Here's an interesting flavor of pushup that lets you dial in a fairly high level of difficulty. The main ideas are to
- force the chest and arms to work outside of their standard grooves
- avoid the bounce aspects of pushups.
The farther you go into the corner (lower and wider), the harder it gets. Start out with some standard pushups and work towards the corners as you warm up. Additional difficulty can be had by
- using your fists for platforms (as shown) or
using a pair of kettlestacks
- adding a weight plate on your back
- supersetting with kettlebell presses
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