Mammalian skeletal muscle tissue is the same regardless of which species of mammal it is in, said Steven Devor, the study's lead author and an assistant professor of exercise science education at Ohio State University.

He and his colleagues studied the effects of aerobic exercise - in this case, galloping on a treadmill - on small sections of skeletal muscle tissue taken from the limbs of retired racehorses. The findings support a "use-it-or-lose-it" philosophy: After 10 weeks of regular workouts, the horses' muscles showed fewer signs of damage caused by exertion, even after the horses worked out at their maximum capacity.

The results apply to humans and are especially important for older adults, Devor said.
"We have to work at keeping muscle mass as we age, otherwise that mass wastes away," he said. "This weakness leaves a muscle more prone to injury even when it's the least bit exerted. Also, joints are less likely to break if the musculature surrounding them is strong."

"According to these results, aerobic exercise training improves the ability of aging skeletal tissue to resist injury," Devor said.

He and his colleagues report their findings in a recent issue of the Journal of Applied Physiology.

Some minor muscle damage is normal after a new or a particularly difficult workout. The pain that often appears a day or two after such exertion is called delayed onset muscle soreness, or DOMS.

"The way to get rid of this kind of pain is to stay physically active," Devor said. "It's ironic, but muscles are most often injured during exercise. But muscles get stronger by repairing this damage."

The current study builds on experiments Devor previously conducted in rats - about 10 years ago, he helped identify the mechanism that causes DOMS.

He was part of a team that found that this damage happens when tiny skeletal muscle segments called sarcomeres - the smallest units of contractile muscle - pull apart as a muscle lengthens.
Contractions that lengthen muscles are particularly damaging to sarcomeres. And lengthening contractions are some of the most common type of contractions humans do - leg muscles contract and lengthen as we sit down or walk and run, and arm muscles contract and lengthen when we lower heavy objects.

The six quarter horses in the current study ranged in age from 23 to 30 years, which made the animals elderly by horse standards. A horse usually lives for about 28 to 32 years. The animals used a treadmill - a long conveyor belt built into the floor of a barn - three times a week for 10 weeks. Each workout lasted about 20 minutes. The horses got little to no exercise during the three months leading up to the study - that way, the animals would have nearly the same fitness level once the study began.

The researchers increased the speed and resistance of the treadmill during each session, and the animals spent about 15 minutes of each workout exercising at a relatively high intensity. Training protocols were updated every two weeks, based on the animal's performance and its response to the given workload.

The researchers examined muscle tissue taken from each horse's forelimb (triceps brachii) and hindlimb (semimembranosus - a large muscle of the thigh, and also the largest muscle the researchers looked at.) Both muscles are used during walking and galloping. The researchers also removed a small piece of the masseter, a muscle that helps the jaw close during chewing. The masseter served as the control.

The researchers removed small portions of tissue from each muscle before and immediately after the first and last treadmill sessions, and also before and after a session during the eighth week of training.

The treadmill was set at the same speed and resistance during that eighth-week workout as it was during the very first workout, in spite of increases in speed and resistance in the weeks between the two sessions. The researchers wanted to see if nearly two months of exercising would better protect the muscles from damage. During the very last workout, the horses ran at their maximum capacity until they reached exhaustion.

Eight weeks of exercise had a considerable effect on the hindlimb muscle, as the degree of muscle damage had decreased three-fold by then. After the first workout, the researchers noted a five-fold increase in damaged sarcomeres compared to the muscle tissue they examined prior to the workout.

"It wasn't serious damage, but the horses probably felt a little sore afterward," Devor said. "A human would definitely notice some soreness if they hadn't been regularly exercising."

After the workout during week eight, researchers measured only a two-fold increase in the prevalence of sarcomere damage in the hindlimb muscle. They saw the same results two weeks later, after the very last treadmill session.

"The muscle had become more resistant to injury by week eight," Devor said. "And it was stronger, too, since the horses worked as hard as they could during the very last treadmill session."

The triceps, however, showed about the same amount of sarcomere damage - about two-and-a-half times more damage - before and after each of the workouts.

"The bigger muscle responded in a positive way to several weeks' worth of conditioning," Devor said. "It suggests that the protective effect of aerobic training may benefit larger muscles more than smaller ones.

"It also suggests that there was less post-exercise pain after the later workouts," he said, adding that the horses could run up to 24 percent longer by the end of the study.
As expected, the masseter, or jaw muscle, was unaffected by the workouts.

"The bottom line is that since the horses had kept up with their training program, there were dramatic reductions in the amount of muscle tissue injuries the animals had by the end of the study," Devor said.

"This suggests that, in older adults, regular exercise may help prevent injuries associated with age-related impairments such as reduced muscle strength, impaired mobility and a tendency to fall."

Devor conducted the study with Ohio State colleagues Kenneth Hinchcliff, Mamoru Yamaguchi and Laurie Beard, all with the college of veterinary medicine, and Chad Markert, formerly with Ohio State's sport and exercise science program. The work was a portion of the doctoral dissertation of Jeong-su Kim, who is presently at the University of Alabama, Birmingham.