Unilateral training – training one limb at a time – is an excellent strategy for maintaining balanced strength levels from one side to the other. Sustaining relatively even strength levels will help to keep you injury free and develop a symmetrically proportioned physique.
Are You Imbalanced?
For the legs, split squat and lunge exercises are the most common choice. Machines offer another option: single-leg extensions and leg curls for example.
The problem with relying on single-leg curls for your unilateral hamstring work is that the hamstrings have two functions (knee flexion and hip extension) and the hamstring curl machine only trains one of these – knee flexion. So you're not fully developing hamstring strength and size unilaterally without a single-leg hip extension exercise.
The most popular hip extension exercise is the Romanian deadlift. Another is the barbell good morning. Neither of these lend themselves to being trained one leg at a time though because balance would be the limiting factor. Consequently, you'd have to use baby weights to stay upright.
That's why I love the single-leg 45-degree back extension. It trains your hamstrings, glutes, and lower back.
The machine also provides a very stable environment to train the hamstrings one leg at a time through hip extension.
The position of your body during this movement means that you can challenge the posterior chain through a greater portion of the entire range of motion than on either a horizontal back extension or RDL. As such, it should be considered a staple movement for developing a strong, muscular, injury-proof backside.