Many powerful karateka have trouble keeping their butts tucked when they extend their arms. The same people often have trouble making large movements from their shoulders instead of their elbows. In many cases the cause of this is simply that from all of the effort they have made to connect their arms to their bodies they have developed overly tight lats. Here are a couple of relatively easy and painless stretches that may help.
Keep in mind that karate people tend to have tight hip flexors from kicking and poor stretching habits and this can also cause or contribute to “shelf butt” . Look for an upcoming post on stretching the hip floexors.