INJURY PREVENTION & PERFORMANCE ENHANCEMENT

Monique Mokha, PhD, ATC, Report Editor

Lower Extremity Stress Fracture in Runners: Risk Factors and Prevention Stephen Magness; Jatin P. Ambegaonkar, PhD, ATC, OT, CSCS; Margaret T. Jones, PhD, CSCS; and Shane V. Caswell, PhD, ATC, CSCS • George Mason University, Sports Medicine Assessment, Research & Testing Laboratory

T

here are more than 16 million runners in the United States who train a minimum 100 days per year.1 Runners can be separated into the following three categories on the basis of training frequency: (a) core participants (>50 runs/yr), (b) frequent runners (>100 runs/ yr), and (c) core runners (>224 runs/yr).1 The core runner category includes most competitive high school and collegiate runners in the United States as well as recreational runners Key Points training for distances Risk factors for lower extremity stress frac- ranging from 5 km to half-marathons (13.1 ture include training load, impact forces, miles). In addition to and fatigue. the high running freRisk may be reduced by minimizing sudden quency per year, the changes in training, alteration of running core runner averages mechanics, and appropriate resistance 31 miles per week.1 training. Given the repeated s t re s s e s a n d h i g h impact loads of running, up to 46% of runners suffer a running-related injury each year.2 Further, lower extremity stress fractures (LESFs) are reported to account for up to 20% of all injuries in athletes, with runners and track athletes experiencing the greatest incidence.3 Among runners, the tibia is highly susceptible to fracture, accounting for up to 55% of all LESFs.4 Combining the high incidence of LESFs in runners with recovery typically requiring up to 8 weeks,5

participation in training and competition can be greatly disrupted. In addition to the pain and disability a runner experiences as a result of an LESF, a high rate of injury reoccurrence (36%) has been documented.6 Because runners repeatedly experience impact forces that are two to three times body weight with each step, load absorption, and fatigue influence LESF injury risk.7 Given the incidence and recurrence rate, understanding of training strategies that can minimize the risk of LESF is important. Thus, the purposes of this report are to (a) discuss risk factors for LESF in runners and (b) describe preventive interventions to minimize LESF injury risk. Because training volume and intensity differs among runner groups, this report will focus on the core runner (>20 mi/wk).6

Risk Factors for LESF The two major classifications of stress fractures described in the literature are fatigue fractures and insufficiency fractures.8 Fatigue fractures are primarily caused by overstress, which primarily affect cortical bone, whereas insufficiency fractures are primarily caused by low bone mineral density.8 Either type of stress fracture occurs as a result of the inability of the bone to adequately adapt to the stress imposed upon it. © 2011 Human Kinetics - ATT 16(4), pp. 11-15

international journal of Athletic Therapy & training

july 2011  11

Bone Adaptation to Stress According to Wolff’s Law, bone tissue adapts to the specific demands that are imposed.8 In response to stress loading, micro-damage occurs within the structure of the bone. In response, osteoclast cell activity predominates (i.e., removal of mineralized bone matrix).8 With adequate recovery time, the micro-damage is repaired through increased osteoblast cell activity (i.e., new bone formation).8-10 As long as the stress loading remains within the limits of adaptation and recovery, increased bone strength will result.9 When a normal balance between osteoclast and osteoblast cell activity is disrupted, however, a pathologic condition can develop.10 Excessive running volume and inadequate recovery time can lead to bone weakness and development of an LESF. A stress fracture is most likely to occur during an early phase of training (i.e., first 40 days) or when a training regimen is substantially altered.5,11

Impact Forces An important distinction should be recognized between internal and external loading. Internal loading is produced by muscles, tendons, and ligaments,5 whereas external loading results from ground reaction forces (GRF) that are generated by impact of the foot against the ground.7 GRFs measured during running provide an approximation of the magnitude of internal load imposed on the body tissues.7 Because determination of the stress imposed on the tibia during running in vivo is difficult to measure, GRFs provide as an acceptable surrogate for representation of the load. Although GRFs act in all three planes, the vertical component is most commonly examined.7 Vertical GRF typically has two peaks in heel-striking runners and one peak in forefoot-striking runners (Figure 1).12-16 The initial peak is the smaller of the two peaks in heel-strikers and is absent in forefoot-strikers. The magnitude of the peak vertical GRF is an important indicator of internal tissue loading. The loading rate is represented by the slope of the initial GRF (i.e., from foot strike to initial peak).6 A recent systematic review provided evidence that both the average and instantaneous vertical loading rates are greater in individuals who have sustained LESFs compared to those who have not.7 However, the magnitude of peak vertical GRF does not substantially differ between the two groups.7 Thus, an elevated loading rate appears to be an important risk factor for LESF.

12  july 2011

Figure 1  Typical vertical GRF vs. stance time. T: the time period during which the VLR is calculated. From Zadpoor AA, Nikooyan AA. The relationship between lower-extremity stress fractures and the ground reaction force: a systematic review. Clin Biomech. 2010. Reprinted with permission from Elsevier.

Fatigue Fatigue may play an important role in the development of an LESF.6,17,18 At the end of a fatiguing run, GRFs have been observed to be greater in runners with a history of stress fracture than runners without a such a history.16 Researchers examining the role of muscle fatigue on bone strain in a canine animal model reported a decrease in the ability of the musculature to dissipate GRFs and a 36% increase in tibial strain.17 The reduced capability of the musculature to dissipate force when fatigued results in greater load on the tibia.18 Muscle fatigue also alters running biomechanics, which has been associated with a 25% increase in vertical GRF.18 Researchers have suggested that runners tend to compensate for elevated vertical GRF by increasing heel strike at initial ground contact.19 Consequently, efforts to prevent LESF should focus on (a) alteration of training regimens to reduce impact loading of the lower leg and (b) development of muscle fatigue resistance in the lower extremity. 20

Preventive Interventions Resistance Training Female runners with a history of LESF have smaller calf girth and lesser lower leg muscle mass than do other

international journal of Athletic Therapy & training

runners.14 Runners who are at risk for LESF should be guided to perform a training regimen that will increase muscle strength, hypertrophy, and endurance. An example of such a resistance training program is presented in Table 1. Various resistance loads (75-95% of one repetition maximum) should be utilized to increase muscle strength (2-6 repetitions), hypertrophy (6-10 repetitions), and endurance (10-15 repetitions). The program should be periodized, with a relatively low load and high volume (i.e., sets × repetitions), initially, and a shift to a relatively higher load and lower volume as the competitive season approaches. Combining exercises for development of strength and endurance in 3-week periodized phases allows for gradual change in the training regimen. Although training volume and intensity can be regulated, finding a balance between training that will simultaneously increase performance and minimize injury risk may be difficult to achieve. Some experts have suggested that training volume should be limited to < 20 miles per week to minimize risk for LESF.21-24

Assuming that excessive loading is avoided, regular running actually increases tibia bone strength. 25 Among competitive runners, there may not be an association between running volume and LESF development.23 Thus, training load should be carefully adjusted to facilitate positive adaptation of the bone structure.

Running Volume and Intensity Stress fracture occurrence is often associated with an increase in training load or a change in training intensity.5,26 Although little research evidence is available to guide progressive increase in running volume, a common approach is to limit weekly increases to no more than 10% per week.27 For example, a 3:1 periodization cycle could be used, i.e., a runner’s mileage would be increased by no more than 10% each week for three consecutive weeks, followed by one week of reduced mileage for recovery (Table 2). Training intensity is another important consideration. Some training regimens involve abrupt changes in the intensity of running, such as transition from

Table 1. Base Training 12-Week Lifting Program Template Day 1

Comments

Dynamic flex warm-up Olympic or plyometric movement Lower body push, upper body pull*

10 minutes 2-3 min rest between sets Pair 1

Upper body push, lower body pull* Circuit: Abs/low back

Pair 2

Prehab Exercises Dorsi/Plantarflexion Eccentric calf lowers (lower on one foot, rise on two feet)

Volume: Phases I-IV I,II: 4-6 reps III, IV: 2-4 reps I: 8-10 reps II: 6-8 reps III: 5-7 reps IV: 2-4 reps Same as pair 1

3-4 exercises

1-3 sets of 10-15 reps; phase dependent

Resistance bands Lower on 4 count

2-3 sets of 10-15 reps 2-3 sets of 8-10 reps

Day 2

Comments

Foam roller warm-up Jump squats

10 minutes 2-3 min rest between sets Pair 1

Upper body push Lower body pull*

Lower body push Upper body pull* Circuit: Glute activation and core

Pair 2 2-4 exercises

Prehab Exercises Toe taps

Fast tempo

Calf raises

Low weight

Volume: Phases I-IV I,II: 4-6 reps III, IV: 2-4 reps I: 8-10 reps II: 6-8 reps III: 5-7 reps IV: 2-4 reps Same as Pair 1 1-3 sets of 10-15 reps; phase dependent 2-3 sets of 30-60 sec 2-3 sets of 15-20 reps

Notes. Recommended intensities as percentages of 1RM are as follows: 2 to 4 reps = 95–90%; 4 to 6 reps = 90–85%; 5 to7 reps = 87– 83%; 6 to 8 reps = 85–80%; 8 to 10 reps = 80 to 75%. *Volume range of pulling exercises: 6-10 reps. Set range: 2-4, phase dependent.

international journal of Athletic Therapy & training

july 2011  13

long distance runs to interval training sessions that are performed several times per week.27 A better approach for minimization of LESF risk is a periodized training regimen that involves gradual increase in intensity and integration of training modes within phases, thereby minimizing the abrupt changes (Table 3). When running volume is substantially increased, the volume of resistance training should be reduced to avoid an increase in total training volume.

Foot Strike, Stride, and Frequency Biomechanical factors can increase LESF risk,15 and modification of running mechanics can reduce the risk level.28 Foot strike, stride length, and stride frequency are relatively easy to assess and modify. A 10% reduction in stride length reduces risk for LESF.29 Making runners aware of stride length may decrease over-striding and generation of high GRFs.13 A simple method to adjust stride length involves having the runner increase stride frequency on a treadmill that is set on a constant speed (i.e., stride length must be reduced at a higher stride frequency to maintain a constant speed). Over-striders typically increase heel-strike impact and loading rate.13 Conversely, most competitive runners tend to land on the forefoot or midfoot, which is

Table 2. Sample Progression for Increasing the Volume of Training Over 8 Weeks Training Cycle in Weeks 1 20

2 24

3 28

4 20

5 28

6 32

7 36

8 28

The program divided into 2 micro cycles each lasting 3 weeks with 1 week of recovery. Training volume is represented in miles per week.

Table 3. Sample Progression of Interval Training for 5k Runner Interval Training Progression Off season

8 × 100 m at race pace following a distance run

Base training

8 × 200 m with 200 m jog at race pace

Precompetitive season

5 × 800 m with 400 m jog rest at race pace

Competitive season

2 × 1mile with 800 m jog rest at race pace

14  july 2011

associated with less ground contact time and a lesser loading rate.13,19 Simply instructing a runner to change the portion of the foot that makes initial contact with the ground may alter running mechanics.30 Video may be used to determine whether or not a runner has changed his or her foot-strike pattern.

Biofeedback For runners with a history of LESF, a running analysis that provides immediate GRF biofeedback may be useful. Research suggests that peak GRF and loading rate can be reduced when this information is displayed for the runner during performance.28

Conclusions Athletic trainers and therapists should be aware of risk factors for LESF, which include excessive training load, elevated impact force, and muscle fatigue. Preventive measures include periodization of training, resistance training, and the alteration of running mechanics. A multifaceted approach may simultaneously reduce LESF risk and enhance running performance. 

References 1. 2009 National Runner Survey. Running USA. 2010. Available at: http:// www.runningusa.org/node/57770. Accessed December 10th, 2010. 2. Jacobs S, Berson B. Injuries to runners: a study of entrants to a 10,000 meter race. Am J Sports Med. 1986;14(2):151-155. 3. Fredericson M, Jennings F, Beaulieu C, Matheson GO. Stress fractures in athletes. Topics Magnetic Resonance Imaging. 2006;17(5):309-325. 4. Milner CE, Davis IS, Hamill J. Free moment as a predictor of tibial stress fracture in distance runners. J Biomech. 2006;39(15):2819-2825. 5. Beck BR. Tibial stress injuries: an aetiological review for the purposes of guiding management. Sports Med. 1998;26(4):265-279. 6. Hauret KG, Shippey DL, Knapik JJ. The physical training and rehabilitation program: duration of rehabilitation and final outcome of injuries in basic combat training. Military Med. 2001;166:820-826. 7. Zadpoor AA, Nikooyan AA. The relationship between lower-extremity stress fractures and the ground reaction force: a systematic review. Clin Biomech. 2010. 8. Harrast MA, Colonno D. Stress fractures in runners. Clin Sports Med. 2010;29:399-416. 9. Burr DB, Martin RB, Shaffler MB, Radin EL. Bone remodeling in response to in vivo fatigue microdamage. J Biomech. 1985;18:189-200. 10. Pepper M, Akuthu V, McCarty EC. The pathophysiology of stress fractures. Clin Sports Med. 2006;25:1-16. 11. Edwards BW, Taylor D, Rudolphi TJ, Gillette JS, Derrick TR. Effects of running speed on a probabilistic stress fracture model. Clin Biomech. 2010;25(4):372-377. 12. Basset EJ, Littlewood JJ, Taylor SJG. Relations between compressive axial forces in an instrumented massive femoral implant, ground reaction forces, and integrated electromyographs from vastus lateralis during various osteogenic exercises. J Biomech. 1997;30:213-223.

international journal of Athletic Therapy & training

13. Lieberman DE, Venkadesan M, Werbel WA, Daoud AI, D’Andrea S, Davis IS, Mang’eni RO, Pitsiladis Y. Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature. 2010;463(7280):531-535. 14. Bennell K, Crossley K, Jayarajan J, Walton E, Warden S, Kiss ZS, Wrigley T. Ground reaction forces and bone parameters in females with tibial stress fracture. Med Sci Sports Exerc. 2004; 36:397-404. 15. Pohl MB, Mullineaux DR, Milner CE, Hamill J, Davis IS. Biomechanical predictors of retrospective tibial stress fractures in runners. J Biomech. 2008;41(6):1160-1165. 16. Grimston SK, Nigg BM, Fisher V, Ajemian, SV. External loads throughout a 45 minute run in stress fracture and non-stress fracture runners. J Biomech. 1994;27(6):668. 17. Yoshikawa T, Mori S, Santiesteban AJ, Sun TC, Hafstad E, Chen J, Burr DB. The effects of muscle fatigue on bone strain. J Exp Biol.1994;188:217-233. 18. Nyland JA, Shapiro R, Stine RL, Horn TS, Ireland ML. Relationship of fatigued run and rapid stop to ground reaction forces, lower extremity kinematics, and muscle activation. J Sports Phys Ther. 1994;20:132137. 19. Caplan N, Hayes P. Foot strike patterns of high level male 1500m runners.  In Proceedings of the 14th Annual Congress of the European College of Sport Science. 2009 24-27 June 2009, Oslo, 30-31. 20. Volpin G, Petronius G, Hoerer D, Stein, H. Lower limb pain and disability following strenuous activity. Mil Med. 1989;154:294-297. 21. Monteiro AG, Aoki MS, Evangelista AL, Alveno DA, Monteiro GA, Picarro Ida C, Ugrinowitsch C. Nonlinear periodization maximizes strength gains in split resistance training routines. Strength Cond Res. 2009;23(4):1321-1326. 22. Esteve-Lanao J, Rhea MR, Fleck SJ, Lucia A. Running-specific, periodized strength training attenuates loss of stride length during intense endurance running. Strength Cond Res. 2007;22(4):1176-1183. 23. Brunet ME, Cook SD, Brinker MR, Dickinson JA. A survey of running injuries in 1505 competitive and recreational runners. J Sports Med Phys Fitness. 1990;30(3):307-315. 24. Macera C, Pate R, Powell K, Jackson KL, Kendrick JS, Craven TE. Predicting lower-extremity injuries among habitual runners. Arch Intern Med. 1989;149:2565–2568.

international journal of Athletic Therapy & training

25. Green DA, Naughton GA, Briody JN, Kemp A, Woodhead H. Assessment of bone strength at differentially-loaded skeletal regions in adolescent middle-distance runners. J Sci Med Sport. 2006;9(3):221-230. 26. Scully TJ, Besterman G. Stress fracture: a preventable training injury. Mil Med.1982;147:285-287. 27. Daniels J. Running Formula. 2nd ed. Champaign, IL: Human Kinetics; 2005. 28. Crowell HP, Milner CE, Hamill J, Davis IS. Reducing impact loading during running with the use of real-time visual feedback. J Orthop Sports Phys Ther. 2010;40(4):206-13. 29. Edwards BW, Taylor D, Rudolphi TJ, Gillette JS, Derrick TR. Effects of stride length and running mileage on a probabilistic stress fracture model. Med Sci Sports Exerc. 2009;41(12):2177-2184. 30. Pohl MB, Buckley JG. Changes in foot and shank coupling due to alterations in foot strike pattern during running. Clin Biomech. 2008;23(3):334-341.

Stephen Magness is with the Sports Medicine Assessment, Research & Testing Laboratory (S.M.A.R.T. Laboratory) at George Mason University in Manassas, VA. Jatin Ambegaonkar is an assistant professor and Co-Director of the S.M.A.R.T Laboratory at George Mason University. His research interests include dance and performing arts medicine, injury prevention, lower extremity injury risk factors, and surface electromyography methodology. Margaret T. Jones is an associate professor in the S.M.A.R.T. Laboratory at George Mason University. Her research interests lie with strength and power development for athletic performance. Shane Caswell is an associate professor and Co-Director of the S.M.A.R.T Laboratory at George Mason University. His research interests include injury prevention in youth and high school sport.

july 2011  15