Previous studies on activated frog muscle fibres (1-3) showed the presence of a Ca²⁺-dependent non-crossbridge stiffness, called “static stiffness” (SS) which preceded tension generation following stimulation. This effect was attributed to a Ca²⁺-dependent stiffening of the titin filament. The experiments reported here were made to ascertain whether the SS was present also in mammalian muscle fibres from either wild-type (WT) or MLC/mIGF1 transgenic (TG) mice in which the local expression of the insulin like growth factor-1 (IGF1), under transcriptional control of Myosin Light Chain (MLC) promoter, induces muscle hypertrophy (4). Mice (3-6 month-old) were killed by rapid cervical dislocation, according to the EEC (Directive 86/609) guidelines for animal care. Experiments were performed at ~23°C on single intact fibres from the flexor digitorum brevis muscles. Fibres were mounted in an experimental chamber between the lever arms of a force transducer and of an electromagnetic motor to apply fast stretches. Sarcomere length (l₀) was measured by means of a videocamera. The results showed the presence of the SS in both WT and TG mouse fibres. SS value was about two times greater and developed faster than in frog, reaching the peak on average 4 ms after the stimulus compared to 8 ms in frog. In WT fibres the peak of SS was 2.75 ± 0.27 % (n = 7) of the maximum stiffness (S₀) measured at tetanus plateau tension (P₀) thus representing a negligible fraction of the total. However 4 ms after the stimulation, when the active tension is still very low (2.57 ± 0.32 % P₀), SS represents about 50 % of the total sarcomere stiffness. SS was about 10 times greater than passive stiffness. Thus due to the increase of the intracellular Ca²⁺ the fibre stiffness rises before tension increasing significantly the mechanical stability of the sarcomere. SS was significantly smaller (2.13 ± 0.15 % S₀; n = 6, P<0.05, t test) in TG fibres. The finding that the static stiffness is present in mouse muscle fibres strengthen our hypothesis that SS is due to a Ca²⁺ stiffening of titin. It is known in fact that mammalian skeletal muscle express the N2A titin isoform whose stiffness is Ca²⁺-sensitive. The lower value of SS found in TG fibres could be due to a lower sensitivity of titin filament to Ca²⁺.