On job sites, we avoid static stretches before heavy lifts. While hunt.pat is spot on about breaks and knee bending, stretching cold hamstrings can sometimes cause more strain. I warm up with five minutes of light cardio, like brisk walking around the parking lot. Then I practice the lifting motion with no weight, bending my knees and keeping my back straight. This gets the muscles ready without the risk of tearing. After years of moving equipment, I've found this routine keeps my back safe.