Background and aims
Hypertonic saline (HTS) injections are used to induce acute experimental pain. Passive stretching, or contracting muscles near the injected region may alter pain perception. The aim of this study was to describe the pain experienced following HTS-injections in the subacromial bursa space (SBS) and supraspinatus muscle belly (SSMb) both at rest and during forced passive shoulder tests and active tasks.

Methods
1-2mL of 5% NaCL was injected into SBS (n=8) and SSMb (n=10; age 22.9±2.9, 5-male) under ultrasound guidance. Participants were then exposed to 8  provocation-tests, in random order, at 20-second intervals. Pain severity (NRS-11) was reported after each provocation-test. Pain location (cm ), referral patterns (body-diagram), depth (cm) and quality (Short Form McGill
Pain Questionnaire) were reported after completion of all tasks.

Results
SBS and SSMb-related pain was experienced for 925±382 sec and 434±47 sec, respectively (p< 0.02); with an average intensity over the first 3.5 minutes of 5.9±1.9/10, p=0.14. Pain intensity reduced by 0.8±0.9and 1.0±1.4 during the Hawkins Kennedy-test and active abduction, respectively (both p< 0.05) for SBS. There was no significant difference between provocation-tests in SSMb (all P>0.07). Both pain models were described as “aching”,SBS was also described as “throbbing”, “sharp” and “heavy”, and SSmB as “cramping”.

Conclusions
Our study demonstrated that the pain experienced differed between these models. The reduction in pain intensity with clinical provocation tests with SAB is in-line with previous studies of HTS-induced acute back pain with stretching/contracting, but in contrast to what is observed in some clinical shoulder pain conditions.

CHARACTERIZING EXPERIMENTAL ACUTE SHOULDER PAIN, AND THE EFFECT OF CLINICAL PROVOCATION TESTS