Sledge Hammer Push Ups work the core in a different fashion. You will need to grip with one palm down and the other upwards. The hand close to the weight will need to be pushing while the other hand is pulling. Balancing the sledge hammer on the weighted side also makes this exercise tremendously challenging for the core. The wider the feet that will be placed a part, the easier this exercise will be. More you advance with this exercise, try on working on closing the gap between two feet.