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At present there are little available data which can explain what structures deform and how changes take place in unipennate cat medial gastrocnemius (MG) during contractions. Therefore the purposes of this thesis were (1) to quantify using sonomicrometry (MG, n = 8) the contribution of tendon and aponeurosis deformation to changes in fascicle geometry during isometric contractions, (2) to determine how fibre pinnation and series elasticity influence length changes in relaxed and activated fascicles during whole muscle excursion and, (3) to identify the functional range of sarcomere lengths in relaxed and active unipennate muscle. When going from the relaxed to the maximally activated state, fascicle lengths decreased on average by 27%, angles of pinnation increased on average by 42%, and muscle height remained constant. Observed changes in fascicle geometry during force production were primarily attributed to deformation of the aponeuroses. A decrease in the pinnation angle and elongation of the series elastic structures were responsible for differences in excursion between relaxed and activated fascicles and the whole muscle. It was concluded that pinnation and series elasticity allow sarcomeres of relaxed and activated fascicles to work on distinctly different limbs of the force-length relation.