Great article in the current issue of USA Weightlifting Magazine by Richard Lansky called Approaching Core Strength From the Weightlifter's Perspective. Although it is an article targeted for athletes involved in the sport of Olympic Weightlifting, I think it has great value for anyone who trains with weights.
Lansky defines the core as "...the gross musculature of the anterior and posterior trunk, hips and mid back," as well as "the smaller stabilizing muscles of these regions." For weightlifters, the core has two major functions: 1) Force transfer and stabilization and 2) Force reduction and stabilization. The first function is the exercise scientist's way of saying, "You can't shoot a cannon out of canoe" and the second function is the corollary,"You can't catch what's shot at you in a canoe either."
In force transfer (for example pulling the bar from the floor, or the jerk drive) any flexion or extension of the spine during weightlifting movements not only puts dangerous stress on the spine, it also dissipates force the athlete is working to apply to the bar. Hip and leg extension provides the motive force and the core must provide an efficient linkage. Any sagging of the core will result in less force acting on the bar.
In the force reduction function capacity (stopping the downward motion of bar in the jerk dip, the front squat/clean and overhead squat/snatch) core flexion or extension also increases injury risk and it will also cause the center of gravity of the bar to move outside the lifters base of support...the feet...possibly causing the lift to be missed in front (core flexion) or behind (core extension). Core instability will also interfere with the athlete's ability to use the elastic and stretch reflex capacity of the lower body musculature to generate force as well as diminish the ability of the athlete to make use of the rebound of the flexing bar.
Many movements that beginning weightlifters learn as "skill transfer"exercises also have a profound impact on the core musculature. These exercises not only address technique development, they condition the lifter's body to "fire up" the appropriate core musculature in a sport specific manner. Lansky recommends that more advanced lifters who may have abandoned these exercises for technique development reconsider including them in their training in the context of core development exercises. Lansky also notes that it is important lifters learn early on to use the Valsalva maneuver to increase core stability via increased intra-abdominal and intra-thoracic pressure.
Besides general core conditioning with the usual suspects of core movements, Lansky recommends the following weightlifting movements for core conditioning:
1) Power Cleans and Cleans + Front Squats
2) Jerk Drives
3) Overhead supports and Jerk Recoveries
4) Power Clean + Front Squat + Jerk
5) Snatch and Clean Pulls to knee height
6) RDL + High Pull
7) Snatch Grip Behind Neck Push Jerk + Overhead Squat
8) Overhead Squat
9) Overhead Stationary Alternating Lunges
10) Overhead Walking Lunges
11) Overhead Step Ups
12) Drop Snatches/Snatch Balance
Lansky particularly likes overhead squats as a way to condition the stabilizing core musculature in a manner specific to the needs of weightlifters. This recommendation reminded me of Dan John's high regard for the overhead squat. The overhead squat and exercises related to it may very well be the best movements going for simultaneously training the linkage between the lower body, the core and the upper body. They aren't easy and they aren't pleasant but easy, pleasant and effective rarely go together, except in infomercials.
I would also add to the above list overhead squats starting from the bottom position...I do these from time to time and they really provide excellent feedback on what is and isn't happening in your own core strength. The Chinese Lifter in the photo above is doing these with a jerk width grip...since he is one of those rare squat jerkers. If you have the shoulder flexibility to do overhead squats with a jerk grip, then by all means do these too. If you don't currently have the flexibility you can ease your grip in over time...varying grip width will vary the training effects as well. One writer has observed that some Chinese Weightlifters can overhead squat from the bottom position more than they can front squat. The Chinese lifters also do snatch grip overhead static holds for time, several minutes in some cases. I've tried this one too and besides being another core wrecker it will give you some valuable feedback about how solid your overhead lockout really is!