Bless him.
This is a good example of the plank. He has a nice long back with a very very small curve in the low back. A very simple exercise that, done well, flattens the abs and can be developed in many ways.
With the plank, technique is everything. The best aid to this is a piece of string tied very tightly around the waist. When you calmly breath out, the string is loose. When you hold the plank position, the string remains loose and you are still able to breath nicely. The principal area of work will be felt in the low abdominals and not in the back or the upper abdominal area.
To build up good technique, it is good to start off on the knees and elbows, still in a nice long line. Use a mirror to check:
> Long Back
> Long back of neck, head in line with back, looking straight down (well it will be when you are no longer looking in the mirror -or at the photographer)
> Elbows under shoulders.
Now tuck under toes, breath out and we have lift off. You should be able to continue breathing out and in. If you find you are holding your breath, or the breath is ‘tight’, stop the exercise and try again. If either of things happen, the string will also have gone tight. If the string goes tight this is a clear sign of poor technique.
At first a ten second hold may be quite long enough. This you repeat, with a five second rest between lift offs, for 3 – 6 times.
It is fine to build up the holding time to a minute, but beyond that it does get boring. Instead of just holding there, I introduce other plank variations that are admittedly harder to do, but much more fun and more productive in useful ab strengthening and flattening.
> Watch out for the body hanging in the shoulder blades. As with a press up, slightly squeeze the armpits so the ribs are encased by the shoulder blades.
> As you lift off, keep the back long. Watch that the backside does not lift nor the ribcage sag.
If all this is frustratingly difficult, then stop eating all wheat for 4 days. After this the exercise will be miraculously easier to do.
Bluntly, a well performed plank is not particularly hard work.