Association of a Subjective Muscle Score With Increased Angles of Flexion During Sitting Trot in Dressage Horses

Journal of Equine Veterinary Science 40 (2016) 6–15

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Original Research

Association of a Subjective Muscle Score With Increased Angles of Flexion During Sitting Trot in Dressage Horses V.A. Walker*, C.A. Tranquille, S.J. Dyson, J. Spear, R.C. Murray Centre for Equine Studies, Animal Health Trust, Newmarket, Suffolk CB8 7UU

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 September 2015 Received in revised form 19 January 2016 Accepted 20 January 2016 Available online 28 January 2016

Dressage requires the horse to be supple through the thoracolumbosacral region, and movement should allow for efficient locomotion and expression, but excessive thoracolumbosacral movement is likely to be detrimental to the soft tissues of the vertebral column. It is not known how development of the musculature relates to thoracolumbosacral movement in the ridden dressage horse. The aim of the study was to investigate the relationship between grading of muscle development and back kinematics. Thirty-five horses (Novice to Grand Prix level) in active dressage training were ridden in sitting trot in a straight line by their normal rider on an artificial surface. Thoracolumbosacral angles were derived from high-speed motion capture. Muscle scores were assigned based on visual assessment and manual palpation of the left and right sides of the neck, abdomen, thoracic and lumbosacral (LS) regions, pelvis, and hindlimbs. Our findings suggest that there is a relationship between muscle scores and kinematics of the back in ridden dressage horses. There was an association between neck trunk, thoracolumbar, LS angles, and dorsoventral difference between withers and tuber sacrale markers and muscle scores. Muscle scores assigned during clinical examination were related to the back kinematics of dressage horses ridden at a collected trot. Observations from this study suggest that thoracic, abdominal, and LS muscle development is important for achieving gait patterns which are desirable, according to equitation texts, at the collected trot. Ó 2016 Elsevier Inc. All rights reserved.

Keywords: Equine Back Kinematics Collected trot Stability

1. Introduction The thoracolumbosacral region (which will be referred to from hereon as back) has several functions during locomotion, including providing support and stability while facilitating movement in three planes: flexion and extension, axial rotation, and lateral bending. During the trot, the back region moves in a sinusoidal pattern with two peaks (flexion) and two troughs (extension) per stride [1–3]. At a slow trot, the back is passively flexed and extended through movement of the visceral mass [2]. The flexion peaks occur during the swing phase when the limbs are not in contact with the ground and the extension troughs occur at the stance phase when the diagonal limbs * Corresponding author at: Centre for Equine Studies, Animal Health Trust, Newmarket, Suffolk CB8 7UU. E-mail address: [email protected] (V.A. Walker). 0737-0806/$ – see front matter Ó 2016 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2016.01.011

are in contact with the ground and therefore supporting the weight of the horse and rider [1–3]. Back stabilization can be achieved through both passive (bone, ligament) and active (muscular) mechanisms [1]. Back extension is actively moderated by the action of the back flexor muscles (e.g., rectus abdominis), and flexion is actively moderated by the back extensor muscles (e.g., longissimus dorsi) [2,3]. The influence of head and neck position on back kinematics has also been illustrated in ridden [4] and unridden horse [5,6]. To provide a stable support platform for the rider, training of the horse should aim to stabilize and improve coordination throughout the head, neck, and back [1]. At collected trot, the horse works with a shorter frame and the hindlimbs become more propulsive compared with working trot and the forelimbs work to elevate the forehand relative to the hindquarters. To achieve this, the neck and back of the horse need to be flexed [7]. The thoracic serratus ventralis and pectorals are important in support

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Fig. 1. Arena set up (A) for testing; showing field of view and runway used. (B) Marker placement for data collection: (1) rostral aspect of the facial crest, (2) wing of atlas, (3) proximal aspect of the scapular spine, (4) over the cranial eminence of the greater tubercle of the humerus, (5) the lateral epicondyle of the humerus over the lateral collateral ligament of the elbow, (6) lateral styloid process of the radius, (7) proximal aspect of the third metacarpal bone at the junction with the base of the fourth metacarpal bone, (8) distal aspect of the third metacarpal bone over the lateral collateral ligament of the metacarpophalangeal joint, (9) lateral collateral ligament of the distal interphalangeal joint (designated coronary band), (10) dorsal aspect of the hoof wall at the level of the coronary band, (11) dorsal aspect of the hoof wall at the level of the coronary band, (12) lateral collateral ligament of the distal interphalangeal joint (designated coronary band), (13) distal aspect of the third metatarsal bone over the collateral ligament of the metatarsophalangeal joint, (14) proximal aspect of the third metatarsal bone at the junction with the base of the fourth metatarsal bone, (15) midtalus, (16) proximal aspect of fibula, (17) medial epicondyle of the distal femur, (18) proximal aspect of the greater trochanter of the femur, (19) ischiatic tuberosity, (20) top of tail, (21) proximal aspect of the tuber coxae, (22) tuber sacrale, (23) spinous process of the fourth lumbar vertebra, and (24) spinous process of the sixth thoracic vertebra.

and elevation of the thorax, and along with the rectus abdominis and external abdominal oblique muscles, they lift the abdomen and enable flexion through the thoracolumbar (TL) and lumbosacral (LS) regions [8–10]. The long back muscles (longissimus dorsi, intercostalis, gluteus medius) are responsible for moderating flexion of the back [2] and facilitating limb movement [11]. Flexion of the LS region is crucial for the caudal weight shift during collection which is facilitated by the caudal pelvic tilt through the action of the gluteal, hamstring, and psoas muscles [8]. It is proposed in equitation texts that this and contraction of the longissimus dorsi and intercostalis and consequential

elevation of the neck through the action of splenius and semispinalis capitis cause TL flexion and the forehand to be elevated relative to the hindquarters [8,11,12]. Although body condition scoring is frequently used as a guide for nutritional advice [13–15], there is no comparable scale for evaluation of muscle development which could potentially be useful in relation to training, nutrition, and detection of orthopedic problems, such as pelvic or hindlimb fracture [16]. Alterations in muscle development may be related to back pain [17,18], lameness [19], conformation [20], rider [21,22], saddle fit [23], and exercise history [24]. Visual and palpation assessment of the posture

Fig. 2. Regions assigned muscle scores (see Table 1): (1) cervical, (2) thoracic, (3) lumbosacral, (4) pelvic, (5) hindlimb, and (6) abdominal. The figure also illustrates some of the marker placements used for measurement of angles and distances between landmarks.

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Table 1 Criteria for assigning muscle scores by visual assessment and palpation in 35 dressage horses. Grading Scores were allocated before exercise, with the horse standing squarely, the head and neck in a straight line, with the nose at the level of the shoulder and the horse attentive. Horses were assigned a score based on “best fit” of the statements given for each score. To be assigned a specific score, a majority (at least 4) of the statements should be true. If the horse fits between scores (i.e., 3/6 for two scores), then a 0.5 mark may be assigned. Definitions Palpation should be done with the hand and fingertips, with the hand flat so that the finger tips do not dig in. Palpation should follow the direction of the hair and be for the entire length of the muscle in the specific region being scored. Pressure should be light, unless specified otherwise. Pressure
Lightdpressure that depresses the area no more than 0.5 cm.
Moderatedpressure that depresses the area between 0.5 and 1 cm.
Strongdpressure that depresses the area more than 1 cm. Muscle tone
Weakdmuscles are soft and flaccid. Provide minimal/no resistance to light pressure.
Slightdmild elastic resistance to pressure, but still quite soft when light pressure is applied.
Moderatedmuscle feels firm but still has a reasonable amount of elasticity (“bounce”), and the muscle can be depressed slightly when light pressure is applied but does not feel soft.
Gooddmuscle body feels firm but is still deformable, not stiff or rigid, when light pressure is applied. Muscle tension
Muscle or a specific area of the muscle is stiff or rigid and has no “give” in it with application of light/moderate pressure Score

1

2

3

4

Neck

Side area concave

Side area flat or concave

Top of neck convex through part of length only, with concave areas Top of neck easily moveable

Top and side of neck Top and side of neck convex through entire convex through parts length of length but not entire length Indentation cranial to Ventral aspect of neck withers concave or flat

Top of the neck flat but not convex and easily moveable Side of neck flat Ventral aspect of neck Cervical vertebrae 1–5 Cervical vertebrae 1–5 concave or flat easily palpable but not visible and easily visible palpable Shelf where shoulder Shelf where shoulder Ventral aspect of the neck No vertebrae visible meets neck meets neck concave or flat Scapula relatively Scapula visible but not Indentation cranial to Cervical vertebrae 1–3 prominent prominent withers vertebrae palpable under strong pressure Muscles have moderate Muscles have weak tone Muscles have weak tone Cervical vertebrae 1–5 tone on palpation on palpation on palpation palpable under moderate pressure Muscle tension on Muscle tension on Muscles have slight tone palpation palpation on palpation Spinous processes visible Spinous processes visible Spinous processes Spinous processes at the tip only (except (including left and (including left and withers) right sides) visible and right sides) visible and easily palpable easily palpable Spinous processes only Articulation with ribs not Dorsal tips and left and Articulation with ribs palpable left and right right sides of spinous easily visible although visible and easily side in dorsal <1 cm processes palpable in palpable palpable dorsal 1–2 cm Muscles concave from Articulation with ribs not Articulation with ribs not Muscles markedly visible or palpable caudal and side view visible or easily concave from side and palpable caudal view, the shape appearing more like a shelf than a smooth curve Muscles have weak tone Muscles have weak tone Muscles slightly concave Muscles flat from caudal on palpation view on palpation or nearly flat from caudal view Muscle tension on Muscle tension on Muscles slightly concave Muscles flat from side view palpation palpation or nearly flat from side view Dipped back posture Mild dipped back posture Muscles have slight tone Muscles have moderate on palpation tone on palpation

Top of neck narrow and easily moveable

Thoracic

5

No indentation cranial to withers No vertebrae visible Cervical vertebrae 1–3 vertebrae palpable under strong pressure Muscles have good tone on palpation

Only dorsal tip of spinous processes palpable except at withers Articulation with ribs not visible or palpable

Muscles level with spinous processes from side view

Muscles convex from caudal view Muscles have good tone on palpation

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Table 1 (continued ) Score

1

2

3

Spinous processes visible Spinous processes Lumbosacral Spinous processes (including left and (including left and right sides) visible and right sides) visible and easily palpable easily palpable

Pelvic

Transverse processes visible and easily palpable

Transverse processes not easily visible although palpable

Muscles concave from side and caudal view

Muscles concave from side and caudal view

Muscles have weak tone palpation

Muscles have weak tone palpation

Muscle tension on palpation Tubera coxae prominent and visible

Muscle tension on palpation Tubera coxae prominent and visible

Tubera coxae easily palpable Muscles deeply concave with a shelf-like appearance from caudal and side view Tubera sacrale prominentdeasily visible and lateral aspect visible and palpable Tubera coxae prominent, visible and easily palpable

Tubera coxae easily palpable Muscles concave in a smooth curve from caudal and side view

Sacrum and/or coccygeal vertebrae definable or prominent Muscles have weak tone on palpation

Hindlimb

Muscle tension on palpation Muscles concave from side view (semimembranosus and semitendinosus region) Muscles concave from caudal view (biceps femoris, vastus lateralis region) Tubera ischia visible and easily palpable

Muscle tension on palpation Tubera coxae easily palpable but not prominent

Muscles flat from caudal view

Muscles flat from side Tubera sacrale prominentdeasily view visible and lateral aspect palpable but not visible Tubera coxae prominent, Tubera sacrale prominentdeasily visible and easily visible and dorsal part palpable of lateral aspect palpable Sacrum and/or coccygeal Tubera coxae visible and palpable but not vertebrae not prominent definable Muscles have weak tone Sacrum and/or coccygeal on palpation vertebrae not definable Muscle tension on Muscles slightly toned on palpation palpation Tubera ischia cannot be Muscles concave from visualized easily side view and/or muscles concave from caudal view. If only one is concave the other is flat. Tubera ischia visible and Tubera ischia easily easily palpable palpable

Greater trochanter of femur visible but not prominent

Greater trochanter of femur prominent

Muscles have weak tone on palpation

Muscles have weak tone on palpation

Muscle tension on palpation

Muscle tension on palpation

Dorsal tips and left and right sides of the spinous processes palpable in dorsal 1– 2 cm Muscles slightly concave or flat from caudal view Muscles have slight tone on palpation

4

5

Spinous processes visible at the tip only, and only palpable left and right side in dorsal <1 cm Transverse processes not palpable

Only dorsal tip of spinous processes palpable

Muscles flat from side view

Muscles convex from side view

Tubera coxae palpable but seen only as a smooth convex region Muscles have moderate tone on palpation

Muscles convex from caudal view

Muscles convex from caudal view and/or side view

Muscles convex from caudal view

Tips of tubera sacrale palpable only

Muscles convex from side view

Tubera coxae palpable but seen only as a smooth convex region

Tubera sacrale largely hidden under muscles and only seen as smooth convex region

Sacrum and/or coccygeal vertebrae not definable Muscles have moderate tone on palpation

Sacrum and/or coccygeal vertebrae not definable Muscles have good tone on palpation

Transverse processes not palpable

Muscles have good tone on palpation Tuber coxae hidden under muscles and only seen as smooth convex region

Muscles convex from side Muscles convex from side view and muscles view or muscles convex from caudal convex from caudal view view but not both, with other being flat Muscles moderately toned on palpation

Muscles flat from caudal Tubera ischii not obviously visible and side view although isolated muscles may be concave Tubera ischii palpable Greater trochanter of with moderate femur palpable with pressure moderate pressure Greater trochanter of Greater trochanter of femur not easily visible femur palpable with strong pressure Muscles have slight tone on palpation

Muscles have good tone on palpation

Tubera ischii not visible

Tuber ischii require strong pressure to palpate Greater trochanter of the femur requires strong pressure to palpate

(continued on next page)

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V.A. Walker et al. / Journal of Equine Veterinary Science 40 (2016) 6–15

Table 1 (continued ) Score

1

Abdominal

Sagging appearance to abdomen

2

3

4

Abdomen is cylindrical in Abdomen lacks shape but Abdomen is more shape with minimal cylindrical than does not have a dropping of the ventral sagging in shape marked sagging aspect appearance Abdomen hangs slightly Abdomen appears to be at Abdomen held Abdomen appears to approximately at level lower than sternum a similar level to the hang below level of the of sternum sternum sternum Dipped back Moderate extension of Slight extension of the Muscles have moderate the back back to good tome on palpation Muscles have weak tone Muscles have slight tone Muscles have moderate Flat back on palpation on palpation tone on palpation Abdomen easily moves Abdomen easily moves Slight resistance to Moderate resistance to on pressure under pressure movement on pressure movement on pressure

and musculature of a horse could potentially give a guide to muscle development, based on the external appearance of the muscle and palpable assessment of the nature of the tissue to confirm muscle tone rather than softer, more spongy adipose tissue [25]. To date, the effect of development of the muscle groups responsible for movement and stability of the neck and back, and the consequential effect on back kinematics, has had limited investigation [26]. The study aimed to investigate the back kinematics of the subjects in normal training conditions; therefore, highspeed motion capture was used. This has been used extensively to determine back kinematics in ridden and unridden subjects at various gaits [27–39] and does not need laboratory conditions. The aim of the study was to investigate the relationship between subjective grading of muscle development (the “muscle score”) and back kinematics. It was hypothesized that muscle score would be associated with flexion of the (1) head to neck (HN), (2) neck to trunk (NT), (3) TL, and (4) LS regions and with dorsoventral displacement of the withers relative to the tubera sacrale (W-TS) in ridden horses. 2. Material and Methods Thirty-five horses (Novice to Grand Prix level; mean height: 167  7 cm; mean age: 9  3.5 years), in active dressage training, were evaluated by an experienced clinician (RM Diplomate of the American College of Veterinary Surgeons) in hand at walk and trot in straight lines and in hand in 5-m circles on a firm surface to ensure that they were classed as <1/8 lameness grade [40]. Each horse was ridden by their normal rider at collected trot sitting (the degree of collection depended on the stage of training) in a straight line marked out with cones (Fig. 1A) at a single venue on a sand and rubber surface. Nine 30-mm-domed markers were placed at predetermined anatomic sites (Fig. 1B) by a single experienced technician (V.A.W.), verified by a veterinarian, according to palpable surface landmarks [41]. Repeatability of marker placement was confirmed using five horses with five overlaid digital photographs of each horse being compared. Each horse stood squarely on a marked point, and the image was acquired from a second marked spot 2-m away from the horse to ensure standard positioning and therefore image

5 Abdomen is cylindrical in shape

Abdomen tends to be held above sternal level Muscles have good tone on palpation Flat back or slight flexion Marked resistance to movement or pressure

acquisition. The maximum difference, center to center, for a given marker was 5 mm, which was considered acceptable based on pilot data. Horses were warmed up, as they normally would be at a competition with that rider, for 12 to 29 minutes (mean: 18 minutes) before testing. 2.1. Ethical Review The study was approved by the Animal Health Trust Clinical Research Ethics Committee (Project Number: AHT19-2013), and all owners consented to their horse taking part in the study. 2.2. Data Collection 2.2.1. High-Speed Motion Capture High-speed video motion capture (Casio EX-FH250) (240 Hz) was used to film each horse from the left and right sides. Each camera was placed 6-m away from the middle of the trot pathway, equidistant from each end (Fig. 1A). The field of view was 5-m wide for each camera. As the horse passed the camera, a stride was recorded and retained for analysis when the stride was correct according to the Fédération Equestre Internationale Rules for Dressage [42], contained the entire stance phase, and was in the center third of the field of view (directly in front of the camera) to minimize the camera/marker angle and improve accuracy of kinematic measures. A minimum of four strides of collected trot were recorded from the left and right sides simultaneously. The mean speed was 2.75 m/s; range ¼ 2.32 m/s to 3.58 m/s. 2.2.2. Muscle Score All horses were assessed before exercise by a veterinarian (R.C.M.) experienced in the evaluation of dressage horses, but blinded to the performance history of the horse. Muscle score was assigned based on visual assessment and manual palpation of the left and right sides of the neck, abdomen, thoracic region, LS region, pelvis, and hindlimbs (Fig. 2). Thus, the muscle assessed included the splenius, rhomboideus, cervical and thoracic trapezius, serratus ventralis, brachiocephalicus, rectus abdominis, external abdominal oblique, latissimus dorsi, longissimus dorsi, intercostalis, gluteals, semitendinosus, semimembranosus,

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Fig. 3. Right lateral images of a ridden dressage horse with markers placed at standardized anatomic landmarks [24]. (A) Stride phases at which each variable was measured (1) right forelimb midstance, (2) right hindlimb protraction, (3) right hindlimb midstance, and (4) right hindlimb retraction. (B) Angles measured in 35 dressage horses ridden in straight lines in collected trot sitting: HNdhead and neck calculated from markers at the rostral end of the facial crest, wing of atlas, cranial eminence of the greater tubercle of the humerus, NTdneck and trunk calculated from markers at the wing of atlas, cranial eminence of the greater tubercle of the humerus and proximal aspect of the tuber coxae, TLdthoracolumbar calculated from markers at sixth thoracic spinous process, fifth lumbar vertebra and tubera sacrale, LSdlumbosacral calculated from fifth lumbar vertebra, tubera sacrale, and head of the tail, and W-TSdvertical distance between withers (W) (sixth thoracic spinous process) and tuber sacrale (TS) markers.

biceps femoris, vastus lateralis, tensor fascia lata, and gastrocnemius. During assessment, all horses stood squarely, on a concrete surface, with the cervical and TL regions aligned and the nose at the level of the shoulder joint. Horses were assessed from the front, back, and both sides. Muscle score was assigned on a scale of 1 to 5, with 5 representing the maximal degree of muscle development (Table 1). Scores were assigned for the left and right sides. 2.2.3. Repeatability Repeatability of the muscle scoring system was assessed by five assessors (three veterinarians and two experienced research technicians) grading a group of 10 dressage horses, seven of which were in active training. All horses were graded on the same day by all assessors. Scores assigned for each horse were compared using a weighted kappa agreement to determine the level of agreement between assessors. For the all regions, moderate (0.60–0.79)

Table 2 Mean  standard deviation for kinematic variables measured from highspeed video recordings acquired from left and right sides and both sides pooled at four phases of the stride of 35 dressage horses ridden in collected trot. Variable HN (degrees)

Side

Left Right Pooled NT (degrees) Left Right Pooled TL (degrees) Left Right Pooled LS (degrees) Left Right Pooled DVDw-ts (cm) Pooled

FL ST 82 83 83 112 112 112 196 197 197 160 161 160 4

            

HLP 6 5 5 7 8 6 3 4 3 3 4 3 1.5

81 82 82 108 107 107 193 194 194 159 160 159 3

HL ST             

5 6 6 7 8 6 4 4 4 4 4 4 1.7

82 83 83 109 109 109 198 199 198 158 159 158 3

            

HLR 5 6 5 7 8 6 4 4 4 4 4 4 1.4

82 82 82 109 109 109 194 194 194 158 160 159 3

            

5 5 5 8 9 7 3 3 3 4 4 4 1.9

Abbreviations: DVDw-ts, vertical distance between withers and tubera sacrale; FL ST, forelimb midstance; HLP, maximum hindlimb protraction; HLR, maximum hindlimb retraction; HL ST, hindlimb midstance; HN, head neck angle; LS, lumbosacral angle; NT, neck trunk angle; TL, thoracolumbar angle.

to very good (0.80–0.90) agreement [43] for muscle score was observed. 2.3. Image Analysis Images acquired were analyzed by an experienced analyst (V.A.W.) using digital image analysis software (Pro Analyst, Xcitex). Camera and software accuracy have been validated previously [44]. Head to neck, NT, TL, and LS angles were measured. The vertical location of the tuber sacrale (TS) marker (y axis) was subtracted from the vertical location of the withers marker to give relative position of the forequarters to the hindquarters (Fig. 3A and B) [22]. All these variables were determined at four points within the stride: forelimb and hindlimb midstance and at maximal hindlimb protraction and retraction (Fig. 3B). Repeatability of marker tracking was determined by tracking all angles three times in five horses. A coefficient of variance of <3% was determined and was deemed acceptable by all authors. 2.4. Statistical Analysis Descriptive statistics were carried out for right and left side kinematic and muscle development grading data. Kinematic data from the left and right sides (four observations per horse per side, total n ¼ 140 per side) were compared using a paired Student t-test for parametric data. Left and right muscle scores (1 score per horse per side for each muscle group, total n ¼ 35 per side) were compared using a Wilcoxon signed ranks test for ordinal data. All analysis was undertaken using statistical analysis software (Analyse It Microsoft version 1.73), with the significance value set at P < .05. For each kinematic variable, HN, NT, TL, LS angles and dorsoventral displacement W-TS, a mean value from the four strides of collected trot, were calculated at forelimb midstance, hindlimb midstance, hindlimb protraction, and hindlimb retraction. This resulted in a single value for each variable per horse for the left and right sides (n ¼ 35 left and n ¼ 35 right) for each of the four stride phases. There

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Table 3 Median and range of muscle scores for left and right sides and both sides pooled for each region scored from 1 to 5 (see Table 1) in 35 dressage horses. Side Neck Median Range Thoracic Median Range Abdominal Median Range LS Median Range Pelvic Median Range HL Median Range

Left

Right

Pooled

4 2–5

4 2–5

4 2–5

3 2–5

3 2–5

3 2–5

3 1–5

3 1–5

3 1–5

4 3–5

4 3–5

4 3–5

4 2–5

4 2–5

4 2–5

4 2–5

4 2–5

4 2–5

3.2. Neck to Trunk Angle Neck to trunk flexion angle was larger at forelimb midstance when muscle scores of the neck (rs ¼ 0.27, P ¼ .03), abdominal (rs ¼ 0.30, P ¼ .01), and LS (rs ¼ 0.37, P ¼ .002) were greater (Fig. 4). Neck to trunk flexion angle was larger at hindlimb midstance when muscle scores of the abdominal (rs ¼ 0.27, P ¼ .03) and LS (rs ¼ 0.31, P ¼ .01) groups were greater. Neck to trunk flexion angle was larger at maximal hindlimb protraction when muscle scores of the abdominal (rs ¼ 0.32, P ¼ .008) and LS (rs ¼ 0.35, P ¼ .003) groups were greater. Neck to trunk flexion angle was larger at maximal hindlimb retraction when muscle scores of the neck (rs ¼ 0.2, P ¼ .04), abdominal (rs ¼ 0.28, P ¼ .02), and LS (rs ¼ 0.34, P ¼ .004) groups were greater. 3.3. Thoracolumbar Angle

Abbreviations: HL, hindlimb; LS, lumbosacral.

were no significant differences between left and right sides; therefore, the kinematic data were pooled for further analysis (n ¼ 70) for each stride point. Muscle scores were compared between left and right sides (n ¼ 35 left, n ¼ 35 right), and no significant differences were seen. The results were therefore pooled for left and right sides for each group: cervical, abdominal, thoracic, LS, pelvic, and hindlimb (total n ¼ 70 per muscle group). The muscles included in each group are listed above. The muscle score data were ordinal. Mean HN, NT, TL, and LS angles and dorsoventral displacement W-TS at each stride point (forelimb stance, hindlimb stance, maximum hindlimb protraction, maximum hindlimb retraction) were tested for associations with muscle score for each group using a Spearman’s rank correlation. 3. Results Mean and standard deviations for back kinematics and median muscle scores are summarized in Tables 2 and 3, respectively. Significant associations between muscle scores and kinematics are presented in Figs. 4–7.

Thoracolumbar flexion angle was smaller at maximal hindlimb protraction when muscle score of the LS (rs ¼ 0.35, P ¼ .04) group was greater (Fig. 5). Thoracolumbar flexion angle was smaller at maximal hindlimb retraction when muscle score of the LS (rs ¼ 0.36, P ¼ .03) group was greater. 3.4. Lumbosacral Angle Lumbosacral flexion angle was larger at forelimb midstance when muscle scores of the abdominal (rs ¼ 0.46, P ¼ .01) and LS (rs ¼ 0.34, P ¼ .04) groups were greater (Fig. 6). Lumbosacral flexion angle was larger at hindlimb midstance when muscle scores of the neck (rs ¼ 0.36, P ¼ .04), abdominal (rs ¼ 0.40, P ¼ .02), and LS (rs ¼ 0.45, P ¼ .01) groups were greater. Lumbosacral flexion angle was larger at maximal hindlimb protraction when muscle scores of the thoracic (rs ¼ 0.46, P ¼ .01), abdominal (rs ¼ 0.50, P ¼ .002), and LS (rs ¼ 0.40, P ¼ .02) groups were greater. Lumbosacral flexion angle was larger at maximal hindlimb retraction when muscle scores of the thoracic (rs ¼ 0.36, P ¼ .03), abdominal (rs ¼ 0.46, P ¼ .01), and LS (rs ¼ 0.37, P ¼ .03) groups were greater.

3.1. Head to Neck Angle

3.5. Dorsoventral Distance Between the Withers (Spinous Process of Sixth Thoracic Vertebra) and Tubera Sacrale (W-TS)

No significant associations were seen between HN angle at any stride points for any muscle group.

Withers relative to the tubera sacrale was larger, indicating greater distance between the withers and tubera

Fig. 4. Right lateral image of a dressage horse ridden in collected trot sitting. Neck to trunk (NT) angle (far left) was larger at corresponding stride phases (1) forelimb midstance, (2) maximum hindlimb protraction, (3) hindlimb midstance, and (4) maximum hindlimb retraction when the regions highlighted by boxes had greater muscle score.

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Fig. 5. Right lateral image of a dressage horse ridden in collected trot sitting. Thoracolumbar (TL) angle (far left) was smaller at corresponding stride phases (1) forelimb midstance, (2) maximum hindlimb protraction, (3) hindlimb midstance, and (4) maximum hindlimb retraction when the regions highlighted by boxes had greater muscle score.

sacrale markers, when muscle scores of the LS group at forelimb midstance (rs ¼ 0.36, P ¼ .04), maximum hindlimb protraction (rs ¼ 0.40, P ¼ .02), hindlimb midstance (LS: rs ¼ 0.36, P ¼ .03), and at maximum hindlimb retraction (rs ¼ 0.35, P ¼ .04) were greater (Fig. 7). 4. Discussion Our findings suggest that there is a relationship between muscle score and kinematics of the back in ridden dressage horses. There was an association between NT, TL, and LS angles and W-TS and muscle scores. No associations were observed between HN angle and muscle scores. Smaller NT angle was associated with greater LS and abdominal muscle scores. This may be indicative of relative elevation of the forehand to the hind quarters, but movement of either the neck or trunk could result in change in NT angle; therefore, alterations need to be evaluated together with other variables. Larger TL angle was associated with greater muscle score of the LS region at maximum hindlimb protraction and maximum hindlimb retraction, which both occur during the suspension phase of the stride. No associations were seen during the stance phase. The flexion peak of the TL region occurs during the suspension phase at the trot, and increased TL angle during this phase of the stride suggests that this region is less flexed during the swing phase in horses assigned greater muscle scores for the LS region. The timing of this may influence its potential effect on the back; if it were during the stance phase, this would mean the back would be more extended during loading, which may be undesirable [2,3,38]. Smaller LS angle was associated with greater muscle score for the abdominal and LS regions, suggesting their development also affects stability of the LS region. Lumbosacral angle was associated with thoracic muscle development during the suspension phase. This includes longissimus dorsi and iliocostalis, which limit passive

flexion of the back [45] at the end of the stance phase and facilitate hindlimb propulsion during the swing phase [11,46]. These actions are particularly important in relation to movement of the LS joint because this is the most mobile joint of the back [8]. Improving stability in this region may be desirable for a more energy-efficient gait and for reducing repetitive overload on the structures in this region [1]. In the horse, the large epaxial muscles (longissimus dorsi, iliocostalis, middle gluteal) tend to produce global spinal stiffness rather than dynamic intersegmental stability [46,47]. So, although the deeper musculature (e.g., multifidus) may be more important for spinal stability, the more superficial musculature also plays a role in overall back support for the rider [46,47]. High muscle scores for the LS region were associated with larger W-TS at all stride points, indicating that the withers are higher than the TS. This alongside the reduced NT angle, observed with greater scores for the LS and abdominal regions, implies that the forehand is elevated. This elevation of the forehand is a key training goal in the dressage horse [48]. Larger TL angle during suspension was also associated with greater LS muscle scores. This suggests that to achieve forehand elevation and limit TL flexion, development of the abdominal and particularly the LS musculature is important. The LS epaxial musculature is responsible for moderating flexion of the back [2] and facilitating limb movement [11]. The abdominal musculature, comprising the external oblique and the rectus abdominis, limits back extension [2]. It is proposed that ridden dressage horses with a high score of the abdominal and dorsal LS musculature may find it easier to elevate the forehand and stabilize the back compared with horses with lower scores. Dynamic stability and optimum back function in humans relies on several trunk muscle groups [49–51], and our findings suggest that this also applies to the horse. Human [52,53] and equine studies [1,46] have highlighted the importance of back stability for reducing risk of injury, which is contrary to the suggestions of the lay

Fig. 6. Right lateral image of a dressage horse ridden in collected trot sitting. Lumbosacral (LS) angle (far left) was larger at corresponding stride phases (1) forelimb midstance, (2) maximum hindlimb protraction, (3) hindlimb midstance, and (4) maximum hindlimb retraction when the regions highlighted by boxes had greater muscle score.

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Fig. 7. Right lateral image of a dressage horse ridden in collected trot sitting. Vertical distance between wither and tuber sacrale (W-TS) arrows signifying that they are evaluated in dorsoventral plane (far left). This distance was larger at corresponding stride phases (1) forelimb midstance, (2) maximum hindlimb protraction, (3) hindlimb midstance, and (4) maximum hindlimb retraction when the regions highlighted by boxes had greater muscle score.

press, which often discusses back mobility and maximum flexibility as desirable for horses’ health and performance. Given the relatively high incidence of back pain in dressage horses [54], understanding more about the relationship between superficial muscle development and back movement in ridden dressage horses may be useful for many equine professionals. Assigning subjective muscle scores may enable a veterinarian or physiotherapist to identify horses which are likely to have a poor ability to stabilize the back. They can then encourage the rider to embark on a strengthening program with the horse to reduce the risk of injury before it occurs or try to minimize reoccurrence. The muscle score used in this study has many limitations, but it may be helpful to identify and monitor alterations in muscle development in dressage horses that could be associated with “poor performance” and associated injury development. It could be used to identify early asymmetry in the horse or imbalances in muscle development between different regions. Use of a flexible curve ruler in further studies may be useful to provide an objective measure of symmetry [55]. However, the timing of the measurements is important because significant differences in measurements may occur before and immediately after exercise [56]. This study had various limitations. Two-dimensional regional kinematics was assessed because this was noninvasive and most suitable for the field assessment. Although data presented here give some indication of back mobility, they do not actually reflect what is happening to the vertebrae directly because of the effect of muscle mass and skin movement on marker location [11]. This may also contribute to the fact that the correlations observed were only very weak to moderate. The grading scale aimed to distinguish between muscle bulk and body fat by including criteria based on palpation and tone to distinguish between firmer toned muscle tissue and softer, more indentable adipose tissue [25]. Based on our experience in clinical cases, we would recommend that body condition scoring [14] is done in parallel with muscle scoring to encourage observers to be aware of the presence of body fat during muscle score allocation. The muscle grading scale was subjective and is only applicable to dressage horses and may not be suitable for horses in other disciplines, although it is likely that it could be adapted. The scale has been evaluated for repeatability, and the correlations observed were only moderate to very good. Descriptions of associations between muscle scores and muscle size determined ultrasonographically [57] are beyond the scope of this study.

We used horses from a range of different competition levels and ages, training history, musculoskeletal strength and coordination and rider skill level which may have introduced considerable variation to our data set. The horse’s level of training will have had an influence on the degree of collection it was able to achieve, which would therefore affect speed and consequently back kinematics. However, this does reflect the general population and therefore supports its application to dressage horses at a variety of levels. The initial information from this study could be used as pilot data to carry out further work in a much larger group of horses (the number based on a power calculation using this data) or within a more tightly defined group. 5. Conclusions Muscle scores assigned during clinical examination were related to the back kinematics of dressage horses ridden at a collected trot. Our findings suggest that greater thoracic, abdominal, and LS muscle scores were associated with larger LS flexion angle during stance and swing and smaller TL flexion angle during swing. They were also associated with larger NT flexion angle and larger W-TS throughout the stride, both of which are indicative of increased elevation of the forehand relative to the withers which is a key goal of dressage training. These muscle groups may be influential in stabilization of the spine and therefore potentially energy transfer and propulsive force from the hindquarters to the forehand of the horse. Increased spinal stability may be desirable for dressage performance. References [1] Clayton HM. Equine back pain reviewed from a motor control perspective. Comp Ex Phys 2012;8:145–52. [2] Audigie F, Pourcelot P, Degueurce C, Denoix JM, Geiger D. Kinematics of the equine back; flexion-extension movements in sound trotting horses. Equine Vet J 1999;30:210–3. [3] Robert C, Audigie F, Valette JP, Pourcelot P, Denoix JM. Effects of treadmill speed on the mechanics of the back in the trotting saddlehorse. Equine Vet J 2002;33:154–9. [4] Weishuapt M, Wiestner T, Von Peinen K, Waldern N, Roepstorff L, van Weeren R, Meyer H, Johnston C. Effect of head and neck position on vertical ground reaction forces and interlimb coordination in the dressage horse ridden at walk and trot on a treadmill. Equine Vet J 2006;36:387–92. [5] Rhodin M, Johnston C, Holm KR, Wennerstrand J, Drevemo S. The influence of head & neck position on kinematics of the back in riding horses at the walk and trot. Equine Vet J 2005;37:7–11. [6] Gómez Álvarez CB, Rhodin M, Bobbert MF, Meyer H, Weishaupt MA, Johnston C, Van Weeren PR. The effect of head and neck position on

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