INTRODUCTION
"Running as an exercise can strengthen the limbs, develop the lungs, exercise the will and promote the circulation of the blood. The clothing should be light, the head bare and the neck uncovered. Care must be taken not to overdo."
-Scientific American, 1883
Running remains one of the most popular ways for Americans to seek fitness. Estimates suggest that 10%-20% of Americans run regularly. Important historical events helped popularize running. Dr. Kenneth Cooper's best selling book Aerobics, first published in 1968, initiated a dramatic shift in the theory of how people could most efficiently reach high levels of fitness (12). His work with United States Air Force trainees demonstrated that aerobic fitness programs led to faster and more efficient training of Air Force Cadets. Before its publication, there only were estimated to be 100,000 regular runners in the United States. After Cooper's book, the military and many individuals built their basic fitness programs around running.
In 1972, Frank Shorter won the Olympic marathon in Munich. This victory inspired running in Americans and caused a rapid growth in marathon participation. Whereas most major marathons in the 1960s had fewer than 1000 competitors, today major marathons register more than 30,000 competitors. Another factor fueling the "running boom" was the publication of The Complete Book of Running by Jim Fixx in 1977 (23). This became a how-to guide for the individual who wished to become a runner. This bestselling book remains in publication today and newer how-to programs, such as the Galloway training method and the Team in Training running schools, have brought large numbers of people to running.
There is strong evidence that running is one of the most efficient ways to achieve fitness and promote long-term exercise, and equally strong evidence links fitness with longevity. Competitors of all ages participate; after 10 yr, 56% still run and 81% exercise regularly (39). The biggest issue with running remains the high rate of injury. Generally accepted numbers suggest that approximately 50% of runners experience injury yearly, and 25% are injured at any given time.
Considering the high injury risk, prevention of running injury is an important issue in sports medicine and general health care. A metaanalysis gives an overview of the general factors known about running injuries (66). These authors noted evidence for the following:
- ▪ Yearly incidence of long-distance running injuries is high with variability of 19.4%-79.3%.
- ▪ Higher running miles per week in male runners is a risk factor.
- ▪ History of previous injury predisposes to subsequent injury.
- ▪ Increased training miles per week appears protective against knee injuries.
There also is one controlled trial that sought to determine the incidence of running injuries and the probable causes. In this study, 115 runners were followed for 18-20 months. During the course of the study, 85% of the runners experienced an injury significant enough to miss 1 d of training. Mileage greater than 40 miles·wk−1 was the strongest factor associated with injury with a relative risk (RR) of 2.88. Daily running and length of long run were possibly related to injury occurrence. The second factor identified was a history of previous injury in the past 12 months. Those individuals had an RR of 1.51 of new injury (5).
Because there are very limited data based on controlled trials of runners, these conclusions explain only part of the complex variables that determine why some individuals can seemingly train for years at very high mileage without injury, while others develop repeat injuries even though they never reach mileage greater than 15-20 miles·wk−1. This article expands upon the individual factors that have been implicated as possible triggers of running injury. These include anatomical variants in individual runners, biomechanical factors that affect running form, use of orthotics and shoes, training errors and total running mileage, strength training and muscle weakness, warm up, stretching, nutritional errors, and psychological factors.
Anatomical Factors and Running Injury
A few specific anatomical factors consistently appear in running injury research. Cavus feet demonstrate higher impact on force-plate studies and often are more rigid. Numerous indirect and some specific studies have linked cavus feet to greater risk of injury. In a cohort study of 70 runners who had either anterior knee pain or were asymptomatic, Duffey et al. found that the injured group had 25% less pronation. The best predictors of knee pain in this study were less pronation in the first 10% of stance phase, arch height, and shoe mileage (17). Most individuals with cavus feet demonstrate supination on gait. In a prospective trial, anatomical factors proved poor predictors of patellofemoral syndrome (PFS), with the exception of genu varus and forefoot supination, both of which are often associated with cavus feet (44). This study supported findings in an earlier comparative study of 15 patients with PFS and 15 control subjects, which showed that the injured group had greater rearfoot varus (52). Another prospective trial of military recruits found that, of 84 cadets, 36 developed PFS. Gait analysis revealed that the injured group had plantar pressure patterns and push off, both demonstrating supination. These studies linking under pronation, supination, genu varus, and rear foot varus with injury all suggest an association with cavus foot shape, although that was not directly measured (64). The higher injury risk is consistent with studies of plantar pressure patterns, which show that the forefoot load patterns are significantly higher in those with cavus feet (61).
Cavus feet also were found in a prospective trial of military recruits to have a much higher risk of overall lower extremity injury, with an RR of 6 noted for individuals with the highest arch height (13). In a study of recurrent stress fractures in which 60% of the affected athletes were runners, cavus feet were noted in 40% of the injured group vs only 13% of the control group (41). Thus cavus feet have been implicated in at least two specific injury problems, PFS syndrome and recurrent stress fractures, as well as overall lower extremity injuries. However, no prospective data indicate that treating cavus feet with arch support, orthotics, or other interventions will lessen running injury risk.
Other anatomical problems have fewer data to support their role in running injury. Leg length inequality has been suspected as a factor in hip, pelvis, iliotibial band syndrome (ITB), and low back injury among runners. One study that found a specific association of leg length inequality with injury was the Korpelainen study of recurrent stress fractures (41). A survey study of injuries in 1505 runners also concluded that biomechanical imbalances such as leg length inequality appear to be a major contributing factor to running injuries (6). "Miserable malalignment" and genu valgum are among anatomical changes often mentioned as increasing risk in runners; however, no studies to support these clinical observations have evolved.
Muscle weakness is suspected of increasing overall injury risk in the lower extremity. Hip muscle weakness has been identified in several studies of injured runners. Niemuth noted both hip abductor and hip flexor weakness in 30 injured runners on the affected leg (49). Hip weakness also has been identified as a factor in specific injuries such as ITB syndrome. In a study of 24 patients with ITB, hip abductor weakness was identified in the affected leg of the symptomatic patients. Rehabilitation exercises led to a 51% and 35% strength gain in abduction in men and women, respectively. This also led to resolution of symptoms in 22 of 24 cases in only 6 wk (25).
Typically, vastus medialis obliqus muscle weakness has been the key deficit suspected in PFS. This is based on clinical observation of atrophy in this muscle and also on multiple trials that demonstrate that quadriceps exercise regimens, either open or closed chain, lead to improvement in symptoms in athletes with PFS (73). More recent research focuses on hip muscle deficiencies as possibly contributing to PFS. A study of 13 injured collegiate athletes demonstrated significant weakness in both hip abductors and rotators not found in the unaffected leg or controls (10). A study with limited subjects does not add strong evidence to prove the association of hip strength with PFS, but other observations of hip weakness in patients with PFS suggest that this merits further study (33).
ECCENTRIC STRENGTH AND INJURY
Because running involves several eccentric contractions, it would seem plausible that eccentric strength training could be beneficial in helping runners avoid injury. While there is evidence for benefit of eccentric strengthening in treating patellar tendinopathy, achilles tendinopathy, PFS, and hamstring strains, there are no prospective, primary prevention studies in runners. A recent systematic review of eccentric exercises for Achilles tendinopathy showed promising results (37). Because Achilles tendinopathy and PFS are common injuries incurred by runners, further study is warranted on eccentric exercises for primary prevention of these running injuries.
ORTHOTICS
Athletic foot orthotics are shoe inserts that replace the removable stock insole. Providers may use either custom-made or off-the-shelf orthotics with the goal of correcting biomechanical pathology, cushioning the foot, and/or prolonging time to muscle fatigue in order to prevent injury or help an injured runner recover (62). They have been in use for over 50 yr (32). Debate exists regarding their mechanism of action and the evidence for their use (3,9,27,29,36,54). However, in the last 5 yr there have been several good studies supporting their use for injury treatment. Double-blinding is challenging in that patients usually can tell the difference between using an orthosis and using a sham orthotic or the stock insole. Many studies have been performed in the past 30 yr showing that patients have high levels of satisfaction and subjective improvement with orthotic use (14,27,48).
There are relatively few articles in which prevention of injury was the focus of study. Most used off-the-shelf orthotics or shock-absorbing insoles for military conscripts. A Cochrane review analyzed the literature regarding the role of orthotics in avoiding stress fractures. They concluded, based on studies in military basic training, that shock-absorbing insoles "probably reduce the incidence of stress fractures" (56). Not all studies showed benefit. One randomized, controlled trial looked at shock-absorbing insoles vs standard mesh insoles issued to military recruits at the initiation of training. There were similar injury rates (outcome was any lower extremity injury) for both types of insoles (72).
Several recent high-quality studies on orthotics bear mention. A single-blinded, prospective trial evaluated treatments for PFS in women. Heat-molded orthotics proved equally effective to office-based physical therapy for PFS at 6- and 12-wk analysis (endpoints were global improvement, pain control, and functional level). The molded orthotic and physical therapy were both more effective than flat insoles. Interestingly, the same study showed that at 1 yr, all three interventions had the same endpoint scores, i.e., not statistically significant (11). A newly published, prospective, nonblinded study compared runners with lower extremity pain, randomizing them with orthotics or no intervention. The orthotics group had significant improvement in pain and in runners' perception of comfort at the 8-wk endpoint (31).
Two Cochrane systematic reviews on orthotics showed that there is a clinically significant decrease in symptoms for painful pes cavus with orthotic use. This also is the case in conditions less commonly seen in runners (juvenile rheumatoid arthritis and painful hindfoot in rheumatoid arthritis). The benefits of orthotics for plantar fasciitis are inconclusive (8,28).
In summation, evidence indicates that orthotics can prevent stress fractures. We found few studies that directly addressed prevention of other running injuries with orthotics; these were in military settings with noncustom orthotics or shock-absorbing flat insoles, and results were mixed. There are many studies that show custom orthotics are an effective treatment for several running-associated injuries, including PFS and pes cavus-related pain. Those of recent years are well designed, prospective, and focus on patient-based outcomes. Based on the clinical experience of the authors, we suspect that future research will show that orthotics can be a preventive tool to avoid many running injuries, but more research and clearer research design must evolve to test this hypothesis.
RUNNING SHOES
In the 1970s, the cushioned, waffle-sole running shoe revolutionized the sport at the same time that long-distance running and racing shifted from a competition among the elite to an exercise for the masses. Sports physicians debate the role of running shoe quality, type (cushion vs motion control), and build with regards to injury prevention and treatment. Traditional thinking has held that using a well-made, cushioned running shoe will decrease risk of injury (42,48).
Most sports medicine providers have accepted the concept that fitting foot/gait type to specific shoe type also decreases injury incidence (e.g., motion control shoe for overpronators, cushion-type shoe for pes cavus/supinators) (35,55). Two studies show that patient-oriented research is needed to confirm current practice. One study evaluated over 3000 soldiers in military basic training. There was no difference in injury rate between the experimental group (fitted to motion control, stability, or cushion type running shoe based on low, medium, or high arch type; the arch type derived from plantar shape assessment by foot imprint) vs the control group (all in stability-type running shoe) (38). A recent article reviewed studies on running shoe type and injury patterns. Per this review, no studies were of sufficient quality to give an evidence basis for recommending a "shoe prescription" for a specific foot type (55). On Medline review of "running shoe and injury" and "running shoe and prevention," there are many studies looking at cushioned running shoe function from a kinematic standpoint, but none were found that look at prevention of clinical injury. A prevention-focused study would pose the challenge of finding a good control, if using a cushioned running shoe is the intervention.
An increasingly popular concept in the media (and literature) is barefoot running (43,70), in which shorter stride and lower-impact midfoot to forefoot strike (as opposed to heel strike in cushioned shoes) are hypothesized to decrease risk for injury. The counterargument is that running on hard surfaces without cushioning will increase risk of stress fracture and associated injury. All articles on barefoot running to date are based on anthropology and theory. It will be interesting to see what develops as injury and prevention studies unfold.
TRAINING ERRORS
One of the most common training errors that leads to injury is excessive mileage (34,40,46,66,71). Jacobs et al. estimated that 60% of running injuries were due to training errors, with half of those attributed to excessive mileage (34). Another study suggested that sudden changes in routine or excessive mileage were possibly associated with up to 72% of the running injuries reported (45). Running injuries also were more common in those who train year-round (71).
Several studies show a correlation between higher mileage and injury rates. Weekly mileage of greater than 40 miles (64 km) in men was associated with a higher risk of injury (OR 2.9, 95% CI 1.1-7.5) (34,46,66,71). Interestingly, in one study, higher mileage training showed a protective effect for knee injuries but caused more thigh/hamstring problems (59). A randomized, controlled trial among young, healthy, male prison inmates evaluated differences among groups who ran 15, 30, or 45 min, 3 d·wk−1 for 20 wk and found that there was a direct correlation between longer running duration and injury rates. The same study also evaluated a second population of inmates who were assigned to run 1, 3, or 5 d·wk−1 for 20 wk. Similarly, as the frequency of running increased, so did the rate of injury. Other studies also have shown that increasing running frequency was associated with a higher rate of injury (34,71). In Walter et al., male runners who ran more than twice per week were at increased risk for injury. These studies did not control for total weekly mileage, which may be a better indicator of injury risk. When Marti et al. evaluated a subgroup of runners who ran the same weekly distance over two, three, or four separate sessions, there was no difference in injury rate (47). A study conducted among military recruits showed that a reduction in a 12-wk basic training running program (280-82 km) resulted in fewer injuries (57).
Unfortunately, much of the information on the association between mileage and/or running frequency and injury was studied in male participants, so few data are available for female runners. While men had a higher incidence of injury as mileage increased, there was conflicting evidence for women.
Experience level has been studied as a potential risk factor for injury, as well, and the results have been mixed. In first-time marathoners, hamstring and knee injuries were more common, but more experienced runners had more foot-related injuries (66). Macera et al. reported that runners with fewer than 3 yr of experience had an odds ratio of 2.2 (95% CI: 1.5-3.3) for injury (46). Another study among that high-school cross-country runners did not show a significant difference in injury rate among those with 0 to 3 yr of experience (53).
Erratic training schedules have been shown to increase injury rates (e.g., a sudden increase in weekly distance or change in type of training such as hill training or interval work). This was evident in studies of military recruits. Those that entered into basic training with a background in running had fewer injuries (58). In contrast, one study by Andrish et al. was a prospective study in which Navy midshipmen were entered into a graduated running program before the normal physical education program (2). Compared with the controls who did the usual physical education program, those who gradually increased their mileage over 2 wk actually had a higher rate of injury. There was, however, a statistically significant higher rate of "shin splints" that developed in those midshipmen who had had no previous physical training than those who did. A study by Jacobs and Bersen found that one third of the runners they studied had either changed their training schedule or shoes just before their injury (34).
It has been postulated that running surface may be associated with injury. With the exception of female runners experiencing higher injury rates when running on concrete, no association was seen with harder surfaces, hilly terrain, or running in the dark (35,46,47,53,66,71). Although Jacobs et al. found a direct correlation with increasing training speed and injuries (35), most studies show no association between training speed and injury risk (40,53,66,71).
In summary, excessive mileage and changes in training schedule are associated with an increased incidence of running injuries. Since each person's body responds differently to the stress caused by running, individualized training programs are recommended. More research is needed to make recommendations for the advanced/elite runner.
Stretching and Running Injury
Perhaps two large metaanalyses provide the strongest evidence to question the value of preexercise stretching for runners. A review of 12 major trials with 8806 runners examined the role of stretching in preventing lower extremity running injuries. The data were insufficient to suggest any benefit of stretching in preventing running injury (75). A comprehensive metaanalysis of articles spanning 1997 to 2002 examined the role of prexercise stretching to prevent sports injuries and concluded that there was no evidence for reduction of injuries (63).
These two large metaanalyses reinforced the results of studies that surprisingly had shown a trend toward a small increase in injury among runners who stretched vs control groups. This seemed to most sports medicine practitioners to be counterintuitive. Typical of the findings was a randomized trial of 421 runners with stretching, warm up, and cool down before and after running. This trial showed a slightly lower injury rate in the control group vs the group that used stretching as an intervention before running (67). Another intervention in a military population showed that 1538 army recruits placed on a stretching protocol vs no intervention showed no reduction in injury risk (51). Similar protocols showed that greater cardiovascular fitness in both male and female recruits was associated with lower injury risk but not flexibility (4). Another study demonstrated that flexibility from stretching was not necessarily a beneficial attribute to runners. In a 4-yr prospective trial, researchers found no evidence that flexibility decreased sports injuries (65). An additional review found that stretching had no result on delayed muscle soreness or injury rate (30).
Despite the scientific literature, the consensus among sports practitioners still leans strongly in favor of stretching. Bob Anderson's book Stretching, which originally targeted the running boom, just published the 30th anniversary edition and remains a popular resource (1). One survey study of coaches reveals how prevalent the belief in stretching remains. The results showed that 95% of coaches felt that preexercise stretching decreased injuries and 73% believed there were no drawbacks (60). This relatively uniform endorsement of coaches raises the logical question of whether the medical literature has flaws that make the conclusions invalid.
If that is the case, perhaps other stretching studies might show better results. Other studies could not be considered primary prevention of injury, but could lend some credence to a scientific basis for pursuing stretching. The most positive of these involved a stretching intervention in plantar fasciitis. This prospective trial lasted 2 yr, and after the first 8 wk, crossed the placebo group into the same treatment protocol. For the first 8 wk, the experimental group used a plantar fascia (PF) stretch, while the control group did an Achilles stretch. Both groups used insoles and Cox 2 inhibitors. After phase 1, the authors noted significant differences in worst pain and pain first two steps on 8-wk follow-up. For phase 2, all patients were given the PF stretch protocol. Follow-up at 2 yr showed that 92% had a high degree of satisfaction (15,16).
Other studies that targeted specific injuries showed mixed results but none as positive as the PF trial. For example a study of PFS focused on stretching and showed that a 3-wk static stretching program to increase quadriceps muscle flexibility in PFS participants demonstrated significant improvement with quadriceps flexibility. However, they found no association of stretching with better pain relief or clinical functional outcomes (50).
Another valid criticism of stretching research is that until the optimal technique of stretching is used in intervention groups, the intervention will be inconclusive and begs the question of whether the scientific literature actually identifies the most effective form of stretching.
Four key types of stretching are noted in most literature. These include:
- ▪ Ballistic stretching, which is a rapid, bouncing, stretch-shortening cycle. Typically, coaches feel that this increases muscle stress and generally is not favored as it is suspected of increasing stretch-induced injury.
- ▪ Passive stretching with a partner helping the individual increase the passive motion limit beyond what can be done without assistance. This type is most useful when the stretching partner is skilled in the techniques, such as an athletic trainer or physical therapist.
- ▪ Contract-relax/agonist-antagonist/muscle energy techniques that use the muscle's physiologic responses in the contraction and reciprocal relaxation cycle. Studies show superior flexibility in laboratory settings, particularly when techniques are mastered.
- ▪ Static stretching is the most common form where an individual lengthens a muscle and holds the position typically for a maximum of 30-60 s with slow buildup.
While all of these techniques have shown the ability to improve flexibility, there are many variations of how stretching is done. One logical challenge in any study relates to whether intervention groups actually use the correct stretching technique.
Stretching research has demonstrated mixed results. Data suggest that too short of a time (less than 10 s) is ineffective and that prolonged stretches longer than 60 s accomplish no more than stretching periods of 10-60 s. However, the traditional ideas about stretching effectiveness often are challenged by research as in the study of Achilles stretching that compared standard static techniques to ballistic stretching. The results showed that static stretching resulted in no change in Achilles stiffness, whereas ballistic stretching decreased Achilles stiffness. Thus the common wisdom that ballistic stretching might be harmful has been studied less in regard to prevention of injury and may be incorrect (74).
Another consideration about stretching is "timing" for optimal effect on prevention of injury. Many athletes have adopted the strategy of stretching after activity rather than before. One study raises the possibility that this might offer injury protection. This prospective study followed a single team for four playing seasons assessing hamstring injury. The authors completed preintervention and postintervention injury tracking. For the intervention program, the athletes stretched after practicing and while fatigued. The authors also did sport-specific training drills with an emphasis on increasing the amount of high-intensity anaerobic interval training. Results favored the intervention in that in preintervention 9 and 11 athletes sustained hamstring injury compared with 2 and 4 after intervention. In addition, competition days missed were reduced from 31 and 38 to 5 and 16 after intervention (69).
Warm-Up
Fewer studies have examined the role of warm-up as an isolated variable in the prevention of running injury. A large cohort study of 1680 runners examined variables associated with injury risk. Warm-up did not appear associated, but in a survey study multiple confounding variables could have affected the results (71). Fradkin et al. assessed the current evidence relating warm-up to injury prevention (24). They analyzed five high-quality studies. Three studies of teenage athletes engaged in team handball and American football demonstrated a benefit of warm-up. In two studies of lower-extremity injuries in recreational runners or in military recruit, no benefit was noted. Overall, they concluded that evidence was in favor of warming-up to decrease injury risk, with no detrimental effects. Whether their conclusions could be extrapolated to runners remains speculative as potential confounders of this research include the variability of the specific warm-up regimens used, the different sports included, and the heterogeneity of the participants as well as the observation that the two studies most specific to running did not show benefit.
Faigenbaum studied warm-up extensively in elementary- to high-school-aged athletes. His group studied multiple protocols and found improvement in a variety of sports activities using various warm-up protocols. School athletes improved in long jump, shuttle run, anaerobic activities, and vertical jump. None of these specifically correlate that closely to recreational or distance running and they did not assess injury risk (18-20). Of interest, some of the evidence that warm-up may reduce running injury risk comes from the stretching study literature. In Van Mechlen's study in which the control group did warm-up whereas the intervention group did warm-up plus stretching, the control group had slightly lower injury rate (68). Further study would have to identify whether warm-up would have lessened injury vs a control group who did nothing before running. The literature at this point simply does not have enough high-quality research to determine whether warm-up helps prevent running injury (21).
PSYCHOLOGICAL FACTORS IN INJURY
Most studies have found limited evidence to link specific psychological factors to running injury. One prospective trial of 182 athletes looked at sports injuries overall and included measures of 16 psychological and psycho-social factors. Three variables found a weak association with injury risk, and they included dominance (OR 1.71), vital exhaustion (OR 1.85), and stressful life events (OR 1.84). These psychological measures were much weaker predictors than total sporting time (OR 6.87) and injury in the prior 12 months (OR 9.41) (68). A paired retrospective and prospective study found that associators (individuals who are aware of their physical symptoms and fatigue while running) vs dissociators (individuals who ignore symptoms and fatigue level) actually have higher injury rates. However, a key confounding variable makes this type of conclusion questionable in that more competitive runners tended to be associators and also tended to train and race more.
Type A behavior seems a plausible risk factor for running injury. A 1-yr prospective trial of 30 runners did not demonstrate a higher injury rate in the Type A runners, but demonstrated a much higher risk of multiple injuries (22). A similar study found injury rates to be similar among Type A and B runners, but Type B runners missed 1 wk more of training due to their injuries (26). Perhaps the two studies detect a common theme that the impatience factor of Type A runners leads them to rush their return from injury. If so, this may explain the reinjury rate in the Fields et al. study. A large cohort study of 532 novice runners also failed to find an association of Type A behavior scores with risk of a new running injury (7).
Sports psychology studies have found significant relationships with increased life stress and injury, but the sports examined tended to be contact or team sports where factors such as anger, concentration, or tension have greater effects on risk. Similarly, tough-mindedness, assertiveness, and aggressive feelings have been identified as potential risks for a number of sports. However, neither the history of life stress nor of higher risk personality traits have been confirmed as predictors of running injury.
CONCLUSION
Running injuries are common, and evidence suggests that 40%-50% of runners experience injury yearly. Many variables potentially contribute to injury. Modifying one or more of these may help prevent running injury.
In spite of numerous studies, strong evidence for prevention of running injury exists only for controlling training errors primarily by limiting total running mileage. While previous injury is clearly a risk factor for subsequent injury, how to lessen this risk remains unclear. Moderate evidence identifies cavus feet as a risk factor, and weaker evidence identifies leg length inequality. Some studies suggest that orthotic inserts might lessen future risk of stress fracture but did not relate this directly to individuals with anatomical variants.
While studies of strength, biomechanics, stretching, warm-up, nutrition, shoes, and psychological factors all raise intriguing questions about both the etiology and the prevention of running injuries, strong evidence that modifying any of these will prevent running injury requires further research.
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