Simply put mastering eccentric isometrics on the foundational movement patterns will do more for your athletic performance and sprinting mechanics than any other form of training. Read more in my book MOVEMENT REDEFINED.
On a related note, many of the lateral movements and movements in the frontal plane recommended by fitness professionals often involve significant external rotation of both hips/feet at the same time. Similar to the above examples this would represent a mismatch of force vectors from a functional standpoint, not to mention potentially deleterious hip positions due to the extreme torque and dysfunctional positions.
There’s absolutely nothing wrong with externally rotating the feet and hips during movement, the caveat being that only one limb should externally rotate at any one time such as during cutting, rotating, or twisting. In fact, during most rotational movements, when one hip externally rotates the other hip should simultaneously internally rotate in order to match all the force vectors of the body with the desired movement.
Simply put, dual external rotation of both hips and legs at the same time represents 3 primary complications: 1) a mismatch of force vectors, 2) a biomechanically faulty position, and 3) a neuromuscular malfunction in which there is a neural misfiring of one or more groups of muscles, with the body attempting to move in one direction and one or more segments/limbs moving in the other.
Its also for this very reason that generally speaking I do not advocate squats or other bilateral movements with the feet externally rotated. Instead, I recommend a relatively straight foot position with neutral hip alignment, neither externally or internally rotated. That’s because the force vectors involved in most traditional bilateral strength training movements such as squats, deadlifts, hinges, and jumps, involve straight and linear force vectors not rotational or lateral vectors. Read more about force vectors and proper squat form and depth here.
However, this principle applies only to biomechanically sound 90 degree positions. If, in fact, the individual is dead set on performing biomechanically flawed movements with excessive range of motion and insists on going significantly deeper than 90 degrees on their loaded squats or other strength training movements, then one could make the argument that externally rotating the hips and feet would, in fact, be a safer and more effective way to perform these functionally flawed and dysfunctional positions.
On a side note, and as a semi-disclaimer, there may be rare instances where dual external rotation of both hips is necessary. However, for a majority of functional movements as well as general fitness exercises that are therapeutic and biomechanically sound, dual external rotation is rarely warranted and unilateral external rotation is optimal. The same is true of internal rotation. Therefore, training dual external rotation or dual internal rotation is likely setting the athlete up for potential injury and compromised activation patterns since movements we perform during training transfer to the field.