Intra- and Inter-Individual Reliability of Muscle Pain Induced by an Intramuscular Injection of Hypertonic Saline Injection into the Quadriceps

Background: Intramuscular injections of hypertonic saline are commonly used to induce experimental muscle pain but reliability data on this technique is lacking. This study investigated the intra- and inter- individual reliability of pain measures from a hypertonic saline injection into the vastus lateralis. Methods: Fourteen healthy participants (6 female) attended three laboratory visits where they received an intramuscular injection of 1 mL hypertonic saline into the vastus lateralis. Changes in pain intensity were recorded on an electronic visual analogue scale and pain quality was assessed after pain had resolved. Reliability was assessed with the coefficient of variation (CV), minimum detectable change (MDC), and the intraclass correlation coefficient (ICC) with 95% confidence intervals. Results: Mean pain intensity displayed high levels of intra-individual variability (CV = 16.3 [10.5 - 22.0]%) and ‘poor’ to ‘very good’ relative reliability (ICC = 0.71 [0.45 - 0.88]), but had a MDC of 11 [8 - 16] au (out of 100). Peak pain intensity exhibited high levels of intra-individual variability (CV = 14.8 [8.8 - 20.8]%) with ‘moderate’ to ‘excellent’ levels of relative reliability (ICC = 0.81 [0.62 - 0.92]), whereas the MDC was 18 [14 - 26] au. Measures of pain quality exhibited good reliability. Inter-individual variability in pain measures was high (CV > 37%). Conclusions: Intramuscular injections of 1 mL of hypertonic saline into the vastus lateralis display substantial levels of inter-individual variability, but MDC is below the clinically important changes in pain. This model of experimental pain is suitable for studies involving repeated exposures.