How Stretching Works, Without Boring You to Death

Let’s keep this simple — because honestly, most of us don’t need a deep dive into molecular biology to understand why our backs feel like bricks.

Think of your muscles like Russian nesting dolls. Big picture: you have a muscle. Inside that are bundles called fascicles. Inside those are muscle fibers. Inside those are myofibrils. And deep inside those live the sarcomeres — tiny little units that actually do the contracting and lengthening.

Within the sarcomeres are two key players: actin and myosin — thin and thick filaments that slide across each other to make your muscles move.

To picture it: hold your hands up, palms facing you, fingers touching. Slide them together, then apart. That’s what actin and myosin do when they expand and contract.

So how does this start?

Your brain sends a signal to your nerves, which then sends a message to your muscles at the neuromuscular junction (fancy name, I know). That signal sparks a release of calcium, which tells actin and myosin to start sliding — a.k.a. your muscles contract.

Example: you pick up a full cup of coffee. That’s a heavier lift, so more sarcomeres are recruited. If the mug’s empty, fewer sarcomeres are needed. Same motion, different effort.

Now apply that to stretching.

When you stretch, not all sarcomeres lengthen — at least not at first. This is also why people think they aren’t flexible or feel stiff. If you yank into a stretch too quickly, your body freaks out a little. It thinks it’s in danger, so it contracts to protect you.

But when you stretch slowly and hold it — 1 to 5 minutes — you’re telling your nervous system, “Hey, it’s cool. We’re safe.” That’s when your body starts to release the tension and your muscles actually lengthen (well, more accurately, your sarcomeres slide apart).

This is why time over tension matters — and why smart, sustained stretching is way more than just bending over and counting to 10.