Sit-ups are superior to crunches in virtually every way. There are precisely zero reasons to perform crunches and there is no scientific or clinical data to suggest that sit-ups are harmful. Crunches provide a fraction of the adaptive stimulus, only work a fraction of the range of motion, and provide a nightmare for quantification. This is precisely why sit-ups are better. Performing weighted sit-ups is done for the same reason that you would perform the bench press as opposed to push-ups. The bench press allows you to add load in a way that you cannot do on the conventional push-up. Unlike the push-up, in the sit-up your hands are free and consequently you can easily hold a weight in them so as to increase the load on the abdominal muscles. The start position for the weighted sit-up has the feet flat and anchored, either by having a training partner hold them or by wedging the toes under something that is heavy enough not to budge. The knees are bent to no less than a ninety-degree angle. The torso and head are resting on the floor. The weight is held in both hands at the level of the shoulders with the elbows completely extended. The arms are perpendicular to the floor. This is the start and the end position. The up position has the hip joint, the shoulder joint, and the weight fully extended and supported overhead, all in line. BibliographyEarle, Roger W., and Thomas R. Baechle. NSCA's essentials of personal training. Champaign, IL: Human Kinetics, 2004. Kilgore, Lon, and Michael Hartman. Fit. Iowa Park, Tex.: Killustrated Books, 2011.