You can hurt your back picking up a piece of paper from the floor if you do it incorrectly. When you are swinging a large piece of iron between your legs, there is absolutely potential for you to injure yourself if you are doing it incorrectly. Most injuries occur if you are too tired or using a weight that is too heavy. Always pay attention to your form and take a rest if you are too fatigued to continue the workout as written without the form remaining intact. Keeping proper form is more important than completing a written number of repetitions or working a certain amount of time. Do not ever sacrifice form to get more repetitions of a movement. If you are unable to complete a certain number of reps with good form that is okay, take a break and finish when you are ready.