Stu provides you with some great advice.
Like Stu, I agree that "10 deep squats of 250 lbs is more impressive than 6x315 at 90 degrees..."
And I am a powerlifter who does the lame 90 degree squats. :)
If I stop half way down aka 90 degrees I am isometrically stopping my descent and then going back upwards. Eccentric-Concentric Movement
Every eccenctric-concentric contains an isometric element, even you full squats do. More on that in a minute.
Stopping at 90 degrees is harder then going down and then back up at least it feels that way to me.Stopping At 90 Degrees
Stopping at 90 degrees is a completely different exercise. ANYTIME you change anything in an exercise, you going to see a drop in strength for a variety of reasons.
Full Squat Momentum
Another factor is that in performing a full squat, you generate momentum in the concentric part of the movement.
The greater amount of momentum you can generate, the greater you changes of completing the lift. Momentum allows you to slide through your sticking point.
The Down Side Of Momentum
One of the problems with momentum is that it does not allow you to develop strength at certain angles within the lift.
That means in performing your full squat, some strength is developed at the 90 degree angle but not as much as if you were specifically training it.
"...throw in some heavy partial reps..."
It feels like I have to "put on the brakes" during the descent and then go back upward. Putting On The Brakes
As I stated above, you have to put on the brakes with any eccentic-concentric movemet.
Force = Mass X Acceleration
In every eccentric movement, the weight on the bar is going to be magnified.
Even in a full squat!
Riding The Brakes
What riding the bakes does is minimize the eccentric force.
Dr Tom McLaughlin's Biomechanics Eccentric Research
McLaughlin measured the eccentric force in lowering the weight in a bench press decades ago.
McLaughlin found that lifter lowering the bar too fast increased the weight (force) of the bar 149%.
300 lb Bench Press = 447 lb Bench Press
That means lifter who allowed the bar to drop too fast, found the weight (force) of the bar on their chest was 447 lbs. (300 lbs X 149% = 447 lbs).
Lifter's who RODE THE BRAKES increased the weight (force) of the bar 112%.
300 lb Bench Press = 336 lbs Bench Press
Thus, lifter who rode the brake on the way down, minimized the weight (forc) of the bar on their chest to 336 lbs (300 lbs X 112%).
The same is true with squats and other movements.
If you allow the eccentric speed to increase too much in a squat, you will end up with your ass tatooed to the floor.
Ride The Brakes!
The take home message is when lifting heavy load, RIDE the brakes when lowering the weight!!!
The Exception To Riding The Brakes
In squatting (other movements as well), the exception to riding the brakes occurs appoximately 2" from when the eccentric motion stops and the concentric contraction begins.
Appoximately two inches from the transition from eccentric movement to concentric contraction, allow the eccentric speed of the bar to increase.
Doing so elicits the stretch reflex a...
The stretch reflex is akin loading a spring. Once you release the spring, you recoil upward with more force/momentum.
Research shows that up to 18% more force is generated when the stretch reflex is evoked.
90 Degree Squats
Two things that will increase your strength at this angle are:
1) Performing Paused Squats from the 90 Degree postion.
2) Training the Stretch Reflex "Recoil". In other word, practice "Recoiling" out of the 90 degree hole position.
I am just really frustrated that the 315 felt so heavy and that i only got to six reps. It motivates the hell out of me and pisses me off.Pissed Off Works
One of the greatest forms of motivation is, "Pissed Off." Harness that and it will make you better.