Stretch-shortening cycle performance enhancement is not affected by initial pre-activation muscle length in permeabilized single muscle fibres

The purpose of this study was to assess whether initial pre-activation muscle length alters stretch-shortening cycle (SSC) performance enhancement in permeabilized single muscle fibres. Single fibres (n = 16) from the psoas major of Sprague Dawley rats (n = 6, 13-14 weeks) were dissected and chemically permeabilized. Fibres were maximally activated (pCa 4.5) while mounted between a force transducer and length controller. For pre-activation at a long length, the fibre was passively stretched from 2.5 to an average sarcomere length of 3.0 µm. After passive force stabilized, the fibre was activated before being rapidly shortened to 2.5 µm at 10 Lo/s to detach all cross-bridges. Activation was maintained and once active force recovered the fibre underwent the SSC protocol. For pre-activation at optimal length, the fibre passively underwent the same protocol as the long length condition. However, activation only began once the fibre was returned to a sarcomere length of 2.5 µm. For SSCs, fibres were stretched from sarcomere lengths of 2.5 to 3.0 µm and immediately shortened back to 2.5 µm at a speed of 0.6 Lo/s for both phases. The SSC effect was calculated by comparing work and power during shortening to an active shortening contraction not preceded by active lengthening. The SSC enhancement (∼55% increase; all P < 0.003) was not significantly different between the two initial pre-activation muscle lengths (P > 0.14). Therefore, initial pre-activation muscle length does not significantly alter the SSC effect of permeabilized single fibres as non-cross-bridges structures, such as titin, were unlikely to be differently modified by the pre-activation starting lengths.