As homosapiens we are a bipedal species. Meaning we walk on two legs. When we walk, run, hop, and bound we are only on one leg at a time. The only activities where we are on both legs simultaneously is jumping and standing. Yet for some reason the vast majority of training programs are built around bilateral movements (having both feet planted and working together). Anything from Squats, Deadlifts, Kettlebell swings, Leg Press, Leg Extensions, Leg Curls, and many more are all staples in many programs. While these exercises all have their merits, it's important that these aren't done at the expense of unilateral (single leg) movements. Here's a quick rundown of advantages of both bilateral and unilateral movements:
Unilateral Bilateral
-Better balance improvements -More time efficient
-Improved durability of foot, ankle, knee, and hips* -Larger increases in muscle mass
-Easier to recover from -More durability in the spine*
-More localized fatigue -More full body fatigue
*Something being better at improving durability also has the potential to be more stressful to that region, especially if progressed or loaded incorrectly. Someone who has an injury to the foot, ankle, knee or hip will want to be more cautious with single leg movements. While someone who has an injury to the spine will want to be more cautious with bilateral loading (especially if weight is placed on the spine).
For general fitness and athletic performance I find that individuals with less than 2 years of training experience do well with 70-80% of their training being bilateral in nature. While more experienced or older clients (over the age of 60) reap more benefits from a split of 60-70% coming from unilateral movements.
Some of my favorite unilateral movements include single leg squats, lateral lunges, and single leg stiff leg deadlifts.
If making substantial changes to your activity levels, or introducing new movements to your training plan, it is always advisable that you do so under the supervision of a trained and experienced professional.