Correlation between stiffness and electromechanical delay components during muscle contraction and relaxation before...

The study was aimed at assessing possible correlations of the electromechanical delay components during muscle contraction (Delay_TOT) and relaxation (R-Delay_TOT), with muscle-tendon unit (MTU), muscle, and tendon stiffness before and after static stretching (SS). Plantarflexor muscles' maximum voluntary torque (T_max) was measured in 18 male participants (age 24±3yrs; body mass 76.4±8.9kg; stature 1.78±0.09m; mean±SD). During T_max, surface electromyogram (EMG), mechanomyogram, and force signals were detected. Delay_TOT and R-Delay_TOT with their electrochemical and mechanical components were calculated. Passive torque and myotendinous junction displacement were assessed at 0°, 10° and 20° of dorsiflexion to determine MTU, muscle and tendon stiffness. The same protocol was repeated after SS. Delay_TOT, R-Delay_TOT and their mainly mechanical components correlated with MTU, muscle and tendon stiffness, both before (R² from 0.562 to 0.894; p<0.001) and after SS (R² from 0.726 to 0.955; p<0.001). SS decreased T_max (-14%; p<0.001) and lengthened almost all the Delay_TOT and R-Delay_TOT components (from +5.9% to +30.5%; p<0.05). Correlations were found only between stiffness and the mechanical components of Delay_TOT and R-Delay_TOT. Correlations persisted after SS but delays increased to a higher extent than stiffness, indicating a complexity of the relationship between stiffness and delays that will be discussed in the manuscript.