The Supine Heel Contraction is designed to train the hamstrings and calf muscles to work together and in a linear pattern
Moving the knee medially just about everyone will use the abductors, the muscles on the outside of the leg to move the knee inward. This means that the knees are unstable and muscle development from exercise and sports will create inefficient use patterns and increase the probability of injury.
In a supine position with one knee bent, move the knee inward against a little resistance. The vast majority of people will use the outside of the thigh to push rather than pulling with the muscles on the inside of the thigh.
After doing the exercise, when adducting the knee, moving the knee towards your midline. The movement is stronger, you will feel the muscles on the inner thigh being used.
The range of motion in the hip and knee normally improve and the knee becomes more stable.