The Side Leg Raise works primarily into our hip abductor muscles - the muscles that help us to bring our leg away from our body, rotate the body at the hip joint and work to stabilise the hips when on one leg. These muscles include the Glutes and Tensor Fasciae Later (TFL) and are particularly important for runners as having weak hip abductor muscles often leads to the knees rolling inwards when running which can increase your risk of IT Band Syndrome - a common running injury.
To set yourself up attach an elasticated band to a stable object and wrap the other side around your left ankle. Your right foot should be in front of the banded foot and placed firmly on the floor. You can hold onto a stable object for support if you find this helpful. From here, lift your left leg out wide, away from your body, pausing at the furthest away point before bringing the leg back in, controlled.
Throughout the exercise you should have your shoulders pulled back and core engaged, which will help prevent any movement across the rest of your body.
If you are finding this difficult you can perform the same movement without the band.