“Compared to training for strength, intensity is going to drop during the hypertrophy phase of a program, with intensity sitting between 50 and 75 percent of the person’s 1RM, the maximum weight he or she can lift for one rep,” says Ava Fitzgerald, C.S.C.S., C.P.T., a sports performance coach with the Professional Athletic Performance Center in New York.
Apply the above concept to your hips. When you sit, your hips are in a "flexed" position. Therefore, the muscles that flex your hips are in a shortened state. You probably spend at least a third of your day sitting down. Think about how much time those hip flexor muscles stay shortened. A lot. Over time, they become tighter and tighter until you look like the old man in the picture. So unless you want to look like that, perform the stretches shown below.

This test measures a participant's ability to stand up from a seated position as many times as possible in a thirty-second period of time.[2] Testing the number of times a person can stand up in a thirty-second period helps assess strength, flexibility, pain, and endurance,[2] which can help determine how far along a person is in rehabilitation, or how much work is still to be done.
Rest your hip. Sore hip flexor muscles can benefit from rest. Avoid the activities that caused your pain. Ice your affected hip for 20 minutes at a time, several times a day. Keep the leg of your affected hip elevated as much as possible 48 hours from the first onset of pain. You can also take over-the-counter pain medications to help control pain.
Learning to activate your glutes is important so that you can strengthen them. Strong glute muscles are extremely important as these muscles can have a major impact on your overall body strength; your glutes support your core, help to support a range of exercises and compound movements, as well as help avoid muscle imbalances which can lead to decreased mobility. 
Expert tip: “Optimum form and range of motion for the squat is very dependent on your physicality, structure, joint integrity, limb length, strength, flexibility, and current condition,” Reames says. Your range of motion is 90° max at the knee. Some guys will be able to go lower than this, but not everyone will squat to parallel, he says. And that’s totally okay. “Stick to form and the appropriate resistance levels for you; the squat is still one of the best and simplest moves for overall strength, building lower-body muscle, core strength, and athletic performance. 

The lower the rep range (and therefore the higher the intensity and the heavier the weight), the more rest there should be between sets. So most of the time, exercises being done in the 5-8 rep range need longer rest periods than exercises being done in the 8-10 rep range, which need longer rest periods than exercises being done in the 10-15 rep range.
The hip flexor muscles flex the hip during swing. They are particularly important for initiating swing91 when walking at slow speeds. Without adequate hip flexion during swing, knee flexion is more dependent on hamstring muscle activity.59 Patients with paralysis of the hip flexor muscles attempt to advance the swing leg by either externally rotating the hip and using hip adductor muscles as hip flexors or by circumducting the leg.59,61 The effects of hip flexor muscle paralysis on gait are particularly evident when walking up stairs or slopes, which requires lifting the leg.

A: No. You should ensure that the squat and hinge motor pattern are both emphasized but other variations (front squat, sumo deadlift, safety bar squat, Romanian deadlift) should be included until you can master technique on the more advanced variations. For more information on exercise progressions and regressions see this article: Train Like An Athlete, Look Like a Bodybuilder.
To combat steroid use and in the hopes of becoming a member of the IOC, the IFBB introduced doping tests for both steroids and other banned substances. Although doping tests occurred, the majority of professional bodybuilders still used anabolic steroids for competition. During the 1970s, the use of anabolic steroids was openly discussed, partly due to the fact they were legal.[9] In the Anabolic Steroid Control Act of 1990, U.S. Congress placed anabolic steroids into Schedule III of the Controlled Substances Act (CSA). In Canada, steroids are listed under Schedule IV of the Controlled Drugs and Substances Act, enacted by the federal Parliament in 1996.[10]

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.