The squat curl is one of the best whole body stability exercises that exist. You can use an upside kettlebell as seen in the video, but the exercise is versatile and can use a dumbbell, plate weight, sandbag, or heavy backpack. This exercise is not to be done fast as it is about maintaining stability with the legs while doing work with the arms. Being smooth and elongating the time under load for this exercise will lead to the greatest benefit for any athlete.