Search the stretch shortening cycle
(SSC). Running and also walking uses the stretch shortening cycle a lot.
Some illustrations to get the idea, some not too related-
The advice to 'stay on your toes' from psv255 and _craze is for calves and quads. To stay on your toes causes the calves to use more stretch shortening than they otherwise might. Quads and hamstrings also use stretched shortening in tennis. To 'split-step' as your opponent hits the ball uses your weight with stretch shortening to stretch your calves, quads and probably hamstrings also.
If you 'have no spring' in your step it sounds as if your calves might be weak and too long. If stretch shortening is not used enough the bones and ligaments then have to stop the motion nearer the end of the range of motion, rough on ankles and knees.
. Do calf exercises.
Strengthen your leg muscles, quads and hamstrings with squats and other exercises. Generally, if you strengthen muscles they tend to be a little tighter and shorter which may be good for you.
As you strengthen the calves stretch the calves so that they do not get too short or tight and get stressed or injured. Stretch both calf muscles, the soleus and gastrocnemius. Soleus requires a stretch with a bent knee. Stretch quads and hamstrings, etc. Information on stretches in earlier threads.
Of course, all muscles use SSC. Another area for exercise involves weak hip muscles such as the gluteus medius
. This muscle holds the pelvis level when one foot is off the ground. It is important for movement and balance but neglected even with lots of tennis. My gluteus medius was weak despite continuously playing tennis. For details search: Chas Tennis gluteus medius
and the internet.
In general, posture issues can cause much heavier loads on joints especially, the knees and ankles. Posture results from the length and strength of all your muscles. Think in terms of complete body, head-to-toe, and not separately the leg or hip down, etc.
Look at your posture in the mirror. Also from the side. Search: posture evaluation
but this is a tricky business so take care with your conclusions.
Take a video of your tennis movement and see what you can pick up. I almost always find clear obvious flaws in my tennis videos.