Bone Strength and Mass: Impacts of Stall Rest and Sprinting

Note: The following article recaps the research discovered through a study funded by the American Quarter Horse Foundation:“Determination of Bone Loss Associated with Disuse in Yearling and Mature Horses." The Foundation fills a vital role by being just one of four private institutions funding research. Make a contribution at www.aqha.com/equine-research. 

By Abigail Boatwright

Confining horses to stalls and pens is a common practice thought to protect them from hurting themselves in turnout, particularly for valuable performance horses and racehorses.

But an American Quarter Horse Foundation-funded study performed by Brian Nielsen from Michigan State University has discovered a surprising side effect to this confinement: penned horses suffer a marked decrease in bone mass and strength versus horses at liberty, noticeable after just two weeks of stalling. Your horse might have good muscular strength because of training, but beneath those muscles, your horse’s bones could be at their weakest because he isn’t turned out.

This revelation doesn’t mean you can’t ever stall your horses. Rather, with a few management changes, your horse can regain and increase bone strength, even while spending time in a stall.

Does Keeping a Horse in a Stall Affect Bone Strength? 

Brian is an animal science professor with a background in racing Quarter Horses and Thoroughbreds. As a graduate student at Texas A&M University in the early 1990s, he worked with a large group of Quarter Horses in race training. After testing their bone mineral content throughout their time together, he noticed a marked decrease in the bone mineral content of the cannon bone in the horses that were confined to stalls.

“At the time, we chalked it up to the idea of bone remodeling, and the idea that bone wasn’t strong enough for the training, so it underwent this bone remodeling process where you had to take away damaged minerals, and put mineral back in,” Brian says.

These horses were getting injured at the time when the bone mineral content of their cannon bones was at its weakest, but it was also the time where they were first beginning to learn how to run at speed in training.

“We had a situation where most of the systems within the horse’s body are presumably getting stronger because they’re in training – the cardiovascular fitness, the musculature system – they were all getting stronger, but the skeleton was getting weaker,” Brian says. 

Brian wondered whether the injuries that researchers were seeing might be related to the fact that the horses were being stalled, instead of living in the pasture. Perhaps the bone was not getting stimulated enough, and that was what was causing the bone to get weaker.

The now-professor of equine exercise physiology performed a research study at Michigan State University in 1996, examining the factors that cause equine bones to weaken. Using long yearlings that had been kept on pasture, the researchers put half of the horses in stalls for the study, while the other half remained outside. The stalled horses were put on a mechanical walker for an hour every day. All of the horses had normal bone growth for yearlings. The researchers monitored the bone mass of all of the horses in the study and found surprising results.

“The horses in the pasture continued to increase their bone mass in the cannon bone, which is what you’d expect,” Brian says. “But the horses we put in stalls had an immediate drop in bone mass, and it remained low for the next three months.”

Next, the researchers started all of the horses under saddle as early 2-year-olds. For the next two months, they went through typical early training – walking, trotting and cantering. Their cannon bones were regularly monitored for bone mass and density.

“Even while being exercised, it was not enough to make the stalled horses’ bone mass increase,” Brian says. “Five months after the start of the study, the horses that were put in stalls had lower bone mass than when we started, despite having been in training. The bone mass stayed lower than what was seen in the pastured horses.”

Stalling a Horse For Extended Periods of Time Can Contribute to Injuries

After more studies, Brian and his research team learned that the key ingredient to healthy and growing bone mass is the horse’s amount of sprinting – short, intense bursts of running.

“It has been really clear that the issue is, when you do not sprint horses or when they do not have access to sprint on their own, they will lose bone mass,” Brian says. “Or if it’s a growing animal, they will not deposit as much mineral as they normally would.”

The science behind these results points to how bones respond to stress.

“Bones like to bend, and if they don’t bend enough, the body says ‘Oh, we’re too strong, so let’s get rid of this extra mineral in here so that we bend more,’ because otherwise, the body thinks it is just carrying around extra mineral that is not needed,” Brian says. “If the bone bends too much, such as when the horse is exerting force on the bones through sprinting, then the body says ‘Oh, we are too weak, we need to make our bones stronger.’ If you have a horse that is kept in a stall and is never allowed to sprint, then you run the risk of that bone realizing it is too strong, so it loses bone mass and will actually become weaker.”

These bone changes might not adversely affect performance for an average horse with a fairly easy job, but for a horse in a high-stress job such as a racehorse, jumper or reiner, extended periods of stalling might contribute to injuries as the horse loses bone mass.

Sprinting a Horse Can Increase Bone Strength

The next step in Brian’s research was testing adult horses to see whether bone mass changes also occur in mature animals.

“We know with human astronauts, research has found that with as little as three days in space, humans have detectable bone loss,” Brian says. “NASA has funded a lot of research into bones, looking for ways to reduce bone loss in astronauts.”

The American Quarter Horse Foundation provided funding for Brian’s study on adult horses, which went through the same process as his earlier research using stalled and turned out young horses, measuring the changes in bone mass.

“It certainly looks to be the case that yes, even with adult horses, bone loss occurs when the horse is confined,” Brian says.

It’s not all doom-and-gloom, however. The body has a remarkable ability to regain some of the lost bone mass, but it takes a proportionately longer period of time.

“If you have a horse that needed stall rest for some reason, it can regain that bone mass, but you need to understand that for a period of time, the bone mass is going to be lower than before you put the horse in a stall and it was not allowed to sprint – under saddle or on its own.”

Brian found that the amount of sprinting needed to maintain bone mass is surprisingly miniscule. Even the small amount of running and playing that an average horse does in the pasture is plenty to maintain its bone mass.

“Every once in a while, you will see horses do a little bit of running around, and that is actually all it takes to stimulate cellular mass,” Brian says.

In a 2003 study, Brian and his team looked at sprinting bull calves to determine the amount of exercise they needed to stimulate bone mass. One group was stalled with no exercise, and the other group was sprinted once a day for about 50 meters (nearly 55 yards), five days a week, for six weeks.

“It was amazing – the calves that sprinted even just that little bit of distance, it made a huge difference in the amount of bone mass, compared to the ones that weren’t sprinted.”

Brian is currently studying how often sprinting is actually needed to stimulate bone growth in calves – looking at the differences between zero, once, three or five times per week, because he understands that, practically, racehorses are not sprinted five days a week.

Turn Horses Out With Other Horses Who Will Encourage Movement

Brian emphasizes that for horses in high-intensity sports, regular sprinting is critical to injury prevention.

“You need to be aware, that when you put a horse in a stall for an extended period of time, you are going to be running the risk of losing bone mass,” Brian says. “So having some turnout is critical.”

The amount of actual running needed is minimal, he says. His study on weanling horses found that just 82 meters (about 270 feet) a day was sufficient to prevent bone loss. That amounts to five to 10 seconds of running. Bones don’t respond to the number of strides, he says, but to the intensity or load of each stride.

“If you turn out a horse that was cooped up in a stall, most athletic horses will naturally turn around and run off,” Brian says. “That’s probably all they needed, one short little sprint to prevent bone loss. Just 20-30 strides is probably enough.”

Another consideration is the horse’s pasture mates. A performance horse turned out with geriatric buddies or lazy broodmares might not get that needed stimulation. So it’s worth putting fellow athletes out in proximity to encourage a bit of running.

Turning horses out has many other benefits besides bone maintenance: mental well-being and healthier cartilage and tendons. But if you’re wondering whether training can take the place of this pasture running, yes, sprinting under saddle can boost bone maintenance, with a caveat.

“Circular training is bad. Be cautionary about high-speed exercise on a circle (such as longeing or round penning),” Brian says. “You will have uneven loading of the cartilage, which will cause you other problems. We really like straight-line speed of 50-100 meters.”

Introduce speed under saddle slowly, gradually upping the distance from just a couple of strides to the desired 100 meters.

Brian understands the concern horse trainers might have about turning out a valuable horse to run every day. But the flipside – risking an in-training or in-performance injury due to weak bone structure – outweighs the danger.

Allow Your Young Horse to Play in Pasture to Prevent Bone Mass Loss Early in Life

Brian charges breeders to also consider changing how they manage young horses.

“Oftentimes, we put horses in stalls because we think we’re doing a good thing for them,” Brian says. “But we are really doing them a disservice – to both their bones and tendons. If you can put your young horse, especially, out on pasture, being a horse, you are going to have a much better chance of your horse being a competitive athlete when he gets older. Breeders can play a role in this. Allowing a horse to be a horse, particularly while he’s a weanling and yearling – allowing it to run around and play – that is your best chance to make a horse that is going to withstand the rigors of training.”

Funding Equine Research: American Quarter Horse Foundation

The American Quarter Horse Foundation fills a vital role by being just one of four private institutions funding research. Since 1960, more than $11.7 has been awarded to various colleges and Universities to further our understanding of equine management and health and improve the overall welfare of our horse.

Our horses give us their all; whether in the thrill of victorious moments in the show ring or in the solace we find in their companionship. The American Quarter Horse Foundation is committed to giving back to the horses we love by challenging the frontiers of knowledge through research.