Kelley used to do three sets of squats one time per week. after three weeks she noticed she was less sore than when she started this exercise. which principle is at work?

The principle underlying this change is called progression. This principle tells us that when your fitness level improves, your body has adjusted to your regular routine; thus, you will feel less sore than when you started the squats three weeks ago. During this time, your muscles have developed and your body has adopted to the stress brought about by your exercise.