I bought some of these years and never got around using them much.
Physiologically, the idea of “balancing” your muscles make no sense to me if applied to the forearms. The muscles for opening and closing the fingers are both inside the forearm. Structural imbalances within the forearm are impossible. It would be equivalent to claiming that training your support grip (e.g. holding on to a heavy barbell) would lead to imbalances unless you trained finger strength too.
I also don’t believe there is that much strength involved. Beginners are usually strong enough to fret chords, they’re just limited by endurance and soft finger tips. Finger speed is mostly control.
If you think that you have strong fingers from playing guitar, try rock climbing. The finger strength needed for the smaller grips is way beyond the fingers strength needed for guitar playing.
In the case of guitar playing if you’ve been playing for a while you don’t need any more strength. It’s all endurance. In strength training, it’s considered endurance if you can do more than 30 reps of a given exercise. And we’re not talking about slowing down but to the point where your limb won’t even move. Now imagine playing until the your left hand stops moving, that would be well over 30 reps.
Long strory short I believe that finger strength training tools for guitar are a gimmick since guitar is primarily an endurance exercise. There is also something known as the “interference effect” between strength and endurance, which means if you want to specialise in endurance, it’s better to avoid strength training.
Another aspect is that strength is the result of muscle size and neuromuscular adaptation. For guitar it mainly comes from neuromuscular adaption, and the thing with this sort of adpation is that highly specific to the motion you’re rehasing. This is why olympian weightlifters are usually much stronger than bodybuilders. They focus all their training on perfecting only 2 lifts by rehearsing the motions over and over, while bodybuilders want to achieve balance for all the muscles of the body. Similarly, for guitar playing, any training “tool” that doesn’t mimic the exact motions of guitar playing will be a waste of time. You’re best training tool is the guitar.
For warmups, stretching before physical activity has been debunked years ago. The whole point of warming up is to increase blood flow, stretching does the opposite.
I personally don’t do any warm up routine. Playing at moderate speed for a few minutes should be sufficient for injury prevention purposes.
Pain is usually a sign that muscle or joint damage has already occured, so it’s very important to avoid any movement that are causing pain, for at least enough time for you to heal. Connective tissue (e.g. “joint”) pains will heal much slower than muscle pain, because they have a lot less blood flow the the tissue. This is why the general advice of “complete rest for 1-2 weeks” is bad, you will heal faster is you keep playing, this will bring oxygen to the tissue and help it heal, the key here is play lightly and avoid the motion that you do know causes pain.
I am by no means an expert on posture or injury prevention for guitar playing, I do have though some knowledge when it comes to strength training, I have a science based approach, and am trying those principles to guitar. One thing that I’ve always hated from guitar teachers is that they have so many dogmas. I’ve been looking for a science based approach to learning and now I’ve finally found something with CtC. But in the case of muscle strength and endurance I do believe the science of of muscle strength and endurance is already out there and can be applied to guitar.
I also find it funny how common it is to hear players make analogies between guitar playing and weight lifting. It doesn’t make any sense at all when you question it.
To prevent shoulder, back, neck pains, etc, posture is important but whole body strenght training helps a lot too.
For example I’ve had neck pains in the right side for a long time due to my profession. I am a dentist and working with a neutral posture never really possible. Kind of like when you look down a left to see your fretting hand on the guitar. Except I don’t have the luxury to take as many breaks when I’m at work.
What has really helped relieve this pain completely was training my upper trap muscles with weigths.