Perhaps you should try some spelling variations:
"performance-enhancing designs" » "performance-enhancing design", "performance-enhancing lesions", "performance-balancing designs"
"performance-enhancing drug" » "performance-enhancing drugs", "performance-enhancing data", "performance-enhanced drug"

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 designs") OR ("performance-enhancing drug")).
• Removing quotes may allow a broader search: (( performance-enhancing designs ) or ( performance-enhancing drug )).
• Adding a wildcard symbol may retrieve word variants: (("performance-enhancing designs") or ("performance-enhancing drug"))*.