Abstract
The sarcomere length non-uniformity theory (SLNT) is a widely accepted explanation for residual force enhancement (RFE). RFE is the increase in steady-state isometric force following active muscle stretching. The SLNT predicts that active stretching of a muscle causes sarcomere lengths (SL) to become non-uniform, with some sarcomeres stretched beyond actin–myosin filament overlap (popping), causing RFE. Despite being widely known, this theory has never been directly tested. We performed experiments on isolated rabbit muscle myofibrils (n = 12) comparing SL non-uniformities for purely isometric reference contractions (I-state) and contractions following active stretch producing RFE (FE-state). Myofibrils were activated isometrically along the descending limb of the force–length relationship (mean ± 1 standard deviation (SD) = 2.8 ± 0.3 µm sarcomere−1). Once the I-state was reached, myofibrils were shortened to an SL on the plateau of the force–length relationship (2.4 µm sarcomere−1), and then were actively stretched to the reference length (2.9 ± 0.3 µm sarcomere−1). We observed RFE in all myofibrils (39 ± 15%), and saw varying amounts of non-uniformity (1 SD = 0.9 ± 0.5 µm) that was not significantly correlated with the amount of RFE, but through pairwise comparisons was found to be significantly greater than the non-uniformity measured for the I-state (0.7 ± 0.4 µm). Three myofibrils exhibited no increase in non-uniformity. Active stretching was accompanied by sarcomere popping in four myofibrils, and seven had popped sarcomeres in the I-state. These results suggest that, while non-uniformities are present with RFE, they are also present in the I-state. Furthermore, non-uniformity is not associated with the magnitude of RFE, and myofibrils that had no increase in non-uniformity with stretch still showed normal RFE. Therefore, it appears that SL non-uniformity is a normal associate of muscle contraction, but does not contribute to RFE following active stretching of isolated skeletal muscle myofibrils.
Highlights
A popular mechanism thought to be responsible for residual force enhancement (RFE) is the sarcomere length non-uniformity theory (SLNT) [24,25]
Individual myofibril results are presented in table 1, and visual representations are depicted in figure 2a–c
The I-state exhibited an average stress of 89 ± 35 nN μm−2, which was significantly less than the average stress produced in the FE-state (121 ± 45 nN μm−2; p < 0.05), thereby exhibiting an average RFE of 39 ± 15%
Summary
A popular mechanism thought to be responsible for RFE is the sarcomere length non-uniformity theory (SLNT) [24,25]. Proponents of the SLNT suggest that upon active stretching along the descending limb of the force–length relationship, muscles develop vast sarcomere length non-uniformities that are not present for purely isometric contractions. These non-uniformities are thought to occur because of instability of sarcomeres on the descending limb of the force–length relationship [7], which causes most of the stretch to be taken up by initially slightly longer, and weaker, sarcomeres, while the initially short, and strong, sarcomeres remain at about a constant length [24,25]. That (i) longer individual SL in the I-state correlates positively with individual SL change between the 3 I- and FE-states; (ii) that myofibrils in the FE-state have greater SL non-uniformity than the I-state; (iii) that active stretching is always accompanied by the popping of some sarcomeres; and (iv) that an increase in SL non-uniformity correlates positively with RFE
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