What is better for bench - a thumb over or thumb under grip?

From a biomechanical standpoint the thumb under or traditional grip is more advantageous. It increases grip, and activates more muscles in the arms, chest and shoulder region. Furthermore, the traditional grip is better from a safety stand point, having your thumbs wrapped around the bar decreases the risk of an injury from the bar slipping and falling during a lift. A thumbs over or "suicide grip" is popular among bodybuilders because it increases utilization of the chest muscles instead of the forearms and shoulder, and for some athletes is more comfortable. The biggest concern with the thumb over grip is the safety issue involved with it. Without your thumbs in place, the risk of the bar rolling off your hand and allowing the bar to land on your chest, face or neck increases dramatically. In IPF sanctioned meets you are required to do a bench press with the traditional thumbs under grip, so if you plan on competing, you should be comfortable with that method. Regardless of which grip you go with, always use a spotter for anything beyond warm up sets.