Tissue characteristics during temporal summation of pressure-evoked pain.

Temporal summation due to repeated pain stimulation of deep somatic structures is facilitated in chronic musculoskeletal pain. In this study, the relation between repeated pressure-induced pain and stress/strain distribution within the deep tissue was evaluated to understand whether tissue characteristics may change during repeated stimulation. This information is important for interpret the pain-evoked responses. The muscle pain intensity was recorded on a 10-cm visual analogue scale (VAS) during ten computer-controlled pressure stimulations (3-s interstimulus interval) at pain threshold intensity. The experimental data validated computer models describing the stress/strain time relationship in the deep tissue during pressure stimulation. VAS scores increased progressively with 2.5 ± 0.7 cm during the ten stimuli (P < 0.02) in contrast to the tissue indentation assessed by ultrasound, which was non-significantly changing leading to a mean of 3.4 ± 0.4 mm. The principal stress peaked in the skin was reduced to 16 % in the underlying muscle tissue and not different during the ten stimuli. The peak principal strain in adipose tissue was 0.12; in muscle tissue, it was 0.108 during the first stimulus and increased by 16 % in the tenth stimulus. In a model of a one-stimulus paradigm, it was found that a VAS increase of 2.5 cm required a 47 % increase in muscle strain. These findings show that the increase in muscle strain during repeated pressure stimulations is not sufficient to explain the VAS increase; the temporal summation of deep-tissue pain evoked by repetitive pressure stimulations is not likely to be fully explained by peripheral tissue changes.