Please explain (biomechanically) why shoulder-in is so beneficial

[Naiad]

I have read many many times about how great shoulder-in is as a movement for development of the horse. I have also seen lots of photos and I understand the motion and steps. And I have read that this movement is really good for the inside hind leg development and getting it to step under (have I understood this correctly?).

But honestly, I still don't get what is going on in this movement that makes it so great.

Can someone take me (a relative newbie at dressage) back to the basics and explain in as much understandable detail as necessary, why the shoulder-in is such a great movement? What is going on biomechanically in this movement, and what does the flexion up front do that is so great for the hind leg development? Please explain the various forces and motion that are going on. Thank you in advance! 

[issywizz]

Read 59 times and nobody has bothered to answer it 

Actually I think JennyB should answer as she is into the more geeky side of biomechanics 

The main thing about SI is that it loads the inside hind which very simply means you are collecting your horse one side at a time which is easier than asking for both hinds to collect at once ( ie piaffe)
This happens because the inside hind comes closer to the CG in the weight bearing phase which is why CORRECT ALIGNMENT is essential 

[Naiad]

Read 59 times and nobody has bothered to answer it  

Actually I think JennyB should answer as she is into the more geeky side of biomechanics  

The main thing about SI is that it loads the inside hind which very simply means you are collecting your horse one side at a time which is easier than asking for both hinds to collect at once ( ie piaffe)

Yay! A reply!     I admit that I chuckled (in a friendly kind of way) at the 10 pages of debate about 3 or 4 track for shoulder-in which shows a high-level knowledge, and yet my simple elementary-level question about shoulder-in remained a most lonely thread indeed!   No worries, but it did make me smile!  (IW - perhaps the 59 people who read it were the other elementary-level dressage people eagerly checking out the question and waiting for the responses to come pouring in - or maybe people just needed more time to reply!)

I had not clued in about loading the one leg vs. two legs. That is really helpful to read. So is loading a hind leg just a necessary step to attain collection - or once you load the leg, do you to some extent actually have collection? I am guessing the former? Naive question, I am sure, but there it is...

[Naiad]

p.s. If you aligned correctly, by default is the inside hind leg loaded, or could you still be front-heavy despite correct alignment? I suppose that this is a very biomechanics-oriented question.

[issywizz]

Honestly I think this sort of question is very difficult to answer hence the lack of answers,I dont feel that I have answered it well either as there is a lot more to be said if anybody geeky enough comes along  
Once the hind leg is loaded and thus closer to the CG then it is already a degree of collection.
Ref the flexion at the front end thats to do with the fact that you cant have correct bend without flexion-you can have flexion without bend but not the other way round-so its part of the alignment that causes the inside hind to come under the CG.

Your last question requires more thought,could be a good subject for debating actually 

[issywizz]

I think the answer is that if everything is correctly aligned then the inside hind is loaded by default

[Naiad]

Thanks for attempting to answer my perhaps tricky questions. I just really want to try and understand in depth *why* I need to work on the shoulder-in, and not just do shoulder-ins because every classical dressage book tells me that I should - do you know what I mean. I am a researcher by profession, so I suppose that I am always trying to get at explanations for everything. 

[Trudi]

  I read the title and didn't open the thread (chicken!!)
I cannot retain stuff like this but when I want check stuff out this place is helpful

http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/BioMech/BioMechBook.html

[shoveltrash]

well i've only just seen this topic!

at the risk of being repetitive (IW's answer was spot-on imho)
the shoulder-in demands the horse to engage his hip joint, hock, and fetlock on his inside hind to remain balanced, which shifts his weight to his quarters and relieves his shoulders and forehand of the load
benefits:
- deepens the hindquarters and elevates the forehand - because the horse has to engage his hindquarters to perform the movement, the weight is lifted off his forehand, which supples both hindlegs and haunches
 
-strengthens the horse's back - the shift of weight to the hindquarters strengthens the back and loins, giving the horse the physical capacity to achieve higher collection and perform more complex moves. the horse will begin to develop a rounded appearance as he becomes stronger and more flexible

next question should be, "list the aids for shoulder-in"
so, to stimulate more discussion ........
your weight should be on your inside seat bone and your body should very slightly be turned to the inside (be careful not to collapse at the waist)

inside hand: softly ask for a flexion to the inside
outside hand: support the horse's shoulder and indicate the direction with the outside rein.

i'm leaving the leg position aiding for someone else

[issywizz]

Oh good something to argue about 

Heather teaches not to think about the weight aids but simply to allow the pelvis to shift as the inside leg gives the aid with the swing of the belly on the girth and have the shoulders turned in.
I think there is some vid of her discussing it on the vid board and I have to say it has helped me to become MUCH straighter in the SI 
She also teaches to have the outside hand on the neck with the fingers gently pressing into the side of the withers ( NOT across the neck though) which I also like as it stabilises the rein aid nicely.
Also have the hands stacked so the inside hand is just above the outside which again stabilises the hands and prevents pulling back or opening too much on the inside rein. ( which given that it should only be being used to ask for flexion would be totally incorrect and unnecessary)
Outside leg passively recieves the Qs behind the girth as the belly swings across.

Im currently pondering what happens collection wise if the horse does SI in a more stretched position,ie; fdo but only 1/2 way down? I am currently experimenting with the idea of 'letting the exercises do the work' so to enter the SI with the horse stretched as described above ( NOT BTV!,although that could also be interesting with a horse like Nico who puts himself BTV?  please experiment Trish and report back  ) and see whether the exercise alone will raise the forehand.

[issywizz]

  MTA interesting link Trudi,thanks 

[shoveltrash]

i will experiment
and hopefully report back with video
actually, my boy likes to put himself more *fdo* than btv i think - i'll know more after my video session today .  i really like the idea of "letting the exercise do the work"

Heather teaches not to think about the weight aids but simply to allow the pelvis to shift as the inside leg gives the aid with the swing of the belly on the girth and have the shoulders turned in.

agree!  and to add, i think that by positioning the legs correctly it places the seat in the right position, weighting that seatbone automatically (provided the upper body is correct as well).

[issywizz]

 
And yes I think you are right re the weight aid but I find if you focus on weighting the seatbone you tend to be more likely to lean and /or twist.

[Emma]

But aren't the effects of SI on the 'outside' of the horse just as important / relevant ?

And I mean outside as in that rein ! 

[Wendy]

To work the outside leg you use travers or haunches in.