When a muscle is stretched, the lengthening reaction keeps it from contracting and helps it to relax. If you’ve read about the stretch reflex before, then you can think of the lengthening reaction as pretty much the stretch reflex’s opposite.

Whereas the stretch reflex contracts your muscles in response to being stretched, the lengthening reaction allows muscles to become more flexible, i.e., longer.

The stretch reflex helps muscles maintain a constant length, and helps avert injury by keeping us from going beyond the normal range of motion. The lengthening reaction protects us from injury, too. In the case of the lengthening reaction, though, it allows our muscles to relax so we don’t tear them during use.

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