The primary function of the ACL is to prevent the anterior displacement of the tibia. The ACL, along with the resistance of the posterior muscles the cross the knee joint (aka the hamstrings), prevents the normal knee from hyperextending.
Below is a breif video that will explain the ACL injury in more detail:
Below is a BOSU Ball. You can find these at your local gym or at places such as FITNESS TOWN. It is a great way to help strengthen the muscles surrounding the knee for stabilization.
The Bosu ball is half of an exercise ball. It has a flat plastic peice on the bottom. It is good for performing balancing exercises as well as squats and lunges. This will work your intrinsic muscles the ones that are underlying your major muscles. When strengthening the intrinsic and stablizers you will help strengthen the quadriceps to help keep the knee in place.
If you are awaiting surgery I would suggest to do at least 3 sets of each exercise for a comfortable timed set. Or as many reps as you feel comfortable doing. The idea is not to injury yourself more but to strengthen what you have.
Start of by trying to balancing on one leg is not too difficult and if this isnt difficult enough try throwing a tennis ball against the wall while doing it. You can also flip the Bosu on the ball side and stand on the flat surface to add more difficulty. Try doing it for 30s then gradually increasing your time.
Squats are also challenging. you can use hand weights or a bar. Start with small weights and move on to larger ones ones you get comfortable on the ball.
Or to change things up if you get bored you can do a Bosu Side Squat. As seen in the video below.
To make squats even more difficult, flip the Bosu with the ball side down, stand on the flat part. Form is the most imporatant thing so focus on that first without any weights. Then move on to dumbells or the bar.
Lunges are great because you can really focus on the single leg that is injured. This is important when only one ACL is injured. To lunge step forward onto the ball part and lunge down and make sure your knee does not go over you toe. You don't want the knee to touch the ground because this could injure the knee. My tip of advice also for this exercise is to not actually lunge forward but focus more in dropping your hips straight down towards the ground. Once this is mastered you can then add dumbells or a bar.
To add more difficulty you can also do backwards lunges onto the bosu and to increase the difficulty if you feel stable enough flip the ball over. Once this is mastered you can then add weight such as dumbells or a bar.
Next on the list you can add in abductors. Now remember you are also working the leg that is stabilized on the ball rather than the abductors. Below is a video for it.
It is also important to strengthen the antagonist muscles as well as the agonist muscles. This will keep your muscles from being improperly balanced. So make sure to work the hamstrings as well as the quadriceps.
A simple exercise on the BOSU is called the hamstring lift, and can be seen below. Don't worry about the triceps part unless you want to add in some triceps work. haha
OPTION 2 (Much more difficult)