The need to alternate routines.

Too much of one activity is a bad thing. I started hitting the pool daily since about 2 months ago, thinking it is better for my back than running. Problem was, I was only efficient at a few strokes, and did mostly the crawl. Then my left shoulder started aching, now I back to alternating running with swimming. Actually, I still swim almost everyday, but careful to alternate different strokes, and working on my weak ones.

Do others find the same problems? I am starting to think, working on different muscle group or doing different activities is better than just one. Is that what they teach in fitness instructor school?

Yes, sort of. You are referring to what is known as an "overuse injury" or a "repetitive stress injury." Athletes of all kinds get these, and those new to an activity can get similar injuries due to a lack of conditioning or proper technique (especially important for preventing swimming injuries).

You can certainly just vary between a bunch of different things you like to do- nothing wrong with that. There are many ways of being a little more purposeful in choosing how you crosstrain, as well.

That said, overuse injuries are not necessarily a direct result of specializing, and crosstraining does not necessarily protect you from overuse injury. Whenever you get into a new activity, it is most important to avoid reinventing the wheel, so to speak, when it comes to creating a conditioning plan. There are few activities that benefit from professional coaching as much as swimming, and I would begin by getting some qualified critique of your freestyle technique, combined with advice on strokes and exercises that can correct the specific issue plaguing your shoulder.

Cameron

I've had 2 swim lessons so far, the pointer on my craw stroke really did help. I am not flexing my elbow, and not abducting my arm enough on the stroke. Hopefully, I will not develop any chronic pain... I am swimming 4 to 6 days a week, and it is going fine so far... knock on wood. Thanks for the input.