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. 2015 Jun 10;6:174. doi: 10.3389/fphys.2015.00174

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Copyright © 2015 Herzog, Powers, Johnston and Duvall.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Force enhancement (FE) based on the sarcomere length non-uniformity theory. Isometric contractions of muscles on the descending limb of the force-length relationship are thought to occur with sarcomeres of uniform lengths (black circle and gray square). However, if a muscle is stretched from a short length (black circle) to a long length (gray square), some sarcomeres are thought to become overstretched (popped—right black diamond) while others are thought to be stretched only minimally (left black diamond), thus producing an average sarcomere length the same as that of the long isometric contraction (gray square), but producing more force because of the more favorable position of the short sarcomeres (left black diamond) and the passive forces of the overstretched sarcomeres (right black diamond).