I have no medical training. And don't have knowledge of back injuries and the tennis serve.
Your back has some extra bend to it in your normal posture. Have your posture evaluated by a Dr.
See Thoracic Spine. You have a lot of Thoracic Extension bending. Search also
latissimus dorsi muscle insertion and origins pictures.
I studied Thoracic Extension during the serve and it might have an unusual function. When Thoracic Extension normally occurs,
as the legs thrust, it shortens the distance between the lat's bone attachments. This reduces the length of the lat and makes stretching it easier. Also, when you straighten your spine with Thoracic Flexion, it lengthens and stretches your lat muscle. How the shoulder joint moves while you are straightening your spine will also affect the length of the lat. In other words. straightening the spine can stretch the lat or directly move the shoulder joint, or both. The last video below shows this sub-motion of the serve with texts.
It may be that your spine is too flexible, bends too much, and then when you straighten your spine something is getting stressed. ? Your normal posture may have shortened the muscles in your back and they get stressed when you straighten your back. ? Search
anterior pelvic tilt pictures You really need a specialist to evaluate your posture and physical therapy to correct posture problems.
Do you have an excessive inward low-back curve? If so, you might be struggling with chronic anterior pelvic tilt. Can you, right now, while you’re reading this, squeeze your butt-cheeks and create a visible contraction without the use of resistance? Can you squeeze each glute independently? If so, y
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Exceptionally flexible back, Thoracic Spine Extension.
Do you have or once had the belief that you want to bend forward very fast for serve speed? I believe that the speed might be secondary to the distance between the lat's attachments. I have not found publications of this view.
Thread with details. Scan thread and look at videos.
WARNING - How your spine is used during the serve can risk injury. Many backs are not capable of much Thoracic Extension. ? http://www.racquetfit.com/articles/Screening/if_you_can%27t_extend_from_your_thoracic_spine_you%27re_squandering_power_and_inviting_injury The extension of the...
tt.tennis-warehouse.com
WARNING - How your spine is used during the serve can risk injury. Many backs are not capable of much Thoracic Extension. ?
RacquetFit instructor Jeff Salzenstein discusses the importance of spine mobility for durability in tennis and how you can assess your thoracic extension using the RacquetFit screen.
www.racquetfit.com
The extension of the thoracic spine is brief and when it occurs can be seen in videos of high level servers.
Thoracic Extension is mid-back, different than the lower back.
There have been posts on Thoracic Extension. TE is probably used in a similar way for the baseball pitch.
Since you have a video you can compare your Thoracic Extension followed by the Thoracic Flexion that straightens the Thoracic Spine.
Compare videos in this post one above the other and single frame. To single frame on Youtube stop and use the
period & comma keys. Always select the video using the
alt key plus the left mouse click, otherwise the video starts playing. You can go full screen and come back down and the video stays on the same frame. Compare you thoracic back bend to the pro's and to other pros in the compilation. For most accuracy select similar videos with similar camera angles.
This last video has 4 sec pauses and text.
Taking a video from the side view would show your back near the end of its motion.
If you have a model server post their video below yours and use the above directions to compare.