You're more likely to get a hip flexor injury if you've had one in the past, you don't warm up properly before engaging in athletic activity, your muscles are already tight or stiff, or your muscles are weak from being overused. If, while exercising, you try to do too much at once in too short an amount of time, you can also put yourself at risk for a hip flexor injury.
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.
By that logic, a 160-pound man should consume around 160 grams of protein a day—the amount he'd get from an 8-ounce chicken breast, 1 cup of cottage cheese, a roast-beef sandwich, two eggs, a glass of milk, and 2 ounces of peanuts.) If you don't eat meat for ethical or religious reasons, don't worry — you can count on other sources, too. Soy, almonds, lentils, spinach, peas, and beans are packed with protein.
Unlike your boobs, your glutes are loaded with muscles that are capable of growing larger, so you can score the curvy butt of your dreams. Obviously, if you want those muscles to grow, you need to work them, says Tony Gentilcore, a certified personal trainer and strength and conditioning specialist, and co-founder of Cressey Performance in Hudson, Massachusetts. To score a bigger booty, he says to start by doing glutes-focused exercises, like the glute bridge and squats, while progressively adding more weight. (Gentilcore says you can do this by holding a barbell or dumbbells on your hips for the glute bridge and by holding a dumbbell in each hand for the squats.)
In the modern bodybuilding industry, the term "professional" generally means a bodybuilder who has won qualifying competitions as an amateur and has earned a "pro card" from their respective organization. Professionals earn the right to compete in competitions that include monetary prizes. A pro card also prohibits the athlete from competing in federations other than the one from which they have received the pro card.[12] Depending on the level of success, these bodybuilders may receive monetary compensation from sponsors, much like athletes in other sports.
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. 
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. 
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