First off, Split Squats are not lunges. This is a stationary movement that's staggered stance in the sagittal plane of motion. The Split Squat will require proper hip, knee and toe (particularly the big toe) mobility in order to execute properly.
If you feel any pain or limitations within the movement such as knee pain in the back leg or limited mobility in the big toe that causes you to compensate by rotating the heel outward, then you will need to address this before you begin with the exercise. In some cases this can be quickly accomplished by foam rolling your Quadriceps and massaging the arch of your foot, then moving into active stretching for each area. In this example, you would perform a Side Lying Active Quadricep and Kneeling to Tall Kneeling Stretch.
Begin with 3 sets of 10-20 repetitions for each leg before increasing the intensity of the exercise with weight or progressions.
The SLRD is a challenging exercise that promotes strength, mobility and dynamic stabilization throughout your core, foot & ankle complex, hamstring & lumbo-pelvic hip complexs. Take your time advancing and your starting goal should be 10 - 15 repetitions that show no compensations. Once you developed sufficient strength and stabilization, progress to barefoot or in a minimalist shoes and begin adding resistance (dumbbells, kettlebells, etc.).
These are great for developing stability, balance and strength of the foot, ankle, knee and hip complex. This will lead to initial enhancements of your ability to accelerate, decelerate and stabilize movement. Develop a fitness base for the basic reach 1st and show no compensations before progressing. Build to 20 repetitions of a single leg reach to 1 cone 2 feet in front. Watch for the knee bowing out or collapsing inward.
How to: From a standing position, place both hands against a wall at shoulder height with feet in a staggered stance position hip width apart and straight. Slowly drop your hips a level, allowing the back legs knee to travel forward as far as you can without the heel coming off the ground. Focus on the stretch coming from the back of the lower leg furthest from the wall. This can be done actively for reps of 3 seconds or held statically for 30+ seconds.
Compensations: Watch for the back heel coming off the ground and the foot collapsing or externally rotating (toes shifting outward).
How to: Lay flat on your back, placing a 55cm ball between your lower legs. Squeeze the ball using your adductors (inner thighs) and slowly raise and lower the ball from the floor, drawing your knees to your chest.
Compensations: Watch for arching of the lower back, pushing the abdominals outward and rounding or elevating of the shoulders.
How to: From a kneeling position, place 2 arms with palms facing one another and thumbs up, shoulder width apart on a stability ball (55, 65, 75cm ball depending on size). Keep arms straight and locked out at the elbow. Slowly begin to round your spine (similar to a cat stretching). Gently drop your head, keeping the ball stationary or minimally rolling it away. Be sure to have a deep full breathe at the top. This can be done actively for reps of 3 seconds or held statically for 30+ seconds.
Compensations: Watch for bending of elbows and rolling the ball towards you.
How to: From a straight arm plank position with a neutral spine and abdomen tight. Slowly draw 1 knee towards your arm. Bring the knee in as far as you can go without rounding the lower back or spine.
Compensations: Watch for your hips dropping and or rotating, they must stay neutral and show no movement. Watch for elevating of shoulders, rounding of spine and dropping of head.