ABSTRACT
The active isometric force produced by muscles varies with muscle length in accordance with the force–length relationship. Compared with isometric contractions at the same final length, force increases after active lengthening (force enhancement) and decreases after active shortening (force depression). In addition to cross-bridges, titin has been suggested to contribute to force enhancement and depression. Although titin is too compliant in passive muscles to contribute to active tension at short sarcomere lengths on the ascending limb and plateau of the force–length relationship, recent evidence suggests that activation increases titin stiffness. To test the hypothesis that titin plays a role in force enhancement and depression, we investigated isovelocity stretching and shortening in active and passive wild-type and mdm (muscular dystrophy with myositis) soleus muscles. Skeletal muscles from mdm mice have a small deletion in the N2A region of titin and show no increase in titin stiffness during active stretch. We found that: (1) force enhancement and depression were reduced in mdm soleus compared with wild-type muscles relative to passive force after stretch or shortening to the same final length; (2) force enhancement and force depression increased with amplitude of stretch across all activation levels in wild-type muscles; and (3) maximum shortening velocity of wild-type and mdm muscles estimated from isovelocity experiments was similar, although active stress was reduced in mdm compared with wild-type muscles. The results of this study suggest a role for titin in force enhancement and depression, which contribute importantly to muscle force during natural movements.
FOOTNOTES
Competing interests
The authors declare no competing or financial interests.
Author contributions
Conceptualization: U.T., J.A.M., K.C.N.; Methodology: U.T., J.A.M., K.C.N.; Formal analysis: U.T., N.A.R.; Investigation: U.T., N.A.R.; Data curation: U.T., N.A.R.; Writing - original draft: U.T.; Writing - review & editing: U.T., N.A.R., J.A.M., K.C.N.; Visualization: U.T.; Supervision: K.C.N.; Funding acquisition: U.T., K.C.N.
Funding
This research was supported by the W. M. Keck Foundation, the National Science Foundation [IOS-0732949, IOS-1025806, IOS-1456868 and BIOTEC 0742483], and the Technology Research Initiative Fund of Northern Arizona University.
Data availability
All data and analyses reported in the paper are available from the Dryad digital repository (Tahir et al., 2019): dryad.3vm2818
- Received February 17, 2019.
- Accepted December 19, 2019.
- © 2020. Published by The Company of Biologists Ltd
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