal anti-inflammatory drugs (NSAIDs) in the 3 months prior to the injury.

Individuals with Acute Low Back Pain

Inclusion Criteria:

• Age over 18 years.

• Pain located between the lower ribs and the gluteal folds (top of the buttocks).

• Pain occurred on fewer than half of the days over the preceding six months.

• Pain intensity $\ge$ 6/10 on the Visual Analogue Scale (VAS).

• Pain related to one or more of the following:

  • Lumbar facet joints
  • Sacroiliac joints
  • Lumbar intervertebral discs
  • Muscles, tendons, or ligaments of the lumbar and sacral region
  • Other bony structures of the lumbar spine

• Discontinuation of non-steroidal anti-inflammatory drugs (NSAIDs) or analgesics:

  • 1 day prior to measurement
  • 48 hours after measurement

Exclusion Criteria:

• Radiographic evidence of inflammatory disease affecting the spine.
• Spinal fracture.
• Significant genetic structural abnormality of the spine.
• Pregnancy.
• Psychiatric disorders.
• Receipt of systemic medication or treatment in the last 3 months.
• Presence of neurological deficits (sensory, motor, or reflexes).
• Receipt of TENS or TECAR therapy within the last year.

Individuals with Chronic Low Back Pain

Inclusion Criteria:

• Episodes of low back pain (between the lower rib margin and the gluteal fold) for at least half of the days during the last 6 months.
• Pain intensity $\ge$ 20% on the Visual Analogue Scale (VAS).

Exclusion Criteria:

• Spinal stenosis.
• Radiographic evidence of inflammatory disease affecting the spine.
• Vertebral fracture.
• Presence of spondylolisthesis or spondylolytic lesion (spondylolysis).
• Significant genetic structural abnormality of the spine.
• Daily, severe low back pain.
• Pregnancy.
• Use of medications that might influence heart rate or blood pressure.
• Psychiatric disorders.

De-identified individual participant data (IPD) that underlie the results reported in this study may be made available upon reasonable request. However, under no circumstances will identifiable personal information, medical records, or any data that could compromise participant confidentiality be provided. All shared data will be strictly anonymized and restricted to numerical outcome measurements to ensure full compliance with privacy regulations and the study's ethics protocol

Beginning 6 months and ending 24 months after the publication of results.

Data will be made available to researchers who provide a methodologically sound proposal. Requests must be directed to the corresponding author and are subject to approval based on the study's ethical and privacy guidelines.

Due to the distinct physical sensations associated with the different interventions-specifically the thermal sensation of TECAR therapy and the electrical paresthesia of TENS-masking of participants and care providers is not feasible. To minimize bias, a single-masked design is employed where the outcomes assessor is blinded to group allocation. Outcome measures are recorded and assigned unique alphanumeric codes by administrative staff not involved in data collection. This ensures that the assessor remains blinded to the intervention received by each participant during the evaluation and data entry processes.