And as for tendonitis type problem…
Start taking a magnesium + calcium supplement. Every health shop in the western world, probably the whole world, will have plenty of “Magnesium + Calcium” tablets.
Quite cheap as well. Some of the tablets are; “Magnesium + calcum + Vitamin D3, + zinc.” Don’t take (magnesium only) tablets. It is much better when taken with calcium, as those two minerals support each other.
Ofcourse, certain foods like brown rice, are stacked with magnesium. 420mg of magnesium a day, is the RDA. But that is for a non-athletic person. Most peeps don’t get that much unless they have a very healthy diet. A medium sized banana has about 30mg of magnesium.
I hesistate to recomend brown rice though, as you don’t want to be eating too many high glyceamic carbohydrates, as you need to stay lean.
You are better off just adding a “Magnesium + calcium” supplement.
The magnesium helps the muscle to relax. This in turn, takes some of the pressure of the tendon. And therefor; the tendon will have less problems.
… And go easy on the stretches, because stretching can cause allmost as much soreness as strength work, if you haven’t stretched fo a while.
Light gentle stretches. Start with light weights for low reps in the quadriceps (way below what you are capable of doing, for the first several sessions. )
Have you tried sit ups with your feet on the wall? Very good for rectus femoris muscle.
Your heel is on floor, with ball of foot on wall. Or if you prefer, your heel is several inches above the floor, touching wall, with rest of foot.
Legs are only slightly bent, when your torso is near ground, and as you curl yourslef up, your legs inevitabley bend a bit more.
“Feet on the wall” sit ups, muscularize the rctus femoris, without you having to do squats.
If it is spacifically the rectus femoris you are concerned with, there are plenty of alternatives to the squat. If however, it is general thigh mass & strength per se’ that you want, then back to a varient of the squat or similar.