July 2016

Lower extremity strength and injury risk in runners

7running-iStock_95379025_LARGEStudy findings regarding strengthening interventions in runners with patello­femoral pain syndrome have been inconsis­tent, perhaps because specific subgroups are more responsive than others. Preliminary research suggests high school runners may make up one such subgroup.

By Lace Luedke, PT, DPT, PhD, OCS, CSCS

Running continues to be a popular recreational activity, with more than 28 million Americans running 50 or more times per year in 2014.1 However, while running confers many health benefits, the injury incidence is high, with reports of 19.4% to 79.3% of runners experiencing injury.2 The knee, lower leg, and foot are the most commonly injured locations in runners,3 and strengthening exercises are frequently used for the treatment of injured runners to address areas of impaired or asymmetrical strength.4-7 Strength training has been associated with improvements in running economy,8 and strengthening exercises are often suggested for prevention of common running injuries.9-13 Although it’s plausible that strengthening interventions may reduce injury risk in runners, the current evidence to support these interventions is lacking.

Strength impairments in injured runners

Most evidence available on the relationship between lower extremity strength and injuries in runners comes from cross-sectional studies on recreational runners. Strength impairments in injured runners are common. In a cross-sectional study of 25 recreational runners, Ferber et al observed that runners with patellofemoral pain syndrome (PFPS) presented with a 30% deficit in hip abductor strength compared with asymptomatic runners.12 In another study, runners with PFPS demonstrated weaker hip abductor muscles than healthy runners, both prior to and after an exhaustive treadmill run.14

In a recent study, high school runners with weaker hip abductor and knee strength values had a higher risk of anterior knee pain than their stronger counterparts.

7running-iStock_24907254GirlsWoods4-copyHip abduction strength deficits have also been reported in the involved limb of female runners with iliotibial band syndrome.15 Similarly, in a cross-sectional study of 60 recreational runners, Niemuth et al found that runners with overuse injuries displayed worse hip abductor strength in their involved limb compared with noninjured runners.16 In athletes with PFPS, less hip abduction and external rotation strength has been reported in the involved limb than the uninvolved limb; the same study reported global hip strength impairments in those with PFPS, compared with age- and sport-matched women without PFPS.17 However, a more recent study on female runners in early stages of PFPS did not find significant hip abduction strength asymmetry, and hip abduction strength in hip extension was not impaired relative to noninjured runners.18

In a cross-sectional study of 169 recreational runners, Duffey et al reported that decreased peak knee extensor torque was a predictor of anterior knee pain (AKP).19 Knee extensor strength was also decreased in recreational runners with chronic low back pain.20 In contrast, Messier et al reported similar knee extensor and flexor strength values among recreational runners aged 16 to 50 years with and without PFPS, but the runners with PFPS tended to have greater knee flexion and less knee extension endurance bilaterally than controls, despite unilateral symptoms.21

Strength impairments have also been reported in runners with calf and foot injuries. In a sample of 12 female marathoners, decreased toe flexor strength was associated with a higher rate of posterior-medial shin pain and a lower arch.22 Lower peak plantar flexion torque than healthy runners was observed at 180°/sec in runners with Achilles tendinitis23 and at 60°/sec and 180°/sec in runners with plantar fasciitis.24

Strength and running injury risk

7runningInWoods-copyWhile strength impairments are often observed in injured runners, whether these impairments are causative factors or secondary to the injury is not clear.11 Prospective studies assessing strength values prior to injury occurrence may help determine which strength impairments are risk factors.

Hip muscle strength and function may be risk factors for knee injuries.25 A lower preinjury hip external-to-internal rotation strength ratio was associated with increased AKP risk in a prospective study of 98 high school distance runners.26 In a study of more than 600 novice adult recreational runners, those with higher eccentric hip abductor strength had a lower risk of developing PFPS.27 In contrast, a study of 77 novice female distance runners reported that hip flexor, extensor, abductor, adductor, external rotation, and internal rotation isometric strength did not predict which runners would experience PFPS during a 10-week running program.28 Knee flexion and extension strength may influence knee injury as decreased isometric hamstrings and quadriceps strength were significant risk factors for AKP in US Naval Academy midshipmen.29

With regard to shin injuries, Verrelst et al found that decreased hip abduction strength predicted the development of exertional medial tibial pain during running in a prospective study of 95 female physical education students.30 In contrast, greater hip abduction strength was not protective against medial tibial stress syndrome or tibial stress fracture in a study of 230 high school runners.31 In a prospective study of 125 collegiate cross country runners, distal strength values didn’t influence shin injury incidence, as reduced isotonic ankle plantar flexor endurance was not significantly associated with incidence of exercise-related lower leg pain.32 Additionally, in a study of 146 collegiate athletes, isometric strength of the ankle plantar flexors, dorsi­flexors, invertors, and evertors was not associated with occurrence of medial tibial stress syndrome.33 In a study that assessed isometric trunk, hip, and knee strength in recreational runners, decreased knee flexor strength was associated with the development of Achilles tendon pain in runners.34

Although strength was not assessed in Bennell et al’s prospective investigation of 111 track and field athletes aged 17 to 26 years, female athletes with stress fractures (most frequently to the tibia) had less lean mass in the lower limb and smaller calf girth than female track and field athletes who did not incur a stress fracture.35,36 Track and field athletes with a 1-cm deficit in corrected calf girth were also four times more likely to incur a stress fracture.35 Bennell et al also found that corrected calf girth and lower limb lean mass were positively correlated with tibial and fibular bone density,37 suggesting lean mass may be protective against stress fractures in runners.

Our recent study

Like recreational running, high school cross-country running is also very popular, with participation increasing by more than 30% in the past decade.38,39 However, 59% to 68% of high school distance runners report a history of running injury.40 In high school runners, the shin and knee have been the most frequently reported injured areas, together representing 48% of new injuries and 59% of reinjuries.40,41 We conducted a study to determine if hip and knee strength was associated with the incidence of shin and knee injuries in high school cross-country runners.

At the start of the interscholastic season, 68 high school cross-country runners (47 girls) were tested for peak isometric hip abductor, knee extensor, and knee flexor strength using a handheld dynamometer. Values were normalized to body mass. During the season, runners were tracked for athletic exposures (practice and competition participation) and occurrences of AKP and shin injuries that resulted in time lost from practice or competition. Runners were categorized by their strength values into weakest, middle, and strongest tertiles for each muscle group; the strongest group was used as the reference group.

Runners in the weakest tertiles for hip abductor, knee extensor, and knee flexor strength were more likely to experience AKP than those in the strongest tertile.42 The incidence of shin injury was higher for runners in the weakest tertile for hip abductor strength than the strongest tertile, but the relationship was not statistically significant. Runners in the weakest tertiles for knee extensor strength and knee flexor strength did not have an increased risk of shin injury.42

In our study, high school runners with weaker hip abductor and knee strength values had a higher risk of AKP than their stronger counterparts. However, we concluded further evidence is needed to determine whether screening for strength impairments and interventions targeting strength impairments will reduce risk of running injuries.

Clinical implications

7running-iStock_39728958-copyThe inconsistent findings in the literature on associations between lower extremity strength and running injury occurrence are likely because running injuries are multifactorial. Anatomy, biomechanics, and training load may all contribute to injury occurrence.

Strengthening may improve runners’ ability to tolerate loads associated with running. Theoretically, screening for hip muscle weakness and adding strengthening exercises may help prevent AKP in female athletes.17 Additionally, strength training of the hamstrings in runners has been proposed to lower the risk of knee and soft tissue injuries,13 as the hamstrings are thought to be important for maintaining stride mechanics due to their role in controlling braking forces at the knee.43

While muscle strength may improve tolerance of loads during running, another reason for inconsistencies in the reported relationships between strength and injury risk may be that strength is typically assessed isometrically. It’s unclear how much of an influence peak isometric strength has on the dynamic task of running, and specifically on prolonged running in the presence of muscular fatigue.44 Schmitz et al found that, while isometric hip strength values were similar between novice and experienced runners, hip internal rotation motion during running was higher in the novice runners,45 suggesting isometric strength may not correlate strongly with muscular control and kinematics during running.44-46

Strengthening exercises are effective for increasing strength. Runners who, in addition to their running, participated in a six-week strengthening program for hip abductors and external rotators increased their strength and reduced hip internal rotation during running.46 Although this intervention appeared to influence injury risk factors, injury incidence after the intervention was not assessed directly.47

In other studies, hip strengthening exercises in runners with PFPS increased their hip strength values without changing running mechanics,48 while heavy strength training in female runners resulted in improved 1RM (one-repetition maximum) for leg exercises.49 Active individuals with PFPS who participated in six weeks of either hip or knee strengthening had similar success rates for improvements in symptoms and function and increases in strength para­meters.50 However, parti­ci­pants in the hip-strengthening group started experi­encing significant improvements in self-reported pain at three weeks, while those in the knee-strengthening group started at four weeks.50

With respect to strengthening exercises for running injury prevention, a preconditioning program implemented with the intent of improving strength and load tolerance in 432 novice runners didn’t significantly reduce incidence of running-related injury.51 The experimental preconditioning program included four weeks of progressive walking and hopping before starting a nine-week running program; injury rates during the nine weeks of running were similar for the preconditioning program participants and nonparticipants.51

7running-iStock_60862808_LARGE-copyIn other sports, strengthening programs added to training have reduced injury rates. A structured warm-up program to improve neuromuscular control, balance, and strength in youth handball players reduced the injury rate in the intervention group (.5 injuries per 1000 player hours) compared with a control group (.9 injuries per 1000 player hours).52 In adolescent soccer players, an intervention group added two to three sessions of plyometrics per week and strengthening to their regular training program for 12 weeks. Over the course of the season, those in the experimental group experienced a lower rate of injuries lasting three or more days than the control group (.70 injuries per 1000 hours of sport exposure vs 2.32 injuries per 1000 hours).53

In elite soccer players, adding eccentric and concentric hamstring strengthening exercises improved hamstring strength and reduced hamstring injury incidence.54 Another study on elite soccer players noted fewer muscular injuries with the addition of functional strength, proprioceptive, core stability, and agility exercises.55 A group of female floorball players participating in a neuromuscular training intervention including balance, hopping, and strengthening exercises had an injury rate that was 33% of the rate for the control group.56

Although each of these programs was associated with decreased noncontact injury rates in their respective sports, the programs included numerous exercises that may have improved attributes other than strength, including neuromuscular control.

Conclusion

The use of targeted strength interventions in runners may reduce injury risk for some types of running injuries, but at this time, the evidence is limited. Considering the scarce evidence on strengthening for injury prevention in runners, any potential interventions may be most beneficial if focused on populations known to have a high risk of running-related injury, such as novice runners57,58 and female high school runners.40,59 Based on available prospective evidence, the addition of hip abduction strengthening exercises to training programs for runners is a low-risk intervention that may reduce the risk of PFPS, one of the most common injuries in runners.27,50 Further research is needed, however, to determine conclusively whether screening for lower extremity strength and implementing strengthening interventions is beneficial for reducing injury risk in runners.

Lace Luedke, PT, DPT, PhD, OCS, CSCS, is a lecturer in the Department of Kinesiology at the University of Wisconsin-Oshkosh.

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