Since I imagine subject is extending his arms forwards I imagine your depth of field is too shallow to cover the whole body.
Depth of field is area that remains sharp in front and behind your focus point. Shallow depth of field means that area is smaller.
The way you can regulate depth of field is by changing your camera's aperture, for example aperture of f8 will give you more detail (bigger depth of field) than for example f2.8. Distance from your focus point also changes depth of field, moving closer to your subject will make depth of field more shallow.
There are many online calculators to calculate your depth of field based on what you're using, one of them you can find here: http://www.dofmaster.com/dofjs.html
Also keep in mind that 1/3 of depth of field is in front of your focus point, and 2/3 is behind. Meaning if your depth of field is 3m, something 2m behind your focus point will be sharp, while something 2m in front won't be as sharp.