Okay, please take this with multiple grains of salt because I’m unable to do quadrobics, but most of your struggles sound like strength and conditioning issues /gen
The hunch (rounded back) sounds like your body bracing itself against discomfort. Usually a human on two legs would “curl up” into a crouched position to improve balance and avoid overstretching muscles, so stretching out (flattening the spine) goes against that practiced instinct. You mention that in some positions (curling fingers) your hands and palms hurt, and that you keep landing on your back legs instead of your arms. That all sounds like your body anticipating discomfort and subconsciously “correcting” your posture, which theoretically can be trained out by strengthening you body so it trusts that it won’t get injured or knocked over/unbalanced by being in those positions.
I think a balance of quadrobics and non-quadrobics-specific exercises would be the best to build your strength & balance so that your body doesn’t feel “weak” and need to curl up/brace itself.
For strength: you’re probably curling your spine (with your legs under instead of behind you) because your distribution of weight feels most comfortable being over your legs, which are already very strong from bipedal walking. To convince your brain to move your legs backwards, you need to show it that your arms are a strong and viable option too. Build arm strength and practice holding your weight without legs whatsoever. That’s why for the back leg form you want, “high planking” (when your arms are straight, at a right angle beneath your shoulders) sounds like a great place to start because it forces you to get comfortable with your upper body’s weight resting on just your arms. Pushups can be the next step up (both of these positions also help to bend your toes forward by resting your leg’s weight on them instead of a flat foot).
The end goal strength and confidence builder if you want to really improve the upper body strength and move your center of gravity forward; try learning to hold your weight entirely on your hands using whatever poses feel most accessible to you. Poses like the Crow Pose, the Firefly Pose, the Crane Pose, the Scale Pose, or even a Handstand if you want to be fancy. (The Crow Pose and Crane Pose are probably the closest to the weight distribution you’re looking for) The goal is to make your body learn that your legs can be an afterthought, not the leading force, so they don’t need to come up and walk ahead of you, they can be behind you until you need an extra boost while running or jumping upward.
For balance: try intentionally unbalancing yourself while doing quads to build the muscles that will help correct your positioning, as well as making your subconscious more confident that you won’t be badly injured if you do end up falling. If falling is common, harmless, and easy to bounce back from, then there’s no need to fear or brace for the landing. Set up mats or pillows and purposefully fail, both at walking and at smaller jumps if it’s legitimately safe to do so. (Don’t push yourself until injury or it’ll just reinforce the “go back to familiar two legged mode” stuff) Practice feeling those impacts in situations where it’s not painful. Purposefully roll or fall sideways while trying to trot forwards. Then practice turning that roll or fall into momentum to roll back up and in the position you were in before, so your body learns to take a hit and keep going. (You can throw in some quadrobics with your legs purposely much wider apart than your arms, to unlearn those close-together-legs you mentioned).
As a bonus, if you have a friend who you share this with, ask them to pelt pillows at you while you try to trot in as straight a line as possible, so your body gets confident with that clean movement being interrupted. Then try swerving and weaving while being pelted. Then try ducking or even little hops. (Don’t try jumping while being pelted, because that’s adding two types of interruptions to your form instead of just one; the goal is to get more confident with jumps interrupting your typical form, not making jumps scarier or harder)
As a last confidence builder to hopefully train out the “hind legs first” mentality when landing jumps, a lot of it is waiting for that mental breakthrough. If you’ve hit the metaphorical wall multiple times in a row (tried to jump arms first but your legs keep coming forward) take a break and renew that trust in yourself by doing easy intentional failing or balance practice before going back to it. Increase the confidence that failing is ok. In fact, try falling from your full, bipedal standing position in safe ways (there are YouTube tutorials on how to do this safely) so that you’re used to falling from a greater height like you would be when jumping. Encourage your arms to be what catches you. Do whatever you need to make it safe and fun (elbow pads or hand pads or even knee pads made for skateboarding, cushions, mats, jumping into a pool first, or onto grass, or on a trampoline), or make tiny (painfully easy) jumps so small you physically can’t bring your legs forward in time, and increase height little by little until your brain realizes you’re already succeeding. Long jumps are actually a perfect way to get more jumping practice and arm-forward muscle memory to prepare you for high jumps. They also have less injury risk, and the skills and confidence carries over.
(In a more subtle way, clothing can also help train your back to stay straight; corset or corset-like garments, shirts or jackets that are tight and stiff around the waist, and genuine support garments and bracers to prevent back injuries and strain are all good options. While waiting for your arms to get stronger, wrist and arm bracers can probably help build confidence in your arm’s ability to hold you up and prevent that frequent pain too. If it feels too artificial, paint and faux fur are always there to add on top for your own custom gear.)
Also. STRETCH STRETCH STRETCH. I have no specific stretching advice other than do it. Look up guided stretching on YouTube, don’t wing it. If you’re having such common arm/wrist/hand pain or ANY neck/back pain, you need to be stretching before every workout. It sucks, and I for one am impatient, but you have to invest in rest. If you feel you might be close to pulling a muscle, take a break and come back to it, even if it’s just for a minute to catch your breath and drink water & electrolytes (yes, the electrolytes are important, the body does not actually stay hydrated on water alone when it’s sweating out salts. Yes, you will get injuries more intensely and frequently that will prevent you from continuing if you don’t hydrate properly. Or get enough nutrients cough cough. This PSA is not at you specifically, but at everyone, including and especially myself.)
I wish you the best of luck, regardless of if anything I said here was useful, and I hope you’ll share updates of your progress!
