I did a workout yesterday with one of those spring bars. Well, four of them. I'm now able to do 30 reps on the 50 kg bar. So I did that to failure. Then I went down the next size, to the 40, the 30, and even the 20. I did them all until I couldn't do anymore.

That's been my routine followed by five days of rest.

Yesterday, I was physically weak, but felt completely amazing.

Ten hours later, things began to feel inflamed. This morning, everything in my upper body just hurts, muscles I didn't even know I had.

After seeing the thread about the guy snapping his bicep, I am not looking for that to happen to me.

My bones also feel achy.

1. Are my tendons okay?
2. Why am I still hurting after having done this routine before?

I did have more coffee than usual yesterday before doing this.

Sounds like you have a case of the DOMS (Delayed-onset muscle soreness) is muscle pain that begins after you’ve worked out. It normally starts a day or two after a workout.

You get it because you worked out too hard and your body isn't use to it.

If you do your routine only once every 5 days, you are not doing it enough. There is no training effect, so your body is not getting use to working out or efficiently clearing out the lactic acid build up in your muscles.

What's a spring bar?  The things you try to bend with the spring in the middle?  Seems like a really odd routine you have.  Is this a supplement to your regular work?

Whatever the case, above post is right.  Going full blast one day a week on one exercise will make you sore every time.

By "spring bar" I mean this thing:

https://www.amazon.com/gp/product/B06XX9BFKJ

When I started, I could only do a few reps on the 50 kg one. Now I'm at 30. My whole back, shoulders and arms are sore the next day, so it's doing something. And my reps are going up.

I've been doing a three week cycle:

Week 1: Bend down
Week 2: Bend up
Week 3: Dumbbell curls

But I've read that pushing-past-failure isn't good habit. I've gotten excellent results out of it, but I'm going back to probably...three sets of 10 each day and doing all three each day, Monday, Wednesday, and Friday.

Anyway, my tendons appear to be fine.