The solution I've found is to rotate the spine bone of the model. So, I manipulate it through bone rather than playing it's vertex animation.

Code:
else if(key_d)
{
  my.pan += 1*(key_a-key_d)*time_step;         	         
				         
  bone_angle.pan = -20;				         
  ent_bonereset(my,"Spine1");
  ent_bonerotate(my,"Spine1", bone_angle); // rotate the bone
}