Whether it’s part of injury recovery or injury prevention, many racehorses will be put on turnout at some point in their career by their veterinarians. For many, it’s an intermediary step between stall rest and return to work.

But what actually is “turnout” and what does it accomplish?

Turnout means different things to different people. Dr. Larry Bramlage, equine surgeon at Rood & Riddle Equine Hospital in Lexington, Ky., oversees the rehabilitation of orthopedic injuries well beyond the surgeries and lameness exams he conducts day in and day out. Sometimes, he sees horses who haven’t made the progress he’d expected by the time of their recheck. Bramlage says it’s important for horsemen and farm managers to know that “turnout” isn’t an all-encompassing term for just anything between stall rest and a return to the track. It has a specific purpose.

“It’s useful for the horsemen to understand what we’re trying to accomplish when we say we need a horse turned out,” said Bramlage. “There are two components. Part of getting a horse turned out is we just want to get him out of the stall for their mental wellbeing, but there’s an even more important skeletal component to what we’re trying to facilitate.”

Horses evolved as grazing animals, and their skeletons are designed around the notion that they should be in motion frequently. Bramlage has a handful of retired Thoroughbreds on his farm and will sometimes watch them amble around the field. He has noticed that if you give a horse a 20-acre field, they will never stand in one place until they’ve eaten all the grass they can reach down to the roots. Instead, they wander slowly through most of the acreage throughout the day, in search of the plants they like best. The movement may appear lackadaisical, but it’s accomplishing an important purpose besides eating.

A horse’s lower limbs are something of a biological and engineering marvel, an observation made repeatedly when describing their injuries and bone diseases. The lower limbs are mostly bone, tendons, and ligaments, with no muscle below the knees and hocks. As such, their circulation is limited, although their blood supply isn’t. Horses can drive whatever blood they need to their lower limbs because of their great cardiovascular capabilities, but the blood’s return in the veins after going through the capillaries is a largely passive system. Blood pressure doesn’t help with this the way it does for humans. In fact, the increased volume in circulation actually increases the need for movement.

To do its job, the circulatory system pushes fluid through the capillaries and out into the tissues. The fluid, called lymph, moves from the tissues back into the circulatory system and returns back to the circulation, some by moving back into the veins and some through the lymphatic system. In most situations, and in most species, it is muscular activity and one-way valves in the veins that gradually work the fluid up the limbs and back into the circulation. Horses evolved to not have valves in the veins of their lower limbs, because they didn’t need them — they were constantly on the move and the activity supported the return circulation.

When you increase a horse’s exercise level, you increase the circulatory demand. Horsemen know that a horse who stands in their stall for a long time will “stock up,” a temporary swelling of the lower limbs improved by adding standing bandages after morning work. The wraps treat the result of the inadequate movement, but not the cause.

When blood circulation is inadequate, so are processes that depend on circulation – including bone remodeling in response to exercise. Bones go through a constant process of remodeling in response to the forces placed on them. This involves either 1) microdamage from exercise which is removed and replaced with stronger bone or 2) a change in size and shape from the bone in response to exercise loads. Healing and remodeling are also part of recovery from an injury. Horsemen have long known the benefit of getting a horse who has been in heavy training in the field for a break.

Turnout is often recommended for the common stress injury in training known as “bone bruising.” The disease has many names, depending on who you ask; it’s sometimes called “palmar osteochondral disease” or “mal-adaptive bone remodeling.” Whatever the name, the issue occurs when there’s unresolved damage to the bone as a result of the stress of training. Tissue damage normally results in pain, heat, swelling and redness, the four cardinal signs of inflammation. This is largely the result of the increase in circulation. Anyone who’s bruised their arm is familiar with these trademarks, but a bruise in the bone is different. Swelling in the bone can become stagnant, especially when you reduce normal movement with time in a stall. This eliminates the normal circulatory support system which depends on the horse’s activity walking around the field grazing to maintain the return of fluid from the distal limb to the main circulation.

“When you put the horse in the situation of increased circulatory demand due to the training and decreased circulatory response due to the time in the stall for months at a time, the required repair response is often inadequate and the damage begins to accumulate in excess of repair over time,” said Bramlage.

This became more apparent when nuclear scintigraphy or “bone scan” came into popular use for horses. Advanced imaging shows accumulation of damage and the repair process which had shown up as multi-limb lameness or a horse’s resistance to training.

Radiographs are able to detect the anatomic signs of bone bruising once it has reached the stage of changing the structure of the bone (more powerful hospital machines are more sensitive than field machines, but both are much improved over initial versions). They now lag behind the actual physiologic process which can be detected earlier with bone scans and now PET scans. Advanced imaging like bone scans and PET scans are great diagnostic tools, but they aren’t great surveying options for this type of issue because they detect increased biological activity in the bone due to normal remodeling as well as actual bone bruising. Diagnosis and monitoring of bone bruising makes use of both old and new imaging technology.

Once a horse has been diagnosed with bone bruising, what then?

“When we began to recognize this was an issue for horses, we started treatment in the wrong direction, thinking that if they had bruises in their bone, they needed rest,” said Bramlage. “As it turns out, if they have bruises in their bone, they need moderate exercise to support their circulation and repair. The solution is to reduce the high speed exercise, but increase the moderate exercise that the horse evolved to depend on: turnout.

“So how much exercise is this? We don’t have scientific proof, but a normal horse (based on wild horse studies) with adequate forage will graze six to eight hours per day if left to its own choice. Based on our experience, two hours per day is not enough. They need more than that. It depends on the severity of the problem, but six to eight hours a day is probably ideal.”

Bramlage said the number of turnout days that produce optimal healing from bone bruising also took some trial and error to confirm.

“Initially when we began to understand the issue, we tried 30 days and more than half of the horses had recurrence of the problem before racing upon returning to training,” he said. “When we lengthened the time to six weeks it helped, but a significant number still recurred prior to racing again. When we extended the time to 60 days of six to eight hours per day in the field, 95 percent of the horses made it back to racing again. If the stage of the disease is more severe it takes longer, but if diagnosed early, this is the program we normally recommend.”

Bramlage said some trainers hoping to keep a horse who is off form housed at the track will put a horse in a sand round pen for an hour. While this helps some, and probably serves as a useful way for the horse to release some energy, it’s not getting them as many steps as six to eight hours in a field. Putting a horse in a dry lot with a hay rack also doesn’t really fit the bill, because it will erase the horse’s need to move much beyond a track from the gate to the hay rack to the waterer. Walking in the shedrow and otherwise keeping the horse stalled isn’t ideal either, because it’s not practical for barn staff to walk the horse for the length of time they’d need to meet the circulatory benefits for repair. Then there’s the question of weather. Some people worry that turnout in winter is problematic for recovering horses, but Bramlage thinks it’s actually better. Unless the ground is covered with snow long-term or is a dry lot, horses will spend more time walking and browsing when grass is less lush.

“When people tell me that they’re going to take a horse who’s finishing the season, let’s say we’re in November, and they’re just going to keep them in the barn and walk them, I just cringe,” said Bramlage. “It’s probably the last thing the horse would choose to heal his wear and tear because you’re making him stand in the stall for 23 hours a day when he really needs to be moving around for the best circulation to support repair.”

While bone bruising is usually happening in the distal (or lower) cannon bones, other parts of the skeleton benefit from the same circulatory boost a horse gets in turnout.

Bramlage realizes this is a practical challenge at the track, and in some parts of the country where grass pasture is difficult to access.

“In an ideal situation we would train in the morning and turn out for a good part of the rest of the day in a paddock to keep the skeleton happy, but you can’t keep shoes on the horse in that situation. So, it doesn’t work very well,” he said.

Complete healing has long-term benefits; in the case of bone bruising, once the issue has healed, it doesn’t typically recur. It can be re-created over time, but normally that is after a significant number of races. Rapid recurrence normally means incomplete healing.

The timeline for turnout in a bone bruising case, or any bone healing, is tricky. Horsemen should be aiming for the horse to heal, but not for the remodeling activity associated with healing to have stopped and reversed to removing strength in the bone rather than enhancing it. Skeletal atrophy due to lack of training begins to be a concern after 60 days. More time isn’t always better.

“If you’re going to start the horse back in training, you really would like to start them back when the bone is most active, meaning it is finished healing but is in the most active remodeling stage.” said Bramlage. “The idea that you need to have totally quiet physiologic imaging like a bone scan or a PET scan makes the horse wait so much longer than he needs to, and much longer than is ideal, in my opinion. Once the healing is done and the remodeling phase has gotten to the point where the blood supply cell populations start to atrophy again, that horse actually does better if you put it back into training.”

That is not always an easy decision.

Turnout used to be a regular part of the season for most racehorses, and Bramlage thinks it has benefits even in cases where a horse isn’t diagnosed with bone bruising.

“If they are getting time and don’t have to be restricted for some specific injury to heal, I like to see them get into the field,” he said. “When we didn’t have year-round racing, every horse got a couple of months in the field every year, that’s probably what the horse would have chosen to get some time off.

“They like to train, but they need to get to a situation where they feel like training.”