Muscles shorten when working out - why do I feel this most strongly in the lower back?

Question

When I follow the normal routine of my gym including stretching after the workout, I strongly feel a shortening of my lower back muscle, to the point where I have a distinct hollow back after working out.

I don't know the english name of the exercise for the lower back, but basically I lie on a platform inclined at roughly 45°, tilt my torso downward till the angle torso-legs is 90° or a bit smaller, and up again. I do (as advised) 15 reps, 3 sets, that's barely doable for me.

I concentrate strongly on my lower back when stretching after exercise, doing stuff I've seen in yoga or pilates, but I still have a stronger hollow back after a workout (or at least it feels like that).

Why is this so? Any advices to avoid long term trouble?

edited to add/clarify: My subjective feeling is that the lower back muscles are shorter (and don't have that feeling that strongly with other muscle) and I see that it affects my posture.

Answer 1

It's called a back hyperextension.

If 3x15 is "barely doable" then you feel it the most in the lower back (I assume you mean when compared to other muscles you've exercised) because you're stressing it near the max of what your body can tolerate. The muscle is engorged with blood ("pumped"), tired and stressed.

It doesn't sound like you're injured, it doesn't sound like there's anything out-of-the-ordinary. You're feeling it because you're at your limit and sensitive to the input the muscle is giving you.

You could also just so a sanity-check postural check to see if there's a large phsyical difference, or if it just feels like there's a big difference. Compare your posture before and after your workout session.

I'd add lower-back self-myofascial release work and possibly massage, and keep an eye on it. It doesn't sound like anything acute or damaging at this point, with the caveat that we can't know that.