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One of the best assistance exercises yet developed.
Often when a trainee experiences a problem with one of the big lifts such as a squat or a deadlift, one of the first questions that gets asked is, "What other exercises should I do to make (insert muscle group here) stronger?" This is a very natural reaction and assistance exercises certainly have their place. However, in many cases, a simpler solution exists. If progress in the squat is lacking, then the answer is probably to squat more instead of doing something else.
Strength is not built overnight, but, instead, is the result of hard work and consistency. Lifts like the squat and deadlift are multi-joint exercises that employ most of the musculature of the body and are renowned for their difficulty. Because of the muscle mass used, the technique required, and the will that must be summoned to complete these lifts successfully, it is not always clear what the weak link is when a lift is missed. At limit weights, something is going to give and you are not going to be able to employ technique alone to pick 600 pounds up from the floor. Until a trainee is already fairly strong, it may not really matter what muscle group, or groups, are impeding progress on a lift. The body responds well to being trained as a system instead of a collection of discrete parts. Instead of looking to greater complexity or more exercises, it will often be a better use of everyone's time to work on the movement in question with renewed focus.
There is probably no movement in the training arsenal capable of building total body strength like the squat. If the squat is a weak point, then choosing a weight that can be handled for three sets of five is a very good place to start. After the training session, the lifter should go home, eat some good food (probably a lot of good food), sleep, and then add a little weight to the bar and squat three sets of five again the next time they train. The process of lifting, recovering, and then lifting a little heavier is a proven method for getting stronger. Note that there is nothing complicated here. The repetition scheme is held constant. No additional exercises are added to bring up weak points. Instead, the only variable adjusted is the weight. Simplicity is good. Training should be as simple possible to drive the progress that is desired.
For many trainees, a simple approach such as the one above is all that is needed for a good long time. As strength levels increase and as familiarity with the lifts increase, additional complexity and exercises will become useful. However, the basic lifts remain the primary drivers of progress throughout a training career. If the goal is to squat more weight, then more time needs to be spent squatting. Any weaknesses that impede progress will be addressed by doing the exercise correctly and loading it in a way that encourages success.