Practically every way of moving and transferring force is done from a split stance position pushing off one foot or leg from the floor. And, the same goes with walking, running, and sprinting. Every step requires one leg to move and transfer force to the other. If one leg is weaker than the other then it cannot absorb nor sustain the force being transferred resulting in a slower movement. Let it be known, the faster you move the stronger, more control, and stable each leg needs to be to optimize speed and power, and, transfer it effectively.
Single-Leg Training Stimulates More Muscles
Single-leg training stimulates more muscles that promote greater muscle strength and stability. If you strength-train one leg, you will notice how it boosts and enhances the strength of the two; improving bilateral strength. For example, you engage a greater amount of fast-twitch fibers and stabilizers in a single-leg squat more than in a bilateral squat. It dives more into proprioception and neural training. Squatting with one leg requires a greater amount of control and stability than with two that activates and strengthens stabilizing muscles and muscles more needed to stabilize and balance your body and movements.
Single leg exercises, as well as single-arm upper body exercises, are also major core stabilizing enhancers. Doing a single leg squat or a single-arm back row or chest press, will dramatically boost your core stability and strengthen anti-rotating muscles too!
Single-Leg Needs to be Stable to Optimize Speed and Power
When you have stability, you have control. And, stability is the key to unlock speed, explosiveness, and power. You see, without balance and stability, the CNS goes into compensation mode. That means that your joints will move into a compensated position and your muscles will remain tight to prevent pain and strain because they cannot handle the force of the movement. The muscles tighten because they are trying to stabilize the motion, slow down the movement, and inhibit the range of motion to protect you from injury. Also, it will slow certain reflexes. When you have stability, it is like pushing off a platform to move. You feel lighter, faster, and more mobile.
For example, if you cannot stand with your knee off the floor or repeatedly lift your knee up and down from the floor without wobbling, there are stability issues. And, when you start to wobble the speed slows down and muscles tighten to stabilize and balance you first. Once there is balance, speed will occur and you can optimize it. If you continue to force and compound speed on top of a compensated unstable strength foundation, you will not attain speed.
Single-leg training balances strength so you can kick, jump, punch, change direction, and transfer force equally on both sides of your body. The more symmetrical strength becomes, the fewer compensations and imbalances you will have. Therefore, if you want to get stronger, be faster, and more explosive, as well as minimize your risk and chances of injury, then do single-leg training.