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.
Protein, you may have heard, is what your muscles are made of. Well, not quite. The amino acids that make up protein are the building blocks of muscle, and your body needs—not wants, needs—these substances during and after training for repair and recovery. It breaks down the protein into the respective aminos, uses them for their various functions, and then you convert what doesn't get used. I break down a lot of tissue in my daily workouts, so balancing it out with adequate amounts of protein throughout the day is important.
I HATE that the resistance training community can be so tribal. I have been preaching to bodybuilders for years about the benefits of powerlifting, or Olympic lifting or kettlebells or even Crossfit style conditioning and many have been receptive. Learn from each other and achieve levels of fitness you simply could not have otherwise. Don’t brush off bodybuilding wisdom…it could be the missing factor in your program.
Do standard/oblique crunches. Lie down on a mat and position both arms behind your head without locking the hands. Bend your knees so that your feet are flat on the ground. Pushing the small of your back into the ground, slowly roll your shoulders off the ground only a couple of inches (not to a full sitting position). Don't use your momentum to help you up; use slow, regulated movements. Repeat 3 x 20.

Your questions kinda ties everything together. You are already aware of “nutrient density” which is AWESOME. This is important because I believe one food may be more “healthier” over another due to its “nutrient density”. The other food is still healthy because it still contains nutrients, but the nutrients could be more dense…so this is why I think one food may be “healthier”, while the other just isn’t as “healthy”. It’s not, not healthy.. Make sense?
A 2001 study at the University of Texas found that lifters who drank a shake containing amino acids and carbohydrates before working out increased their protein synthesis more than lifters who drank the same shake after exercising. The shake contained 6 grams of essential amino acids — the muscle-building blocks of protein — and 35 grams of carbohydrates.

Reaching failure matters because tension matters. Failure is when the muscle cannot generate anymore force. Which means one applied (worked out) enough tension through the muscle to just want to give up. And guess what? Carrying groceries can do this for some. Why? We are all at different strength levels. So don’t worry about other people and what they can or cannot lift. I say this because FAILURE can be achieved with either light weights or heavy weights.
Muscle imbalances are quite common among strength athletes and are arguably the most common cause of their injuries. Many times this is due to a “weak link” in the kinetic chain of muscles that activate during their activity. Identifying the “weak” muscle and being able to feel, isolate and contract that “weak” muscle makes correctional exercise and rehab much easier. Bodybuilding training, with its focus on “feel” rather than movement, helps to train and develop the mind to muscle connection. This comes in handy when you need to train a muscle imbalance with correctional exercise and, in the case of injury, for rehab.
This exercise is challenging and will certainly be beneficial even without weights. If you can execute perfect form with your body weight, you can make the exercise harder by adding weights to your hands, by stepping on an unstable surface with your front foot (balance disc, foam pad), or you can place your rear leg on an unstable surface such as a physioball. You should be able to comfortably perform 2-3 sets of 10 reps on each leg before advancing this exercise.
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