There was a great question this week in the American Viola Society group- is weight training setting myself up for injury as a musician? People gave lots of cautionary advice, but the simple and most direct answer is "It Depends." Weight training is a large class of potential exercises, modalities, and approaches towards exercise. For some, it might consist of using Nautilus machines at the gym, for others, free weights and benches, for others olympic weightlifting, crossfit, orange theory or HIIT workouts. It's hard to make a blanket statement like "weight training will (or won't!) hurt you" without knowing:
1) What sort of weight training program you are doing right now
2) How your body currently is doing in terms of imbalances, deficiencies, etc.
3) What your history of injuries is
4) How much you are practicing your instrument on a daily and weekly basis.
When starting any new physical activity or even when starting to practice an instrument after a hiatus we have to look at frequency of the same activity in a given week, intensity of the activity, duration of the workout, your other movement habits, and how you were training before. I'm not a big fan of going from zero activity level to 6 AM bootcamp intensity overnight, especially in terms of upper body strength, adaptation, and potential for injury. That jump in intensity doesn't guarantee someone an injury but there's a high chance that the rigor of the workout will far exceed their tolerance (both in terms of muscles,connective tissue, and physical capacity) and that form and quality of movement may be compromised to achieve a specific goal. (i.e., do these reps 10 times in a minute- the desire to finish the task may exceed the ability to execute the move with quality form). However, I absolutely think that one can work gradually in a weight training context to gain strength, stability, and whole body health.
If you haven't done weight training before, I wholeheartedly suggest finding a coach or instructor whose approach is not only on physical results but also quality, pain-free movement, gradual change, and is aware of your specific issues. That doesn't mean you can't participate in a group class environment, but if you have had shoulder impingement (or carpal tunnel or tendinitis or whatever), in the past, your trainer (or physical therapist!) should know and be working with you to restore your movement in a sustainable way. Many group class fitness instructors cannot give this kind of attention in a large setting, or some may not have the knowledge or resources to do so. One thing to remember is that most musical instruments require a softer, supple hand position, which is a very different motion than extreme wrist extension and grip strength to execute pull ups/hold a barbell. Your body is most likely more adapted to music making than grip strength, and for many musicians (myself included!) that can be an area of deficiency and weakness, as well as shredded blister/calluses on the palm.
If you want to start training, or you feel that you need a tune up, finding a coach can be challenging if you don't know what to look for. Biographies and credentials will give you an idea of things, but look for a mission statement about their teaching, approach, and tools. Someone with a weekend group fitness training or short certification won't have the knowledge or resources that other trainers may have. There are many highly trained intelligent movement educators out there- it's just a matter of finding them!
In terms of training itself, it can be tempting to overwork the front of the body (pecs, anterior deltoids, etc.) and if you're sitting and practicing a lot (carrying a weight on the front of your body hours a day), your back body needs as much if not more strength to be balanced. Strengthening the rotator cuff, lats, and back extensors is critical, not just in a weight training context, but in whatever activity you're doing. (Don't forget your lower body either, but that's a conversation for another day) If you're experiencing pain in any context, take a look at how you're training, what particular sequences you're doing, and see a professional for evaluation. Physical therapists can be a great resource in this process! Pain is incredibly complicated and doesn't always signify tissue damage, but it does signal that something may not be working as well as it could be in the body, whether that's in relation to music making or weight training. It is absolutely possible to workout regularly and be a professional, amateur, or enthusiast musician, and you deserve the opportunity to do it pain free!