Dynamic stretching (stretching while in motion), is the most efficient way to lengthen and strengthen the hip flexor muscles. An exercise that you can do that will engage the core and all of the lower extremity muscles while focusing on the hip flexors is called the Single Leg Split Squat. Remember that the rectus femoris crosses the hip and knee joint. This exercise involves both the hip and knee joint, making it an appropriate exercise to train the rectus femoris as well as the iliacus.
The volume here isn’t excessive. You’ll do 11 sets total for large muscle groups (the one exception being shoulders, for which you’ll do 15) and train each bodypart once a week. Reason being, to pack on tons of mass you need ample recovery time. Doing endless sets in each workout can easily put you in a catabolic (muscle-wasting) state in which lean tissue is broken down, not built up. Gaining 10 pounds of muscle in such a short period requires the right balance of adequate volume to rest and recovery.
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.