Decrease of the electrical potentials measured on the surface of the knee and produced by cartilage compression during successive loading cycles

Electroarthrography (EAG) is a new technique formeasuring electrical potentials appearing on the knee surface during loadingthat reflects cartilage quality and joint contact force. Our objective was toinvestigate the evolution of EAG signals during successive loading cycles. Thestudy was conducted on 20 standing subjects who shifted their body weight toachieve knee loading. Their EAG signals were recorded during 10 successiveloading cycles, and during a subsequent sequence of 10 cycles recorded after a15 min exercise period. Multiple linear regression models estimated theelectro-mechanical ratio (EMR) interpreting the ability of cartilage togenerate a certain potential for a given ground reaction force by taking intoaccount this force and the center of pressure displacements during unipedalstance. The results showed that the EMR values slowly decreased with successivecycles: during the initial sequence, the correlation coefficients between EMRvalues and sequence numbers were significant at 3 of the 4 electrode sites (P<0.05); for the post-exercise sequence, the EMR values still decreased and were significantly lower than during the initial sequence (P<0.001). The reduction of EMR values could arise from muscle activity and habituation of the stretch reflex, and also from the time dependent electromechanical properties of cartilage. In conclusion, refraining from physical activity before the EAG measurements is important to improve measurement repeatability because of the EMR decrease. The electromechanical model confirmed the role of EAG as a natural sensor of the changes in the knee contact force and also improved EAG measurement accuracy.