Epaxial Musculature, Motor Control, and its Relationship with Back Pain in the Horse: Objective Clinical Physical Therapy, Pathological, and Imaging Studies

1. Introduction

Equine back pain often presents with more than one lesion or problem area, including the presence of limb lameness. Osseous lesions of the thoracolumbar (TL) spine and the lumbopelvic complex are widely recognized as significant causes of equine back pain, poor/loss of performance, and altered back and limb kinematics.^1-10   Improved diagnostic capabilities combined with increased clinical and research interest have raised the awareness of the importance of equine back pain and the ongoing need for research into rehabilitation strategies and a long-term management of chronic and recurrent back pain in horses, as is the case in the human. Specific causes of back pain that have been identified include muscle strain^2,11  ligamentous lesions,^2,12,13  fractures of the TL and/or lumbo-pelvic complex,^14-19  vertebral body osteophytes and spondylosis,^2,18,20,21  osteoarthritis and ankylosis of the inter-transverse and/or lateral inter-transverse joints^18,22-24  impingement of the dorsal spinous processes,^9,25-28 sacroiliac disease,^2,18,29-31  degenerative intervertebral disc disease,^3,5,28,32,33 and, more recently, osteoarthritis of the synovial intervertebral articulations (facet joints) as a source of pain and dysfunction.^2,8,9,18,34 

Haussler et al.¹⁸ and Stubbs et al.⁸ highlighted the potential underdiagnosis of TL vertebral or pelvic lesions in postmortem studies of Thoroughbred racehorses euthanized for reasons unrelated to back pain. An alarming rate of osseous lesions was reported in the mid-caudal thoracic and lumbar regions (T9-sacrum), with all specimens having moderate to severe lesions, many of a degenerative nature and/or related to excessive cyclical loading such as stress fractures of the facet joints. Stubbs et al.⁸ reported that 77% had evidence of severe osseous pathology at various sites in the TL spine and pelvis. However, the relationship between the changes observed at necropsy examination and the presence of pain or loss of function has to date not been fully established in the horse. Pain in the TL region of a horse is very complex because it can originate from various pain receptor structures.

Stubbs et al.⁸ utilized ultrasonography of the deep epaxial muscles in the horse (multifidus and the sacrocaudalis dorsalis complex) to demonstrate a relationship with muscle function and unilateral osseous pathology. This is a validated technique widely used in human back pain and motor control models to establish a functional relationship with muscle size/function and the presence of unilateral osseous pathology. Within the clinical equine literature, it is frequently suggested that gross muscular dysfunctions occur secondary to underlying bone pathology in horses with back pain, hypothesizing this to be a result of an underlying lesion in the spine and/or peripheral joints, with pain and or inflammation causing reflex inhibition of motor neurons resulting in weakness and atrophy of associated muscles.³⁵

2. Motor Control and Back Pain

Dynamic control involves a spectrum of control strategies during locomotion, ranging from muscular co-contraction causing stiffening to more dynamic control strategies that involve carefully timed muscle activity and movement. An essential element of motor control is accurate internal and external receptor activity, especially regarding proprioceptive feedback on joint position during locomotion, which can be altered due to pathology. Much of the research in human musculoskeletal disorders in recent decades has focused on the neurosciences including neuromotor control, dynamic stability, pain, and the interrelationship between pain and function. Research in this area has allowed major advancements in the prevention and treatment of major problems in people including pelvic pain³⁶ and lower back pain^37,38. In human back pain research, it is now recognized that the central nervous system’s control of the muscular system is probably of greater importance to the muscular system’s ability to satisfy the needs of spinal movement and stability. The central nervous system must plan suitable strategies of muscle recruitment, co-ordination, and levels of activity to meet the demands of internal and external forces and initiate appropriate responses to unexpected disturbances of movements and function. Research in these areas for horses is clearly warranted and in the very early stages of development.

A key area of research in the human and animal models has shown that the deep local muscles of the TL region (transversus abdominis and the segmental lumbar multifidi) play vital roles in modulating the stiffness of the lumbar spinal segments and pelvic joints during limb and lumbo-pelvic movements.^36,39-42 It has also been shown that the central nervous system pre-programs activity in these trunk muscles in preparation for limb movement. For instance, the transversus abdominis and multifidus activate prior to limb movement, regardless of direction. This serves to increase dynamic segmental stiffness for spinal segmental support prior to loading,^38,43-45 with the multifidi contributing two-thirds of the total increase in spinal stiffness imparted by muscular action.⁴⁶ In vivo studies in pigs confirmed that the multifidi are also a major stabilizer of the quadrupedal lumbar intersegmental motion.⁴⁷ Back pain patients display delayed activation of the transversus abdominis and multifidus, depriving the painful and injured spinal segments of timely support. This dysfunction continues even once the pain subsides.²⁸ In both acute and chronic low back pain, ipsilateral reduced cross-sectional area (CSA) of multifidus was observed at the same intervertebral level where pathology was present, suggestive of inhibition, and the onset of these changes is present within 24 hours of injury.^49-52 In a recent study, the density of multifidus/erector spinae was associated with facet joint osteoarthritis, spondylolisthesis, and disc narrowing.⁴¹ This also occurs within 3 days after a unilateral experimental lesion to an intervertebral disc in pigs.⁵⁰ Further, multifidus does not automatically resume its normal function following recovery from or resolution of an episode of acute back pain.⁵³ Specific physiotherapeutic interventions (exercises) are required to restore the size and function of multifidus after an episode of acute back pain in people, and these interventions reduce the rate of recurrence of injury from 84% in untreated control subjects to 30%⁵³ in patients who received specific interventions.

In the horse, the epaxial muscles including longissimus dorsi, iliocostalis, and middle gluteal all have multiple compartments and a large variation of muscle architecture and fiber type, thus potentially there is marked variation in dynamic functional motor control, most of which is currently unknown. Multifidus, the most medial epaxial muscle group, has five distinctive multi-pennate fascicles crossing 1 to 4 intervertebral discs, emanating from each spinous process attaching to the mammilary process of the articular pillar of every TL vertebrae and sacrum.⁷ Interestingly, sacrocaudalis dorsalis lateralis appears to be a morphological extension of multifidus, attaching to the last 2 to 3 lumbar dorsal spinous processes and lateral sacrum, from where it continues with sacrocaudalis medialis into the tail. Preliminary biomechanical modeling suggests that the horse’s multifidus function is comparative to the human, where the primary movement, if acting alone, is “posterior “(dorsal) sagittal rotation. However, the hypothesized primary function is preparatory dynamic stability, whereby the muscle action is to limit accessory motion during flexion and coupled rotation, as is the case in the human.⁷ The deepest fascicles of multifidi in the human and pig are predominantly type 1 fibers (as is the horse, unpublished equine data). [...]