Passive stretching-induced changes detected during voluntary muscle contractions.

Stretching exercises are known for reduction of musculoskeletal stiffness and elongation of electromechanical delay (EMD). However, computing a change in stiffness by means of time delays, detected between onset of electromyographic (EMG), mechanomyographic (MMG) and force signals, can reveal changes in subcomponents (Δt EMG-MMG and Δt MMG-FORCE) of EMD after stretching. In our study, the effect of stretching was investigated while quadriceps femoris (QF) muscle performed isometric contractions. The EMG, MMG, and Force signals were recorded from rectus femoris (RF) and vastus medialis (VM) during five voluntarily isometric contractions at 15°, 30°, and 45° of knee flexion angle, while the leg was positioned on a custom-made device. Subjects in both intervention and control groups underwent same recording procedure before and after stretching. No difference between the baseline repeated contractions (before stretching) was ensured by ANOVA for repeated measures while a difference between PRE and POST was analyzed and concluded based on the effect size results. The EMD did not change; however, subcomponents (Δt EMG-MMG and Δt MMG-FORCE) showed differences within RF and VM muscles after stretching. The 30° knee flexion angle appears to be a position where isometric contraction intensity needs to be carefully monitored during rehabilitation period.