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.
Are you tired of stretching or rolling out tight hip flexors with no improvements? If you are a runner, weight lifter, or team sport athlete, chances are you have heard a teammate or friend complain about having “tight hip flexors”. Either that or you yourself have had to deal with the problem. The problem with these tight hip flexors is that people will stretch, stretch, and ……..stretch some more (until the cows come home) and get NO RELIEF. This keeps people in a perpetual cycle of ALWAYS stretching their tight hip flexor with no real permanent relief.  The cycle looks something like this:
The high levels of muscle growth and repair achieved by bodybuilders require a specialized diet. Generally speaking, bodybuilders require more calories than the average person of the same weight to provide the protein and energy requirements needed to support their training and increase muscle mass. In preparation of a contest, a sub-maintenance level of food energy is combined with cardiovascular exercise to lose body fat. Proteins, carbohydrates and fats are the three major macronutrients that the human body needs in order to build muscle.[24] The ratios of calories from carbohydrates, proteins, and fats vary depending on the goals of the bodybuilder.[25] 

Just because your hip flexor region feels sore doesn’t necessarily mean the muscles there are tight — in fact, they might need strengthening. This is where that sports science debate we mentioned earlier comes into play. It’s important to identify whether you’re tight or if the muscles are weak. Again, the Thomas Test will help you identify if you’re maybe stretching something that actually needs strengthening.
(10) Exercising - you talk about building muscle - this comes from breaking down the muscle and building it back up with protein. A surplus is not needed for muscle growth, protein is. I always say stick with 100g minimum so you’re consistent. 100g is 400 calories. Muscles need glucose to perform, so I would eat enough carbs to fill your glycogen levels to prepare for your next training. Then eat fats to cover the rest of the calories whether it’s a surplus or deficit. You can build muscle and lose weight in the same day, just not at the same time (I’ll explain in point 10). Building muscle = breaking down the muscle and rebuilding it with protein. Losing weight = a deficit. Tell me why this can’t happen? Some fear muscle loss during deficits. No. Eat protein. Eat a little more. Some think surpluses are needed to build muscle. No. A surplus leads to fat gain. Even if the excess calories come from protein. Everything has a number. Figure out what fits for you. This is why point 9 is important.
The bulk of the gluteal muscle mass contributes only partially to shape of the buttocks. The other major contributing factor is that of the panniculus adiposus of the buttocks, which is very well developed in this area, and gives the buttock its characteristic rounded shape. The gluteal muscle bulk and tone can be improved with exercise. However, it is predominantly the disposition of the overlying panniculus adiposus which may cause sagging in this region of the body. Exercise in general (not only of the gluteal muscles but of the body in general) which can contribute to fat loss can lead to reduction of mass in subcutaneal fat storage locations on the body which includes the panniculus, so for leaner and more active individuals, the glutes will more predominantly contribute to the shape than someone less active with a fattier composition.[citation needed] The degree of body fat stored in various locations such as the panniculus is dictated by genetic and hormonal profiles.[citation needed]
Expert tip: To make it harder, inch yourself close to a wall so your trunk is about a foot away. Bring what would be your “planted” leg on the wall, foot flat against the surface. From here, raise your “non-working leg” up into the air, keeping your knee bent. As you raise up into the glute bridge, you’ll drive your heel against the wall, raising your hips. You can also use a bench. And like the standard bridge, you can up the ante by adding weight. 
Yes, you can pack on size while only doing bodyweight exercises – I did it while traveling the world – and gymnasts train mostly with bodyweight movements. However, this can feel like playing Halo on Legendary difficulty. It can be done, but damn it can be challenging – especially for lower body movements. If your sole goal is to get bigger as fast as possible, access to a barbell for squats and deadlift is almost a requirement.
Prison food isn’t as bad as people think. Prisoners often get three meals a day. Meals need to meet a certain amount of calories. You don’t need that much protein to build muscle, but prisoners can buy protein powder (and also extra food like oatmeal). Prisoners aren’t underfed in most western countries. The diet may not be optimal, but it’s sufficient to build muscle.
Remember my special answer: here it is… MOUTHTAPERS exist. People out there will tape their mouth closed during sleep so that they can breathe their nose. Even during the day sometimes. Why? Because the nose is directly connected to the diaphragm while the mouth is connected to the chest. Sure, increase chest mobility and your lungs may be able to expand more which will allow more air to be held, BUT HOW CAN YOU HOLD THAT MORE AIR IF YOUR DIAPHRAGM is weak? You won’t. So everything I just talked about must be done through the nose. Notice yourself breathing with your mouth and SWITCH right away. It takes work and energy. How you breathe during the day rolls over to how you breathe during the night.