The problem is that these muscles aren't designed to be prime movers—they're designed to support the action of the glutes. Inability of activating the glutes can result in low back pain (low back muscles compensating), hamstring strains (overacting hamstrings), hip pain (resulting from hamstring-dominant hip extension) and knee pain (poor glute medius strength).
Premkumar et al present evidence that the traditional “red flags” for ominous causes of back pain can be quite misleading. The correlation between red flags and ominous diagnoses is poor, and prone to producing false negatives: that is, no red flags even when there is something more serious than unexplained pain going on. In a survey of almost 10,000 patients “the absence of red flag responses did not meaningfully decrease the likelihood of a red flag diagnosis.“ This is not even remotely a surprise to anyone who paid attention in back pain school, but it’s good to have some harder data on it.
Grade III (severe): A complete tear in your muscle that causes severe pain and swelling and you can't bear weight on that leg, making it difficult to walk. You've also lost more than 50 percent of your muscle function. These injuries are less common and may need surgery to repair the torn muscle. They can take several months or more to completely heal.