The following comes from not just my experience with teaching yoga and with anatomy but also from many mistakes made over the years. I've been through many sports injuries and helped many students through them, and unfortunately have learned many things the hard way. As I like to say, I know the surefire ways to fail.
If you're starting Insanity, pay close attention to the Digging Deeper section of the DVD. Form should always come first, but to do that, first you have to understand exactly what the form should be and why, and Beach Body only barely touches on that.
Insanity is loaded with plyometrics (jumps). Each time you land, the shock of that landing is ideally absorbed by a) the floor; b) your shoes; c) your feet; d) your ankles; e) your knees; f) your hips; and g) your lower back. In other words, most of the compressive force you generate with the jump goes right back into your body. To avoid damaging yourself, you want to spread the force out as evenly as possible.
SURFACE AND SHOESI stupidly practiced high-impact aerobics in my early 20s on a concrete floor overlayed with industrial carpet. This type of floor absorbs no shock, and it's no surprise I had terrible knee trouble and quickly. I have witnessed these same injuries in many yoga students over the years. The surface you're practicing on is vital (as is the type of shoes you wear). Even if you have a less-hard floor, it's a good idea to use a plyo-friendly mat too. I use the black mat by Manduka and highly recommend it. It also has a lifetime warranty (of which I'm sure Insanity is a very good test). Your shoes will also lose shock absorbency over time, so make sure your crosstrainers still have good life. Runners are known to replace their shoes every six months.
LANDING and CENTER OF GRAVITYWhen you land on the ball of your feet first, you help distribute the shock more effectively throughout your feet and ankles as well as your legs -- there are more joints that can "give" -- if you strike heel first, the shock goes directly into your knees and to a greater degree. It's vital to land ball-first, and to do that you need to have your core engaged to keep your weight more forward. This is what Shaun means when he talks about keeping your core tight and your weight forward. When you let your belly go, you tend to lean backward a little to keep your balance, and you're more likely to land the same way, striking on the heel and sending shockwaves directly up. In the same vein, keep your knee over the ankle when squatting, as Dig Deeper indicates. Jutting the knees out over the toes puts stress on the knees.
KNEES AND ELBOWS UNLOCKEDShaun notes throughout to keep your knees soft. This allows some give in the joints when you land. You can probably see where I'm going with this... locking a joint prevents soft tissue from absorbing shock and leads to bone-on-bone pressure, and when you apply force to that, you have instant injury. Equally important is to protect your elbows by keeping them very slightly unlocked anytime you're in the plank position.
WOMEN AND HIPSMen have a very fixed, stable pelvis with femurs (upper thigh) extending straight down to the knee joint with no angle. This means that when men run, the downward force is evenly distributed. Women, however, have a wider pelvis and this means that there is an angle from our hip to knee joint. Right out of the starting gate, we're at a disadvantage with unequal pressure on the knees. Unfortunately, this just means we need to be extra cautious.
BODYWEIGHTWhen you walk, you put four times your bodyweight in pressure on your knees. When you run, this becomes tenfold. This is why the supposed ideal weight for runners is 100 pounds for the first 5 feet and 5 pounds for each additional inch. That's really light, but not as far as your knees are concerned. If you're overweight and you're jumping, you're placing a huge burden on knees that are already bearing tremendous force. So if you're only 20 pounds overweight, you're putting 200 EXTRA pounds of pressure on each knee during, say, log jumps. Now consider how many times you strike down over the course of a 40-minute workout.
This is where a little bit of what ails you can be the cure. Plyometrics can help strengthen your knees when done correctly and wisely. Strengthening the muscles and supporting structures in and around the knee joint will allow you to endure more compression over time, but be careful -- if you're completely out of shape, be aware that your joints probably have very little protection around them and injury becomes likely. Knee injuries take a long time to heal, and knees are unfortunately a weak point in human anatomy in the first place, even without other strikes against them.
JOINT HEALTH and FLEXIBILITY TRAININGFinally, with all the compression on your joints, it's vital to lengthen everything back out to bring everything back into alignment and allow healthy fluid exchange. Never skip the stretching, and ideally, stretch more. When you bend forward to stretch your hamstrings, make the back as long as possible (most people round the back to allow them to reach farther, but they put a lot of stress on their spinal discs). Just soften your knees to the point that you can make your back long. With straight legs, tight hamstrings will pull on your low back and force it to round. Also know that tightness in one area will be felt two joints away. If your hips are tight, they will pull on your knees. An imbalance in flexibility between the quads and hamstrings will pull your back out of alignment. This topic is complicated, but can be summed up this way: stretch. A lot. Do it while your muscles are warm, and never skip it or consider it wasted workout time.
SUMMARYIn summary, pay close attention to proper form at all times. Don't try to match the participants' speed and never take your mind off what you're doing, even for a second. All it takes is one tiny misstep. Pay attention to your choice of shoes and flooring, and seriously reconsider taking on Insanity if you're very out of shape. However, if you understand the mechanics of your joints before starting, you can head off a lot of problems. Even though I'm 10+ years older now than I was when I first abused my knees, I am now practicing Insanity pain-free. If you have any questions, please post and I'll do my best to answer. Good luck with this program and thanks for following me!