Running Science
Page 54
There are five anatomical hot spots for overuse running injuries:1, 5
The knee area (25 to 30 percent of all running injuries)
The calf and shin (20 percent of running injuries)
The iliotibial band, the sheath of muscle and connective tissue that runs down the outside of the leg from the hip to just below the knee (about 10 percent of injuries)
The Achilles tendon (approximately 10 percent of overuse problems)
The foot, including the plantar fascia that runs along the bottom of the foot (another 10 percent)
Iliotibial band problems appear to require the longest recovery period following the occurrence of injury compared with the other four areas. In this case, recovery is defined as the adaptive and healing processes that eventually produce a return to normal training. The good news is that approximately 65 percent of runners report that they are running pain free 8 weeks after the initial occurrence of an injury.6 The bad news is that this finding means that 35 percent of runners are on the shelf for longer than 2 months when an injury occurs, which is an extended recovery process.
Another sobering fact is that the occurrence of an injury is associated with an increased risk of future overuse injury; runners who have previously been injured are more likely to get hurt again over any relevant period compared with runners who have been injury free.5, 7, 8 In fact, science reveals that the absolute best predictor of running injury is a prior history of injury. About 50 percent of injuries occur in the exact spot where a prior injury had occurred, suggesting that weakness of the injured area is a root cause of running malady.9
Despite increased knowledge about training and sports medicine and the advent of sophisticated, allegedly injury-preventing running shoes, rates of overuse running injuries have not lessened over the past 30 years. There is some evidence that the frequency of knee injuries among runners has actually increased over the past three decades. This suggests that the fundamental causes of running injury have not been understood or properly addressed by runners and coaches.
Risk Factors for Injuries
The risk factors associated with running injury have been explored in various research studies. Many coaches and runners believe that males have higher injury rates than females, but research indicates that male and female runners have very similar frequencies of injury per hour of training. This is not true among high school cross country runners, however. Research reveals that high school girls running cross country have an injury rate of about 19.6 injuries per 1,000 athletic exposures (AEs), where athletic exposure is defined as a workout or race.10 This equates to one injury per 51 sessions or races, which would mean that nearly every young woman on a typical team would be likely to suffer injury over the course of a 12-week season that would usually include 60 to 72 workouts or races. Male high school cross country runners have an injury rate of approximately 15 injuries per 1,000 AEs, or one injury per 67 workouts or races. Compared to the general population of runners, high school girls also have significantly higher rates of types of injuries from which recovery takes longer than 15 days.
Training speed, racing speed, running surface (e.g., running on concrete instead of trail or track), and body weight (i.e., having a greater body mass) are often cited as risk factors for running injury, but scientific research indicates that they are not linked with a heightened frequency of problems. There is controversy about whether foot-strike pattern—landing on the heel versus striking the ground with the forefoot or the middle of the foot—has an effect on the risk of running injury.11
Newcomers to the sport of endurance running are significantly more likely to be injured compared with experienced runners who have been in training for 3 or more years.11, 12 This suggests again that running-specific weakness is a risk factor for injury and further implies that many runners would profit from undertaking a systematic program of running-specific strengthening prior to embarking on an actual running program.
The part of the body at risk of injury depends to some extent on preferred race distance. Marathon runners are more likely to develop foot problems such as plantar fasciitis than runners favoring shorter competitions, and middle-distance competitors have higher risks of back and hip difficulties. Sprinters injure their hamstrings much more frequently compared with endurance runners, and sprinters have about double the rate of injury per hour of actual training.1 For endurance runners, an extremely good injury predictor is simply the distance run during the previous month of training. For example, a runner who engages in high-volume training in November is at significantly increased risk of injury in December.13
Among the general running population, the actual rate of injury is about 1 injury per 150 to 200 hours of training; this does not conflict with the high school runner data because high school runners tend to be more inexperienced and lacking in running-specific strength. This means that total training distance is generally a solid predictor of injury: The more miles or kilometers a runner accrues per week, the higher the risk of injury in any relevant time period.5, 8 The increased risk may be an exponential rather than a linear function of distance: Scientific investigations have found a marked upswing in injury risk when runners logged over 40 miles (64 km) of training per week.2, 12
As mentioned, the best predictor of injury is a prior history of injury. Scientific investigations suggest that the second-best predictor may be the number of consecutive days of training without a recovery day.14 This finding reinforces the idea that recovery days can stop cumulative stress on muscles and connective tissues and promote the kind of rebuilding processes that can keep injuries at bay. A runner who runs 6 miles (9.6 km) per day Monday through Friday and rests on Saturday and Sunday is posting 5 consecutive days of training per week and could probably reduce his or her risk of injury by running just 4 times a week (e.g., Monday, Wednesday, Friday, and Saturday) for 7.5 miles (12.1 km) per session. In the latter case, weekly distance run would be the same as for the 5-day plan, but the number of consecutive days of training would fall from 5 to 2, and injury risk would be reduced because of the extra recovery time. In a similar vein, the serious or elite runner who runs every day could probably reduce the risk of injury—with no negative impact on fitness and quite possibly a positive one—by taking one complete day of recovery each week.
Psychological factors can play a role in producing running injury.15, 16 Research suggests that individuals who score high on inventories measuring exercise dependency and Type A behavior patterns tend to have higher rates of injury, especially stress fractures of the tibia. The scientific work suggests that runners who depend on regular running to manage stress-related mood states are at greater risk, perhaps because they are more apt to conduct higher-volume training and are more willing to train through pain than runners who do not depend on running for psychological relief.
Marathon training and racing are particularly risky as they are usually performed without appropriate running-specific strength training. About 16 percent of marathon entrants suffer a significant new injury during the month preceding their marathons, and approximately 18 percent become injured during the race itself.17
Although the information about injury rates among endurance runners is somewhat bleak, there is good news: Research suggests that running injury rates can be cut by at least 25 percent—and probably more.18 Two effective strategies for cutting the risk of injury are to optimize recovery between workouts and to upgrade running-specific strength. These practices are discussed thoroughly in chapters 14 and 21.
Acute Health Risks Associated With Endurance Running
Although running reduces the risk of heart attack, runners are not immune to cardiac problems. The act of running actually increases the likelihood of a heart-related difficulty during the exertion (compared with resting), especially if there are any underlying disorders in the cardiovascular center. Running beyond one’s usual limits can also produce an occasionally life-threatening disorder called rhabdomyolysis, in which muscle and kidney fun
ction can be drastically disrupted. Running strenuously or for very long periods can also increase the risk of developing an infectious disease, and exercising in a cold or hot environment is associated with various risks.
Risks to the Cardiovascular System
According to legend, one of the first endurance runners in recorded history—Pheidippides—dropped dead shortly after a 21-mile, 1,470-yard run from the plain of Marathon to the agora of Athens in 490 B.C. No autopsy was performed on the Greek messenger, and it is possible that his death could have been caused by dehydration, heat stress, or an unsettling encounter with the god Pan in the mountains north of Athens as described in some early accounts of this first marathon. A more likely cause of death would have been myocardial infarction, or heart attack.
Such exertion-related sudden deaths are not as uncommon as many runners believe. One scientific study suggested that the death rate during marathon running is about one fatality per 50,000 participants,19 which would make marathon running far less safe than traveling in a commercial airliner. This investigation followed a total of 215,413 runners who competed either in the Marine Corps Marathon from 1976 to 1994 or in the Twin Cities Marathon from 1982 to 1994. Three of these runners died during their races, in all cases after the 15-mile (24 km) point of the competition, and one succumbed shortly after completion of the event. All four deaths were attributed to heart attacks.
Other research suggests that the death rate associated with running might be somewhat lower. One study found that in male runners between the ages of 30 and 64 who have not been diagnosed with heart disease, there is approximately one death per each 800,000 person hours of running or jogging.20 This finding essentially means that if 800,000 apparently healthy middle-aged males began running the London Marathon, one of them would die during the first hour, another during the second, a third within the third hour, and so on.
There have been eight cardiac deaths at the London Marathon during its 27-year history, a rate of one death per 3.4 years, or one death per 80,000 completed marathons;21 this is somewhat lower than the 50,000 calculated from the Marine Corps–Twin Cities inquiry. Overall, a male marathon runner is about seven times more likely to die during or shortly after a marathon compared with engaging in nonrunning activities over the same period.22 Female marathon runners have a significantly lower risk than males, but their actual mortality rate has not been determined.
While such numbers are troubling to some runners, it is important to note that the rate of one death per 800,000 hours means that an individual runner’s risk is quite low. A healthy, middle-aged male who runs for 1 hour each day can expect to die while running once every 2,192 years (800,000 hours/365 hours of running per year = 2,192 years). Individuals who run 2 hours per day have a risk of dying while running about once every 1,096 years. Viewed in this light, many endurance runners believe that the risks of cardiac death are acceptably low especially since the overall risks of heart disease and myocardial infarction are diminished by endurance training.
When cardiac deaths occur during running, they are usually not random events caused by fleeting disturbances in heart function. Postmortem analyses characteristically reveal that something was wrong with the dead runner’s heart prior to the fatal run. In the Marine Corps–Twin Cities study, three of the four runners who passed away had atherosclerotic coronary artery disease (i.e., narrowing of two or three key coronary vessels) even though they had been symptom free prior to their races. The fourth individual had an anatomical defect related to the origin of his left main coronary artery; this runner was also symptom free going into his race.19 Out of the eight deaths at the London Marathon, five individuals were found during autopsies to have coronary artery disease, and the other three suffered from hypertrophic cardiomyopathy or idiopathic left-ventricular hypertrophy, enlargements of the heart wall.22 Only one of the eight runners had reported symptoms of heart disease to his physician or family before the race.
Perhaps up to 50 percent of individuals who have heart attacks while running do experience some warning signals during the weeks leading up to the attacks even though they do not necessarily report these symptoms to friends and family,22 so it is important for endurance runners to monitor themselves closely during training. Premonitory symptoms of heart trouble, for men and women, include chest discomfort, squeezing sensations in the chest, throat tightness, pain that radiates into the left jaw or left shoulder and arm, unusual fatigue, a sudden unexplained decrease in performance capability, and heart palpitations. Discomfort that appears during running and then disappears afterward is of particular concern.22
Exercise Stress Tests: Effective Screening?
Findings concerning cardiac problems and death during running suggest that screening marathon entrants for heart troubles prior to the race would be a good idea. For middle-aged and senior male runners, it has been suggested that exercise stress tests might detect existing heart disease and thereby decrease the number of deaths on race day. About 34 percent of physicians who run the Boston Marathon believe that individuals should undergo an exercise stress test before beginning a marathon training program.23
While this appears to be a reasonable concept, exercise stress tests have their own set of problems. Up to 63 percent of those who fail such exams have completely normal cardiovascular systems.24 The rate of such false positives can be even higher among endurance runners because of the natural thickening of the heart walls in response to endurance training; this thickening produces changes in EKG signals which can be interpreted as being abnormal.
Stress tests themselves are by no means risk free. The death rate associated with taking a stress test has been estimated to be as high as one per 20,000 tests25 and as low as one per 500,000 tests.26 If the true rate is greater than one per 80,000, undergoing a stress test would be more risky than running a marathon, the event for which the stress test is supposed to reduce risk.
Finally, the majority of individuals who die during running because of heart troubles would have completely normal stress tests even if the tests were administered the day before death occurred.27 Some experts believe that stress testing can detect only 20 to 25 percent of the likely victims of sudden, running-related death. The one London Marathon fatality who had undergone a stress test prior to the race had received a negative test result—an all-clear indication to run the competition.21
Rhabdomyolysis
Long training runs, marathon competitions, and ultramarathon running can produce significant damage to leg-muscle cells as the exercise is being conducted. This process of muscle destruction can produce a condition called exertional rhabdomyolysis. In many cases, the damage is moderate and is resolved gradually during the days following the workout or race even when there is considerable harm to the muscle fibers as in a 246K (153 mi) race like the Spartathlon.28
When the damage is quite significant, however, potassium can pour out of muscle cells into the blood and surrounding tissues and interfere with normal heart function. Muscle proteins, including a unique protein called myoglobin, are also released into the blood through ruptured muscle membranes. Inside muscle cells, myoglobin acts as a storage depot for oxygen. When it is dropped into the blood in extra quantities as a result of rhabdomyolysis and reaches the kidneys, myoglobin can break down into a toxic chemical called ferrihaemate that injures kidney cells. The damaged renal cells may then fail to eliminate the rising tide of potassium ions.29
Myoglobin’s accelerated appearance in the kidneys can also lead to myoglobinuria, the presence of excess amounts of myoglobin in the urine. Runners can identify this condition without medical testing; in myoglobinuria, the urine is dark purple, resembling the color of Coca-Cola. Excessive quantities of myoglobin in the kidneys can produce acute renal failure.30 The risk of kidney failure appears to be increased if the runner is dehydrated or taking analgesic medications, including nonsteroidal antiinflammatory drugs such as ibuprofen.30
The risk of serious rhabdomyolysis increases as a f
unction of the distance run. The Comrades Marathon, a 90K (56 mi) race, averages about one case of significant kidney damage per year.31, 32 The regular marathon distance is not immune to the problem, and deaths from rhabdomyolysis have been reported in association with marathon running.29
Risk factors for developing serious rhabdomyolysis during running include having a recent viral illness, experiencing dehydration, using analgesics and nonsteroidal antiinflammatory medications, and engaging in running that is significantly longer than usual. Running in hot weather also appears to increase the risk. The best strategy for avoiding rhabdomyolysis is to stay within usual limits of intensity of volume and to avoid runs that are far more extended than usual. The runner who ordinarily runs 5 miles (8 km) per day and suddenly decides to run a half marathon, especially if it is in the heat, would be a prime candidate for significant rhabdomyolysis.
Running and Infections
Scientific research suggests that strenuous running can worsen the effects of a bacterial or viral infection. Although it has been nearly wiped out through vaccination, the polio virus provides an example of how strenuous exercise worsened an infection. A series of papers published in the British Medical Journal in the late 1940s linked intense physical activity with the severity and extent of paralysis suffered by individuals with polio. The research suggested that heavy exertion weakened the resistance of motor neurons in the spinal cord and offered polio viruses increased opportunities to occupy and destroy these muscle-controlling cells.33