Injury to skeletal muscle fibers of mice following lengthening contractions

TLDR

The study tested whether lengthening contractions cause more muscle fiber injury than isometric or shortening contractions. Mice underwent a controlled contraction protocol (isometric, shortening, or lengthening) with the extensor digitorum longus muscle attached to a servomotor, followed by recovery and in vitro force measurement of the isolated muscle. Lengthening contractions caused acute fiber degeneration (≈37%) and a 78% drop in force, whereas isometric/shortening contractions had minimal effect; muscle function largely recovered by 30 days, confirming that lengthening contractions uniquely induce significant injury.

Abstract

We tested the hypothesis that lengthening contractions result in greater injury to skeletal muscle fibers than isometric or shortening contractions. Mice were anesthetized with pentobarbital sodium and secured to a platform maintained at 37 degrees C. The distal tendon of the extensor digitorum longus muscle was attached to a servomotor. A protocol consisting of isometric, shortening, or lengthening contractions was performed. After the contraction protocol the distal tendon was reattached, incisions were closed, and the mice were allowed to recover. The muscles were removed after 1–30 days, and maximum isometric force (Po) was measured in vitro at 37 degrees C. Three days after isometric and shortening contractions and sham operations, histological appearance was not different from control and Po was 80% of the control value. Three days after lengthening contractions, histological sections showed that 37 +/- 4% of muscle fibers degenerated and Po was 22 +/- 3% of the control value. Muscle regeneration, first seen at 4 days, was nearly complete by 30 days, when Po was 84 +/- 3% of the control value. We conclude that, with the protocol used, lengthening, but not isometric or shortening contractions, caused significant injury to muscle fibers.