Be sure the coast is clear. Yes, you can bend your knees or take any other position you are comfortable in. Brain centers that regulate urination include the pontine micturition center , periaqueductal gray , and the cerebral cortex. You can even do three if you like, whatever makes you comfortable. Conversely, afferent input causes contraction of the sphincter through excitation of Onuf's nucleus, and contraction of the bladder neck and urethra through excitation of the sympathetic preganglionic neurons. Before you begin, make sure you have privacy.