Are Your Horse's Bones Tough Enough?

Skeletal injuries--those involving bones and joints--are a major concern for all athletic horses. The usual outcome of these injuries is a lameness problem that hampers a horse's training and competition program or, in some cases, is so severe that the horse can no longer be used for any athletic purpose. In fact, by a wide margin, musculoskeletal injuries are the most common cause of poor performance and wastage (where wastage refers to a loss of training days, either temporary or permanent) in the equine industry.

For this reason, researchers, veterinarians, and horse owners are keenly interested in developing training programs and other preventive methods that help minimize skeletal injuries, thereby improving the health and well-being of the athletic horse. Fundamental to this quest for a decrease in the rate of skeletal injuries is an understanding of how bone and cartilage adapt to the rigors of exercise and how different training methods and dietary practices influence these processes.

In this article, we'll review the basic elements of bone development (modeling and remodeling), how exercise and physical conditioning modify these processes, and whether different training methods and diets can modify bone strength--recognizing that current knowledge in regard to this last question is fairly limited and there is a need for a great deal more research in this area.

Bone Structure and Function

The skeleton provides structural support for the body, protecting internal organs (e.g., the ribs protect the heart and lungs in the chest) and housing the bone marrow, where red and white blood cells are produced. Bone also functions as the body's reservoir of calcium and phosphorus. Although you might view bone as a static, rigid organ, it is actually a dynamic tissue that is constantly undergoing change, mostly in an attempt to maximize its strength in the face of changing demands.

The basic structure of bone includes three types of cells and an extracellular matrix--this is the material that resides outside the cells and gives bone its overall strength. The extracellular matrix is composed of several types of protein, including the structural protein collagen, a whole host of growth factors that are important in bone growth and remodeling, and the mineral component (termed calcium hydroxyapatite). Importantly, about 70% of the skeleton's strength is due to the mineral content.

The three cell types are called osteoblasts, osteoclasts, and osteocytes. Osteoblasts are the bone-forming engines; they direct the formation and hardening (mineralization) of bone. Osteoclasts do the opposite; the main role of osteoclasts is to break down (reabsorb) bone. Osteocytes appear to control the level of osteoblast and osteoclast activity; these cells can sense changes in bone loading and initiate an appropriate modeling or remodeling response.

The relative balance of activity by osteoblasts and osteoclasts will govern whether there is a net gain or loss in bone mass.

Bone Development, Modeling, and Remodeling

Long bones such as the metacarpus (cannon bone) have three parts (see "Bone Modeling in the Young Horse" above right). The middle portion of long bones is called the diaphysis, and is a hollow tube formed by dense bone. At either end of the bone is an epiphysis, which is the area responsible for formation of the joint (articular) surfaces of bone. In the growing foal, the diaphysis and epiphyses are centers of bone formation (ossification). Bone formation itself takes place by a process of endochondral ossification--in which soft cartilage cells are transformed into hard bone cells.

Between these two zones (at each end of the bone) is the metaphyseal growth plate (also called the physis). This growth plate allows the bone to lengthen during growth. When the bone reaches maturity (i.e., stops growing in length), there is complete closure of the growth plate as the two centers of ossification become joined.

This growth (lengthwise) and shaping of immature bones is also termed bone modeling. This process allows bone to maintain its shape and proportions as it grows. Modeling is also important as the bone adapts to an increase in functional demands such as the rigors of exercise. More on that later.

On the other hand, bone remodeling is a repair process that occurs on a continuous basis. Remodeling also occurs during fracture repair. Under normal circumstances, more than 5% of the total bone mass is "turned over" each year by the process of remodeling. This turnover also plays a role in maintaining the calcium and phosphorus balance in the body. Remodeling first involves the removal of some bone over the bone surface followed, at the same site, by the formation of new bone. This new bone is eventually mineralized, or strengthened by the addition of calcium and phosphorus.

Gain From the Strain

It has long been recognized that bone size is related to the amount of strain to which it's subjected. In fact, as far back as 1663, Galileo Galilei noted a positive relationship between body weight and bone size. In the 19th Century, a German researcher named Julius Wolff took these observations further and developed mathematical laws (known as Wolff's Law) describing the relationship between mechanical load, or strain, on bone (or an area of bone) and changes in bone structure and strength.

The greater the load (or strain), the larger the bone mass--modeling results in the addition of new bone. But the reverse is also true--decreased load will result in a reduction in bone mass, emphasizing the dynamic nature of bone and its ability to adapt to its local environment.

These are important concepts when considering how bone reacts to athletic training. Appropriate exercise training should stimulate bone modeling, improve skeletal strength, and, in theory, reduce the probability of exercise-associated skeletal injuries. On the other hand, long breaks from training, either voluntary or forced by an injury, will "unload" the skeleton and lead to a decrease in bone mass and strength.

Studies in horses have shown increases in the density of the cannon bone and third carpal (knee) bones during training. This is an important adaptation to the stress of training because bone density or bone mineral content--a measure of the amount of mineral in the bone--is an important determinant of bone strength. However, the intensity of training has an important bearing on this response.

Low-intensity exercise (e.g., trotting) results in minimal changes in cannon bone density, whereas training at higher speeds (galloping exercise) more significantly increases bone density. In treadmill studies, short periods of galloping at speeds greater than 27 mph (43 km/hour) were associated with an increase in the density of the cannon bone (4-5% change). This means that some intense exercise is necessary to elicit a beneficial response in bone.

This bone modeling response also results in a preferential enlargement of the anterior (front) aspect of the cannon bone in Thoroughbreds during intense exercise training. Strengthening this area might reduce the risk of "bucked shins." Other studies have shown similar changes in the anterior aspects of the knee bones and of the sesamoid bones after four to five months of rigorous exercise training.

Currently, an incremental training program that gradually increases the length, speed, and repetition of galloping is recommended for enhancement of bone strength. Initial gallops should be at low speeds (such as a canter), with an increase in speed after periods of three to four weeks. The number of repetitions can also be increased in a similar manner. Even then, most young racehorses will develop some degree of shin soreness ("bucked shins," more on this later), so close monitoring is required to prevent worse injury, particularly as training gallops approach maximum speed.

Use It or Lose It

Whether horses are kept outside or confined to box stalls could also affect bone strength. Several studies have documented a substantial decrease in bone mineral content (of the cannon bone) in horses kept in box stalls. In one study, conditioned horses were taken out of training and restricted to daily 30-minute walking sessions over a 12-week period. For the remainder of the day, the horses were housed in box stalls. Overall, bone mineral content decreased by 0.45% per week during deconditioning (5% over the 12 weeks).

Similarly, confining yearling horses to box stalls (with limited daily exercise) results in a decrease in the mineral content of the cannon bone compared to yearlings kept at pasture. Interestingly, this difference in bone mineral content was maintained when both groups of horses were subsequently conditioned for eight weeks (trot and gallop exercise five days per week).

Whether horses kept at pasture are at lower risk for skeletal injury compared to horses kept in confinement is not known. Still, the enhancement in bone mineral content associated with "free exercise" speaks in favor of allowing horses access to the great outdoors as much as possible.

These findings also have important implications for management of horses re-entering training after lay-up time. Long lay-ups could result in a substantial decrease in bone mineral content (and strength) that must be gradually regained during subsequent conditioning. The longer the horse's lay-up, the more gradual the increase in the volume and intensity of training needed to minimize the risk of excessive strain on bone.

Modeling Gone Awry

There is evidence that not all of the bone modeling or remodeling that occurs in response to exercise conditioning is beneficial. For example, it is now thought that chip fractures involving the carpal (knee) bones in racehorses are not an acute traumatic event, but rather the end result of a negative remodeling balance that leads to a loss of bone mass and strength (termed osteoporosis). The weakened area of bone is then predisposed to fracture. Unfortunately, just why this osteoporosis occurs is not yet understood.

Bucked shins (also known as "shin soreness") can also be regarded as bone modeling gone wrong (see X ray on page 86). This painful condition is extremely common in young racing Thoroughbreds and Quarter Horses (and occasionally Standardbreds) worldwide. As expected, with the onset of galloping exercise, additional bone is deposited on the front portion of the cannon bone--which should ultimately result in improved bone strength. However, early on this new bone appears to be prone to microfractures similar to the stress fractures that can occur in human athletes during training.

Stress fracture of the bottom portion of the cannon (distal condyle) is another common injury in racing Thoroughbreds that might be the result of unbalanced bone modeling. Recent research has found that training results in an uneven increase in the density of the condyles. The resultant variation in bone density might predispose the horse to stress fractures in the weaker areas under the strain of normal training and racing.

Clearly, we have much to learn about the effects of conditioning on bone, particularly in relation to common bone and cartilage injuries that occur in horses. This information will be critical to the development of training programs that minimize the risk of injuries.

Calcium for Strong Bones

There is no doubt that an adequate supply of the minerals calcium and phosphorus is critical for maintenance of bone mineral content and strength. As mentioned, bone strength is very much dependent on mineral content. Calcium and phosphorus comprise 70% of the mineral content of bone. However, "How much is enough?" is more controversial--particularly for young horses entering training (18-24 months of age). Younger horses in training are undergoing more bone modeling than mature horses and might be more sensitive to an inadequate supply of calcium or phosphorus, or a calcium:phosphorus (Ca:P) imbalance.

For adult horses, the 1989 National Research Council (NRC) recommends that the total diet contain 0.35% calcium and 0.25% phosphorus. For 1,100-pound (or 500-kg) horses in moderate to intense conditioning programs, this would equate to 1.4 ounces (45 grams) of calcium and 1.05 ounces (30 grams) of phosphorus per day (with a Ca:P ratio of 1.5:1).

A study completed in the 1970s found no additional benefit when a 0.6% or higher calcium diet was fed. However, a more recent study of horses in training found higher bone mineral content of the cannon bone in horses fed a 0.7% calcium diet compared to the NRC recommendation of 0.35% calcium.

At a time of increased bone modeling and remodeling, it is thus possible that the added dietary calcium might allow greater bone mineralization and strength.

The Ca:P ratio is also important for maintenance of appropriate bone mineralization. At the minimum, the Ca:P ratio should be 1:1, with a preferred value of about 1.5:1 (as stated above). "Calcium and Phosphorus" on page 84 shows the calcium and phosphorus contents of some common horse feeds. Note that all cereal grains are low in calcium and fairly high in phosphorus, wheat and rice bran are very high in phosphorus, and alfalfa hay is 1.0-2.0% calcium while grass hays are 0.3-0.4% calcium. Commercially available concentrates for performance horses are typically 0.6-0.8% calcium and 0.4-0.6% phosphorus. When planning your horse's diets, consider all the components in order to optimally balance the Ca:P ratio.

Even when the diet appears to have adequate calcium, high dietary phosphorus (such as from bran) can impair calcium absorption from the intestine, with adverse effects on bone development such as bone demineralization and increased risk for skeletal injury. An example of such a diet is one containing grass hay (about 0.3% calcium) in combination with large amounts of a "straight" grain such as oats (0.35-0.4% phosphorus) and some wheat bran (1.2-1.3% phosphorus). A better approach is to feed a commercial concentrate (that has added calcium and the right balance of calcium and phosphorus) or provide a calcium supplement (e.g., a mix of 66% dicalcium phosphate and 33% limestone). Also, avoid feeding excessive high-phosphorus bran (wheat or rice), particularly in young horses.

At the other end of the spectrum is a high-alfalfa diet. Typically, the Ca:P ratio of these diets is 3:1 or more. However, these high Ca:P ratios do not appear to be a problem with respect to bone health and certainly ensure that adequate calcium is provided in the diet.

Work with your veterinarian and equine nutritionist to develop exercise and nutritional programs that will maximize your horse's bone strength, minimizing his risk of skeletal injury.


Wheat bran




Rice bran




Grass hay
Commercial concentrates


* Note that forage values will vary depending on the maturity at the time of harvest and the local growing conditions.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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