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.
^ Burd, Nicholas A.; Yang, Yifan; Moore, Daniel R.; Tang, Jason E.; Tarnopolsky, Mark A.; Phillips, Stuart M. (2012). "Greater stimulation of myofibrillar protein synthesis with ingestion of whey protein isolate v. Micellar casein at rest and after resistance exercise in elderly men". British Journal of Nutrition. 108 (6): 958–62. doi:10.1017/S0007114511006271. PMID 22289570.
Partial range of motions train partial ranges while full range train full range. Simple as that. They both help each other. Isometrics are great. I do them all the time. I add them in however I can. Holding it on the bottom. Holding it on the top. Or just in the middle. Constant tension vs time under tension are one in the same. When constant tension cannot be applied, it’s okay because one can just keep holding the weight so time can apply tension. Either way, Tension is caused until the weight is dropped. Will their be enough tension for growth? You tell me?
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.