Bones, joint cartilage, ligaments, and tendons make up your horse’s skeletal system, which provides structural support for your horse’s body.

Most horse owners pay close attention to the role of joints and soft tissue health in preserving soundness and comfort. But bone strength is just as critical for supporting mobility and performance in horses.

Bone is a dynamic tissue that responds to impact and loading forces. Research shows that exercise and nutrition influence bone density in horses.

This article will review bone development, remodelling, and adaptation to training. Keep reading to learn how to support equine bone health with proper feeding and training.

Bone Development in Horses

Skeletal growth occurs rapidly during the first two years of a horse’s life. Studies show the average thoroughbred reaches 98% of its mature height by 24 months. [2]

Horses evolved for early locomotion to elude predators. As a result, newborn foals have distal limb bones similar in size and structure to mature horses. [2]

However, the skeleton of newborn foals only contains 17% of the bone mineral content (BMC) of adults. BMC is the concentration of calcium and other minerals in bone. Mineral content is responsible for 70% of bone strength. [3]

While skeletal growth slows significantly by age two, maximum BMC is not reached until the horse is six years old. Bone is a dynamic tissue and BMC constantly changes throughout the horse’s life through remodelling. [3]

Bone Formation

The bone formation of the appendicular skeleton, including the limbs, occurs through endochondral ossification. This process transforms cartilage cells into bone cells and occurs primarily in utero before birth. [1][2]

Longitudinal growth after birth occurs at the physis, or growth plate. Cartilage cells remain in growth plates to allow the bones to continue to grow until the growth plate ossifies. Some foals can suffer from a developmental disease of the physis called physitis. [4]

Mature bone contains three types of cells and an extracellular matrix. This matrix has inorganic and organic components. Collagen comprises the organic portion, while crystalline mineral salts and calcium comprise the inorganic part. [5]

Types of Bone Cells

Bone cells include osteoblasts, osteoclasts, and osteocytes: [5]

  • Osteoblasts are responsible for hardening bone by laying down the extracellular matrix.
  • Osteoclasts break down old bone so osteoblasts can replace it with stronger bone.
  • Osteocytes maintain bone strength while modelling or remodelling occurs.

Bone Remodeling

Bone remodelling happens when minor damage occurs to mature bone due to aging or stress. Several hormones regulate this complex process. [6]

In a healthy horse, osteoclasts remove the old or damaged bone tissue and trigger the other cells to repair it. Osteoblasts rebuild bone by laying minerals and collagen over the area to strengthen it.  [6]

Scientists estimate that horses replace 5% of their total bone mass through remodeling each year. Bone tissue is in a weakened state during this process. Injuries can occur if the horse’s bones are subject to excessive loads that damage the bone quicker than it can be replaced. [6]

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How Exercise Affects Bone Strength

The remodelling process allows the bone to respond to the forces acting upon it. Exercise is one form of stress that can cause minor damage and trigger remodelling. The body responds to increased strain by replacing the damaged bone with stronger tissue. [7]

When horses are fed a well-balanced diet, exercise is the most significant factor affecting bone strength. Research shows free exercise and appropriate conditioning programs can increase bone density in horses of all ages, but it is especially important for young horses. [7]

Horse owners must be careful not to overload the skeletal system. Exercise only improves bone strength if horses have adequate rest periods to lay down stronger bone tissue. [8]

Impact and Loading

During motion, the horse’s stride has a contact phase with the ground, which can be further divided into the impact, loading, and break-over phases.

The impact phase occurs immediately when the hoof hits the ground. This contact causes rapid deceleration that sends a force through the limb. The hoof initially absorbs the force and transfers it to the bone and joints. [9]

Bones are under constant load while supporting the horse’s body weight. But the impact during exercise increases that load.

Hoof balance, conformation, and speed affect the impact and loading forces on the horse’s limb. The horse’s limbs must endure a force equal to three times his body weight at a gallop. The greater the load, the more bone remodeling takes place. [9]

Bone Adaptation

Adaptive responses to exercise can modify bone mineralization and density to decrease the risk of injury. Tolerable levels of microdamage trigger an adaptive response, but an inadequate repair can lead to damage accumulation. [8]

Balance is key when designing an exercise program for your horse. Bone requires rest periods for tissue repair to adapt to increased loads. [10]

Repetitive stress increases the risk of injuries when bone is repeatedly subjected to the same forces within a short period. [11]

Studies show short durations of high-speed exercise increase bone density more than repetitive workouts. Alternating more challenging efforts with lower-intensity exercise and rest periods allows the bone to remodel. [12]

Initial conditioning programs should begin with low-speed, long-distance exercise. This type of exercise develops cardiovascular fitness without overloading the skeletal system. Walking on different surfaces can also stimulate adaptive bone changes. [13]

Conditioning Young Horses

Exercise at a young age can significantly influence bone strength as the horse matures due to dynamic structural changes to the skeletal system.

Research supports the benefits of appropriate exercise programs for young horses and describes the negative effects of restricting free exercise in growing horses. [14]

One study found that weanlings raised in full or partial pasture turnout had less bone mineral loss than those confined to stalls. [15]

Another study observed that young horses subjected to additional imposed exercise as juveniles had stronger bones than horses raised on pasture turnout without extra training. [16]

This research suggests that pasture turnout and careful conditioning from a young age could reduce the risks of future injuries in performance horses. [14]

Bone Loss During Stall Rest

Sometimes, horses must be confined for extended periods due to an illness or injury. But research links stall rest to significant decreases in bone density. [17]

One study found that bone mineral content decreased by approximately 0.45% per week in highly conditioned horses on stall rest. Increased dietary calcium did not prevent bone loss during deconditioning. [17]

This research suggests that even relatively short periods of limited activity can predispose horses to injury when returning to work. Horse owners should slowly introduce exercise to their rehabilitation program to avoid maladaptive bone disorders.

Non-Adaptive Bone Remodelling

Skeletal injuries usually do not occur because of a sudden abnormal force on a healthy bone. Instead, many bone injuries result from the accumulation of chronic damage, fatiguing the bone over time to the point of failure.

The highest joint loads occur at the fetlock, the most common site of subchondral bone disorders. Subchondral bone refers to the bone tissue underlying the cartilage of a joint. [24]

Repetitive training without appropriate conditioning and rest can cause microcracks in the subchondral bone. Fatigue injuries occur when this microdamage accumulates faster than the horse can repair with remodelling.

Young horses in training are often susceptible to nonadaptive bone remodelling. Horses who are still growing experience bone remodelling from both growth and exercise. [24]

Sometimes, repetitive trauma can lead to bone edema visible on x-rays. Regular