Knees, like shoulders, are very complex joints. The two parts of the limb, the upper and lower, are linked in the middle by a common joint, the knee. Although that enables the knee to be stabilized by the tendons of both the thigh muscles and the lower-leg muscles, the double protection also increases the potential for problems. Add to that the ligament and cartilage structures, and you can begin to appreciate the many variables inherent in movements that involve the knees. What's more, quite often an injury that emanates from an exercise that strongly involves knee action causes pain elsewhere.
There's a long muscle that runs the length of the lower leg along the lateral, or outside, of the tibia. It's called the peroneus longus, and it comes together at the knee with the other muscles of the front of the lower leg. Their job is to turn out the foot, stabilize the ankle and to some extent stabilize the knee. Bouncing at the bottom of full squats very often strains the peroneus longus. When that occurs, the muscle draws in fluid and swells along its entire length. It's one of the most persistently painful conditions I've seen in my many years of training. In addition, it takes a very long time to recover from the problem, often six to eight months. Most of the people who experience it aren't aware of the cause until the injury is well advanced. So, if you don't use proper form on exercises that involve the knees, it can cause injury anywhere in your lower body.
Along with the idea of using proper form goes the principle of starting with a proper warmup. More than half of your muscle mass lies between your waist and your knees. For that and other reasons I prefer a longer warmup for my legs than what l do for my upper body. I ride a Lifecycle for 12 minutes at level 9. Not only does that provide a good warmup, but the resistance is such that I also get an optimal aerobic workout, 12 minutes being an ideal time for high-level aerobics. If you don't get off the bike sweating profusely and with a thigh pump, you need to work harder.
The only other warmup I do for lower body is to march in place, holding onto something stable while I lift my knees one at a time as high as possible. You must forcefully lift your knees all the way up to your chest, if possible. That way you completely flex not only your knees but your abdominal area as well. Do 25 slow, forceful reps with each leg so you feel it through the entire movement.
I only do two stretch movements for the legs. The first is simply to bend forward with your knees locked or, if that's uncomfortable, slightly bent. Grab your ankles and pull your head to your knees for a count of 25. That will effectively stretch your hamstrings as well as the muscles of your lower back. You do the second stretch in the deep lunge position, with your front knee well in front of your ankle and your upper body erect, not leaning forward. The rear foot cannot remain flat on the floor, so you should be balanced on the ball of your rear foot. This position stretches the quadriceps and the calf muscles as well as the Achilles tendon attachment. I perform the marching in place with the two stretches as a circuit, doing three sets of 25 for each movement or position.
That said, it's time for some advice on proper exercise form for leg work as well as tips on recognizing potential problems and preventing injuries.
The single best thigh exercise, without question, is the squat. It may, in fact, be the finest exercise in all of weight training. No other movement has the ability to build incredible strength and endurance not just in the obvious area, the thighs, but in the entire body. No other exercise has the ability to change your metabolism so positively that all bodyparts gain. No other exercise can take a natural Middleweight and make him a powerful natural Heavyweight.
The only way to negate all the good things that squats can do for you is to do them incorrectly. First of all, don't bounce at the bottom. As with any other pressing exercise, you must perform squats under control throughout the rep. I like to stop completely for a count at the bottom. Second, don't turn your feet out too much, as it will create torque, or twisting, when you come up with a heavy weight. Third, don't stop your squat descent at parallel, as it requires the quadriceps' attachments to make a static contraction. The tendons are asked to first check and stop the weight's descent and then act as the fulcrum when the quadriceps start taking the weight back up. That's incorrect! I don't give a damn what any football coach or doctor, ignorant of kinesiology, has told you. Let the weight continue down till your thighs are below parallel, at which point the fulcrum moves back up the thigh and into the belly of the muscle, where it belongs.
Finally, forget the knee wraps. Knee wraps simply usurp the work of the tendons and ligaments as you go to below parallel. Use them long enough, and you'll find you're incapable of squatting anything like the same poundage without the wraps. Do you wrap your elbows when you do presses? Do you wrap your ankles when you do calf work? Wraps are nothing more than a crutch.
Now that I've covered how not to squat, let's talk about the right way to do it. Everyone's structure is different, and the variables require you to make adjustments in how you position yourself to ensure the most effective application of the workload in the exact area of the muscle you wish to work. For example, you may be long-waisted, your thighs may be disproportionately longer than your lower legs, or your ankles may be stiff. Some squatters can't remain erect as they go below parallel. They pitch forward and their lower backs become more directly involved than their thighs. If that's your problem, try a wider stance. Try to squat between your legs, rather than over them. If you still lean forward considerably in the low position, put a block under your heels. Try a one-inch board. If you're still unable to keep your back out of it, go to a two-by-four. Lifting your heels like that tilts your pelvis forward, so you must squat erect to counteract the forward tilt.
I don't like leg extensions, certainly not as a first movement in a quad workout, and I don't like them for precisely the same reason I don't like French presses for the triceps. They create a disadvantaged lever, placing a great portion of the resistance on the attachments, rather than the belly of the muscle. If you feel you must do leg extensions for shape or to rehabilitate a loose knee, put them last in your quad routine, and don't do many.
Leg presses, hacks and sissy squats all pose a common danger. Because of the position of your body, with your torso well back from your thighs and your knees taking an excessive amount of the workload, your knee joints open quite wide at the bottom, fully extended position. That allows the cartilage to be pinched as your quadriceps shorten and your knee joints close. The problem occurs most often with sissies and hack squats and front squats done on a block.
Leg presses also open the knee joints quite wide at the bottom, but they offer excellent options for working the thigh muscles. Again, however, you must know how to use a leg press properly. You can use the 45 degree angled leg press to work three of the four areas of the thigh, the extensors (anterior), the flexors (posterior) and the adductors (medial). Which muscles you work is determined by your foot placement on the platform and the closeness of your thighs to your chest at the bottom of the movement. Not only can leg presses be excellent for the adductors, but when you stop at the bottom with your legs in a specific position and your toes turned out, they become a great first movement for the leg biceps, or hamstrings, far better at promoting growth when combined with leg curls than are stiff-legged deadlifts.
The strength and development of the abdominal and spinal erectors are of great importance to the leg work you're doing, especially squats. When your abs and your lower back are strong, you can maintain a solid position in all of your exercises, no matter how heavy the weight, and maintaining a good position is of primary importance in avoiding all lower-body injuries. Consequently, it's a good idea to start your leg workout with very steep situps. I like to use five sets of 20, 18, 16, 14 and 12. Then at the end of your leg workout do six sets of five reps of regular deadlifts. Note that you should be stronger on the deadlift than you are on the squat.
Keep in mind that if you do a lot of hard, heavy squats, your thyroid will respond to the heavy heart action and slow your metabolism, and you'll grow faster. Also, if you're presently recovering from an injury, don't let that stop you from training the areas that aren't injured. When your body is stressed because of hard exercise, your adrenals produce cortisone, a much better-quality cortisone than you can get from your doctor. The cortisone supplies your entire system, including the injured areas. It also greatly reduces recovery time. For example, if you've injured a shoulder or an elbow, don't let that keep you from working legs or at least riding an exercise bike hard for 15 minutes. It really helps.