A New Experimental Model of Transient and Short-lasting Muscle Pain in Humans Based on Diathermy
Condition: Musculoskeletal Pain; Musculoskeletal Injury
MeSH terms: Musculoskeletal Pain; Myalgia; Hyperthermia
Study type: Interventional

Summary: 
      This study evaluates the use of short-wave diathermy (SWD) as an novel experimental model to
      induce transient and intensity-controlled muscle pain by heating muscle tissue.
    

Detailed description: 
      Application of short-wave diathermy All researchers, students and postdocs involved in
      experimental application of SWD will be properly trained by the Non-Ionizing Radiation
      Research Group of the National University of Entre Ríos. The application of SWD will be
      carried out by a CEC M-8 short-wave thermotherapy unit (CEC Electrónica, Córdoba, Argentina).
      The device has capacitive applicators, that will be situated around the muscle to be
      irradiated on top of a cotton towel, in order to absorb perspiration and avoid undesired
      heating effects. Afterwards the emission mode (continuous or pulsed) will be selected, and
      application of SWD will start. The intensity of SWD will be gradually increased until the
      volunteer perceives a warm sensation, and once familiarized with this perception, the
      intensity will be increased until a sensation of constant but tolerable pain is evoked. This
      sensation will be maintained throughout the duration of the SWD application, estimated in
      approximately 10 minutes. In case the volunteer feels excessive discomfort or does not
      adequately tolerate the application of SWD, the device will be turned off immediately and the
      experimental session will finish.

      Experimental pain model assessment

        -  Model direct effects: maximum and average pain ratings will be assessed through a Visual
           Analog Scale (VAS). The VAS ranges from 0 to 10, where 0 represents no perception, 3
           represents the pain threshold (i.e. the minimum irradiation intensity that elicits pain)
           and 10 represents the tolerance threshold (i.e. the irradiation intensity that elicits
           an intolerable pain sensation). Duration of pain will also be recorded, in case the
           experiment has to be interrupted before SWD application is completed and to assess
           potential cases in which pain outlasts SWD application. Furthermore, delivered RF power
           will be measured using a Bird 43 RF wattmeter (Bird Technologies, OH, USA).

        -  Pressure stimulation: pressure stimulation will be applied through a Somedic algometer
           (Somedic SenseLab AB, Sweden), directly over the muscle being examined, using a 1 cm2
           round tip. Pressure will be gradually increased from 0 kPA until a maximum of 1000 kPa
           at a rate of approximately 30 kPa/s. Pressure pain threshold (PPT) will be defined as
           the force at which the pressure sensation becomes painful. Pressure tolerance threshold
           (PTT) will be defined as the force at which pressure sensation becomes intolerable. If
           any of the threshold are not reached until the pressure is increased to 1000 kPa, the
           this value will be considered as the corresponding threshold.

        -  Pinprick and tactile stimulation: pinprick stimuli will be applied on the skin over the
           muscle being examined using a set of pinprick stimulators, consisting on a needle with a
           0.25 mm tip linked to calibrated weights, ranging from 1 g to 50 g, in order to apply
           forces between 8 and 500 mN approximately. Dynamic tactile stimulation will be applied
           by stroking a cotton tip over the skin. Pinprick sensitivity (PS) and dynamic tactile
           sensitivity (DTS) will be quantified using the VAS described above.

        -  Electrical stimulation: electrical stimuli will be applied to the skin over the muscle
           being examined using a concentric electrode, in which the cathode is made of 16
           stainless steel blunt pins with 0.2 mm diameter tips and 1 mm length, and the anode is a
           concentric stainless steel ring of 20 mm diameter. The electrical stimulus will consist
           on a rectangular pulse of 1 ms duration, generated by a Biopac STMISOLA constant-current
           electrical stimulator (Biopac Systems Inc., California, USA). Stimulation intensity will
           be increased from 1 mA in steps of 0.5 mA until a pain sensation is evoked. This
           intensity will be defined as the electrical pain threshold (EPT). Once the EPT is
           determined, repeated stimulation will be applied referenced to this intensity in order
           to record somatosensory evoked potentials (SEPs) through surface electroencephalography
           (EEG).

        -  Motor responses: motor responses will be recorded from upper and lower limbs. For the
           upper limb, two sets of experimental motor tasks are planned, and the overall aim is to
           measure changes in precision in the execution of the motor tasks due to pain. The first
           one will be a simple task, basically consisting on the use of a joystick to move a
           cursor to a two-dimensional moving target displayed in a computer screen. Specific
           details of the implementation of the optimal setup for this motor task and the required
           software will be provided by collaborators from the Physiology of Action Lab at
           Universidad de Buenos Aires. The second motor task will involve more complex movements
           and haptic feedback, provided by a Phantom Omni haptic device (SensAble Technologies,
           Inc., Massachusetts, USA). The haptic device can provide controlled levels of force
           feedback (up to 3.3 N) and a precise mapping of the trajectories employed in the motor
           task in three dimensions (resolution: 0.055 mm). Specific details of the implementation
           of the optimal setup for this motor task and the required software will be provided by
           collaborators from the Robotics Research Group at National University of Entre Ríos. For
           the lower limb, the motor task will consist on a balance task performed over
           high-resolution pressure sensing platform that will be able to record differences in
           weight loading between left and right leg and variations in the center of pressure
           (CoP). The platform is a recent technological development from the Electronic
           Prototyping and 3D Printing Lab at the Faculty of Engineering of National University of
           Entre Ríos, and it has a sensing area of 430 x 320 mm, with a resolution of 4 pressure
           sensors per cm2 and a measurement range from 1 to 785 kPa.

        -  Brain responses: EEG will be recorded using a Neuroscan SynAmps amplifier (Compumedics
           Ltd., Victoria, Australia) during the planning and execution of motor tasks, in order to
           evaluate changes in movement-related cortical potentials (MRCP) in relation to
           SWD-induced pain (Jochumsen et al. 2015). Additionally, resting-state EEG will be
           recorded before, during and after SWD application, in order to assess potential changes
           in functional connectivity (FC) (Mayhew et al. 2013). FC analysis will be performed by
           collaborators from CNAP at Aalborg University.

      Sample size considerations

      The experimental design is an interventional, pre-post study design (Thiese 2014), in which
      each participant acts as his own control. Sample size calculation will be performed taking
      into account the expected effect size that the model will have on the primary outcome (PPT).
      Several experiments have shown that PPT in the wrist extensor/flexor muscles (e.g. extensor
      carpi radialis longus) is around 350 ± 150 kPa (mean ± standard deviation), whereas PPT for
      the ankle dorsiflexor muscles (e.g. tibialis anterior) is around 600 ± 250 kPa (Fischer 1987;
      Delfa de la Morena et al. 2013). On the other hand, there is no existing information on the
      expected size of the difference in PPT due to the application of SWD, so this value will be
      approximated taking into account reference values of differences in PPT generated by other
      experimental models of pain, such as the injection of hypertonic saline solution or delayed
      onset muscle soreness. In these cases, PPT is usually reduced between 10 and 30% during the
      effects of the model, so an average of 20% difference will be considered in order to
      calculate the sample size for these experiments. Considering a probability of making a type I
      error (α) of 5%, a statistical power (1 - β) of 80%, and an estimated correlation between
      measures of 0.8, the sample size required to detect the aforementioned difference is 16
      volunteers for the experiment in the upper limb and 17 volunteers for the experiment in the
      lower limb. In order to account for an unexpectedly larger variation, 20 subjects are
      expected to be recruited for each experiment.
    

Intervention name(s): Short-wave diathermy
Intervention type(s): Radiation
Intervention description(s): The application of SWD will be carried out by a CEC M-8 short-wave thermotherapy unit. The device has capacitive applicators, that will be situated around the muscle to be irradiated on top of a cotton towel, in order to absorb perspiration and avoid undesired heating effects. Afterwards the emission mode (continuous or pulsed) will be selected, and application of SWD will start. The intensity of SWD will be gradually increased until the volunteer perceives a warm sensation, and once familiarized with this perception, the intensity will be increased until a sensation of constant but tolerable pain is evoked. This sensation will be maintained throughout the duration of the SWD application, estimated in approximately 10 minutes.
Intervention model: Single Group Assignment
Intervention model description: The SWD applicators will be placed in the desired position, around the forearm for the upper limb experiments, or below the knee for the lower limb experiments. The distance between applicators will be adjusted until the desired pain sensation is obtained according to the subjective criterion of the volunteer. This sensation should be mild or moderate pain, but tolerable for the duration of the application. Once the first application is completed, the volunteer will be allowed to rest for a period of 10 minutes, and the assessment will be repeated to test for possible after effects. After it is verified that the muscles recovered their normal temperature and the pain sensation has completely disappeared, the variables related to changes in motor responses under rest conditions will be recorded. Then SWD will be applied a second time using the same parameters as in the first application. Once the registration is complete, the experiment will be finished.
Masking: None (Open Label)
Primary purpose: Basic Science
Keywords: Short-Wave Diathermy; Transient muscle pain; Non-invasive pain model