Perhaps you should try some spelling variations:
"performance-enhancing design" » "performance-enhanced design"
"performance-enhanced drugs" » "performance-enhanced drug", "performance-enhance drug", "performance-enhanced drought"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("performance-enhancing drugs") OR ("performance-enhanced drugs"))) OR ("performance-enhancing design")).
• Removing quotes may allow a broader search: (((( performance-enhancing drugs ) or ( performance-enhanced drugs ))) or ( performance-enhancing design )).
• Adding a wildcard symbol may retrieve word variants: (((("performance-enhancing drugs") or ("performance-enhanced drugs"))) or ("performance-enhancing design"))*.