Stretch-shortening cycles mitigate single muscle fibre power deficits following a two-week immobilization of the rat hindlimb.

Muscles undergo stretch-shortening cycles (SSCs) and following active stretching, work, and power during shortening are amplified compared with pure shortening contractions (i.e., SSC effect). We investigated the effects of immobilization on SSCs and the contribution of residual force depression (rFD) and enhancement (rFE). Single muscle fibres (n = 120) were chemically permeabilized from the rat soleus and medial gastrocnemius of control and cast (14 days unilateral immobilization). Fibres were maximally activated while mounted between a force transducer and length controller. For SSCs, fibres were actively lengthened from sarcomere lengths of 2.5-to-3.0 µm and immediately shortened back to 2.5 µm (all 0.6Lo/s), work and power during shortening was compared to a shortening contraction not preceded by active lengthening. rFD and rFE were assessed using active shortening (3.0-to-2.5 µm) or lengthening (2.5-to-3.0 µm) contractions, respectively, both 0.6Lo/s, with steady-state isometric force compared to a fixed-end isometric contraction at the same length. Cast fibres had smaller cross-sectional areas compared to control (both P < 0.05). Compared to control fibres, cast soleus showed ~ 6% greater rFD% (P = 0.0027) but no difference in rFE% (P = 0.55), whereas cast medial gastrocnemius showed ~ 5% reduced rFE% (P = 0.001) and unaffected rFD% (P = 0.09). Cast fibres demonstrated SSC performance enhancement (~ 120% and ~ 210%, all P < 0.001), however, the enhancement was less in the cast compared to control medial gastrocnemius (~ 88% vs ~ 120%, P = 0.022), while the soleus was not (both P > 0.86). Across muscles the SSC effect mitigated deficits in absolute work and power in the cast compared to control. Therefore, SSCs can mitigate contractile deficits at the cellular level after immobilization.