“Sitting all day can lead to issues like poor blood flow and tight hamstrings because your knees stay bent, while the top of your hamstrings are stretched due to your hips being flexed,” says Justin Kraft, CPT, a certified personal trainer and the founder of Aspire 2 More Fitness in Denver, Colorado. In my own training and with my clients, I have been able to substantially improve performance along with preventing injuries by putting a heavy focus on hamstrings,” Kraft says, adding that he’s trained runners who have drastically improved their times by prioritizing hamstring stretches. Otherwise, he says static hamstring stretches (holding a stretch for a set amount of time) can be done either after a workout or anytime during the day or evening.