Include cardio training. Good cardiovascular health improves blood flow, a requirement for muscle growth. Doing cardio also improves your cardiovascular fitness, which allows you to use your muscle gains for various sports and activities. The standard recommendation is 150 minutes of moderate cardio each week, or 75 minutes of vigorous cardio, or an equivalent combination of the two. A good place to start would be doing 30-60 minutes of cardiovascular activity every other day or 3 times a week. Examples of cardio include running, biking, swimming, and any sport that involves constant movement.
Most problems with the hip flexors, however, don't originate in a lack of strength but in a lack of flexibility. To understand how these muscles lose their flexibility, imagine someone with a broken arm, her bent elbow encased in a plaster cast. When the cast is removed after six or eight weeks, the soft tissues around the elbow (muscles, tendons, ligaments, and even skin) will have shortened, and the elbow won't straighten out. It will take patient stretching over several weeks to restore the range of motion. Similarly, if the hip is constantly kept in a flexed position—like sitting—for hours every day, day after day, the hip flexors will shorten and shrink, limiting your ability to fully extend (straighten) the hip.
Alternately, try King Arthur's Pose: Starting on hands and knees with your feet near a wall, put your right shin straight up the wall and bring your left foot forward so it's under the left knee. Place your hands on your knee and lift your spine straight up while taking your tailbone down. You should feel a strong stretch on the front part of your thigh.
Do standard squats with a weighted bar. Place enough weight on a bar and rack so that it's a little lower than shoulder height. It should be heavy enough that doing a squat is difficult, but not impossible. If you're a beginner, this may mean using a bar without any weight to start with. Duck under the bar and stand up so that the bar rests comfortably on your trapezius muscles, just below the neck. Keep your knees slightly bent and your legs slightly wider than shoulder width. Lift the bar up off the rack and move backwards one step.
Glutes are king as far as the hierarchy of your muscles is concerned. “For athletic performance, overall lower body power, strength, speed, and everyday optimum function, honing strength in and activating your glutes should be a top priority in all exercise programs,” says Robert Reames, C.S.C.S., Gold’s Gym Fitness Institute and Pear Training Intelligence System’s weight control coach.
Stand with your feet slightly wider than shoulder width with a kettlebell about a foot in front of you. With your weight in your heels, hinge at your hips while lowering your hands to the kettlebell handle. Grab the kettlebell with an overhand grip, “Hike” the kettlebell back between your legs, catching the force of the moving kettlebell with your hips. Exhale as you swing the kettlebell forward by thrusting your hips, straightening your legs, and squeezing your glutes and abs. Once the kettlebell reaches chest height, inhale as you allow it to fall, and guide it back to the “hiked” position.
The harder an exercise is – both in terms of technicality and physical/mental demand – the more rest there should usually be. So exercises like squats and deadlifts should have more rest between sets than exercises like leg extensions and leg curls. And exercises like various bench presses, shoulder presses, rows and pull-ups should have more rest between sets than bicep curls, tricep extensions, chest flies and lateral raises.
As I mentioned earlier, the exercises that come first in your workout (aka primary compound exercises) should usually be done in the 5-8 rep range. Exercises in the middle (aka your secondary compound exercises) should usually be done in the 8-10 rep range. Exercises done at the end of your workout (which is typically where isolation exercises belong) should usually be done in the 10-15 rep range.