Brief Answer:
several points.

Detailed Answer:
Let us start with , Yes, they can cause muscle aches. AND this is rare and testable. If the muscles are affected, they ache because they are breaking down. The distribution (a lot of big muscle groups) is the type that these drugs would cause. To see if they DO cause it one can 1) stop the drug and 2) immediately get a blood test for muscle breakdown (CREATININE KINASE) if there isnt muscle breakdown it is like saying you cant drive your car due to a car crash and there is no dents no scratches.....

Then, more reasons not to take them. They dont work (zetia has no evidence of saving lives better than using white out to lower your cholesterol.... the numbers change, but there is no decrease in heart effects...at most 2% and that took so much effort to find that it might show the creativity of the writers/researchers more than the drugs effects).
http://www.medscape.com/viewarticle/821215
http://www.medscape.com/viewarticle/835030

gemfibrozil Works. BUT
http://www.healthline.com/health-news/to-statin-or-not-to-statin-120514
unless the triglycerides are very high, OR if you are at fairly high risk of heart disease, you probably wont benefit from them. If you are say, 12 years old. You arent going to have a heart attack. Gemfibrozil doesnt make sense.

so, checking the Creatinine kinase (maybe ALSO myoglobin) to see if muscle is breaking down. That is a drug effect most likely. If not, then it probably is NOT a drug effect even if it goes away with stopping the drug it could be a coincedence. More of a point is that zetia doesnt do much and if you do not have very high triglycerides (which are plain dangerous and give pancreatitis if not treated) then the risk versus benefit of these (and any other) drugs have to be balanced and mostly they arent that important unless you have high risk of heart disease (having had a heart attack or a stroke puts you at sky high risk and at least the gemfibrozil is a very good idea).

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