ohh, uh, like the excersize? I admit that I had look it up, rather than off the top of my head, but I found an article on it.

Problems with plank exercises arise from the static nature of the “correct form”. The criteria for excellence in planking is the ability to hold the straight body posture without any wavering or “buckling” for long periods of time. None of these criteria are part of day-to-day functional activities, or consistent with athletic movements

Proponents would counter these facts with arguments that as long as clients are building strength there is no harm in utilizing them as a major component in training regimens. However, training muscles to work in an isometric fashion, with no joint motion and no muscle, tendon, or fascia lengthening results in an inefficient and ineffective musculoskeletal system.