Correlation between stiffness and electromechanical delay components during muscle contraction and relaxation before and after static stretching

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.