IlPrincipeBrutto
Hi Mr. Mu,
in every rep your hips move first, raising until your legs are almost straight. Then you start moving your shoulders, which means most of the work is done by your back.
Also, you seem to bring the bar back down too close to our shins. Look at the height of your hips when you start the very first rep, and then when you start the following reps. In the first reps the hips are almost fine, but in the other they are higher; this means your legs are taken out of the lift, and your back ends up doing the work.
Last but not least, it looks t me like you have the bar resting at the bottom of your fingers, just curving them to make a sort of hook. Make sure you grab the bar properly, with the full hand, and squeeze it tight.
Hope this helps,
IPB