of body fat. For most exercises, like pull-ups or pushups, it is just as effective as a weighted vest in adding resistance from gravity. Limitations in flexibility forward or backward or to the side in arms may or may not be present depending on the pack as they vary greatly. A standard form of military and fire fighter training is not only be able to carry a backpack, but to march and run with one loaded down with a concrete "marble". Part of SWAT training is to be able to do pull-ups wearing a heavy pack. Hip drags Hip drags are not as useful due to their weight, but are small attachments which add drag during many movements in swimming by making the swimmer less streamlined. This added resistance allows the swimmer to build up more force, and swim faster when they are later removed. Belts Different from dip belts, weighted belts are affixed to the body and do not use suspended weights or swing. They are useful, like weighted vests, in having weight close to the core and mimic very well the additional weight one might have from body fat stored in the abdomen and lower back. The advantage over other core weights is the flexibility that is freed up by not constraining the upper body, the disadvantage being limited core flexion. There tends to be some overlap in certain larger weighted vests and the region covered by certain larger weighted belts near the upper abdominal muscles. Upper body Upper-arm Upper arm weights are another method of adding weight close to the core beyond that of a weighted vest, or to add resistance to the shoulders with less danger to the elbows than wrist weights or the added biceps/triceps strain. A problem with these is similar to thigh weights in that those with large biceps, triceps, or fat around the region may experience tightness, and the tightness required can be straining in exercises that involve elbow extension, making it more suitable for leg and core movements. Wrist Wrist weights are used in place of holding dumbbells (or to supplement them) or other hand-held weights as a way of adding leveraged resistance to the movement of the arms. Like ankle weights, practicing explosive movements is not advised, and if done, should be approached gradually, with acceleration before full extension, and also reduced gradually, to avoid over-extension injuries. Wrist weights are