June 2012

Foot strike in runners: Influence on injury risk

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Different foot strike patterns are associ­ated with different means of attenuating and redistributing forces during running, which may have implications for injury risk and rehabilitation. Researchers, however, have yet to fully examine and understand those implica­tions.

By Peter Larson, PhD

Foot strike patterns among runners are variable, but surveys of shod runners demonstrate that most make first contact with the ground somewhere on the heel (a rearfoot strike).1-3 Conversely, studies of habitual barefoot runners show that the landing typically involves either simultaneous contact of the heel and forefoot (a midfoot strike) or initial contact on the lateral forefoot, with the heel coming down shortly thereafter (a forefoot strike).4,5 These contrasting observations have precipitated a great deal of debate about how foot strike type is related to the forces experienced by the body during running, and how these might in turn relate to injury risk.

I have observed anecdotally that during running each foot comes into contact with the ground approximately 75 to 100 times per minute, which corresponds to a step rate or cadence of 150 to 200 steps per minute (cadence varies considerably among individuals, and with running speed). When a runner’s foot makes initial contact with the ground during the landing phase of the gait cycle, the body rapidly decelerates during the collision and a shock wave is transmitted through the body.6 The body has built-in mechanisms to attenuate or diminish the magnitude of this shock wave, including the bones, ligaments, and cartilage in the feet, legs, hips, and vertebral column as well as the muscles and tendons that attach to the skeleton.

Running shoes and running surface can also play a role in attenuating the impact shock associated with initial contact of the foot with the ground.7 Given the repetitive nature of the movements and resulting forces involved in running, structures involved in attenuating shock and stabilizing joints are subject to repeated mechanical strain. This is why these forces have been of interest to those studying the causes of running injury.

Ground reaction forces

Figure 1. Hypothetical vertical ground reaction force plot for a shod heel strike.

Forces applied to the foot, which is the only part of the body in direct contact with the ground during the gait cycle, initiate the shock waves passing through the body during running. The ground reaction forces (GRF) associated with foot contact have been studied extensively and are typically broken into the following components: anterior-posterior, horizontal, and vertical. Of these, the vertical GRF is the greatest in magnitude and has received the most attention in comparisons among foot strike types.

Figure 1 depicts a hypothetical vertical GRF curve for a heel-striking shod runner. The vertical (Y) axis displays the value of the vertical ground reaction force as a function of body weight, and the horizontal (X) axis shows time in milliseconds. The curve depicted on the graph shows how vertical GRF changes from the point of initial contact of the foot with the ground (time 0) to the point at which the foot leaves the ground on toe-off (about 300 ms). For a heel-striking runner there are typically two distinct force peaks.5,8-10 The impact peak, sometimes called the passive peak, is the vertical ground reaction force resulting from initial contact of the heel with the ground. Full body weight is not being applied at this point. The active peak, sometimes referred to as the propulsive peak, is the vertical GRF that occurs at roughly midstance, and its magnitude typically reaches two to three times body weight.11

Much of the debate surrounding the relationship between foot strike and injury risk focuses on the initial impact peak since its magnitude (and presence) varies with footwear condition and foot strike type. The vertical impact peak is typically present in heel-striking runners, though its magnitude tends to be smaller and the rate at which the vertical impact force is applied (vertical impact loading rate) tends to be reduced in runners wearing shoes with a cushioned midsole compared with barefoot heel strikers.5,9,12,13 Because both impact peak and loading rate increase dramatically in barefoot heel strikers, it seems reasonable to assume that heel-striking runners will benefit from the impact-reducing effects of a cushioned misdsole. It should be noted, however, that that existence of a link between impact peak magnitude and injury risk has been the subject of considerable debate, and studies specifically comparing injury rates of barefoot and shod heel strikers are not available.14-17

In contrast to heel strikers, the vertical impact peak is often absent in forefoot-striking runners (and sometimes in midfoot strikers) regardless of footwear condition, and loading rates are comparable to or sometimes even lower than those observed in heel-striking runners wearing cushioned shoes.5,8-9,12,18 For example, Lieberman et al5 reported that in the majority of barefoot forefoot-strike runners, rates of loading were approximately half those of shod rearfoot-strike runners. The hypothesized reason for these changes is that instead of absorbing impact through a collision between the heel of the foot or shoe and the ground, forefoot-striking runners instead absorb impact through mechanisms such as compression of the medial longitudinal arch of the foot, eccentric contraction of the triceps surae, and stretch of the Achilles tendon.19

Given these differences, it is important to note that even if impact loading rates are similar in shod heel strikers and barefoot forefoot strikers, external forces of similar magnitude are likely applied internally at the level of individual tissues in different ways as a result of variation in how the foot and lower limb are oriented at contact. For example, a forefoot strike dorsiflexes the foot and ankle, and the triceps surae functions to slow the heel down as it is lowered to the ground after initial contact. In contrast, a heel strike plantar flexes the ankle and the tibialis anterior functions to slow down the anterior portion of the foot as it is lowered to the ground. Differences like these could lead to variation in susceptibility to specific injury types among runners employing different classes of foot strike.

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Some studies have reported results that deviate from patterns discussed above. For example, Laughton et al19 found no significant difference in loading rates between rearfoot and forefoot strikers and increased tibial acceleration in forefoot-striking runners. However, they looked at natural rearfoot strikers asked to switch to a forefoot strike pattern rather than natural forefoot-striking runners, and, furthermore, they instructed runners to run with a “toe-strike” and not let the heel touch the ground. In my observation, natural toe running without heel contact is extremely rare among runners, and the authors point out that running with this style of gait could have caused artificial stiffening of the leg, leading to an increase in tibial shock.

Foot strike, impact, and injury

There is much debate regarding the existence of a relationship between foot strike, impact characteristics, and running injuries. A recent study demonstrated that cross-country runners who habitually use a forefoot strike suffered about half as many injuries as those who were rearfoot strikers.20 It is unknown whether this relationship can be generalized to a more recreational running population or whether it might apply to runners who are actively attempting to modify their running form. Regarding the latter, published case reports have described metatarsal stress fractures suffered in association with transitioning to minimalistic shoes that attempt to simulate barefoot running by encouraging a modification of running form.21 As such, it is perhaps premature at this point to use injury studies conducted on habitual forefoot strikers as a basis for recommending gait modification for habitual heel strikers.

Studies looking more specifically at impact characteristics have linked higher vertical impact loading rates to injuries like lower extremity stress fractures and plantar fasciitis.22-24 Others suggest that there is no relationship between impact forces and injury and that runners need not worry about impact characteristics.16,17 For example, Nigg pointed out that internal joint forces at impact are much smaller than they are during the midstance phase of the gait cycle when the entire weight of the body bears down on the lower limb.17 He concluded that “from an impact perspective, the ‘cushioning’ aspect of shoes needs not take injuries into account.”17

Studies that have actively attempted to modify foot strike in subjects as a way of treating injury are rare. However, a recent small case series of three runners demonstrated that gait retraining incorporating foot strike modification (heel to nonheel strike) reduced both vertical impact peak and vertical loading rate, and also reduced pain in runners suffering from patellofemoral pain syndrome.25 Furthermore, another recent study26 used a gait retraining protocol involving foot strike modification to treat patients with chronic exertional compartment syndrome.

The authors recruited 10 patients diagnosed with anterior compartment syndrome who were candidates for fasciotomy and put them through a six-week gait retraining protocol (including advice to adopt a forefoot strike; all were heel strikers initially). After completion of the gait-retraining protocol, all patients exhibited reduced postrun intracompartmental pressure compared with post-run pressure measured prior to the intervention. After the intervention, patients were also able to run significantly longer prior to pain onset and pain reported on a visual analog scale was significantly reduced. At one-year follow-up all patients had avoided surgery and were able to participate in sports without limitation.

Interpreting the results of these studies is complicated somewhat by the fact that interdependence among various aspects of the running gait is common. As such, it is difficult to know whether a change in foot strike alone can explain positive outcomes or whether altered stride length, stride rate, or some other factor might have been equally or even more important.27

Looking beyond foot strike

All too often runners equate form change with foot strike change. This is a mistake. It must be emphasized that foot strike is just one modifiable aspect of running form to consider when attempting to treat or prevent injuries to the lower extremity, and one for which we still have very limited outcome data. Although differences in patterns of force application among the different foot strike types seem to be fairly consistent, exceptions to general patterns do exist.19 Some individuals can exhibit a high impact loading rate with a forefoot strike, while others can exhibit a low impact loading rate with a heel strike.5,28 Thus, suggesting a modification to foot strike in isolation could have unintended consequences, particularly if a runner adopts an exaggerated form of a recommended strike type due to a lack of understanding of proper mechanics.

An approach that can address the above concern is gait retraining using real-time visual feedback.29,30 In this method subjects are typically not told specifically how to alter their running form, but rather are presented with a visual display of a surrogate variable linked to increased injury risk (e.g., tibial shock magnitude for those with a history of stress fracture) and are encouraged to run in such a way that the variable is reduced. If auditory cues are provided, they are often nonspecific and include such things as advice to “run quietly” or “run softly.” This puts the focus on reducing an outcome through whatever means work best for the individual rather than on a specific and conscious attempt to change biomechanics.

A complete analysis of running form is beyond the scope of this review, but in order to emphasize that modifying foot strike in isolation is shortsighted, it is worth mentioning that other aspects of running form such as stride length can heavily influence loads applied to the limbs during running. For example, studies have shown that increasing step rate at a given speed (which leads to a reduction in step length) can alter muscle activity in the lower limb, reduce loads applied to the knee and hip, reduce peak hip internal rotation and abduction torques, and reduce peak knee extension torque.6,31-33 Furthermore, increased step rate has been shown to be associated with a reduced foot inclination at contact (i.e., flatter foot placement even if still heel striking), so interdependence between various aspects of running form should always be kept in mind.31

Concluding thoughts

Forces and their application to body tissues clearly change with changes in foot strike, and modifying foot strike is one tool that can be used in an attempt to treat some types of running injuries. However, knowledge is still limited about the risks associated with modifying foot strike and how best to transition runners from one form to another. Furthermore, clinical outcome data are limited, and few data exist comparing the relative merits of various approaches to form change for any given running injury. Thus, any attempt to modify foot strike, or any other aspect of running form, for therapeutic purposes should be undertaken carefully and with recognition of the relative risks and rewards.

Peter Larson, PhD, is an associate professor and chair of the Biology Department at Saint Anselm College in Manchester, NH.

REFERENCES
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  2. Hasegawa H, Yamauchi T, Kraemer WJ. Foot strike patterns of runners at the 15-km point during an elite-level half marathon. J Strength Cond Res 2007;21(3):888-893.
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  15. Hreljac A. Impact and overuse injuries in runners. Med Sci Sports Exerc 2004;36(5):845-849.
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  17. Nigg BM. Biomechanics of Sports Shoes. Calgary: Topline Printing; 2010.
  18. Nilsson J, Thorstensson A. Ground reaction forces at different speeds of human walking and running. Acta Physiol Scand 1989;136(2):217-227.
  19. Laughton CA, McClay Davis I, Hamill J. Effect of strike pattern and orthotic intervention on tibial shock during running. J Appl Biomech 2003;19(2):153-168.
  20. Daoud AI, Geissler GJ, Wang F, et al. Foot strike and injury rates in endurance runners: a retrospective study. Med Sci Sport Exerc 2012 Jan 3. [Epub ahead of print]
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  26. Diebal AR, Gregory R, Alitz C, Gerber JP. Forefoot running improves pain and disability associated with chronic exertional compartment syndrome. Am J Sports Med 2012;40(5):1060-1067.
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2 Responses to Foot strike in runners: Influence on injury risk

  1. Dennis Finch says:

    I am an inventor of a new pointe shoe for ballet and have take many dance classes. I am a fore foot striker when running. Most western style dancers land fore foot first. Pointe shoes are noisy and have no cushioning. The same may be said for ballet slippers and jazz shoes. Modern dancers are barefoot. Also ethhic styles, like African, Brazil. I can feel the difference in fore foot compared to heel strike running. I have never attempted a heel first landing from a jump in dance. I have with sneakers with foam soles. It all makes sense to me.

  2. teri green says:

    Very informative article. Some runners are using a heel wedge to change the position of heel strike that will alter the gait cycle to their benefit.
    Thanks again
    Teri

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