Experimental muscle pain: A quantitative study of local and referred pain in humans following injection of hypertonic saline

Objectives: In the present quantitative and methodological study we systematically investigated the variability and sensory aspects of experimental muscle pain induced by hypertonic saline as proposed by Kellgren. Methods: Four experiments were carried out: 1. intra-individual variability in saline-induced muscle pain, 2. the influence of saline concentration [0.9%, 5%, 11.5% and 20%], 3. the influence of infusion time [20 and 100 seconds] and volume [0.1 mi and 0.5 mi] and 4. the influence of tissue type [periosteum, muscle, subcutis]. The infusion of hypertonic saline was standardized using a computer-controlled infusion pump. Pain intensity, quality, distribution and cutaneous sensory changes were assessed. Results: It was possible to obtain reproducible intra-individual [variability < 15.6% +/- 54.3] scores of pain intensity, quality, distribution and cutaneous changes. There was a significant correlation between pain intensity and the infused volume [R = 0.51, P < 0.001] and concentration [R = 0.76, P < 0.0001] of hypertonic saline. The onset of pain was correlated to the infusion time [R = 0.57, P < 0.001] and concentration [R = -0.40, P < 0.02]. The pain offset and peak time were significantly [P < 0.05] influenced by the concentration and infusion time. The infusions close to the periosteum caused a higher pain intensity compared to infusions in muscle and subcutis. Infusions in subcutis produced a ''scalding'' pain quality that was different compared to the ''drilling'' and ''taut'' quality of muscular pain. Cutaneous hypoesthesia to pin-prick and touch close to the injection site were the most commonly observed changes in sensation. Conclusion: The findings indicate that the hypertonic saline muscle pain model is useful when studying basic physiological aspects of human muscle pain.