The Problem With Imbalances
If you have weak glutes (and on a top of it the non-dominant is much worse than the dominant one), the other joints such as knees and ankles will suffer from the extra forces during the movement. They're not built for that and can easily suffer with injuries.
More: Why a Tight Psoas Will Hurt Your Tennis Game
Often, knee, ankle and feet issues come from the hips. Even the shoulder and elbow problems often originate in the hips. Here you see the tremendous importance of having strong glutes. And it looks great too.
Your Pre-Season Training Plan
Focus on squats, lunges, step-ups, deadlifts, kick backs, glute bridges, and similar.
Your core—including the front, back, sides, and "insides"—has to be functionally strong to be able to transfer all the forces between your lower and upper body during the stroke production and to support your entire body in a good athletic position during the movement. Focus on different planks in front and side planks, stabilizing movements such windmills, squats and deadlifts and back extensions.
The stretching and myofascial release part of your fitness training should focus on all the muscles that feel tight. You'll know that they are tight because you won't want to do the stretches. It will feel painful and uncomfortable and you'll want to get out of the stretching position. This is a sign that you need to stretch the muscle group. Hold the stretches for at least one minute while breathing deeply and trying to relax.
Focus on Your Weakest Point
The other muscles that feel pretty good don't need as much attention. Always focus on your weakest points and make them better. Remember that your entire kinetic chain is as strong as your weakest link.
When you stretch and perform myofascial release regularly, you will eventually enjoy it a lot. It can feel almost as good as a pleasant massage, only you have to be more proactive.
Feeling strong, lean, flexible and fit will give you a new confidence on the tennis court and in life. Your tennis game will improve tremendously and the best thing is that you will avoid all the pesky pains and injuries.
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