De la Gueniniere lists the benefits of the shoulder-in as including suppling the shoulders, preparing the horse for collection by teaching him to bend his hocks under himself, teaching him to move away from the rider’s leg, and cross both fore and hindlegs.
We now know that there is another very important benefit. The shoulder-in relaxes the horse. In May 2020, a study was published in the Journal of Veterinary Behaviour (see citation at end of article) that demonstrated scientifically that the shoulder-in helps horses to relax and to pay attention in their training.
It is only recently that science has been applied to equestrian pursuits. Equestrians are naturally conservative and do things because that’s the way they have always done it. Science helps us evaluate what we do. I think that this is a really important finding that can help all of us train more intelligently. We want our horses to be more relaxed and therefore happier in their work. Relaxed work is more beautiful. Relaxed horses learn faster, and are softer and easier to ride.
So, if we know that the shoulder-in, (and possibly other lateral work, where we bend and therefore soften the horse) helps relax the horse, then we can use the shoulder-in and other lateral work in the warm-up to start with relaxation before we teach new or harder things. We can return to the shoulder-in and other lateral work if a horse becomes tense because the work is harder, or even if the environment is stressful. Any time the horse becomes tense we could use the shoulder-in and lateral work to relax.
For example, if you are teaching the flying change and he becomes confused and tense, just change the subject to shoulder-in, get him relaxed and around your inside leg before you ask again. You are at a show and he isn’t coping well with the noise around him, so warm up with lots of lateral work to make him relaxed in his body and in his mind. You have been doing a jumping exercise and he starts to run and get tense, just go back to shoulder-in and get him soft and paying attention and then go back to the jumping exercise. Teach him that being relaxed is the starting point. Remember that the horse is a flight animal, and so learns best when relaxed. A tense horse just wants to run away.
So, what is the evidence that the shoulder-in relaxes the horse? Well the researchers in France took 40 competition horses in work and who had been trained in the shoulder-in, and tested them twice — once with an amateur and once with a professional. A great deal of data was collected. Biomechanical data included frequency and regularity of strides. Physiological data included heart rate variability (HRV), which is well recognised as a physiological indicator of relaxation in humans and animals (Read more). Behavioural data included playing with, and chewing the bit (PCB) and looking around, as a measure of the horse’s concentration. Playing and chewing the bit is considered a normal and desirable behaviour associated with relaxation. Data was analysed for statistical significance.
Each testing session comprised of a normal warm-up of 5-10 minutes riding normally around the arena. Then horses were ridden for 10 minutes in a straight frame, 5 minutes on each rein, and a standard time at each gait. Then they were rested before doing a standard 10-minute workout of lateral working the three paces, 5 minutes on each rein and standard time at each gait.