Abstract : This thesis is devoted to the modelling of transient response of muscle fibers submitted to fast mechanical loadings. At the nanometer scale, the muscle fiber contains actin and myosin filaments grouped to form contracile units called 'sarcomeres'. Myosin filament is an assembly of molecular motors that periodically attach and detach to the actin filament in presence of ATP. During this attachement-detachement process, myosin undergoes a force generating conformational change called the 'power-stroke' whose characteristics can be revealed by the transient responses following fast mechanical loadings. We propose an innovative mechanical model of a half sarcomere that links the characteristics of myosin to the response of the whole fiber. Unlike existing models, using a discrete approach, this model is based on the definition of a continuous energy landscape that takes into account a mean field interaction between the molecular motors. This system presents radically different responses under imposed length and imposed force conditions. We particularly emphasize a difference in the kinetics, also observed experimentally. We show that the half-sarcomere is inherently unstable which explains the sarcomere length inhomogeneities observed recently on myofibrils.