Typically this sway is due to extensive asymmetry in the lumbosacral region, but can also be a result of lumbothoracic or cervicothoracic regional asymmetry. Therefore, when you are treating with FES and you obtain functional movement, you will see the asymmetrical lateral movement through the pelvis during the flexion and extension of the pelvis. If you watch carefully during riding of these horses, you will also typically see this asymmetrical movement when they are asked to “engage” their hindquarters more. For example, at the base of a fence watch as the horse gathers to spring. In addition, when you question the rider, many times the rider will tell you that these horses drift to the same side as the sway during riding. It is harder to see the asymmetry when the rotation is more subtle, so during walking, for example, you may not notice it initially.
What I have found clinically, for example if the horse sways to the left, then typically the right side musculature of the lumbosacral region is hypertonal pushing the lumbosacral region to the left. Of course this is a simplification, and most likely there is a combination of hypertonal and hypotonal musculature on both sides of the spine which causes this abnormal movement during hindquarter rotation. However, I do find that if I start by simplifying my focus, so to speak, to lengthen the musculature on the concave side of the sway, the horses decrease their level of sway. FES works to increase tone in hypotonal regions as well as decrease tone in hypertonal areas to produce symmetrical functional movement when the stimulus is placed symmetrically over the spine. Therefore, it can be harder to determine rhetorically if it is the improvement in hypo or hyper tone which is the most important aspect to produce the improvement that you see. Most likely it is the combination of the increase in tone of some muscles together with the decrease in tone of other muscles that improve the symmetry, and therefore reduces the sway.
If there is no noticeable twist in the lumbosacral area, then this sway can be more of a systemic issue dealing with proprioception. You can see this in horses with advanced Lymes, EPM or other systemic issues such as systemic muscle diseases. The neuromuscular connection is not being made appropriately, and the “sway” reaction is a very normal feeling for these horses when lumbosacral movement is initiated either by their own neuromuscular connection or by FES. It is very interesting to observe these horses because in most situations the horses do not exhibit any discomfort, even when the sway becomes severe and the horse is at risk of loosing their balance.
The improvement in rotation, and therefore reduction in the sway pattern, typically happens progressively over time with the use of FES, for horses that have a asymmetrical muscular development with no systemic muscle disorders, and in 3-4 treatments to the thorax and si regions you should observe at least a 50% reduction in sway. If you do not see an improvement after 3-4 treatments to the thorax and si regions, then I suggest to discuss with the attending veterinarian any suspicions of a neuromuscular muscle disorder.
Below is a typical symmetrical pelvic rotation in response to FES (gray horse).
The chestnut horse shows is a typical sway pattern observed in some horses showing an asymmetrical muscular development. This sway resolved over a sequential FES treatment protocol of 5 treatments to the si region and 5 treatments to the thoracic region over a period of 3 weeks.