Performing single-limb exercises is important to help athletes achieve what I refer to as structural balance. Structural balance means that every muscle is in balance with other muscle groups. This definition extends not just to agonist/antagonist muscle pairs (such as the hamstrings and quadriceps) but also to pairs of limbs (such as the right leg and left leg). It’s not enough just to have the appropriate strength ratio between the hamstrings and quadriceps; to achieve structural balance, the strength of the quadriceps and hamstrings on the right leg should be equal to the strength of those of the left.
An excellent exercise to achieve and maintain structural balance is split squats. Properly performed split squats should make you sore not only in the glutes but also in the hamstrings, quadriceps, and adductors. I have seen many sprinters, jumpers and bobsledders add inches to their already well hypertrophied legs by supplementing their squatting programs with lunges or split squats. I also like to use them if the athlete’s lower back has not yet recovered from a squat or deadlift session.
Another advantage of unilateral split squats is that they stretch the hip flexors very well. This translates to greater stride length and faster times in short-term speed conditions.
Split squats are also superb at preventing lower back ailments. For example, for technical events in alpine skiing such as slalom, the sport itself takes a toll on the quadratus lumborum and the hip flexors. Within only one season of introducing split squats into the general preparatory training phase of elite technical skiers, lower back injuries went down dramatically.
Split squats provide plenty of leg training without overloading the spine. In my opinion, in lower body dominated sports like soccer and American football they are the best prevention against groin pulls. To read more about the value of unilateral training, read Tip 130: Strategic Programming - Use Unilateral Lifts After Large Muscle Group Lifts for Optimal Anabolic Response