If you have a bad back due to spinal complications, then don't lose hope, you can still build the mass of your legs. To do so in a safe manner, use leg exercise machines that have a slightly reclined bench. This position helps to support your back so you can perform the exercises in a comfortable manner. Consult with your physician, however, before doing any machine leg exercises to ensure you are healthy enough to start a resistance-training regimen.
A large bulk of your leg mass comes from the front thigh muscles known as the quadriceps. These muscles are the rectus femoris and vastus medialis, lateralis and intermedius. The seated machine leg extension is the best exercise to pinpoint these four muscles and work them almost exclusively. You must ensure the machine you are using is not upright, but rather slightly reclined. To perform the exercise, sit on the machine with your back against the reclined seat support, slide your feet under the foot pads and then extend your knees until your legs are straight. Hold this contraction for two seconds and then lower to the starting position. You can also do this exercise unilaterally, or one leg at a time, to better focus on each individual leg.
The largest muscle group of your legs -- not your quadriceps -- is known as the hamstrings. The semimembranosus, semitendinosus and biceps femoris -- short and long heads -- make up this muscle group. To build these muscles without compromising your bad back, do the seated machine leg curl. Once again, you need to use a machine that has a reclined seat. This is absolutely vital to avoid stressing your back. To execute the seated leg curl, sit on the machine, place your back against the reclined seat, lift your legs and put your calves over the leg pads. Your legs should be straight in this position. Now bend your legs as much as possible by engaging your hamstrings. Once your legs are past the 90-degree angle, hold the contraction for two seconds and straighten your legs to return to the starting position. Similar to the leg extension, you can do this exercise unilaterally to work one leg at a time.
Total Leg Training
To work the quadriceps and hamstrings with the same movement, along with the triceps surae of the lower legs, perform the seated machine leg press. Sit on the machine with your back against the reclined seat, and position your feet shoulder-width apart on the rectangular machine platform with your legs straight. Bend your legs to 90 degrees and then extend them until they are back to the straightened position. You can perform several variations of this exercise: close-foot stance to emphasize the vastus lateralis, wide-foot stance for the vastus medialis, high-foot stance for the hamstrings and low-foot stance to emphasize all four quadriceps muscles. You can also do any of these variations unilaterally to focus on a single side at a time.
Jog on a treadmill to start and end your leg workouts for safety reasons. Each of these should last between five and 10 minutes. The warmup should be done at moderate intensity, and its purpose is to increase blood flow to your legs. The cool-down should be more of a low-intensity jog or slow walk to lower your body temperature closer to a resting range.