Pressure-induced muscle pain and tissue biomechanics: a computational and experimental study.

Pressure algometry is widely used to assess deep tissue sensitivity. In this study the relation between pressure-induced pain in humans and stress/strain distribution within the deep tissue is evaluated. A three-dimensional finite-element computer model was used to describe the stress/strain distribution in tissues of the lower leg during pressure stimulation. The computer model was validated based on data recorded by computer-controlled pressure-induced muscle pain in eight subjects. An indentation of 7 mm was painful for all subjects and at this level data were extracted from each simulation. Simulations were performed with different stimulation sites (muscle, near-bone), probe diameters (5, 10, 15 mm), and probe designs (flat, rounded). The principal stress peaked in the skin and was reduced to about 10% in the underlying muscle tissue. The principal strain peaked in adipose tissue and was reduced in muscle tissue to 80% with the 15 mm probe and 66% with the 5mm probe. The large diameter probe evoked a strain peak in adipose tissue at 0.12 (flat probe) and 0.24 (rounded probe); in muscle tissue 0.10 and 0.20 respectively. The human pressure pain thresholds with the rounded probe were significantly lower compared with the flat probe (p<0.05). The results suggest that pressure-induced muscle pain is mainly related to muscle strain and most efficiently induced by large rounded probes, while smaller and flat ones mainly activate superficial structures. The relatively low stress in the deep tissue suggests that the mechanosensitivity of nociceptors in the deep tissue is lower compared with nociceptors in the superficial tissue.