Starting Position: Stand with your feet approximately hip width apart. Depress and retract your scapulae (pull your shoulders down and back) without arching your low back, and "brace" (engage your abdominal/core muscles) to stiffen your spine.
In preparation to step forward, slowly lift the right foot off the floor, stabilizing your body on the stance (supporting) leg. Avoid any sideways tilting or swaying in your upper body and try not to move the stance (supporting) foot. Hold this position momentarily before stepping forward. The raised (swing) leg should initiate contact with a heel strike first, slowly transferring your body weight into the leading (forward) foot placed firmly on the floor. As you load into this leg, avoid any sideways tilting or swaying in your upper body and try not to move the stance (supporting) foot.
As you lower yourself forward into the lunge position, maintain the abdominal bracing and focus more on dropping your hips towards the floor rather than driving your hips forward. This will help control the amount of forward movement of your shinbone (forward tibial translation) over your foot. Continue lowering your body to a comfortable position or until your right (front) thigh becomes parallel with the floor and your tibia (shinbone) is in a slight forward lean. While in the lunge position simultaneously bend forward at your hips, while maintaining a flat back and contracting (squeezing) the glutes of the left (backward) leg, this will help increase the stretch in the left hip flexor muscles.
Hold this stretch position for 15-30 seconds at a time for a total of 2-4 repetitions; try to move into the stretch a little deeper with each repetition, but be sure to keep your body in alignment. Complete all repetitions on one side before alternating to the other leg. Firmly push off with your front leg, activating both your quads and glutes (thighs and butt muscles) to return to your upright, starting position.