The ASISes are good indicators of the tilt of the pelvis. On the side of the pelvis with the extended hip (the back leg), the iliopsoas will try to pull the pelvis and lumbar spine down and forward into an anterior tilt. To counter this, use your fingers to show the ASISes how to lift up. Hold this posterior tilt as you bend the front knee, keeping the back knee straight and the back heel grounded. Feel the iliopsoas lengthen and visualize the spine lifting out of the pelvis.
Intensive weight training causes micro-tears to the muscles being trained; this is generally known as microtrauma. These micro-tears in the muscle contribute to the soreness felt after exercise, called delayed onset muscle soreness (DOMS). It is the repair of these micro-traumas that results in muscle growth. Normally, this soreness becomes most apparent a day or two after a workout. However, as muscles become adapted to the exercises, soreness tends to decrease.
At the same time, this also doesn’t mean that primary compound exercises can never be done for more than 8 reps, or that secondary compound exercise can’t be done for 5-8 or 10-15 reps, or that isolation exercises can’t be done for less than 10 reps. Everything can be done in every rep range. However, these are the rep ranges that each type of exercise is best suited for, and where it should ideally be done most of the time.
As stated before, one of the primary hip flexor muscles is the psoas major. This muscle plays a role in core stabilization (something that is needed during running, squatting, and sitting) due to its attachment site at the spine. If there is a lack of core stability or poor movement patterns during these tasks then the hip flexor can become overworked/tired/fatigued (think what happens when your co workers or teammates don’t do their job, you have to pick up the slack and work harder, bringing you more stress and fatigue). It is when the hip flexor becomes fatigued that the sensation of tightness sets in. This is because the hip flexor has to “work harder” to compensate for other muscles not doing their job.
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.