Bone is made of a mix of inorganic minerals and organic material including collagen (mainly type 1), which creates a highly responsive and adaptable compound (1). For those interested, the mineral is a hydroxyapatite component, made of Calcium and Phosphorus, and it is laid down into the collagen (1). For those uninterested, suffice to say Calcium and Phosphorus provide the major mineral/non-living contents. Bones provide the valuable levers for the locomotor system in all vertebrate animals. It is also a crucial reservoir of minerals, providing a buffering system for the body in times of growth/repair/response (1, 4).
Bone tissue consists of cortical bone (a.k.a compact) and Trabecular bone (a.k.a cancellous). https://www.youtube.com/watch?v=Ei4seya3dOg -> This video, (up to the 2-minute mark) offers a clear explanation of the structure of long bones in the body. Bone is essentially a tube of cortical bone with bone marrow (+trabecular bone) on the inside, covered by growth-plates, more bone and cartilage, enclosing each end. The surface of bones is covered by a tissue called periosteum... this is what connects/is continuous with tendon tissue and muscles.
Consider this: If you roll up a piece of paper in to the shape of a cylinder and stand it up next to a folded cube of paper... the cylinder will be a lot harder to crush than the square, which is one of the many beauties of the structure of ours, and our horses' bones.
Crucial built-in safety margins have evolved in the structure of bones: weight of the distal limb is minimised to reduce momentum and therefore excess force applied to the limb during movement, even the alignment of the bone cells intentionally offers effective support of weight and force (1).
Bones are subject to a constant balance of RESORPTION and DEPOSITION, performed by osteoclasts (resorption) and osteoblasts (building/deposition). As a mineral bank, sometimes resorption occurs at a greater rate than deposition, which frees required minerals to the body (3). Alternatively, increased mineral deposition occurs in order to strengthen the bones under stress or to adjust for a surplus and maintain mineral homeostasis (3). Bone is a piezoelectric material, which means when physical stresses are applied, it responds and signals for increased deposition to occur in order to maintain safety margins and full function (4). The response of bone can be short-term or long-term, making it highly adaptive (1). Short-term response is activated by mechanical pressure, which despite not being completely understood could be likened to the muscle “swell” that comes with a workout.
Despite these safety mechanisms, a level of damage is expected to occur, and surprisingly is necessary to increase bone density (2). Training and improvement generally occurs outside of a comfort zone… How far outside that comfort zone you can go depends on the following, and even then there are physiological limits:
a) your horse’s conditioned fitness (are they in full work or box rest?)
b) your horse’s technical ability/training knowledge
c) their natural strength/predispositions (referring to conformation etc.).
Big injuries occur outside of that small range outside of that comfort zone. In the instance of bone, too many microfractures as a result of a sharp increase in workload or work on a surface that was too hard can overwhelm the precious damage-repair cycle of bone, and could result in a major fracture instead (1).
(5): https://europepmc.org/article/PMC/3743123 In short this picture demonstrates the comfort zone, which sort of sits in the “physiological window”. This shows the increased mechanical loading which eventually results in overload and failure.
Expected/common micro-damage is caused by repetitive, cyclical loading (as found in repetitive loading of the limb during movement) (1). Damaged portions of bone will be targeted by osteoclasts and will be resorbed into the matrix; to be followed by osteoblasts laying down the new bone in the same arrangement as the osteons found in fresh/undamaged bone (1). It takes a few weeks for extensive mineralisation to occur to regain strength and density to the affected area (1).
If none of my words made any sense, they are essentially summarised by this picture: picture A = bone that has adapted to exercise conditioning, B = bone that has not had exercise applied. In summary: bone is highly responsive, it can increase density under increased load, or stay the same and even reduce under reduced load. Too much load and it may be unable to maintain function/structural integrity and will break. picture credit: (1)
Now consider this: The cardiovascular system undergoes a significant loss of fitness after 4-6 weeks of rest (2). Bone density takes 12 weeks of reduced rest to result in significant loss (2). Severe injuries commonly require a lengthy period of box-rest/hand-walking (though obviously varying lengths between type and severity of injury); however, bone density is rarely mentioned as a target of the rehabilitation process (personal experience as well as research)… The old english methods of “legging up” horses by trotting them on hard ground is a great way to stimulate osteoblast activity and increase bone density. With a huge word of CAUTION that this should not be done for more than a couple of minutes at a time, a couple of times per week. Otherwise you risk tipping the delicate balance against your favour and causing increased damage.
QUESTION: Have you ever considered bone remodelling/density/strength in your training and/or rehabilitation program? Do you think you should?
Bone remodelling can occur not only in the length of the bone, but also in the tuberosities at the ends of bones, which can and do change with pressures. The difference with this remodelling is that the pressures from other bones come in to play, as well as the behaviour of cartilage. As such the directional behaviour of the joint can become affected or can affect the new bone remodelling. This plays a role with osteoarthritis, which is a huge topic on its own and should be gracing your screens in 2 weeks’ time ;)
AS always, comments and ideas are welcome!
Thanks for reading!
A. E. Goodship and R. K. Smith, "Skeletal Physiology: responses to exercise and training, Role of SKeleton," in Musculoskeletal System, Elsevier, 2004, p. 81.
A. J. Kaneps, "Practical Rehabilitation and Physical Therapy for the General Equine Practitioner," Veterinary Clinics of North America: Equine Practice, vol. 32, pp. 167-180, 2016.
P. Katsimbri, "The biology of normal bone remodelling," European Journal of Cancer Care, vol. 26, no. 6, 2017.
P. Yu, C. Ning, Y. Zhang, G. Tan, Z. Lin, S. Liu, X. Wang, H. Yang, K. Li, X. Yi, Y. Zhu and C. Mao, "Bone-Inspired Spatially Specific Piezoelectricity Induces Bone Regenration," Theranostics, vol. 7, no. 13, pp. 3387-3397, 2017.
A. Robling and C. Turner, "Mechanical signaling for bone modeling and remodeling," Critical Reviews in Eukaryotic Gene Expression, vol. 19, no. 4, pp. 319-338, 2009.