Following information courtesy of CrossFit Oakland:
The deadlift can be a brutally difficult movement. It gets pulled off the ground from a dead stop, hence its name. Unlike a squat, or a bench press, where the weight is first lowered prior to being driven back up, the deadlift requires the lifter pull from the floor without the benefit of an eccentric contraction.
What is an eccentric contraction? I am glad you asked. An ecccentric contraction is where a muscle lengthens under a load. It is, for most purposes, the negative portion of a lift. Aside from being one of the primary causes of delayed onset muscle soreness (DOMS), eccentric contractions also help to initiate stronger concentric contractions, where the muscle shortens under a load and presumably does the work in which we are most interested. The eccentric contraction initiates the stretch reflex, sometimes called the myotactic reflex. As the muscle lengthens under a load, the nervous system is stimulated to encourage a more powerful contraction. We do this unconsciously when we jump. Try jumping without dipping down immediately prior to the movment. Even if you start with bent knees, you still will want to quickly bend both the knees and hips further before jumping. Doing this lengthens both the quadriceps and hamstrings to allow for a higher jump. That’s the stretch reflex at work.
Let’s return to how this applies to the deadlift. In a properly done deadlift, there is no eccentric contraction. The weight gets pulled off the floor and then is lowered back down and comes to a complete stop before the next pull occurs. Each repetition of a deadlift starts with an uncooperative bar. It doesn’t want to move and lifter must summon the requisite amount of will to make it happen.
In timed CrossFit workouts, the idea that the bar must stop on the ground is generally not followed. Not only does the bar not stop, it is often actively bounced off the ground. Perfoming the lift in this way now provides for an eccentric contraction. Additonally, the elastic collision between the rubber bumper plates and the floor imparts energy back into the bar making each repetition easier. This sounds good so far. The lift is easier, times get faster, and power output increases, right? Yes, but something is lost in the process, too.
The problem with bouncing the plates off the ground is that the lifter has now found a way to avoid getting stronger in the critical part of the movement where the bar breaks from the floor. To deadlift safely, the spine must be held in rigid extension while force is applied to the bar. Bouncing the plates off the ground all but prohibits the necessary setting of the back and encourages rounding instead. It shaves time off a workout, but robs the spinal errectors of necessary work that will make them stronger. An inability to pull even moderately heavy weights while maintaining spinal extension is often the result.
So, what to do? If your goal is to compete in CrossFit workouts, then you are going to need to learn to perform deadlift repetitions quickly and that means not pulling from a dead stop. However, if all you do is touch and go, or, even worse, boucing deadlifts, then you owe it to yourself to start training the deadlift as it is meant to be trained – from a dead stop. The bar should not be moving and the back should be locked in extension prior to every pull. The workout will take longer. It will be harder. It will build a stronger back with a decreased risk of injury. It will also build character.