Recreational athletes rarely pay attention to ankle flexion, the degree to which their ankles can be bent or flexed, moving the knee downward. The scientific term for it is "dorsiflexion."
The movement is used constantly in nearly every sport, especially when running.
Here is a huge athletic secret: If you increase your degree of possible ankle flexion, you will also increase your speed, whether it's on a court or a field or even riding a bike. There are even specific exercises designed to increase the bend of your ankle.
Many years ago, I was involved in a little-known sport called speed skiing. This involved the racers boot-packing an avalanche chute or an extremely steep mountain trail where the speed skiing race was to be held. The boot-packing by the racers made the race course snow absolutely flat, with no bumps or depressions. As the racers climbed the steep hill, they wore skis that were between seven and eight feet long. Today's skis average a little more than five feet.
The secret to success in this sport was making yourself "small to the wind," with as tight a tuck as possible. The fastest I had ever gone was 92 mph, which is really slow in speed skiing. The reigning world champion told me that to improve my speed, I needed to flex my ankles more. He suggested that I get into the tuck position and have someone put a heavy weight plate on the middle of my back, holding the weighted tuck as long as possible.
I did that exercise with a 100-pound weight plate on my back, for over a month. When my dorsiflexion was then measured, it had increased by nearly two inches, thus putting my knees several inches closer to the snow. In the next race, I achieved my goal of hitting 100 mph, going 100.4 miles an hour. (That's still not fast; the current world record is 158.4.)
To start increasing your own dorsiflexion, start by bending one knee toward the floor as far as possible, without lifting your heel. If you do a lot of sitting, your calf muscle and tendons may need to be stretched out. You may actually gain more ankle flexion from just stretching the muscles and tendons of the lower leg. There are two muscles in the calf, the gastrocnemius and the soleus, each with a tendon at the top and another tendon at the bottom. That means stretching four tendons must be stretched out to make them each more flexible.
Another exercise: Sit on the floor with your unbent legs stretched out in front of you. Put a towel under the ball of one foot, grabbing hold of each end of the towel, and gently pull until you can feel the stretch in your calf. Flex your toes back, toward the body.
You can also use a jump rope. As you come down from each jump, try to land on your mid-foot, rather than the ball of foot. So come down just behind the ball of the foot, furling the toes upward. As you do these exercises and concentrate on adding increased flexion to the ankle joint, you'll gradually see an improvement in your sports or activity performance.