Glute deactivation/amnesia. Devil's advocate question.
The question is at the bottom of this post.
Force and motion are two sides of the same coin. If a single object undergoes a change in motion, there is only one net force (with a specific direction and magnitude) that could have produced that change.
In the context of human movement, where we are dealing with a system of bones that rotate about joints, there is only one set of net torques that could have conspired to produce a particular pattern of movement. Note that even if a bone rotates with uniform circular motion (i.e. net torque of 0), gravity is often part of the equation, and a force must be generated by muscles to counteract the torque due to gravity.
If you tweak the net torques, the shape of the movement, across time and space, will change.
Accordingly, if a particular movement pattern requires a certain amount of force to be generated by any particular set of muscle fibers, then it is simply impossible for that movement pattern to occur without the contribution of those muscle fibers.
No reasonable and sane person who understood these principles could object to this. And, with a bit of common sense, and physical demonstration, they may be convinced that the best way to shape a movement pattern into a desired form is to practice the movement pattern itself, with the help of verbal, tactile, and visual cues, rather than to do some muscle activation corrective exercises.
But, they might say, what if you could achieve the same set of net torques with multiple possible patterns of muscular involvement? For example, if you had two separate muscles whose respective attachment points were offset by a small amount (e.g. muscle A's attachment points are a couple inches proximal of muscle B's attachment points) such that the direction of force is equivalent for each muscle. Then, there are an infinite combination of firing patterns that could produce the same net torque at the joint.
Or, more realistically, what if you had two separate muscles which, while having different angles of application, could still provide the same net torques. For example, tension in the hamstrings between the ischial tuberosity and an anchored tibia produces a torque in the acetabulofemoral joint about a different axis than does tension in the gluteus maximus between the ilium and the femur (the glutes pull more "sideways" compared to the hamstrings). Yet, if the joint is "stable", either due to constraint forces provided by the interaction of the head of the femur within its socket, or due to the contribution of stabilizing muscle groups (e.g. internal hip rotators), the net torque at the acetabulofemoral joint could be identical, across multiple combinations of glute and hamstring contributions.
To this, one might respond as follows:
If the muscle is lengthened during the eccentric phase of the movement, then it will contract during the concentric phase as it shortens. So, during the eccentric phase of the squat, if the glutes lengthen (and they cannot help but lengthen), then they will contribute accordingly during the concentric phase.
At this point there is one remaining objection. Well, they might say, just because a muscle shortens doesn't mean it was actually firing. If I flex my elbow joint, and then have someone extend my elbow, the triceps have gone from a lengthened state to a shortened state without tension being generated. Similarly, the glutes may shorten without producing their own tension - they may be "spotted" by the hamstrings on the way up in the squat.
My answer to this would be: I don't think that's how the neuromuscular system behaves. Movement is arguably one of the most fundamental tasks that mammals have to carry out. As such, our phylogeny and ontogeny have ensured us that we will use the appropriate muscle groups when solving a problem such as hip extension. In the absence of severe pathology (e.g. central or peripheral nervous system, or muscle lesion), the glutes will fucking contract when they're supposed to. In fact, it would be an extremely challenging thing to do, if not downright impossible, to contract the hips without "proper" glute activation.
Rip, my question to you is whether there is a better answer to this objection than the appeal to evolutionary common sense.