Greetings Realize.
You say that you are pushing through the strain on the muscles?
I may be wrong, but it sounds like you may be 'trying too hard'
Take a look at how you are doing QiGong.
Are you letting the energy flow through you or are you forcing it? It should be an effortless process. Make sure you are as relaxed as possible and allow the energy to move through you as effortlessly as light would. Visualize that you are an empty vessel and that your breath draws in fresh, sparkling energy from the universe to your Dantien. Feel how easily it flows between the cells and atoms of your body--renewing you in every way.
When doing the standing QiGong, make sure there is 0.0% tension in your body at all times. When moving the energy, allow the old denser energies to dissipate and bring positive energy into your Dantien.
Get to know the exercise "7 Steps to New Life" from SFQ Level 1 very well. I would suggest doing that a lot. This exercise will definitely assist you in strengthening the tendons and ligaments in the legs.
~Z