I was about to begin my set today of stiff leg deadlifts, I bent down to pick up the bar and felt fine in the starting position, standing upright. I began to lower the weight to my feet and felt this excruciating pain in my lower back so I dropped the weight there and then.

This was the same weight I was lifting last week and managed it ok. It wasn't ridiculously heavy, I can manage 15 reps with good form. But today on my first rep, that happened.

Any reason why?

I had to abandon the whole exercise and continue with the rest of my workout, I don't like skipping exercises.

Deadlifts are extremely technical lifts and have massive potential for injury. Chances are you have been lifting with weight too heavy. Watch various tutorial vids on youtube and start extremely light.

Some key things to do, alternate grip, one hand underhand the other overhand. Keep your hips close to the ground and drop your knees, most people have their backs horrible rounded when they start a deadlift and thats a one way ticket to a slipped disc from their spine. Look UP, it helps keep the spine straight. And when you begin the lift, push from the legs and the leverage the weight back using your upper body like a fulcrum.

I have had lots of experience with deadlifting and with horrible lower back pain associated with doing bad form. I fixed my technique, went light and had better gains and no pain. Another thing that can be contributing to your improper form is a potential lack of leg strength and development, the legs begin the deadlift and the back takes over after a certain point, if your legs arent too strong, your back will be forced to take the weight from the very starting position where it is at its weakest angle and highest probable position for injury.

Dont do deadlifts for a couple weeks and start off doing light reps when you decide to go back to it. The pain is the worst I have ever felt where I barely move out of bed without being in immense pain and to the point I could not even breathe. Be very cautious when going heavy on deadlifts.

Something also to consider is doing rack pulls, where the weight is deadlifted in a squat box with the safety rack bars parallel to the height of the knees so the legs are taken out of the exercise.

You dont want to be this guy http://www.youtube.com/watch?v=P0ySblouCKs

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Pain is Temporary, Pride is Forever

lol at the video

As someone who study mostly exercise science in college I don't recommend straight leg deadlifts at all. There are much better exercises to develop the lower back, glutes and hammys.

On a second note you asked why it happened on light weight? Well when I was seeing a chiropractor him and I were talking about lifting injuries and the majority of people who saw him that were lifters hurt them selves just bending over. some just from picking up a pencil. The reason why because even the slightest shifting of the vertebrae in your spine can cause a pinch on some of the nerve endings and cause serious pain.

I know I have had several back injuries from dead lifting in the past, mostly from going to heavy, or like you said doing one rep and then BAM pain. personally I don't do them much any more, like I said there are other exercises to develop those muscles especially when it comes to "straight leg" dead lifts, those are probably the most dangerous lifts along with "good mornings"

one more thing if you continue to do the straight leg dead lifts stretch out your hamstrings first, if your hamstrings are really tight it will cause more tension in your lower back.

Straight leg deadlifts are the best exercise for hamstring development due to the unique way the hamstrings insert into the pelvis and knee. But straight leg deadlift is not TRULY straight leg, doing the exercise with just the slightest bend in the knee is more safe and effective.

_________________
Pain is Temporary, Pride is Forever