I get that some movements are slower but thats not what i am asking about, its when the lift gets significantly slower after some reps, is that considered technical detoriation?
No
The Technique of the Movement is not necessarily altered by the speed of it.
An individual can perferm a Movement with good technique with a fast or slow speed.
The Speed in which the Movement is performed has to with...
The Muscle Fiber Type Being Trained
There are three primary Muscle Fiber Types.
1) Type IIb/x "Super Fast" Muscle Fiber
These Muscle Fiber are Explosive and primarily generate Speed and Power.
When innervated with an Explosive Movement, they are innervated for approximately 10 seconds.
After around 10 seconds, they are no longer involved in the Movement.
2) Type IIa Fast Twitch Muscle Fiber
These tend to be primarly used in Maximum Strength Movements.
There duration is a little longer that 10 seconds but not that much.
3) Slow Type I Muscle Fiber
These are the Endurance Muscle Fiber Type.
The Size Principle
This is involved in the shift of Muscle Fiber Type in a Movement.
In an Explosive Movement the Type IIb/x "Super Fast" Muscle Fiber are primarily involved for around 10 seconds. They are then exhausted and are not longer involve; they stop working.
The workload is then shifted to the Type IIa Fast Twich Muscle Fiber. The speed of the movement is slower but continues.
After a litlle longer that 10 seconds, Type IIa Fast Twich Muscle Fiber are exhausted and no longer involved.
The workload is shifted to the Slow Twitch Muscle Fiber. Speed and Power drop like a rock.
Technique and Muscle Fatigue
Technique failure occurs in a Strength Movement (Maximum Strength, Power and Speed) due to Muscle Fatigue of the Type IIb/x and/or Type IIa Fast Twitch Muscle Fiber.
Continuing In A Muscle Fatigued State
Here is what happens...
1) Once Muscle Fatigue occurs Technique deteriorate quickly.
Continuing to push a Movement in a Muscle Fatigued State develops poor Technique.
2) The Training Objective Is Altered
a) Speed and/or Power Training
Once the Type IIb/x "Super Fast" Muscle Fiber are exhaused, they are not longer being trained nor developed.
The workload is shifted the slower Type IIa Fast Muscle Fiber; Maximim Strength is being Trained, not Power and/or Speed.
b) Once the Type IIa Fast Twitch Muscle Fiber are exhaused, they are not longer being trained nor developed.
The workload is shifted to the very slow Type I Slow Twitch; Endurance is being trained and developed at the expense of Maximum Strength, Power and/or Speed.
Training Percentage Strength Training Protocols
This is a good article that beaks down the Training Percentage that need to be used to develop certain Types of Strength.
From 0 to 100: Know Your Percentages!
Each training percentage, from zero to 100% of your 1RM, has a distinct effect on speed, power, and hypertrophy. Here’s how to use that info wisely.
www.t-nation.com
Summary
1) Perfoming a Movement at a Slower Speed doesn't necessarily mean that Techinique is altered.
What it means is the a different Type of Strength is being devleoped.
2) Muscle Fatigue
This is the determinate factor that causes the deterioration of Technique.
Continuing to push/pull a Movement when Muscle Fatigue happens, ensures Poor Technique is devlopes and the Type of Strength Training has been altered.