We reported previously that the force transient produced by a fast stretch, applied to a skeletal muscle fibre during the early phases of a twitch contraction, was followed by a period, lasting several milliseconds, during which tension remained constant at a value well above isometric tension (Bagni et al. 1994). Such a maintained increase of force was not expected on the basis of cross-bridge kinetics, since reversal of the power stroke should quickly restore the original tension (in about 1 ms) by relieving the strain produced by the external stretch on series elasticity (Ford et al. 1977). This suggested to us that the excess tension, referred to as static tension, was not arising from stretched cross-bridges, but from some unknown elastic sarcomere structure whose stiffness (static stiffness) increased upon activation (Bagni et al. 1994). Several other observations were consistent with this hypothesis: static stiffness (1) started to increase during the latent period about 2Ð3 ms after the stimulus; (2) followed a time course clearly distinct from that of tension; (3) was practically unaltered when 1Ð3 mM of 2,3-butanedione monoxime (BDM) reduced twitch tension by more than 90% (4) showed no correlation with myofilament overlap when sarcomere length was altered; (5) reached its value with no delay at the end of the stretch, even when stretch duration was too short to allow significant cross-bridge cycling. A complete analysis of the static stiffness properties under steady-state conditions was performed recently by studying the force responses to stretches applied during tetanic contractions (Bagni et al. 2002). In these experiments BDM at 1Ð8 mM concentration was added to the Ringer solution to inhibit tetanic tension (by as much as 98 %). This reduced the number of attached cross-bridges and allowed isolation of the response of the static stiffness to stretch. In addition, unlike fully activated fibres, fibres in BDM were not damaged by the stretches and retained their sarcomere length homogeneity. Stretches of various amplitude (up to 40 nm hs^-1) and duration (0.4Ð0.8 ms) were applied to the fibre at tetanus plateau and at different times after the start of stimulation. Static tension increased linearly with stretch amplitude in the whole range tested and was independent of stretch duration (up to 30 ms), as expected from a pure Hookean elastic response. As in twitches responses, static stiffness changed following the start of stimulation and was not correlated with tension. Static stiffness reached a peak about 10 ms after stimulation, when tension developed by the fibre was still very small, and it decayed within 100Ð200 ms to a constant level lasting until the start of relaxation. The mean stiffness measured at tetanus plateau in six fibres at a mean tension of 0.0360 (± 0.0038 S.E.M.) times tetanic tension in normal Ringer (P₀), was 1.24 X 10^-3 (± 0.12 X 10^-3 S.E.M.) P₀ (nm hs)^-1. This value corresponds to less than 1 % of the total fibre stiffness measured at plateau in normal Ringer solution. Data from twitch contractions showed that static stiffness development followed a time course roughly similar to that of intracellular Ca²⁺ concentration, suggesting a possible correlation between the two parameters. This hypothesis has been tested by measuring static stiffness in the presence of various agents, such as Dantrolene, Methoxiverapamil (D600), and Ringer made with D₂O, which have all been shown to depress twitch tension mainly by reducing calcium release with no direct action on cross-bridge formation. Experiments were also performed in hypertonic Ringer (up to 1.6 normal tonicity). Similarly to BDM, hypertonic Ringer depressed twitch tension without affecting calcium release. Data from five fibres showed that, relative to normal Ringer, hypertonic solution (1.4 T), reduced tension to 0.270 (± 0.045 S.E.M.) while static stiffness was left practically unaltered (reduced to 0.980 ± 0.030 S.E.M.). On the contrary a similar twitch reduction (0.2 ± 0.038 S.E.M.) produced by Dantrolene (at 10 µM concentration) was accompanied by a substantial static stiffness reduction to 0.64 (± 0.0383 S.E.M.). In general, the results showed that static stiffness was almost unaffected by BDM and hypertonic solutions even when twitch tension was strongly inhibited. Twitch inhibition was instead accompanied by a static stiffness reduction when deuterium oxide or Dantrolene or D600 were added to the bath. These results suggest that static stiffness is modulated by intracellular calcium concentration.

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