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Quantifying Posture : The research so far

The effects of posture on equine musculoskeletal health have been recognised subjectively and visually and are now becoming quantified through quantitative analysis and mathematical model research methods (Fureix et al. 2011, Lesimple at al. 2012, 2016, Seneque et al. 2018, Tabor et al. 2018, 2019, Gellman and Ruina 2021). This research, to establish repeatable and reliable measurement techniques for quantifying posture, has enabled statistical correlation between environmental variables and postures. For example, Fureix et al. (2011), Lesimple et al. (2016) and Seneque et al. (2019) used geometric morphometrics (GM) to measure the impact of environmental factors on head and neck carriage and spinal contour and suggest links to back pain. Fureix et al. (2011) expressed the importance of quantifying global posture to create universal descriptors, enabling effective communication between practitioners, however, neither this study nor the subsequent studies of Lesimple et al. (2012,2016) or Seneque et al. (2019) included limb posture as part of their global posture assessment.

Mannsman et al. (2010) and Clements et al. (2021) defined a camped under posture as when the hind limb is placed further forward than the normal vertical axis and the centre of muscle mass of the hindlimb. These studies suggested a link between dorso-plantar hoof balance, posture and pathology in the hind limb and trunk, however, made no quantification of this posture or suggestion of how this posture could be relevant to the pathologies suggested. Mawdsley et al. (1996) established a scoring system for limb positions historically defined as conformations, such as camped under, or cow hocked, producing high levels of inter-observer agreement, and creating categorical, quantification of limb position (Fig.1), this technique is still mainstream, but lacks the ability to measure amplitude of variation in detail.

Fig.1 Authors representation of Mawdsley et al. (2010) showing a scoring system for one of the linear measurements, camped under. Different conformations are scored between 1 and 7 as extremes with 4 as an ideal. However, while this is good for categorising limb position, more objective measurements would allow for more accurate analysis against other variables.


The definition of camped under as a conformation has been questioned by recent research (Mannsman et al. 2010, Pezzanite et al. 2018, Clements et al. 2019, Gellman and Ruina 2021), however a lack of more accurate objective measurements into limb position, and if they are changeable, has created a paucity into possible establishment of it as a posture.

Sharp and Tabor (2022) established metatarsal angle as a measure for hind limb posture (Figure 2), suggesting camped under as a posture rather than a conformation. The study found a link between how broken back the hoof pastern axis (HPA) was with severity of camped under posture, the more broken back the HPA the more angled the metatarsal toward the horse’s trunk, with correction of HPA with farriery intervention creating a more vertical metatarsal. The study found that the change in limb posture wasn’t occurring in the hock or stifle region. This suggested changes in limb posture from hoof balance occurring higher up in the pelvic region and therefore could prove to be responsible for the anecdotal links between negative plantar angles and sacro-iliac pathology and over-riding spinous process’, following the suggestions of Mannsman et al. (2010) of concurrent pathology along the dorsal myofascial line (Figure 2).

Figure 2. Sharp and Tabor (2022). Metatarsal angle was used as a measure of camped under severity. A. Changes in metatarsal angle pre and post farriery intervention. B. No change in hock angle shows no change in stifle angle due to reciprocal apparatus. C. Changes in limb position suggested as coming from higher up in the hind limb.

 Gellman and Ruina (2021) used mathematical models to discuss the physics of different standing postures in the cranio-caudal plane, calculating that non-vertical metatarsals/carpals would require increased muscular effort to maintain standing.  It could be inferred from these studies that static posture is a functional link between hind hoof balance and the associated musculoskeletal pathologies in the hind limb, linked by recent research (Mannsman et al. 2010, Pezzanite et al. 2018, Clements et al. 2019, Sharp and Tabor 2022), as it is suggested that camped under posture increases load on the heels (Pezzanite et al. 2018), creates increased muscular activity (Gellman and Ruina 2021) and abnormal load of the musculoskeletal system. However, none of these studies created a repeatable and reliable measuring technique for quantifying limb posture, as a part of whole horse posture, to enable statistical analysis of limb posture as a variable. A method of measuring whole horse posture is important to objectively measure meaningful change in health status during a course of treatment (Tabor and Williams 2018) and enable an objective language between professionals in practice. Whole horse posture measurements will allow a new field of research to quantify posture more holistically and add to the existing literature beginning to quantify the impact of environmental factors on head and neck carriage and spinal contour and the effects of hoof balance on limb posture.


1.    Critical analysis of methods


2.1 Quantification of Posture


Many studies on global posture have been based on subjective assessment of emotional state and therefore have been qualitative or mixed method in nature. Although Reefman et al. (2009) quantified ear and tail postures in sheep, by camera recording and measuring their angles in relation to other body parts, the meaning of these angles was qualitative, suggesting them as an indication of emotional valence, and that an increase in ear posture changes, was associated with more stressful situations. The interpretation of an ethogram with only two catalogued areas, and its data, is a subjective assumption of the sheep’s emotional state (Reefman et al. 2009). While focus on areas of the animal suggesting emotional state can make indications of welfare, emotional state does not necessarily correlate with musculoskeletal health and performance. Ethograms using a catalogue of observed behaviours with strict definitions to indicate pain in the horse, has been validated, showing excellent repeatability, however, these have largely focused on facial expressions during ridden work, and can not correlate behaviours with specific pathology (Dyson 2021).  Therefore, ethograms have limited use in quantifying links between static posture and musculoskeletal health. Torcivia et al. (2021) described an ethogram also containing more global postures associated with indications of pain, such as camped under and base narrow, but the assessment of these postures remains subjective observations with a lack of quantitative measurements. While Mawdsley et al. (2010) suggests some quantification of these postures, being able to measure the amplitude of variation helps create improved objectivity (Fureix et al. 2011), and normally distributed data, enabling statistical analysis against other variables. While ethograms are useful for establishing expressions and postures associated with pain, objective measurements of musculoskeletal condition and position can enable a functional diagnosis that identifies impairments to performance (Tabor and Williams 2020).

Studies outlining objective measurement of posture have utilised GM, taken from digital photography, to quantify equine head, neck, and spinal posture, as well as measure limb joint angles, (Fureix et al. 2011, Lesimple at al. 2012, 2016, Seneque et al. 2018, Tabor et al. 2019) as this method has been shown to have a high level of descriptive and statistical power (Adams et al. 2004). GM is a multivariate method of measuring and analysing the shapes of objects (Polly 2018), it creates a mathematical description of biological forms, enabling easier statistical analysis. Using a series of pre-determined anatomical datum points, homologous between subjects, marked with physical markers (Fureix et al. 2011) or digitally (Seneque et al. 2018), and their relationship with one another, an outline of posture or conformation can be visualised, quantified and comparisons made (Fureix et al. 2011, Poly 2018, Seneque et al. 2018).

Fureix et al. (2011) used eight markers to assess neck and head carriage and the croup area (Figure 3) and study the postural differences between two cohorts of horses with different management regimes.

Figure 3. Authors representation of the markers used by Fureix et al. (2011). The eight landmarks. Landmarks were stuck onto the horse’s right side and placed in relation to skeletal or muscular cues on: the nasal bone under the eye, 2 cm in front of the zygomatic process (landmark 1); the temporo-maxillary joint (2); the atlas (3); the trapezium cervical ligament (4); the cervico-thoracic (5); the thoraco- lumbar (6) and the lumbo-sacral (7) junctions and the first coccygeal vertebra (8). Photographs were taken perpendicularly from the horse performing various behaviours, and data of landmark coordinates were extracted from photographs using GM software and analysed by generalised Procrustes analyses, allowing to describe and to analyse global body posture variation.

Group one had more natural conditions, living in social groups with year-round turnout, and group two were kept in boxes, fed three times a day and riding lessons between 4-12 hours a week. Group one comprised of only 6 animals while group two comprised of 63 horses. This difference in group sizes creates a question of statistical power in answering the question of identifying postures in relation to living conditions, and therefore limited its ability to extrapolate its findings of postural variation between groups, to the wider population.

Lesimple et al. (2012) used the head and neck markers from Fureix et al. (2011) (Fig.3), chiropractic evaluation and surface electromyography (sEMG) to study the relationship between head and neck carriage and back pain. This study compared two groups of 9 horses from the same groups as Fureix et al. (2011), although this created even groups, the power number would make it difficult to extrapolate the suggestion that group two were more prone to have concave necks and back disorders than group one, to the general population.   


Figure 4. Authors representation of the marker placement of Lesimple et al. (2012) adapted from Fureix et al. (2011). Markers were placed at the cervico-thoracic junction (Marker 1, M1); the trapezium cervical ligament at the level of C3 (Marker 2, M2); the dorsal aspect of the wing of the atlas (Marker 3, M3); the temporomandibular joint (Marker 4, M4) and on the rostral aspect of the facial crest (Marker 5, M5). Angle α: Formed by the segment (M1–M2) and the horizontal plane running by the withers’ basis (lowest point of the withers, between withers and back), represents the neck’s elevation. Angle β: Formed by the segment (M1–M2) and the segment (M2–M3), represents the neck’s curve. Angle Ơ: Formed by the segment (M2–M3) and the segment (M3–M4). Angle δ: Formed by the segment (M3–M4) and the segment (M4–M5). The last two angles were pooled (angle ϒ) to represent the M3– M5 angle (between the atlas and the rostral aspect of the facial crest).

Lesimple et al. (2012) compared the angles between anatomical points (Figure 4) isolated to the head and neck but made no quantification of limb posture, considering Gomes-Costa et al. (2015) showed horses with a narrower stance had an increased postural sway, indicating increased muscular activity and the theoretical suggestion of the same in camped under horses (Gellman and Ruina 2021), the study falls short in forming a holistic picture of the potential variables creating the increased sEMG readings and back pain.

Seneque et al. (2018,2019) used 85 horses from 11 different riding schools and 9 different breeds, giving a better representation of the wider population, although this didn’t allow for representation of other disciplines. More datum points were added to Fureix et al. (2011) and Procrustes superimposition was utilised. Procrustes Superimposition, a method of removing degrees of freedom, by simply overlaying similar shapes over one another once they have been scaled, translated, and rotated to minimise shape differences (Poly 2018), can use the geometric information to visually quantify differences in posture, regardless of size, orientation, and position of the subject (Seneque et al. 2018). Seneque et al. (2018) used semi landmarks, in addition to the anatomical points of Fureix et al. (2011). Semi landmarks are useful for creating a more accurate homologous curve that can be compared between subjects if the points are corresponding (Polly 2018). Limitations of Procrustes superimposition are that when scaling, due to centralising and minimalization, differences are distributed across all datum points, so while it can express differences between points there can be difficulty in locating which points changed (Polly 2018). This may create issues in intra-individual, longitudinal, geometric morphometric studies, however, in a comparative study into global outline between subjects, this is less of an issue. Analysis of the datum points and creation of the semi landmarks and Procrustes superimposition was calculated using sophisticated software (Seneque et al. 2018), which may not be freely accessible or practical for use in daily practice, this could affect the use of the method as a standard tool for communication between professionals and limit it to a research method until further research can test a simplified version for in the field use.

Seneque et al. (2018) tested three different methods for the assessment of topline posture, using just anatomical landmarks, using a mixture of anatomical landmarks, and sliding semi land landmarks, and by using only sliding semi landmarks with a single anatomical reference. Seneque et al. (2018) suggested that the third method produced the most precise description of the topline by revealing the true shape of the croup, neck, back, loin and tail head (Fig.3), however the second method reduced the effect of neck positions on the measurements.




Figure 5. Authors representation of the three methods tested by Seneque et al. (2018). Method 3 creates the most accurate representation of the horses topline, also creating a homologous curve that enables different animals toplines to be superimposed and compared through statistical analysis, irrespective of size.

A method for measuring thoracolumbar (TL) posture used a series of landmarks to assess TL angle, TL depth and TL surface area (Tabor et al. 2018) (Fig.5).

Figure 5. Authors representation of Measurements of Tabor et al. (2018) to assess TL angle, TL depth and TL surface.

Tabor et al. (2018) found that the only repeatable measurement taken from this protocol was TL angle. Considering the increased accuracy of semi-landmarks, and the ability to compare sections of Procrustes superimposition between semi-landmarks (Seneque et al. 2018), this angle would be better represented using a homologous curve that could also be compared between individuals.

Existing studies have focused on topline and spinal contour (fureix et al. 2011, Seneque et al. 2018, Tabor et al. 2019) in isolation. Positioning of the horse for postural measurements has not been standardised, Tabor et al. (2019) used standardised positioning of the head in a vertical position, and feet adjacent and not adjacent, while Lesimple et al. (2012) allowed free positioning of the head, applied restraint, and stated that the feet were positioned on a line. However, the images presented in their paper question this and show different limb positions (Fig.6).

Fig. 6 From Lesimple et al. (2012) showing differences in limb placement despite the study claiming all feet were positioned in a line.

 In addition, Seneque et al. (2018, 2019) made no comment on limb position while the images clearly showed a camped under posture (Figure 7).

Figure 7. From Seneque et al. (2018) showing the location of seven grey landmarks. The image clearly shows a camped under posture, however this was not quantified or mentioned and could prove to affect topline.

 Seneque et al. (2021) highlighted that biomechanically, the head, back and neck work as a kinetic chain and move together, Tabor et al. (2019) highlighted the influence of foot and head position on the outcome measurements. These studies suggest manipulated positions, although necessary for standardisation, could create influence on postural measurements and that postures of certain areas are affected by others. Furthermore, there is limited research into the influence of topline posture on limb position and vice versa. Collectively these points question the accuracy of measuring isolated areas without also quantifying their relationship with the wider system. One unpublished and preliminary study correlating global posture with dysfunction in three major areas (Gellman et al. 2014), suggested abnormal compensatory posture, defined as a camped under stance with a high head carriage, was associated with issues with dental occlusion and the temporomandibular joint, the upper cervical muscles (Surrounding the poll) and the feet. It is unclear as to whether this observational study used geometric morphometrics or made any quantitative measurements of the global posture, or the postural observations were purely subjective.



2.   Conclusion


Future research should make objective measurement of posture as it is important to be able to statistically analyse it as a variable in research and create a repeatable and reliable descriptive language between practitioners to standardise and objectify outcome measures. Current studies have claimed to have objectively measured global posture; however, they have measured topline and failed to appreciate the possible effects of limb posture. Future research into posture should, therefore, take a global viewpoint and include limb posture, as different areas measured can affect one another. Much larger samples should be studied with a wide variety of breeds and disciplines to enable extrapolation to the wider population. Further research should standardise the handling protocol for global posture research. Higher numbers of datum points should be tested for greater accuracy in displaying global posture and comparison between subjects.   






3.   References


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