The old song, Dem Bones, got it right when it says, "Ankle bone connected to the leg bone, Leg bone connected to the knee bone." Our joints exist in relationship to one another, so what happens at one effects the other. This is significant for hikers who experience knee pain. While weakness at the hip may contribute to knee stress, limited ankle range of motion can further tax the knee.
Tight ankles limit the bend in the knee as you step forward on an incline.
When you shift your weight forward and step, your quadriceps don't have much range to extend and can't generate as much power. Furthermore, your knee reaches full extension sooner than it should and you even risk hyperextending your knee as you step forward.
When you ankle is flexible, your knee has more freedom of movement and can bend further toward your toes as you prepare to shift your weight and step up. The quadriceps muscles generate maximum force at about 50-60 degrees of knee flexion. Therefore, your climb will be more powerful with more flexion in the knee.
As you shift your weight and step forward, your knee remains slightly flexed, thus generating power fully throughout the step to propel you forward and up along an incline.
How to improve ankle range of motion
To increase ankle flexibility, use a foam roller on your calf muscles while seated on the floor. Then, using a strap or belt wrapped around your toes, extend your leg and pull your foot and toes toward your nose. Hold for 30 seconds and repeat four or five times. Repeat the same maneuver for another four to five reps, however, this time bend your knee. When you feel comfortable in sitting, try standing on the edge of a step and letting your heel hang down with both the knee flexed and straight.