Consistency is the key

The importance of repeatability cannot be overstated. When you appraise someones ability to perform a particular exercise or movement it is absolutely imperative that your set up positions for testing are the same each time.

If you watch someones overhead squat and the first time you check their technique you cue them to have their toes straight forward; they struggle down, get halfway through and  their hands come forward and knees collapse inward, then when you re-test them after some intervention and their feet are turned out 5-10degrees and they dramatically improve, you cannot compare the two squats.

These three are all examples of unloaded overhead squats, but they are all very different mechanically, and place very different movement requirements on the body.

Overhead Squat? yes, but narrow stance and straight feet

Overhead Squat? yes, wide stance, wide grip (not good form)

Overhead Squat? yes again, but feet turned out this time. 

The difference between feet straight and feet 10degrees turned out, changes the entire assessment. Slightly externally rotating the feet, changes the position of hip to enable easier internal rotation, it brings the adductors into a position where they can act as a hip extensor (as opposed to adductor), it changes the strategy required at the ankle to allowing the arch to collapse slightly and fascial tension to change. All of these as result of slightly turning out the feet!

Thomas Myers (anatomy trains)
is definitely onto something.

The body needs to be thought of as an interconnected web of tissue, whereby an action in the toes can impact on the stability or mobility of the low back, a slight change of knee position could be implicated in the stability of the shoulder or neck. If your focus is too narrow when looking at the way you or your client moves you will get stuck trying to address limitations that might be caused by something in a different region all together. Conversely if you go too broad and don't approach your movement appraisal in a systematic way to ensure that each time you appraise something you do it exactly the same way, you will never actually know if what you are doing is working.

It is understandable that as clinician, trainer or coach we want to show our clients that what we are doing is helping them improve. However, if you are re-testing your client or even yourself and you are allowing modified positions from test to test, you will never know if the corrective exercises or training strategies you are using are working or not. You are only cheating yourself and your client, and eventually you or your client will pay a price either through injury or limitations in performance.

So, make sure you use objective measures and use a systematic approach to technique/movement appraisal. Whether you use Gray Cooks' Functional Movement Screen or another movement assessment standard it doesn't really matter just make sure your test re-test approach is clear and repeatable.