Perhaps you should try some spelling variations:
"performance-enhancing design" » "performance-enhanced design"
"performance-enhancing drugs" » "performance-enhanced drugs"
"performance-enhancing drug" » "performance-enhanced drug"
"performance-enhancing data" » "performance-enhancing a"

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-enhancing drug") OR ("performance-enhancing design"))) OR ("performance-enhancing data")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"performance-enhancing drugs\") or (((((\"performance-enhancing drug\") OR (\"performance-enhancing design\"))) or (\"performance-enhancing data\"))))".
• Removing quotes may allow a broader search: (( performance-enhancing drugs ) or ((((( performance-enhancing drug ) OR ( performance-enhancing design ))) or ( performance-enhancing data )))).
• Adding a wildcard symbol may retrieve word variants: (("performance-enhancing drugs") or ((((("performance-enhancing drug") OR ("performance-enhancing design"))) or ("performance-enhancing data"))))*.