The main cause for flawed form! The legs straighten too soon, which is most often caused by weak stabilizing capabilities in the hamstrings. It can also be caused by having the weight forward over the front part of your feet. This then may cause the bar to swing slightly away from the body out of its proper straight line upward trajectory. And this will invariably cause the lifter's back to round from its proper straight position. The ideal position at the beginning of your DL is to have your body shaped like the number 4! The slanted line of the number represents your back. The vertical line represents your shins and arms, and the horizontal line represents your thighs.
At the START of the DL from the floor, it is permissible for the shins to be slanted slightly forward so the quads can push off from the floor. Also, to keep the weight back over the heels, try curling your toes down in your shoes at the moment of push off. Then, once the plate stacks have left the floor, concentrate on having your hips as far to your rear as possible, with your back straight, and get your shins as vertical as possible before the bar passes your knees.
My two main assistance exercises when training for the DL are the Romanian Deadlift for the start, done one day per week, and the DL lockout, from about 4 to 5 inches below knees, for the finish. Full deadlifts are done on a second day. For speed or CAT (Compensatory acceleration training) I recommend not doing the DL, but rather Olympic lifting movements like power snatches, power cleans, or High Pulls.
I've found a good sound program for deadlifting is a modified version of the great John Kuc's DL training schedule. On a 10 week cycle, start by working up with a few warm up sets. Then do 4 sets of 3 to 4 reps at 80% for a work weight. By the fifth week do 2 sets of 3 to 4 reps with about 88% for you maximum work weight. Then drop back the sixth week to around 82% and work up about 3% per week to 92% for 3 reps on the 9th week, which should be one week out from trying your limit 1RM in a contest.
The fore mentioned can be shortened to 8 weeks, and should also work well for other compound movements like squatting and pressing. Below is the exact periodization schedule that I've used successfully on compound movements. It works well for 3 to 5 reps.
1. .63X6, .77X4, .82X4X3
2. .63X6, .77X4, .84X4X3
3. .63X6, .77X4, .87X4X3
4. .63X6, .77X4, .85X4, .89X4, .87X4
5. .63X6, .77X4, .85X4, .90X4, .87X4
6. .63X6, .77X4, .85X4, .92X4, .87X4
7. .63X6, .77X4, .87X4
8. work up to 1.03 of previous best
For the Romanian DL, weight is NOT important. It's more important to keep your hips back, your back straight, and your shins vertical at all times during this exercise. However, as a rule of thumb, I like to do 3 reps on the RDL with about 65% of my 1RM of my regular DL. The DL lockouts are done by feel usually after RDL's.
All the best with your training. JVA
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