How do I strengthen wooden chair legs?

Add supports across the bottom of the chairs, tying them together. You can use 1x1 or 1x2 or spindles.

You can buy a kit to inject a very strong glue into joints in furniture. Then you'd want to clamp them tight until the glue dries.

I would add L brackets to each of the inside corners.

If the joints are coming apart, I squeeze some carpenter's glue in the joint, then use a rubber mallet to tap it fully in. (or a hammer with a a wash cloth over the area to be tapped in)

I have in the past, put a wooden dowel support across the legs if wobbly. Drill 2 holes the diameter of dowel partially into the chair legs, then use carpenter's glue to set them.

There are a lot of videos on YouTube for any type of problem in repair chairs, check them out.... https://www.youtube.com/results?search_query=how+to+add+a+dowel+support+to+a+chair

here is some info

https://www.youtube.com/watch?v=m2dpo3q7JPU

Tightening up the chair will help a lot: https://www.youtube.com/watch?v=xBkce8meWkU

You need to take the legs off clean all the old glue off then put new glue then put them back together then stand them on there legs so they are even

What style of legs are they and how are they attached to the frame? With pegs, with hardware?

Please post a picture of the underside of the chair so that we can see what you have to work with.

If you can unscrew them, apply wood glue and screw back

If you can take the chair legs off then you can re glue and use new fasteners. Here is a shot vid that may help you out...

https://www.youtube.com/watch?v=xBkce8meWkU

Hello. I had some wobbly wonky thrift store wooden chairs and I turned the chair upside down placed sand and super glue into the joint cracks. It firmed up the legs very nicely. Hope this helps.