Test-Retest Reliability of an Experimental Model of Shoulder Muscle Pain

High frequency oscillations of non-ionizing electromagnetic fields range can heat deep tissues in a well-localized region. Recently, a new SWD-based model showed to be a promising tool for investigating muscle pain in humans. The main advantages of the model are its noninvasiveness, the ability to control stimulation parameters, and the convenience of the time frame in which pain and hyperalgesia are developed. This study assess the reliability of SWD pain model to induced transient and intensity-controlled muscle soreness on shoulder in healthy volunteers.

This Test-Retest Reliability study comprises a sample of 31 healthy volunteers. Volunteers will be invited to participate in the study through social networks and e-mail.

Experimental pain model assessment

The radiofrequency electromagnetic fields can be produced in the range of 3 kHz to 300 GHz, but frequencies around 27 MHz (short-wave) are preferred to heat deep tissues. Deep tissue heating by means of RF is a safe and extensively used technique in palliative treatment of pain and as a healing agent in soft tissues. This study assess the reliability of SWD to induce soreness. The study will be conducted in two experimental sessions on different days, at least 48 to 72 hours apart each other in order to avoid carryover effects. In each experimental session, several measures will be performed to assess different physiological parameters, including: pressure pain thresholds (PPT), specific self-report questionnaire, isometric muscle strength, pain distribution and myoelectric activity.

In the first session (Test), participants will be interviewed to collect anthropometric data and health history. Basic upper quadrant physical examination will be performed to check inclusion and exclusion criteria. Before and after the application of SWD, several assessments will be performed. Among the different assessment, myoelectric activity of infraspinatus and deltoid muscle will be recorded during isometric arm lifting task. The isometric task will consist in maintaining the arm in 90 degree elevation in the scapular plane (arm flexion) for 8 seconds. This task will be repeated 5 times with 15 seconds of rest between each trial. Finally, a fatiguing motor task will be performed, consisting of flexion and extension of the arm executed cyclically in a range of motion between 30 and 90 degree in the scapular plane. To ensure fatigue, volunteers will perform the task with an additional weight. The size of the weight will be calculated according to each volunteer body mass (1.25 kg in participants under 68 kg and 2.5 kg in participants over 68 kg). The arm flexion and extension speed will be guided using a 60 beats/s metronome. Same process will be repeated in the second session (Retest).

Sample size considerations:

All the reliability assessment methods used in this study are related to the within-subject standard deviation (SD_w). For a test-retest experiment, the SD_w can be calculated as〖 SD〗_w=(SD_diff)⁄√2, and the LoA can be reformulated as bias ± 1.96 〖SD〗_w √2. The precision to estimate SD_w depends on both the number of subjects, n, and the number of observations per subject. A sample size of 31 subjects is required considering a confidence interval of 25% at either side of the estimation of SD_w, and for m=2 observations per subject.

Eligibility Criteria:
Inclusion Criteria:

* Understanding of the content and scope of the experiment, and compliance with the experiment's instructions.
* To have signed the informed consent.

Exclusion Criteria:

* Pregnancy.
* Previous history of neurological or musculoskeletal disorders or chronic pain.
* Previous history of addictive behavior, defined as abuse of alcohol, cannabis, opioids or other drugs.
* Previous history of thermosensitivity disorders.
* Previous history of mental illness.
* Presence of fever, tuberculosis, tumors, infectious processes, or acute inflammatory processes.
* Implantation of peacemaker or metallic prosthesis.
* Use of analgesics within 24 h prior to the experiment.