Our previous results (Colomo et al. 1998) showed that the optimal sarcomere length (SL) for force development in intact muscle fibres, activated with submaximal contractures, shifted to a sarcomere length longer than would be expected on the basis of myofilament overlap. These results, obtained under ‘fixed-end’ conditions are consistent with the evidence obtained in skinned fibres showing that the optimal SL for force development shifted at larger values when the [Ca²⁺] in the myoplasm was lowered. It could be objected, however, that because of a possible non-homogeneous activation, even in submaximal contractures significant fibre portions could still be at full or near full activation. If this were the case the tension creep generated by these fully activated portions could be responsible, at least partially, for the increase in tension observed at long SL. To clarify this point we determined the lengthÐ tension relationship under isometric conditions and during slow fibre shortening (shortening velocity was 10Ð15 % of maximum velocity), a condition which has been shown to eliminate the tension creep (Morgan et al. 1991). Single fibres, isolated from the interosseus digiti IV muscle of Rana esculenta (frogs were killed by decapitation followed by double pithing), were mounted between a force transducer and a length-control motor. Experiments were made in ‘fixed-end’ conditions at SL between 2.10 and 3.10 mm, at a temperature of 18 °C. SL was measured directly by a laser diffractometer, or was calculated from fibre length. The shape of the isometric lengthÐtension relationship in both maximum tetani or maximal contractures ([K⁺]_o = 190 mM), was similar to that already described in the literature for ‘fixed-end’ conditions. In contrast, when the tension was measured during shortening, the lengthÐtension relationship had the shape expected on the basis of filament overlap extrapolating to zero at 3.55 mm SL. In submaximal contractures ([K⁺]_o = 30Ð48 mM, and isometric tension of 0.2Ð0.4 P₀), in both isometric and shortening conditions, the ascending limb of the lengthÐtension relationship had a similar shape: it increased with sarcomere length reaching a peak at 2.80Ð3.00 mm. In both conditions the maximum peak tension corresponded to the value expected on the basis of filament overlap. This result shows that the shape of the isometric lengthÐtension relationship, in our experiments with submaximal contractures, is not affected by tension creep. We conclude that: (i) submaximal contracture is a suitable model to obtain, in intact fibres, conditions of partial and homogeneous activation (low internal Ca²⁺); (ii) the increase in apparent sensitivity of tension to [Ca²⁺]_i that occurs in skinned fibres at long SL also occurs in intact fibres.

All procedures accord with current National guidelines.