The human body has a tendency to sacrifice quality of motion for quantity of motion. That means that if an athlete wants to perform a movement which is lower, higher or more weight (quantity of motion) than they are capable of with good technique, they will sacrifice the good technique (quality of motion) to gain what it thinks is a desirable outcome.
So, if you are trying to achieve a new 1 rep maximum on the back squat 
exercise and the weight is more than you are used to doing, your body will likely have the tendency to sacrifice the quality of the good parallel squat by only going down to a half squat position and then coming back up. Or, some athletes may lean forward excessively, exposing your low back to injury. In this case, the body is sacrificing the quality of a good squat to achieve more quantity (weight).
In this example, you will sacrifice long term performance by limiting your range of motion during the squat. This can lead to flexibility deficits and decreased power output because specific muscles probably aren’t getting activated. It can also lead to injury.
Long Term Improvements
Sacrificing quality may lead to short term improvements, but will almost certainly have long term detrimental effects. This can be avoided by always focusing on the quality of your movement during any drill or exercise. You will be more successful if you focus on performing very few quality focused repetitions, rather than dozens of reps, where you may fatigue and perform poor reps.
This is due to the influence your training has on the software of your body. If you consistently train in poor movement patterns, you will develop motor engrams which are not desirable. It will then become twice as hard to correct those movement patterns because your body will have a pre-programmed idea as to how to perform certain movements, which are likely not the patterns we would like to show during a game. If you’ve ever heard the phrase “old habits die hard,” this is what they were referring to. By focusing at the beginning and paying close attention to detail, you can build good engrams and habits.
