Tactile Acuity in Experimentally Induced Acute Low Back Pain

In the saline injection group, low back pain will be induced via 1.0 ml hypertonic (5% NaCl) saline bolus injection which is a commonly used model of acute low back pain. Saline will be injected into the right (or left) lumbar longissimus muscle 50 mm lateral to the spinous process just above (5 mm) previously marked point. Injections will be performed by a physician under ultrasound imaging guidance to ensure that each single injection will be equally placed at a depth of 30 mm. The side for the pain induction will be randomised across subjects. The place of needle insertion and corresponding point on the opposite side of the body will be covered by placing a small piece of adhesive plaster.

In the sham-injection group (placebo comparator), a real needle will be shown to the participants to imitate and produce the anticipation of a pain experience. A pinprick sensation will be produced by a weighted stimulus applied perpendicularly to the skin just above without piercing the skin. A stimulus of 512 mN will be used to produce a pinprick sensation and activation of cutaneous nociceptors. Adhesive tapes covering stimulation points will also be provided.