Perhaps you should try some spelling variations:
performance contract » performance contracts
mental performance » senegal performance, annual performance, entry performance
improving thermal » improving maternal, improving rural, improving human
improving mental » improving maternal, improving social, improving rural

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

• Performing a fuzzy search may retrieve terms with similar spellings: (( improving mental performance evidence. ) or ( improving thermal performance contract. ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( improving mental performance evidence. ) OR ( improving thermal performance contract. )).
• Adding a wildcard symbol may retrieve word variants: (( improving mental performance evidence. ) or ( improving thermal performance contract. ))*.