The squat is known to be the most powerful generator of leg power, strength and size.  Many trainers often say that the squat is the most important leg exercise, but this strategy can often lead to muscular development problems as I myself found out the hard way. The first big issue lies with the fact that during a two legged squat the force of the weight compresses the body from the barbell to the shoulders down through to your feet to the ground. This has the negative effect of placing the majority of the bending force on the mid region. If like many people, you have a poor level of strength in the lower abdominal region you will not be able to use your core strength to offset this effect and much of the strain may end up being transferred to your lower back. A lot of lifters choose to use squat belts which does alleviate the problem but also compounds the absence of strength in the lower abdominal area. And so if unlucky you may do some harm to your lower back muscles or worse slipped discs. The second big issue is the disproportionate effort placed on the quadriceps muscles by comparison to the hamstring muscle. Even when many try to offset this problem by using other exercises that are more hamstring focused they find that they all involve a lot of lower back effort. It is impossible to do so many exercises that involve lower back work and this generally leads one exercise and muscle inevitably suffers. The result of this is the much increased chances of knee injuries like anterior cruciate ligament damage, strains or tears. The third issue is the level of friction on the knee joint during the movement. Because of the way the joints move during these exercises there is a large amount of friction that usually results in a shortage of sinovial fluid in the knee (dry knee joints) and if untreated this can permanently damage knee cartilage. So what is the solution to this problem? One legged squats focus on one leg reducing weight needed and therefore reducing compression which also uses more of the Hamstring as it has a much more balanced, extended natural range of movement. In regard to the knee problem it is better to use a reverse one leg lunge because the movement does not place much pressure on the knee. During the reverse the  knee does not go over the toes (maximal point of friction). But it is all in all easy to argue that the reverse lunge is a more effective functional squat and one to prioritise on your leg day! And for the most functional effective hamstring dominant squat Hammerstep is one to look at, check it out on www.inno-v8.com.