If you're talking free weights and playing guitar, then I can tell you from experience that the two don't mix well. Because you have to grip the bars so hard to stabilize the weight, then your guitar playing will suffer tremendously. You will end up having to choose between lifting and playing guitar.

If you use machines instead of free weights, the machines will stabilize the weight and you can lift with your palms partially open, reducing the gripping force and this will be better on your hands. You will also need to lower the resistance.

I used to work out a lot and found that my playing was taking a hit. I had to move to machines and back off quite a bit in order to do both.

Hope this helps.

Bob

Bob
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http://soundcloud.com/bobcflatpicke/music-in-the-mountains