The aim of this thesis has been the understanding at the molecular level of stretch activation in muscles. Stretch activation is a feature common to all types of muscle. Such a mechanism is particularly important in cardiac muscles in vertebrates and in indirect flight muscles in insects. Here, the three-dimensional structure of Lethocerus troponin C isoform (F1), recently discovered to be crucial in regulating stretch-activated contraction, is presented. The interactions between F1 and the other subunits of the Lethocerus troponin trimer are also analyzed. Finally a model for stretch activation in muscle, based on the experimental data produced during this thesis, is proposed.