To Grind or Not To Grind
Grinding and Not Grinding out a Deadlift are dependent on various factors.
The Reason NOT To Grind Out The Deadlift.
1) The lower back is quickly and easily overtrained, as per Dr Tom McLaughlin's research. That is also why Powerlifters train the Deadlift only once a week at most; anecdotal data.
2) Deadlift Technique (all lifts are altered) when the muscle in the movement are fatigued.
They key to increasing your Deadlift is to perform train it for single repetition, with load of 85% plus, focusing on technique. Once fatigue set in, STOP. Continuing past that point ensure you will develop poor technique.
Use Auxiliary Exercise that are similar in nature to the Deadlift to increase Strength.
Reasons To Grind It Out
If the Deadlift is being used as a means of increasing Strength, at some point, you need to infrequently ramp of the intensity; meaning Grind Out some Reps.
...if you need to grind it you are probably compensating and will injure yourself.
Compensation and Injury
Muscle fatigue in a movement lead to compensation in technique in the Deadlift, all exercises. However, the greater the intensity, the greater the muscle fiber recruitment. Muscle fiber that aren't trained, don't grow.
Injury
Any exercise that elevates the intensity of the movement to the point of "Grinding It" out increases the risk of injury; however, that doesn't mean you will injure yourself.
I learned that in this lift the setup determines the explosiveness more than anything else.
Not Quite
Technique is important in all movements.
However, it is far from being the most important component of explosiveness.
Strength Is The Foundation of Power
Strength is the primary factor for Power.
Initially, Novice Lifter will increase Power by increasing their Limit Strength (1 Repetition Max).
Thus, a 500 lb Deadlifter is doing to display more explosiveness than a 300 lb Deadlifter.
With that said, as you move past being a Novice Lifter, Power Training combined with Limit Strength Training is vital to increasing explosiveness. This has been addressed in other post.
all the force is directed upward
Not Quite
In the Deadlift, the bar move up and back at the same time.
Pulling The Weight Back
One of the keys to pulling a Personal Best Deadlift is to keep the bar as close to your body's Center of Gravity (COG) as possible.
To do that you need to pull the bar back on top of you, drag it up you legs.
The farther away the bar is from your Center of Gravity, the more the bar weight is magnified (Torque).
Secondly, dragging the bar up your legs is somewhat like lifting a weight in a Smith Machine. Your legs are the "Guide Rods" that you are sliding it across; providing greater stability. That is one of the reason everyone can lift more a Smith Machine than a Free Weight Movement.
Kenny Croxdale