A great niche to target would be assistance for people who have a muscle group they don't use because of long-time postural issues or simply never having learned to use it.
For an issue like this, it doesn't work to simply do an exercise that supposedly works the muscle. Even if the muscle is the limiting link in the lift, if your body isn't using it, it won't get stronger. For example, my mid-upper-back was lax because of typical nerd posture. Even though it limited my squat, years of squatting didn't force those muscles to do anything. I just had a really weak squat that relied on other muscles compensating. Then one day I ran across a suggestion to do bat wings and hold at the top for 20-30 seconds, and I became close friends with some muscles in my back that I barely knew existed.
A web site showing you how to discover each muscle or muscle group in your body (bat wings for deep mid-back, glute bridge for glutes, etc.) would be AMAZING, and I bet people would get referred there all the time from places like r/Fitness where people go for advice.
Do you really need exercises to target niche muscles ( can understand if you are doing rehab post injury or actually bodybuilding to enter a contest)? Most folks who want to be fit, only need to focus on compound movements ( bench/incline presses, squats, deadlifts, pushups, pullups) which target all muscle groups.
In general I agree that compound movements are the best, but in my personal experience most people have postural weaknesses and habitual movement patterns that they just might not be aware of that even compound movements have a hard time addressing. When you start exercising your body may overcompensate for these weaknesses by recruiting muscles that might be less than ideal for the movement, which can lead to injury (which happened in my case).
For me personally, one of those postural imbalances turned into an injury (shoulder), and I found doing exercises that targeted a very specific muscle (infraspinatus) really helped in my recovery.
I think everyone could benefit from spending some time exploring their body by targeting muscles that may be easy to ignore in larger compound movements, if only for bringing awareness to them (which should benefit the main movements in the long run by making them more efficient).
I agree about posture, see too many people with postural issues, almost makes me want to tell them to go see a physical therapist/personal trainer. Body awareness, I realized is only possible if you have been athletic all your life, for most people that awareness is just not there.
Yoga is great - it stresses strength, flexibility and balance. And the complex movements bring attention to those "weak" muscles. I do it once a week with weight training and it has improved my flexibility and kept me injury free.
Absolutely, once you start working on your own postural issues you start noticing it in everyone around you as well. I do think anyone can gain that body awareness though, it's not just for those that have been athletic all their lives. I didn't come to having a physical exercise practice until my early 30s and the amount of internal body awareness I've gained in the past few years has been amazing. It really deepened my seated meditation practice as well.
Yoga definitely is great and I practice asana 5x per week. I will say that my injury came up during an asana class, though, and it wasn't until I really targeted my weak scapular muscle groups that it got better.
I'd wager many of us need rehab despite any apparent injury, but rather due to years of abusing our bodies by sitting still for the majority of our waking hours, and sitting with poor posture. Specific targeted exercises at the start can get people into a condition where the compound movements and other exercises are even feasible (that is, sustainable or less likely to cause injury).
The comment you are replying to explicitly gives an example where compound movements weren't generating results, because of compensation by other muscles. This is the sort of thing a real trainer would provide (but a big-box 'trainer' might well never notice, even if they had you doing compound lifts).
The thing is - there would not be a one-size-fit-all exercise for each muscle group; you would rather need an exercise for each combination of A, B in "muscle A is weak but surrounding muscle B is strong"
I'm not sure if the B dimension affects exercise choice. If it does, it could be covered by providing a short list of exercises to try for activating each muscle group A. That way the user doesn't have to figure out which muscle(s) B are compensating, which could be challenging for a lot of people who need this information. In my case, I'm still not sure exactly how my body was compensating for the muscles in my back that weren't being utilized.
I think that there are fewer likely culprits than you might think. Most lifting coaches end up heavily relying on only a few cues per exercise, because there aren't that many common pathologies. So something like this could be useful if couched in the normal "not medical advice" sort of way. Yeah, this might not apply to you, but if you find that you e.g. squat with too much forward lean, here is a small set of exercises to augment your workout that you should try for 6 weeks and see if it helps.
Something like that would seem useful to a lot of lower-mid weightlifters who don't have a real lifting coach.
For an issue like this, it doesn't work to simply do an exercise that supposedly works the muscle. Even if the muscle is the limiting link in the lift, if your body isn't using it, it won't get stronger. For example, my mid-upper-back was lax because of typical nerd posture. Even though it limited my squat, years of squatting didn't force those muscles to do anything. I just had a really weak squat that relied on other muscles compensating. Then one day I ran across a suggestion to do bat wings and hold at the top for 20-30 seconds, and I became close friends with some muscles in my back that I barely knew existed.
A web site showing you how to discover each muscle or muscle group in your body (bat wings for deep mid-back, glute bridge for glutes, etc.) would be AMAZING, and I bet people would get referred there all the time from places like r/Fitness where people go for advice.