The phrase "impaired ability" means that someone's skill or capability is reduced or not working as well as it should be. It suggests that a person is struggling to do something they could normally do easily. Full definition
By altering the genes in male mosquitoes, researchers have just impaired the ability of these insects to reproduce. (sciencenewsforstudents.org)
The study participants overwhelmingly (79 percent) chose reduced pain over improved mobility - no matter how much the condition impaired their ability to stand and walk. (sciencedaily.com)
The doctors engaged in heavy drinking that sometimes impaired their ability to function. (religion-online.org)