How many legs do you run on at a time? Just one.

This is more balance stuff, from Mike Robertson. This is copied from his article Choosing functional exercises.

This is what really convinced me about single leg training - the idea that most of the time when we are walking or running we are actually balanced on one leg.

From a functional anatomical standpoint it is absolutely critical to be on one leg. Why? Think about this for a moment. How many legs do you run on at a time? Just one. Have you ever had an athlete pull their hamstrings (bilaterally)? The muscles that support the lower leg in single leg stance (quadratus, glute medius, and adductors) are not nearly as active in double leg exercise. The final step up the continuum is to perform a one-leg squat while standing on an unstable surface. Now the athlete must engage the prime movers, stabilizers and neutralizers while dealing with the additional proprioceptive input provided by the instability of the pad.

The interesting thing about functional exercise is that it seems to make some old
school coaches nervous. I don’t know why science or progress makes people nervous.
Opponents of the concept of functional training are consistently trotting out poorly done studies that attempt to show functional training as a fad. Recently I was told that functional exercise is fine for rehab and will help restore proprioception but, that it doesn’t work with healthy athletes. I will tell you that my experience does not bear this out. In six years of professional or Olympic-level soccer (4 women’s, 2 men’s seasons) we had no ACL tears. This is obviously anecdotal but, powerful nonetheless. The concepts described in the functional continuum can be applied to any region of the body. To use an overused cliché, “Think outside the box”. Don’t do what you have always done. Don’t do what everyone else does. Don’t copy powerlifters or weightlifters, they are training for their sport, not yours. Many of the concepts of powerlifting or Olympic-style lifting can be applied to a sound strength program but, remember that sport is different because we so infrequently have two feet in contact with the ground. You don’t have to take exercises like squats or deadlifts out of your program but, compliment them with assistance exercises that are higher on the functional continuum. There is nothing wrong with good old two-legged strength training.

Some suggestions:
Instead of leg extensions try split squats or another single leg squat variation. Why? Split squats incorporate balance, flexibility and single leg strength.

Instead of leg curls try a single leg straight leg deadlift.
Why? The hamstring is more of a hip extensor than a knee flexor. In fact The hamstring is actually a resistor of leg extension in sprinting.