The year was 1922 and the magazine was known simply as Strength Magazine. In one of the issues, something surprising was written: "Squatting on one leg will do more for you than squatting on the two legs together". It was written about again in the 60's and later scientifically discussed in the 80's and 90's. The term used to describe this is the bilateral deficit.
The bilateral deficit is when the summed unilateral work (adding the weight used on the right leg with the weight used on the left leg) is greater than the bilateral work (example is performing a back squat with both legs on the ground). So, lets say you can squat 200 pounds. After a little bit of training, you should be able to squat more than 100 pounds on one leg. For the sake of simplicity, lets say you can do 105 pounds on the right leg and 105 on the left. When you add those two numbers together, they equal 210 pounds. Not sure if I need to write this but 210 is greater than 200. So, because of the bilateral deficit, you are able to lift more weight per leg. When both legs are on the ground, the force given out by each leg is roughly 80% of what it is capable if you were in a unilateral stance. That is one benefit of training on one leg. The next benefit involves your health.
Respected strength coach Mike Boyle ran into a few problems. No matter how hard he worked on improving squat technique, some of his athletes complained of back pain. This got to the point that almost 1 out of 5 of his athletes were not feeling 100% after a squat session. After serious thought, he asked himself “What if the way we had always done it was wrong?” The first thing he did was eliminate the back squat from his training programs and perform front squats only. This helped a little but the issues were still there. He then removed the front squat from training. The results so far have been very positive.
As a sport coach, the best way to win games involves having your best players on the field. When those athletes are hurt, other players are asked to do things that they aren't accustomed to. So not only are you missing your best player(s), but now you have athletes out of their natural position. If these injuries occur in the weightroom, you will not be employed very long as the teams strength coach. It is also your job as the strength coach to reduce the incidence rate of injuries during competition. Everything you do in sport is on one leg (unless you are on the crew team). Running is basically a single leg plyometric exercise as you explode off of one leg to stride. Jumping to catch a pass or grab a rebound is often done on one leg. With that being said, it would make sense to train the legs unilaterally. Improvements in strength and stability on one leg should carry over to less injury and improvements in performance.
I am slowly beginning to make the transition into this type of training with the athletes I work with. For some younger kids, performing some bilateral squats like the Goblet Squat are used as an incredible teaching tool. Same could be said for the Hex Bar Deadlift. But, once they show proficiency and strength in some bilateral exercises, I will then progress it to more single leg work. I do not see much of a benefit by loading a teenagers spine with 300-400+ pounds while performing a squat. The desire to increase weight before mastering form is another common issue with younger athletes...typically male athletes. By removing or limiting some of these aforementioned exercises, your team should be much healthier. After all, we want to have more athletes on the field as the other team. We do not want to have the best power lifters out there. It is my desire to produce great athletes who are good weightlifters and not the other way around!