There are plenty of jokes about the strange relationship between rock musicians and their bodies.
A young Iggy Pop became a legendary concert draw through ritual self-mutilation and total physical self-destruction; the landscape of the remaining 1980s metal stars is littered with recently-developed beer bellies and poorly disguised receding hairlines. The likes of bloodied and bruised Jesus Lizard frontman David Yow, wildly muscular and possibly evil Glenn Danzig, or any of the walking medical experiments from Poison may be extremes of what people in musical circles do to themselves, but even on much smaller scales the tales of fried arms and shot vocal cords are too numerous to count.
Performance is certainly rewarding in its own right, but the physical side effects are very rarely discussed save for the occasional cautionary tale from an old-timer who broke something important. That's not to say every aspiring musician needs to adopt a Henry Rollins-esque powerlifting and protein shake regimen, but the physical demands of playing and performing with an instrument can take a serious toll on a body that's not ready for it.
To that end, each type of instrument applies its own particular stresses on the body. Music can't - and probably shouldn't - be reduced to a passive endeavor, but there are some simple things anyone can do to keep the body in something resembling peak jam-kicking condition. Some things simply can't be avoided due to either genetic predisposition or the nature of the instrument, but with the right approach any kind of damage is absolutely containable.
You Play: Guitar, bass, or any type of string instrument
You Need to Develop: Forearm, triceps, and shoulder strength
Recommended Preemptive Training: Any type of forearm grip strengthener will provide not only the low-intensity workout to improve muscle to the elbow, and also offers a highly portable exercise tool. At a more advanced level, standard free weight curls and lat (latissimus dorsi / upper back) pulls will work both the upper arms and the back muscles which support the bulk of your instrument. Cello and upright bass players get the added burden of not only playing a considerably heavier instrument, but also having to balance it while playing. Most of the weight of the instrument will be up against the force of your upper body, so it's important to keep up the condition of your pectoral (chest) and deltoid (shoulder) muscles. It's also crucial to actually get those muscles loose before either workouts or playing sessions, so set aside at least 15 minutes to stretch your body out before hitting the iron. You'll want to be sure to get yourself loose without actually forcing your muscles to stretch beyond their normal relaxed state, which can lead to pulls or eventual tearing. The best approach is to brace any of your limbs against a stationary upright object and turn your body away from it just to the point where you can feel yourself being pulled in that direction. Hold that position while counting to ten, release, then repeat with incrementally wider stances until your body can't possibly be extended any further. Not only will the limb itself be loosened, but the supporting muscles in the back and torso will follow suit by virtue of their attachment.
You Play: Piano or keyboards
You Need to Develop: Forearm and triceps strength
Recommended Preemptive Training: Since you're most likely sitting down to play the piano, there's not as much stress on the leg muscle groups (unless you really want to wail on the sustain and reverb pedals). Focus primarily on developing forearm strength, as they'll need to support themselves in your attempts at avoiding carpal tunnel syndrome. Good posture should theoretically stave off any back problems brought on by hunching over the keys, but it never hurts to add some rowing or crunches to strengthen the lower back.
You Play: Vocals or any wind instrument
You Need to Develop: Abdominal muscles and respiratory strength
Recommended Preemptive Training: Sit-ups, crunches, and oblique scrunches that tighten up your midsection will put your diaphragm in better shape to project to your heart's content. At the same time, cardiovascular exercise (bike riding, running, walking, et cetera) will improve your ability to exert your vocal force longer, and dedicated breathing exercises ranging from note sustenance to holding your breath underwater can put any singer in a better position to maintain their desired level of control over their pitch and delivery in extended sessions or a live setting. Conversely, it is just as important to avoid (or at least minimize) engagement in activities which can be detrimental to your general throat and lung condition. This includes the obvious vices of smoking and drinking, but it's a lesser-known fact that milk and lactose-based drinks actually promote the build-up of phlegm. No one's saying cut out the milk, but avoiding those milkshakes around gig day might go along way in keeping your voice clear (and your microphone clean).
You Play: Drums
You Need to Develop: Pretty much everything
Recommended Preemptive Training: For a total assault on the body, there's really no instrument more exhausting or demanding than the drum kit. Bass drums require serious calf strength, and those just happen to be the most difficult group in which to develop muscle mass. Lateral drumming motion across the kit also means the entire torso is at work, and there's also the built-in demand for good posture to avoid long-term back problems. Full-out leg presses and work at all levels of a cable crossover should give a solid start, but even decades of prodigious playing and focus aren't enough: witness Dream Theater drummer Mike Portnoy or ex-Pearl Jam drummer Jack Irons seeking forearm refuge in tubs of post-practice ice water in their bands' respective Live at Budokan and Single Video Theory home videos.
As with any exercise program, it is recommended you consult with a licensed physician or trainer before beginning and always put safety before vanity. While there's no guarantee that stretching, muscle conditioning, or strength training will make anyone a better musician, it should also go without saying that the performer in shape is the performer less likely to be complaining they're too sore to rock.