Clinical Outcomes, Viscoelastic Properties and Central Pain Mechanisms After Eccentric Training in Neck/Shoulder Pain

A handheld electronic pressure algometer with a 1-cm2 contact probe will be used. Pressure pain thresholds (PPTs), as the minimum necessary pressure force to evoke pain, will be assessed bilaterally over the middle point of the upper trapezius muscle belly, and the muscle bellies of the extensor carpi radialis brevis and the extensor carpi ulnaris. A 30-second break will be used between assessments to prevent bruising, and an average of the two measures over each site will be calculated for the statistical analysis. Pressure algometry is reliable in healthy individuals, and in neck pain.

A computer-controlled cuff-algometer (Aalborg University, Aalborg, Denmark) will be used to evaluate distal hyperalgesia at the lower legs by means of pain detection (PDT) and tolerance threshold (PTT). A single 13-cm-wide cuff (VBM, Sulz am Neckar, Germany) will be wrapped around the gastrocnemius muscles, at the level with the maximum circumference, on both sides. The cuff pressure will be increased by 1 kPa/s, with the maximal pressure set up at 100 kPa. Participants will be told to rate their pain intensity on a 10-cm electronic Visual Analogue Scale (VAS), and to press a button to release the cuff inflation when the pain was unbearable (PTT). PDT will be considered as the pressure corresponding to the first VAS rating more than 1 cm. This protocol is highly reliable and an examiner-independent procedure.

TSP will be collected from the painful/most painful side, and the same side matched on dominance for controls. Ten consecutive cuff pressure stimuli (1-second duration, 1-second break interval) will be delivered by inflating the cuff chamber at the same intensity than the PTT . During intervals, a non-painful pressure of 1 kPa will be applied. Participants will have to continuously rate their pain on the electronic VAS without returning to zero during breaks. TSP will be defined as the difference between the mean VAS score from the eight to the tenth stimulus (VAS-II) and the mean VAS value from the first to the fourth stimulus (VAS-I) (e.g., VAS-II minus VAS-I). A higher value indicates enhanced TSP.

Experimental tonic pain will be evoked on the contralateral leg (at the gastrocnemius muscle) with a continuous cuff-induced painful stimulus, which will be set at 70% of the PTT on that side (conditioning stimulus). Simultaneously, PDT and PTT will be evaluated (test stimulus) on the leg corresponding to the painful/most painful side (NSP group), or the dominant side (control group). The CPM-effect will be defined as the difference between PDT during versus before conditioning. A lower CPM value indicates a less efficient central pain inhibition.

The maximal voluntary contraction (MVC) will be measured with a custom built dynamic shoulder dynamometer (Aalborg University, Aalborg, Denmark). Participants will be in a seated upright position with back support and no feet support, and, when required, a corselet will be used to prevent uneven loading of the spine. First, participants will be instructed to raise and lower both shoulders at the same time as much as possible. After that, they will be told to place their shoulders in a relaxed position, and when the pad of the dynamometer is lowered to this level, they will be verbally encouraged to perform a shrug shoulder force in isometric condition for 3 seconds. Recordings will be made twice (2-min break between every test) on the painful shoulder, and the average value will be used for further analysis.

The biomechanical properties of the upper trapezius will be collected using a handheld myotonometer device, the MyotonPRO, (Myoton AS, Estonia). The MyotonPRO is small, non-invasive, and it has shown good validity and high reliability in larger and smaller body muscles. This device delivers a multi-scan mode of three brief mechanical impulses, each one shortly followed by a quick release, to record the damped natural oscillation of the muscle using an accelerometer.

The upper trapezius muscle creep will be collected using a handheld myotonometer device, the MyotonPRO, (Myoton AS, Estonia). The MyotonPRO is small, non-invasive, and it has shown good validity and high reliability in larger and smaller body muscles. This device delivers a multi-scan mode of three brief mechanical impulses, each one shortly followed by a quick release, to record the damped natural oscillation of the muscle using an accelerometer.