In graded index multimode fibres the transmitted light intensity profile across the face of the fibre is used to monitor the axial strain to which the fibre is exposed. Fibres embedded in concrete, moulded metal and epoxy resin structures were used to detect the strain experienced by the host matrix via the perturbation of the optical power distribution across the cross-section of the fibre core due to stress induced refractive index changes and the resultant exchange of power between optical modes in the waveguide. Intermodal power coupling coefficients were evaluated theoretically for such fibres - in terms of the stress induced refractive index change - and found to be substantial for moderate strains of the order of a few hundred microstrain. The technique cannot differentiate between compressional and extensional axial strains. The system resolution is estimated to be 100 microstrain and the range is unlimited (measurements were made up to a maximum of 2000 microstrain).