• Lars Arendt-Nielsen and Thomas Graven-Nielsen.
• Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7D-3, Aalborg, Denmark.
• Clin J Pain. 2011 Jun 1;27(5):414-24.

ObjectivePressure algometry is used for assessment of pain sensitivity. In this study the relation between tissue characteristics and pressure pain thresholds was investigated.MethodsThree-dimensional finite-element computer-models were developed to simulate the tissue stress and strain distribution during pressure stimulation on muscles with different hardness (I, II, III, IV (hardest)) and subcutaneous adipose tissue thickness (normal and thicker). The computer model was validated based on data recorded by computer-controlled pressure-induced muscle pain in 8 and 16 partecipants, respectively.ResultsThe experimental pressure-indentation curve fitted the outcome of the FE model (R>0.73). Stress and strain were extracted from the models at a known painful pressure stimulation level. PPT and PPTO were not significantly different in subjects with normal and thick adipose tissue in accordance with the simulation model where the strain in muscle tissue was comparable in the two conditions. The strain in adipose tissue was larger in subjects with thick adipose tissue compared with normal adipose thickness. In relaxed muscle (hardness I) the principal strain peaked at 0.12 in the adipose tissue, was reduced to 0.07 in the muscle tissue and 0.05 in the harder muscle. Significantly higher PPT and PPTO were recorded in harder compared with softer muscles (P<0.02).DiscussionThe pressure pain sensitivity of the deep layer is related to the amount of muscle strain, which is affected by the muscle hardness and the thickness of adipose tissue. This is clinically relevant as these two factors are not taken into consideration when pressure pain assessments are performed in clinical routine.

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