The Often Forgotten Adductors
Try touching your ceiling. (After a brief warmup, of course.) Assuming your ceiling is higher than 8 ft., you likely had to take drop your hips slightly before leaping into the air. Why did the dip allow you to jump higher than a non-dip hop? Two words: stretch reflex. A stretch reflex is a muscular stretch immediately followed by a contraction. The stretched muscle stores elastic energy, and it is then released upon contraction.
You use this same stretch reflex during the squat (and most other lifts, for that matter). As you squat, a number of muscle groups work together as they stretch then contract -- the hamstrings, the glutes, the quads, and ... the often forgotten adductors.
The adductors, the muscles that line the inner thigh, play a vital role when extending the hips. Their importance shouldn't be neglected. But how do you garner their full force -- or, how do you put them into a stretched position? Answer: (1) Point your feet out at a 30-degree angle. (2) Shove our knees out over the line of your feet. As the thighs rotate outward to mirror the feet, the adductors elongate; and during the descent, they become fully stretched. After reaching full depth, a bounce out of the bottom occurs and the stretched adductors contract sending you to the upright, finished position. (If you were to squat with straight feet, would your adductors fire? Absolutely. But to a much lesser degree.)
Angles, positions, and placements matter when lifting. This is just one example of how a tweak here and an adjustment there result in greater muscular activation.