My son and I are just starting out cycling and he's been complaining of soreness in the back after a 20mile ride.
He's a shorter human at 5'3" and we got him a late 80's puch in a 48 to 49cm range. Top bar gives him 1.5in clearance and I've repositioned handlebar height so that he isnt riding too horizontal. The boy is fit and bikes to school daily, so me at 45 y.o and 3 herniated disc felt no pain. What other factors might cause back soreness?

Back soreness usually signals core strength issues. A lot of people have weak lower backs, I don't think youth is a guarantee that isn't the case. Cycling uses the lower back.

Those bikes were really long IIRC. Shorter stem might be a better fix than raising the handlebars.

Thanks for the quick response, I'll look into it, now that I think of it, not sure how bent his elbows are. So yes, if his arms have no bend then We'll def need a shorter stem.

His muscles are adjusting. From the way you're wording it, that's what I'm understanding.

Sit ups and stretching the hamstring will go along way. Break a good sweat before stretching.
I'm not licensed for this.

Lower back? Or upper back between the shoulders? I'm assuming a Puch road bike, but they made a BMX and other stuff too.

On a road bike, sitting too upright is a mistake in my opinion. That will put more weight on both your shoulders and upper back as well as the saddle. For me, leaning forward lets the power my legs are developing lift some weight off the saddle and the fact my body won't bend much more takes some weight off my wrist and arms.

As for his pain, I don't know. But when doing longer rides than your body is used too, muscles will squawk for awhile till they get used to the new norm.

If I don’t stretch after I ride my back will bother me. Why I don’t stretch after every ride, don’t ask me.

Oh you have that!

Let's say for arguments sake. Do 40 or 50 jumping jacks. Now you're sweating.
Your muscles are hot. Now you can stretch and don't bounce. After that continue
with your workout.

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Have someone check the fit, just to be sure; however, I've learned all people who fit seem to have different opinions. Perhaps riding more upright using a different stem? As others mentioned, improving the core helps. I have a bad back issue which derailed be from cycling last summer. Since then I've been doing TRX which has helped a lot with core improvements.

Here's my primer on how to check a fit: How can I fitting my bike

Fit doesn't have to be exact, but the above guidelines should prove helpful, just to see if anything is obviously amiss One thing not mentioned there, but useful in this case, is to check that he is not rounding his back. The back should be more-or-less straight from the top of the shorts to the base of the neck.

The real issue, as has been stressed above, is fitness. Many people think the perfect bike fit will eliminate pain. It will not. The only thing that eliminates pain is perfect fitness. And even that doesn't work forever, just take a google and look at the faces of RAAM finishers.

How to get the back fit? The best way is to ride lots. Ride until it hurts, rest it for a day, then do that again. After not all that many repetitions, the pain won't start for many more miles. That can be eventually increased to 100s of kilometers.

Of course off-bike exercises help too, and will shorten the time to fitness, but nothing makes your back as fit as cycling, lots of it.

Doing specif back strengthening exercises can only help. There are many videos showing such exercises.
https://www.google.com/search?client...xytMPhbmFkAs39
I have my own such series I do to begin my day but they do the same function as the model in the video.

Since my initial post, we've logged almost 200 miles and I'm happy to say he is pain free. ON the other hand, I am now experience back soreness, which I attribute to my hamstring tightening/swelling and pulling on my lower back. A few basic stretches really alleviates it though.

Thanks all

Great! Weird isn't it, how the leg muscles seem to pull on everything below the shoulder blades. It's called the "posterior chain," all connected. My wife and I did 50 miles on our tandem yesterday, with a lot of that being "spirited riding." Cars would pass us, seeing only a bicycle, not having any idea we might be doing 25, and then having issues with the oncoming. I have built-in reflexes to head for the ditch in the event of a head-on happening in front of us.

Back hurt like the devil yesterday, but OK today, glutes still a bit sore today. I think we'll take an hour's walk, today being unusually sunny.

That is true

Did you cange the setup?

Yes. I went and installed the Genetic 16 Driser road bar Google reviews.
Also, I'm trying out a StopShock seatpost by Redshift
I was concerned I would lose power transfer with the stopshock but haven't. It does make the ride (long rides) somewhat more enjoyable. The roads in my area are not in good shape.
I do think TRX is really helping. I do 10~13 movements (15 reps) in two sets minimum, sometimes more every other day.
-B