i recently took a lecture on back injuries. the main subject was a study by the universty of washington (i believe) that took xray pictures of athelets doing deads. it was the first time that an injury has been filmed (xray) while it happened. to make a long story short the injury happened from not "locking " the back . the study found that it really wasnt a matter of having your back "flat" but rather keeping the back from having any movement during the lift. this was a relative study, not how far can the back bend before the lift is compromised.of course as far as form goes ,having your back "flat" is technically better form. judging from your vid ,as was stated previously, i would work on that aspect. ill try to find a link to the study, pretty interesting.
