Bone was once thought of as inert endoskeleton, but accumulated knowledge assuaged that belief. Bone is a very dynamic tissue highly sophisticated in its adaptability to increases and decreases in the demands of exercise. It will enlarge to withstand additional load and is capable of reducing its mass when loads regress. As such, bone is the one tissue that is capable of regenerating itself and tuning its anatomy to its needs.
The neonate is born with a highly adaptable skeletal model. The phenotype is genetically predisposed but the biomechanical loads that the skeleton experiences with the work the horse is asked to do dictate refinement to the most functional skeleton. Unlike organ systems such as the lungs, where training causes no change in organ size, bones can enlarge to double or triple thickness and tune the enlargement to the best shape to neutralize the demands placed on it. The well-knownchanges in a horse’s shins are the most obvious example of this.
The talented racehorse carries skeletal adaptation to an extreme. Race training literally molds the juvenile skeleton into a racehorse skeleton; a horse is not born with a racehorse skeleton. The skeleton is also tuned for horses of other uses, but the racehorse demands the most of the bone because of the repetitive cyclic load the bone must endure during exercise.
It is advantageous for the horse to carry as little skeleton as possible because skeletal load is deadweight during high-speed exercise. The skeleton’s function during racing and training is to carry the weight of the horse and the rider. Redundant bone strength adds extra pounds and demands extra work to carry those pounds during competition, reducing the horse’s competitive advantage. That is why a Thoroughbred is sleek and light and doesn’t look the same as a draft horse.It is ideal for the racehorse to carry the minimum skeleton necessary for traversing the distance of the race, but to carry sufficient skeleton to complete the race without permanent damage.To accomplish this there is great rationale in work specific adaptation of the skeleton to meet the demands it experiences, but to maintain a minimum of redundancy. That is why a Thoroughbred racehorse is the best example of a horse built for speed over a distance.
High-level exercise does this by causing minute damage that the bone senses, responds to, and then makes the skeleton stronger. The micro-damage, stimulated by exercise, causes over-compensation by the bone via the modeling and remodeling process, strengthening the bone mass and changing the bone’s shape to make the bone stronger and prevent the micro-damage from occurring again in subsequent exercise sessions. If the exercise is then increased again, the bone is overloaded again and over-repairs again further strengthening and modeling the bone to neutralize the load. This stepwise stimulus of hypertrophy by “overload” then “over-repair” is the basis for training in all tissues but is particularly important in bone.
Bone senses load by determining tension and compression. Compression deformation always dictates bone formation to increase strength, whereas tension tends to create remodeling to alter a bone’s shape. The compressed surface of the bone strengthens the bone internally by adding to the bone density, and models the external surface of the bone by adding bone mass to the most heavily loaded areas.This shapes the bone to best resist the stress of training. The dorsal surface of the cannon bones and the caudal surface of the tibia are very active examples of this bone modeling to neutralize ongoing bone deformation. Over time the modeling/remodeling process alters the bone to the shape and strength that best prevents deformation upon loading. At this point the load is balanced on the long axis of the bone for the horse’s workload. The modeling process then slows or ceases as long as the exercise load is not altered, but the remodeling has to continually occur to repair “wear and tear” which results during all exercise.
The bone’s ability to respond to training is, however not an unlimited resource. There is a maximum rate of response that the bone can generate. It takes time for the osteocytes (the bone cells) to increase the density of the existing bone and for the osteoblasts (also bone cells) to change the size of the bone. The “art” of training is to use the hypertrophy response to produce a continually stronger athlete, without progressing too rapidly for the bone to adapt, causing injury.
The adaptation of bone to training is different than most tissues because it trains more to the level of work, rather than the volume. Bone requires only a limited number of loads in a specific exercise periods to stimulate its response. Experimentally this has been determined to be approximatelythirty-six similar cyclic loads.As the level of exercise increases the tolerable volume of work the bone can withstand decreases and excess work results in damage and lameness. There is some individual variation in the ability to respond to training, as with most biologic systems. Some individuals can mount the response at a rate that virtually precluded their bone being overloaded, and some individuals cannot respond fast enough for even the most moderate levels of training. Overload faster than the bone can respond results in lameness. The non-bone tissues such as the heart and lungs are more sensitive to the volume of work to train, not changes in level of work. So the two differing demands must be balanced to train a racehorse. The art of reading and understanding this biologic variation is the basis for a trainer to properly dose training.
In summary bone is not a purely structural inert material; it is a very dynamic support tissue that adapts to the work it is asked to do. Most lameness is the result of damage in excess of the horse’s ability to respond to the exercise stimulus. Understanding of the horse’s bone response to exercise and modulation and adaptation of exercise programs to individual horses and to their response to the stimulus greatly aids in the avoidance of injury in the equine athlete
By Dr. Larry Bramlage, DVM, MS, Dipl. ACVS, surgeon and shareholder at Rood & Riddle is world renowned for his innovations in Equine orthoscopic surgery.