It is common to let go of the belly and/or the butt, especially once you have your arms overhead. It takes concentration and dedication to squeeze it all down in a position that challenges your shoulder flexibility. But it is critical to your safety, your efficiency, and your long-term improvement. Lock it down, hold it in and it will pay off.
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Workout
Five rounds for time of:
Run 400 meters
20 Overhead walking lunge steps
10 Toes-to-bar
2 Rope climb