The molecular structure of Bombyx mori silkworm silk fibers is investigated in situ upon externally applied tensile stress using synchrotron X-ray diffraction, while the molecular vibrational response is investigated using cold neutron time-of-flight spectroscopy. The aligned silk fibers are therefore exposed to a tensile force along the fiber axis generated by stretching machines adapted to X-ray and neutron scattering, respectively, and the stress-strain curves are measured in situ. The applied force in both cases is sufficient to reach the yield point of plastic deformation. In the case of neutron spectroscopy, different regions within the hierarchical silk structure are masked by selective deuteration. The X-ray studies confirm the assumption that most of the deformation upon extension of the fibers is due to the amorphous regions of the silk. The neutron results indicate that the externally applied force is not reflected by any noticeable effect on the molecular vibrational or diffusional/reorientational level in the range accessible to neutron time-of-flight spectroscopy. This observation of unaffected molecular dynamics is in agreement with a model of entropy elasticity.