An investigation into the test-retest reliability of the pain response to hypertonic saline injections and the impact of added muscle contraction

Background/Aims: Intramuscular Hypertonic Saline (HS) injections induce pain that resembles exercise-induced pain. The reliability and the impact that parallel exercise may have on this pain sensation is unestablished. Therefore, the aims of this research were to assess the test-retest reliability of this model's pain response and the influence of additional muscle contractions to the pain experience, in terms of both Pain Intensity (PI) and Pain Quality (PQ). Methodology: 8 male and 6 female participants (25 ± 5 years, 172.9 ± 8.5 cm, 71.9 ± 12.7 kg) completed the two studies. Study 1.1 assessed test-retest reliability with 3 separate visits, in which 1 ml of 5.85% HS was injected into the right vastus lateralis and differences in PI and PQ were measured. In Study 1.2, participants attended 3 separate visits, where they completed an isometric exercise task with 3 separate 10-second contractions at different intensities (10%/15%/20%). This was done with either HS, a placebo or no injection as control. Results: Study 1.1: Intraclass Correlation Coefficient scores for all PI measures indicated at least 'moderate' to 'good' test-retest reliability (0.68 - 0.814). Cronbach's Alpha scores for all PQ measures indicated 'acceptable' to 'good' test-retest reliability (0.806 - 0.933), except for the affective dimension (0.397 - 0.601). Study 1.2: Paired samples t-tests revealed no differences between exercise and rest, for any of the PI measures or PQ measures, except for the Present Pain Index (PPI) of the Long-form McGill Pain Questionnaire (P = 0.048). ANOVA analyses revealed no differences in PI or PQ measures between contraction intensities. Discussion: In Summary, HS provides a 'moderate' to 'good' reliable pain response, except for the affective dimension of pain. PI response is not affected by the addition of exercise or exercise intensity. PQ response is only affected in terms of different descriptive words, when exercise is introduced.