Many researchers believe that there are risks of extremely serious complications, including death, when a runner trains during an acute viral infection particularly if the infection is produced by a Coxsackie virus.34 This virus, when unchecked, has a tendency to invade the heart muscle where it can potentially produce arrhythmias. Some reports suggest that athletes who engage in prolonged physical exercise during an upper respiratory system infection have a significantly increased risk of irreversible heart muscle damage.35
Given such evidence, Randy Eichner, MD, team physician for the University of Oklahoma, believes that infected runners should perform a “neck check” before deciding to perform a workout.36 Eichner’s neck check works in this way: If symptoms of illness are primarily above the neck (e.g., runny nose, scratchy throat, sneezing), it is reasonable to train at a moderate intensity and increase the level of effort if symptoms ease during the session. If symptoms are below the neck (e.g., fever, aching muscles, mucous-producing cough, vomiting, or diarrhea), resting, not running, is the prudent thing to do.
There is evidence that prolonged running can impair immune system function and increase the risk of illness in healthy runners. Marathon-type running produces negative changes in various components of the immune system, including the lungs, skin, upper respiratory tract, mucous linings of the digestive and respiratory systems, peritoneal cavity, blood, and muscles.37 The numbers and functioning of special immune system cells—natural killer (NK) cells, T lymphocytes, neutrophils, and macrophages—are also altered in response to marathon running or extended training sessions. An open window of immune dysfunction may last from 3 to 72 hours after a marathon or prolonged workout, thereby increasing the risk of infection.37
Various strategies have been proposed to thwart the negative changes in the immune system that can occur as a result of prolonged running. The ingestion of carbohydrate-containing beverages (i.e., sport drinks) during extended running may be the most effective strategy.37 Intake of such drinks seems to control the production of stress hormones that are linked to increased susceptibility to illness. Unfortunately, consumption of sport drinks is unable to control the suppression of antibody production and the NK cell and T lymphocyte activity that is common after challenging exertion. Marketers of sport supplements have promoted the use of glutamine, vitamin C, and bovine colostrum as immune-system boosters, but there is no scientific evidence that the use of such agents actually decreases the risk of illness in runners.38
Researchers have looked for other ways to preserve immune function during prolonged running. Workouts lasting less than 60 minutes have a smaller negative effect on the immune system than longer sessions,39 so it would appear that higher quality training, with an emphasis on recovery days, would be better for immune system function than repeated days of prolonged workouts. Mental stress, inadequate intake of calories, quick weight loss, and poor hygiene also impair the immune system, so they should be avoided, as much as possible, during periods of challenging training.39
Running in Hot Environments
In a hot or warm, humid environment, running capacity can be dramatically reduced,40 and the risk of hyperthermia induced by heat stress can increase. Hyperthermia is defined as an abnormally large—and potentially damaging—increase in core body temperature. Hyperthermia, and quite possibly even a too-rapid rate of increase in core temperature, can cause the central nervous system to reduce neural drive (i.e., neural output to muscles), thus lowering running velocity and overall intensity of effort.41 Nausea, dizziness, a loss of rational thinking ability, and a reduction in sweat rate can occur once core temperature surpasses 40 to 41 degrees Centigrade (104-105.8°F; cellular damage, especially to the nervous system, can take place at core temperatures above 43 to 44 degrees Centigrade (109.4-111.2°F). There is little evidence to suggest that older runners are at greater risk of hyperthermia compared with younger athletes.42, 43
The key strategy for preventing hyperthermia is to avoid running for any extended period of time in an environment to which one has not become physiologically adapted. Physiological adaptations to exercising in the heat include advanced rates of sweating, more-efficient sweating with sweat emitted more prominently from sweat glands all over the body rather than from smaller areas such as the arm pits, a quicker onset of sweating during exercise, and re-distribution of blood flow to augment blood flow to the skin. These adaptations promote the maintenance of a sub-40-degree core body temperature during hot-weather running, but the magnitude of the responses depend on the environment in which training is conducted and take time to be produced. For example, it takes 7 to 14 days to be acclimatized to 85-degree weather, and that acclimatization process must include daily, slowly progressing training at 85 degrees. Working out regularly at 75 degrees, even though such conditions might be warmer than usual, does not provide adequate acclimatization for hotter conditions.
Running in Cold Environments
Running in a cold environment for an extended period of time can increase a runner’s risk of hypothermia, a physiologically significant decrease in core body temperature. In such a situation, a runner’s rate of heat production during running simply cannot keep up with the rate of heat loss to the environment, and body temperature steadily falls. Unfortunately, a drop in core temperature can impair judgment, leading to an especially heightened risk of cold injury.
Heightened levels of body fat do not enhance running performance, but they do decrease the risk of cold injury during running.44 For this reason, female runners usually tolerate cold temperatures better than males and have a lower risk of hypothermia during cold weather running.45
Women tolerate cold-weather running more effectively than men and have a lower risk of hypothermia.
Loren Homes/Accent Alaska.com
Risk factors for hypothermia include reduced air temperature, wetness of clothing and skin, wind, exhaustion, sudden slowing of running pace, and the appearance of clouds on a previously sunny day. Many runners are surprised to learn that hypothermia can occur under relatively warm conditions. For example, even on a relatively warm spring day a sudden chilling rain can produce hypothermia relatively quickly, especially if one stops running.
The best ways to reduce the risk of hypothermia are to wear adequate clothing with wicking properties that minimize skin moisture, carry a waterproof jacket on days when there is a risk of rain, and avoid longer than usual runs under cold conditions. Longer runs that may require a slowing in pace or even stopping because of fatigue can heighten the risk of hypothermia. On windy cold days, it is better to go out against the wind and come back with the wind. This will help prevent slowing down during the second half of the run.
Conclusion
Running improves fitness and overall health, but it also carries with it certain risks. A key problem is the currently high likelihood of overuse injury. The risk of getting hurt as a result of running training can be lowered by upgrading running-specific strength training and enhancing recovery processes. Chapter 40 explores some techniques used for lowering the likelihood of running injuries.
The adoption of running as a long-term, almost daily form of physical activity lowers the risk of heart attack, but the specific act of running, either during a workout or race, is linked with a momentarily increased risk of cardiovascular mortality; thus, runners should be acutely aware of symptoms and signals of cardiac disorder and work closely with their primary care physicians when such warning signs appear. Periods of extended or intense running may also increase the chances of rhabdomyolysis and infection, but these potential consequences of exercise can be minimized when a runner trains prudently, increasing volume and intensity of exertion only moderately from session to session. The many positive health effects of running will be outlined in chapter 41.
Finally, running in hot and cold environments increases the risk of hyperthermia and hypothermia, respectively, but the risks of such disorders can be lowered through the use of special training techniques and strategic
adjustments of clothing. Acclimatization to warmer weather requires progressively prolonged training sessions carried out under the warmer temperature or increased humidity conditions. To avoid hypothermia when exercising in cold weather, do not expose wet skin to suddenly cooler air, do not slow down in cold weather, and do not get exposed to sudden bursts of wind during an extended run.
Chapter 40
Prevention of Running Injuries
Training is the primary producer of running injury. For any individual runner, there is a level of training beyond which injury will occur. This injury threshold varies dramatically between runners:1 An elite Kenyan runner might surpass his or her ability to stay healthy with a weekly load of 25 quality miles (40 km) and 100 total miles (161 km) while a novice runner could cross over the injury threshold with just 10 total miles (16 km) and 1 quality mile (1.6 km) per week. The limit undoubtedly rises for each runner as strength and fitness improve. To avoid injury during a progressive training program in which volume or intensity are increasing, every runner must find ways to lift the limit as high as possible and avoid crossing over the threshold. Runners should use mechanisms that prevent training stresses from outpacing adaptive processes in muscles, tendons, ligaments, cartilage, and bones.
Evaluating the Effectiveness of Flexibility
Despite the popular perception that enhanced flexibility helps to limit the risk of running-related injury, scientific research does not support this connection. One inquiry found that both high and low levels of flexibility were associated with a greater likelihood of injury in individuals undergoing strenuous training.2 A key problem in this area of research is that there are many ways to measure flexibility. It can be
static (depending on the end of range of motion [ROM] at a joint),
dynamic passive (a measure of stiffness or compliance of the muscles and connective tissues when they are at rest),
dynamic active (stiffness or compliance of the muscles when they are attempting to contract),3 or
running specific (the extent of range of motion of the ankle, knee, and hip joints during the act of running).
Running-specific flexibility is seldom measured even though it would appear to be the key flexibility variable associated with running injury, and the links between the other forms of flexibility and running-specific flexibility are unknown. It would be possible, for example, for a runner to have high static flexibility (the most commonly measured variable) and yet run quite stiffly. Overall, there is no scientifically based prescription for flexibility training for running, and no convincing assertions can be made about the link between flexibility and running injury.3
Stretching is often touted as an injury reducer, and research reveals that it can expand static flexibility.3 Given the uncertain relationship between flexibility and injury, however, it is not surprising that research concerning the effects of stretching on injury has produced mixed results. Two studies have linked stretching with a reduced risk of lower-limb injury. In one of these studies, preworkout stretching correlated with a higher chance of injury while postworkout stretching was associated with a lower risk.4 In the second investigation, the performance of three stretching sessions per week for the hamstrings cut the hamstring injury rate by about 42 percent over a 13-week period during the basic training of army recruits.5
Four other inquiries have found that stretching has no impact on injury rate;6-9 in three of these studies, stretching was conducted immediately before training sessions. It is possible that stretching, especially when it is performed postworkout, might have a small effect on reducing injury rates among runners, but this effect is often swamped by training excesses that overcome stretching’s protective action. Stretching might raise the injury threshold a little way but not enough to prevent injury from occurring in the majority of runners who stretch. If this is true, it would explain why stretching is seldom linked with protection against injury in scientific studies.
In one study, 159 Dutch runners were taught how to warm up, cool down, and stretch effectively while a second group of 167 similar runners received no instruction in these activities at all.9 The warm-up and cool-down consisted of 6 minutes of very light running and 3 minutes of muscle-relaxing exercises; the stretching was carried out twice a day for 10 minutes at a time, with an emphasis on increasing the flexibility of the hamstrings, quadriceps, and calf muscles. Over a 4-month period, injury rates were identical in the two groups, averaging about one injury per 200 hours of running.
A subsequent study actually found that preworkout stretching was linked with a higher rate of injury compared with no presession stretching while postworkout stretching was associated with lower injury rates.10 The mechanisms responsible for these findings are uncertain although it is certainly possible that stretching after training optimally prepares muscles and tendons for the quiescent, or rest, period that generally follows workouts. There is also some evidence from research conducted with chickens that postworkout stretching increases amino acid uptake by muscles and consequent protein synthesis, effects that should promote better recovery.11
Stretching after workouts is still a good idea, however. It relaxes muscles for the quiescent activities that follow training sessions, and there is evidence that stretching may improve carbohydrate uptake and glycogen synthesis in muscles.
Eccentric Strengthening Versus Flexibility Training
Hypothetically, flexibility training could be excellent for reducing the risk of muscle damage. It could extend the point at which muscles stop elongating in response to strain and thus delay reaching the point at which muscles begin to be torn apart in response to the forces being placed on them.12 To find out whether flexibility training or eccentric strengthening could do a better job of preventing training-related injury, researchers from the Norwegian School of Sport Science in Oslo and the University of Iceland in Reykjavik worked with a large group of athletes.13 For the research, male soccer teams were recruited from the highest-level Icelandic and Norwegian leagues. About 14 teams from Iceland participated in the study over a 4-year period, and 14 Norwegian teams took part during a 3-year research period; the number of players varied from 18 to 24 athletes per team.
The flexibility training includes a variety of traditional and partner-assisted stretches. The eccentric strength-training program involved one simple exercise: the Nordic hamstring drill. This exercise is performed as follows: Two partners kneel one behind the other and facing the same way. The front partner keeps the torso straight, staying extended at the back and hips. The back partner leans over just enough to hold onto the feet of the front partner. The front partner leans forward with a smooth movement, keeping the back and hips extended and working to resist forward falling for as long as possible by activating the hamstring muscles. While the front partner descends, the back partner maintains pressure on the front partner’s lower legs or ankles to keep the front partner from falling over. When the front partner’s hands and chest reach the ground, he or she forcefully pushes up and back with the hands to return to the kneeling position with the torso upright.
A key progression in the study with this exercise was to withstand the forward fall for a longer period of time; another was to increase the speed of the starting phase of forward motion. In addition, the partner in back added difficulty to the exercise over time by pushing on the backs of the front partner’s shoulders during the forward movement while the front partner resisted this additional pressure. Three sets of 12, 10, and 8 repetitions, respectively, were used per training session, and the Nordic hamstring training was carried out about three times per week.
During the subsequent season, flexibility training did not reduce the risk of hamstring injury at all while the eccentric strength training significantly cut the rate of hamstring malady. Overall, the rate of hamstring injury was 65 percent lower among the teams that employed the eccentric strength program compared with teams not using eccentric strength training. Hamstring injuries that did occur were also less seve
re when eccentric strength training had been used. This study suggests that the regular performance of challenging eccentric exercises can be protective against injury.
Such a finding is not overly surprising since eccentric actions appear to be the most stressful types of actions on the hamstrings during running. Electromyographic (EMG) analyses during high-speed running have shown that hamstring muscle activity is highest during the late swing phase of gait when the hamstrings are working eccentrically to decelerate the forward movement of the leg.14 Unfortunately, few endurance runners conduct systematic eccentric strengthening for their leg muscles, perhaps explaining why injury rates are so high during endurance training. As outlined in chapter 39, about 65 percent of endurance runners sustain a significant injury during a year of training, and as many as 93 percent of marathon runners are hurt over the course of a year.
In a related study, Roald Bahr and four colleagues from the Oslo Sports Trauma Research Center at the Norwegian University of Sport and Physical Education and the Stabaek Clinic in Bekkestua, Norway, researched the question of whether eccentric strengthening would be better than concentric strengthening for purposes of reducing the risk of injury. In contrast with eccentric actions, concentric activities involve force production and simultaneous shortening by muscles; eccentric actions involve force creation and synchronous elongation. Bahr and colleagues worked with 22 competitive soccer players, 10 of whom were from the first-division national club Stabaek Fotball and 12 of whom were from second- through fourth-division teams. At the beginning of the study, all athletes underwent basic tests of hamstring flexibility and strength as well as quadriceps muscle forcefulness. None of the 22 players were suffering from prior hamstring strains at the start of the study.15
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