Author: Dr. Nate Jones, PT
Progression and Tissue Tolerance to Load
The single most important factor for avoiding pain and injury with running is likely how running is progressed. The human body is remarkably resilient and adaptable - given the right progressions, conditions, and genetics, it can lift over 1000 pounds off the ground or run more than 100 miles without stopping. When a stimulus is applied to the body, the body responds by trying to improve itself in relation to that stimulus, whether this is lifting heavy, sprinting, or running a marathon. If the stress of the stimulus is too low, there is no reason for the body to adapt. If the stress of the stimulus is too high, either in a single session or over time, the body cannot adapt quickly enough and excessive tissue breakdown and subsequent injury can occur. Adaptation to exercise is highly individual, with some people demonstrating more propensity for strength versus endurance, some people adapting well to both, and some adapting very slowly (and in some cases barely at all) to either. In addition, the starting points for tolerance to different types of loads are very individualized - depending on personal genetics, training history, nutrition, and many other factors, some people may be able to run a 10k with no prior training and remain injury-free, whereas others may find themselves injured running significantly less.
There are various strategies for running progressions, and a multitude of programs available for runners to use. Unfortunately, even some of the most popular ones do not seem to influence injury risk when applied. The classic 10% rule, in which running volume progression is limited to 10% increases a week, has been demonstrated in a single study to be no better than a less supervised running program, resulting in a 20% injury rate in novice runners.5 Another study followed novice runners over approximately a year and a half as they participated in a structured training plan to take them from a 10k to a marathon, and 85% experienced an injury at some point in the training.6
An attempt at preconditioning, a strategy which has worked well to decrease injury rates in many team sports,27-32 did not work in novice runners33 - however, the preconditioning program was likely such a low load that no adaptations occurred (self progressed walking and hopping with no supervision for four weeks). Preconditioning programs that have been effective in high school volleyball, soccer, softball, cross country, elite ice hockey, elite soccer, and even military recruits tend to include some form of high-intensity strength work or incline running as well as plyometrics - in other words, they are hard enough to promote muscle, bone, and connective tissue adaptation, but they keep actual impact volume low and don’t overwhelm the adaptive capacity of the body.
Looking at the preconditioning programs which are effective in other sports, the answer seems to be to stimulate musculoskeletal adaptations using strength and impact training before running is used to elicit further adaptations. However, even though injury rate is lower in more experienced and elite runners than novice runners,4 it is still relatively high. Assumedly, these populations have stronger bones, muscles, and connective tissue derived from running, but there is still an imbalance between the load of running and the ability of the body to tolerate and adapt to that load. This is where proper progression and the idea of acute on chronic workload come into play.
Using Acute On Chronic Workload To Progress
Every person’s body is used to a certain amount of work. If more than this amount of work isn’t done, the body has no need to adapt, and it maintains homeostasis. If less than this amount of work is done for a long enough time, the body will get rid of unnecessary adaptations and become used to the lower amount of work. In training and athletics, this idea is called a chronic training load. The acute training load is the training load done most recently - the stress a body is currently experiencing. Comparing the current stress to the long term stress a body is used to is the acute on chronic training load.
In runners, chronic training load may be calculated as simply as just averaging the time run each week (miles could also be used, but most models use time), otherwise known as external training load. However it likely becomes a more useful model if perceived effort, a form of internal training load, is added into the mix. This would be done by multiplying time run by the rating of perceived exertion (RPE) on a 1-10 scale each session, with a 1 being equivalent to sitting on the couch and a 10 being the hardest run that could possibly be done.
For example, a single week might look like this:
Monday: 30 minute run at race pace, RPE 8 -- 30 minutes x 8 RPE = 240
Wednesday: 60 minute steady run, RPE 6 -- 60 minutes x 6 RPE = 360
Friday: 40 minute run with strides, RPE 7 -- 40 minutes x 7 RPE = 280
240+360+280 = 880. The units are arbitrary - call them whatever you want! From here on out in this article they’ll be referred to as Minutes*Effort Load (MEL).
For each week, the Minutes*Effort Load would be calculated, and then the last month of work would be averaged together.
Week 1: 880 MEL
Week 2: 920 MEL
Week 3: 900 MEL
Week 4: 960 MEL
880+920+900+960 = 3660 MEL
The average Minutes*Effort Load for the month would then be 3660 MEL divided by 4 weeks = 915 MEL.
The chronic training load for the last month in this example would be 915 Minutes*Effort Load. The next step to make this useful for running progression would be to compare the chronic training load to the acute training load, or the MEL a person has most recently undergone.
The current training week would be compared to the average of the past month:
If week 5’s MEL is 1060, then:
1060 MEL divided by last month’s MEL of 915 = a ratio of 1.16
A recent systematic review from February of 2020 including 27 studies examining various sports (American football, soccer, rugby, basketball, Crossfit, cricket, hurling, and endurance sports) concluded that an acute to chronic workload ratio between 0.8 and 1.3 resulted in a fairly low injury risk, and a ratio of 2.0 or higher resulted in a significantly higher injury risk.34 Of the studies included, only one had been published looking at endurance sports - running, cycling, triathlon, swimming, and rowing (running-related injuries accounted for over half of the total injuries in the study!).35 This study used a more complex version of the acute to chronic workload ratio giving more weight to more recent weeks of training, but the authors also concluded that a 0.8 to 1.3 ratio was likely needed to promote positive training adaptations while maintaining a low injury risk. Interestingly, high acute training loads from 14 days prior were better associated with injury risk than 7 days prior - this may demonstrate the adaptation time necessary to see the negative or positive effects from a week of training. In addition, training loads that were too low also resulted in an increased injury risk - beneficial adaptations may have been lost and resulted in greater frailty.
With attentive record keeping of time run and perceived effort, and a fairly simple series of calculations, the acute training load can be compared to the chronic training load. This should reduce injury risk while still allowing for enough load to produce positive adaptations. Acute on chronic training load is an easy way to personalize a training program (high accessibility), it takes into account perceived effort (which should be a result of pace, running environment, and personal factors), and while it has not been compared directly to other progressions such as the 10% rule, it likely has an advantage over them. A good starting point acute:chronic workload ratio is between 0.8 and 1.3.
CAVEAT: The acute to chronic workload ratio, just like everything else, will likely have to be individualized. Some people may be able to deal with a higher ratio, and others may need less. 0.8 to 1.3 is a starting point, not the end-all be-all ratio for everyone.
Appropriate running progression is likely the single most important factor for avoiding running related injury, but it’s not the only factor. Foot strike patterns, running form, and footwear have all been examined as well.
Foot Strike Patterns And Shoes Or Barefoot
A foot strike pattern refers to the way in which a person’s foot initially contacts the ground while running. Foot strike can generally be grouped into one of three basic patterns - a forefoot strike, a midfoot strike, and a rearfoot strike, depending on whether the first, middle, or last ⅓ of the foot hits the ground first.
Multiple studies in distance running populations from recreational to elite runners have found between 75% and 95% of runners who habitually wear shoes are rearfoot strikers.36-39 On the other hand, a now-classic study examining habitually shoe-wearing and barefoot adults from the USA, recent shoe wearing adults from Kenya, and habitually barefoot and shoe wearing adolescents from Kenya found that 88% of barefoot adolescent Kenyans ran with a forefoot or midfoot strike, 91% of the recently shod Kenyan adults ran with a forefoot strike, 75% of habitually barefoot Americans ran with a forefoot strike, and 83% of the shod Americans still ran with a rearfoot strike.40 The running speed was self selected, and the Kenyan population in the study (the Kalenjin people) is one renowned for their distance running prowess. However, a study from a different Kenyan tribe (the Daasanach people - not known for their running prowess, but still comprised of traditionally barefoot runners) a few years later showed that 72% of their barefoot runners used a rearfoot strike, with 24% using a midfoot and only 4% using a forefoot strike.41 In the Daasanach population, as running speeds were increased, the strike type changed, so that at sprinting speeds 60% of runners used a midfoot strike or forefoot strike (significantly higher rate of midfoot striking) compared to a rearfoot strike.
In the Kalenjin population, many of the barefoot Kenyan runners were running under a 5 minute/mile self selected pace, and in the Daasanach population as the pace increased to sub 5 minute miles, the incidence of rearfoot striking decreased (but was still over 40%). It does appear that even in shod populations, as running speed increases, the incidence of mid and forefoot striking increases as significantly as well,42 and in studies examining elite distance runners, mid and forefoot striking becomes slightly more prevalent in the higher ranking athletes,36,38 although rearfoot striking is still by far the most common.
So at this point, the vast majority of research, excepting one of the most elite running populations in the world, shows that rearfoot striking is the most common type of foot strike, whether wearing shoes or not. However, rearfoot striking has been criticized as more likely to result in injuries, as it has an initial ground reaction force peak (impact peak) as the heel hits the ground, then another ground reaction force peak as the leg accepts and pushes back against the weight of the body (active peak). Forefoot running, at initial glance, does not seem to have an impact peak, and thus has a slower rate of loading. However, it is likely that the impact peak just occurs closer time-wise to the active peak, and is thus hidden inside the active peak - the impact is still experienced (probably to a lesser extent), but it is harder to measure.43,44
Both Kenyan population studies showed that, based on data from their subjects running over a force plate, the barefoot runners decreased the impact force with the ground. The Kalenjin population study authors posited that this was due to the forefoot strike specifically. However, the model used to examine the ground reaction force (impact with the ground) likely affects these results. A study examining forefoot versus rearfoot strikers (wearing shoes) using a 3D model and taking into account multiple types of forces (shear, vertical, and others) found that the peak forces experienced were actually greater in habitual forefoot strikers than in habitual rearfoot strikers due to higher posterior and medial forces.45 When the study authors had the runners change their foot strike to the opposite one, the habitual rearfoot strikers did decrease their loading rates and ground reaction forces in the transition to forefoot striking (likely as a result of having to focus on a novel form), but the forefoot strikers transitioning to rearfoot did not change the magnitude of the forces experienced.
Rearfoot striking, whether wearing shoes or not, is by far the most common form of foot strike. Forefoot striking becomes more common in more elite runners, but rearfoot striking is still more common in these populations except for the Kalenjin people. Fore and mid foot striking may not necessarily decrease the total amount of force experienced compared to rear foot striking - the force is probably just experienced in different areas with different foot strikes.
Do different foot strike patterns present with different injury rates?
As with most other running variables, there is research showing that foot strike patterns may influence injury rates and other research showing that it has no effect. A study looking at a collegiate cross country team found that rearfoot strikers did have a higher injury rate than forefoot strikers.47 However, another study investigating foot contact angles in runners with and without injury history48 and a third study examining foot strike patterns in U.S. Army soldiers49 found no correlation between the foot strike patterns and injury history. An excellent paper by Hamill and Gruber from 2017 examined the reasoning generally put forth for changing the foot strike pattern from a rearfoot to a forefoot strike:
“(1) it is more economical; (2) there is a reduction in the impact peak and loading rate of the vertical component of the ground reaction force; and (3) there is a reduction in the risk of running-related injuries.”43
After examining the available evidence at the time, the authors reached several conclusions: first, the current research does not support the idea of improved running economy with forefoot striking (although the body of evidence is small). Second, the impact peak of the ground reaction force (often thought to be a significant contributor to injury in runners) may be smaller in forefoot runners but is still present. In addition, the impact peak may not necessarily be associated with increased injury rate - there are multiple studies showing a link to injury rate, many other studies showing no correlation, and a couple studies showing a decreased injury rate with increased impact peaks and loading rates. It is likely that an increased impact peak can contribute to running related injuries in the context of other factors, such as training history, and progression, and tissue capacity and ability to adapt to the load as a result of those factors, but the impact peak cannot be viewed in black and white terms as good or bad. Finally, the authors believed that the current body of evidence was too small to come to conclusions about foot strike patterns affecting injury rates - at the time of the review, the only study directly examining the issue was the collegiate cross country runner paper mentioned earlier.
A narrative review published in 2020 by Hoenig et al came to similar conclusions about the inability to establish a causal relationship between foot strike patterns and injury rate due to lack of high quality evidence.47 They also included the results of several studies that showed higher mechanical loading of the ankle joint in forefoot runners and higher loading of the knee joint in rearfoot runners, and posited that this could result in a higher ankle/forefoot injury rate in forefoot strikers and a higher knee injury rate in rearfoot strikers.
However, just like the impact peak, higher or lower forces cannot be looked at in black and white terms - individual context and ability to adapt to these forces must be taken into account. A study published in June of 2020 looking at recreational runners, half with a history of knee pain and half without, found no difference in foot strike patterns between the two groups,48 but a study looking at barefoot versus shod runners found a higher foot and ankle injury rate in barefoot runners and a higher knee and hip injury rate in shod runners.25 Studies examining strength and tissue changes when runners transition to barefoot or minimalist shoe running have found increased plantar flexor strength, increased Achilles tendon size and stiffness, and increased foot arch height.49-51 The participants’ bodies adapted to the increased loads through the foot and Achilles tendon by becoming stronger, which circles back around to proper progression to allow for adaptation being the most important variable to avoid injury.
Finally, even if impact forces are problematic, they can likely be modulated without moving away from a heel strike. In one study, when runners were instructed to run with an obvious heel strike, a subtle heel strike, a mid foot strike, and a forefoot strike, the impact force decreased from the obvious heel strike to subtle heel strike.52 Interestingly, mid foot striking created the same force as the obvious heel strike, and forefoot striking significantly decreased the total force experienced, although peak force was the same between all groups.
Current evidence does not seem to support changing foot strike patterns to generally decrease injury rates, and while it has been posited that different foot strike patterns may result in different injuries, the evidence is still mixed on this front. Proper progression to increase tissue tolerance will have an overwhelmingly larger effect. However, as the load on specific tissues does change with different foot strike patterns, and injury is a result of load exceeding tissue capacity and adaptive ability, individuals who have appropriately progressed but still find themselves injured may benefit from trying different foot strike patterns with the understanding that stressing tissues differently will require progression as well.
Step Rate, Overstriding, Frontal Plane Deviation, and Vertical Oscillation
While foot strike patterns may not significantly change the total force experienced, and manipulating foot strike patterns may not be the best strategy to reduce running related injuries, there are several running form issues that may have an effect and are easier to change. Frontal plane deviation is the idea that, when looking at a runner from the front or back (running toward or away from the observer), their legs move too far left and right and may cross over the body’s midline. Overstriding is when a runner’s strides are too long and their legs impact the ground in front of their center of mass. Vertical oscillation is the up and down movement of a runner.
Injury-wise, overstriding has been shown to be associated with female runners with bilateral compartment syndrome.53 Other articles have investigated the forces experienced at different stride lengths, and have shown that as stride length increases while running at the same speed (i.e. step rate decreases), forces experienced at the shank (lower leg), knee, hip, and lumbar spine significantly increase.54-57 As step rate increases and stride length decreases at a given running speed, the force experienced through all of the lower extremity joints decreases per step. In addition to increasing the force experienced by all joints, longer strides create a larger “braking” effect, in which the runner is slowed down more by the leg impacting the ground per stride than with shorter strides.57,58 This will likely decrease running economy and performance, so regardless of whether or not overstriding increases injury risk, it is likely a poor running strategy.
A greater vertical oscillation, i.e. bouncing up and down, increases the loading rate (the two important types being average vertical loading rate and instantaneous vertical loading rate) the lower body experiences during running. The vertical loading rate is the speed at which force is applied to the body, and higher rates in runners have been found to be associated with stress fractures, plantar fasciitis,59,60 and other injuries. In addition, greater vertical oscillation will decrease running economy and decrease performance, as excess force applied in an upward direction is not force applied to forward movement. Like overstriding, excessive vertical oscillation is likely a poor running strategy.
Frontal plane deviation is another factor that is widely disputed in the literature. A systematic review from 2015 found that iliotibial band (IT band) injury risk in female runners was associated with increased hip adduction in the stance phase of running (i.e. the leg moving further across the body);61 however, the evidence available to synthesize the review was extremely limited. Hip abductor strength, which biomechanically should control the hip adduction angle and subsequently frontal plane deviation, has also been investigated in a systematic review. The review concluded that based on available evidence, IT band injury risk may be associated with hip abduction weakness, but there wasn’t enough high quality evidence to conclude anything about patellofemoral pain, shin splints, stress fractures, or Achilles tendinopathy. Another systematic review in 2019 also examined hip strength, and found that higher hip abduction strength was actually predictive of injuries (based on a single study); however the authors speculate that runners with a higher BMI likely have higher hip abduction strength but also may be at increased injury risk, and that hip abduction strength is likely not causative of injuries.62
How Can Overstriding, Excess Vertical Oscillation, and Frontal Plane Deviation Be Avoided?
Attempting to cue runners to change individual aspects of their form can be difficult. Luckily, there seems to be a single strategy that effectively eliminates overstriding, excess vertical oscillation, and frontal plane deviation: taking faster steps. Step rate manipulation does not necessarily mean running faster or slower - cadence can increase or decrease at the same running speed. Faster steps at the same speed mean a shorter stride length, which decreases overstriding. A study in 2015 by Lieberman et al found that 85 strides per minute (or 170 steps per minute) decreased the angle from the hip to the foot at impact (meaning the foot landed closer to the center of gravity) which substantially reduced loading rate through the leg and was metabolically the most efficient.58 Taking into account the standard deviation of the participants, approximately 162-178 steps per minute is likely a good range for running economy and minimal injury risk. Many studies have found that increased stride rate substantially decreases various forces through the feet, ankles, knees, hips, and low back.54-58, 64-68 One of these studies examining step length in female runners with and without patellofemoral pain, besides finding a 31% decrease in patellofemoral forces with a 10% shorter stride length, found that the total patellofemoral force over a mile of running decreased 7.5% with the 10% shorter stride length.68
Increasing step rate results in a decreased stride length at any given running speed, which consistently improves running form and reduces potentially injurious forces through every lower extremity joint.
To summarize thus far:
Overstriding, excess vertical oscillation, and frontal plane deviation may potentially be injurious due to increased forces and likely decrease running performance. Increasing step rate improves all of these factors. Additionally, step rate modulation appears to be substantially more effective than altering foot strike patterns for total force reduction. 162-178 steps per minute may be optimal for most runners.
Resistance Training To Reduce Injury Risk
While this article is not necessarily the right place for an in-depth discussion on all aspects of resistance training, it has been repeatedly shown that resistance training can be extremely beneficial for injury reduction across multiple populations. A 2018 meta analysis examining 6 studies, one with military recruits and 5 including soccer populations from high school to professional, found consistently reduced injury rates with strength training.69 Resistance training has an incredibly low injury rate compared to running, especially with the type of training most likely to benefit runners: bodybuilding. Bodybuilding, which is training to improve muscle size, and powerlifting, which is training to improve maximal strength in the squat, bench press, and deadlift, respectively have injury rates between 0.12 and 0.7 and 1.0 and 4.4 injuries per 1000 hours of training.70 Increasing tissue capacity is likely to be the largest benefit of resistance training to reduce injury risk, which bodybuilding and powerlifting style training are effective for. In addition, other tissues in the body, such as tendons and bones, respond positively to the same stimuli provided by bodybuilding and powerlifting.71,72 The stronger all these tissues are, the more impact forces they will tolerate before failure, which should result in less running related injuries.
In addition, a decent body of research has accumulated showing performance benefits with the addition of resistance training in endurance runners and other endurance sports. A 2017 systematic review found that strength training improved running economy, time trial performance from 1.5km to 10km, and anaerobic speed qualities in most studies, and it did not have a negative impact on VO2max, blood lactate levels, or body composition.73 A study examining female recreational runners found that replacing as little as 30 minutes of running with strength training a week significantly improved performance in a maximal treadmill test compared to a group that only ran.74 Subsequent research has further solidified the performance benefits to distance runners, duathletes, and triathletes in regards to running economy, biking economy, and energy cost.75-77
There are well established links between resistance training and injury reduction in sport. Bodybuilding and powerlifting style training will improve tissue capacity in muscles, tendons, and bones, hopefully reducing injury risk. Strength training also improves performance in running and other endurance sports.
Running may have a high injury rate compared to other exercise modalities, but it also has one of the lowest barriers to entry. The health benefits of regular exercise are indisputable, but injury may cause runners to stop running, and oftentimes runners will not replace the loss of activity with other exercise.78 Running injury rates are likely so high because of the high musculoskeletal forces as a result of impact compared to the perceived stress of the activity. The single most important factor for avoiding running related injury is proper progression, and using the concept of acute on chronic training load is a simple and effective way to progress. Foot strike patterns may influence which tissues experience high forces, but they are likely not a hugely important factor with running related injuries. Slowly increasing step rate to 162-178 steps per minute is an effective strategy to reduce running form deviations that increase impact force and decrease performance. Finally, strength training will improve tissue capacity, leading to reduced injury risk and improved running performance.
The Relationship Between Acute: Chronic Workload Ratios and Injury Risk in Sports: A Systematic Review: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7047972/
Training Load and Baseline Characteristics Associated With New Injury/Pain Within an Endurance Sporting Population: A Prospective Study: https://www.ncbi.nlm.nih.gov/pubmed/30427240