This series may be a bit of a break away from the typical Barbell Medicine article, as many of our readers are primarily interested in strength sports and barbell training. It is adapted from a talk given at the American Physical Therapy Association Combined Sections Meeting in January 2019. While it is great that so many individuals are interested in barbell training as their primary means of physical activity, there are many other beneficial ways to be active. If someone wishes to focus on endurance activities (e.g., running, rowing, cycling, swimming, triathlon) there is still good evidence for the utility of supplemental resistance training for both performance outcomes and injury risk reduction.
Background
Resistance training for endurance athletes is an important topic, as some 60 million individuals participate in running alone. According to a systematic review from 2007, the prevalence of injury in these individuals varies from 19.4% to 79.3%. Van Gent 2007 This is despite a trend towards teaching what is considered “proper” running mechanics, appropriate dosing of training, and an awareness of other physiological influences on injury risk. If injury risk were just a matter of proper biomechanics and running technique this number should decrease with the integration of this new information, but it appears that anywhere from 1 to 4 out of five runners are dealing with some form of injury at any given time.
A recent systematic review demonstrated little correlation between what are deemed “biomechanical risk factors” and injury. Ceyssens 2019 Another systematic review showed a 27% injury rate per year among novice runners, 32% per year among long-distance runners, and 52% per year in marathon runners. Kluitenberg 2015 The lower rate among novice runners is thought to be due to lower total training volume. Similarly, a 2015 meta-analysis by Videbaek et al estimated the pooled risk at 17.8 running-related injuries per 1000 hours of training for novice runners and 7.7 injuries per 1000 hours in recreational runners. Videbaek 2015
Injury Risk Factors and Treatments
Among the established risk factors for injury in runners, the biggest predictor of future injury is a history of prior injury. Van der Worp 2015, Saragiotto 2014 Since this is a non-modifiable risk factor, it does not give a clinician, coach, or athlete a target to reduce the risk of injury. Common treatments for running-related injuries include stretching, gait retraining, early mobilization, bracing, orthoses, gradual return to activity, eccentric exercises, heavy slow resistance training, and a host of non-efficacious modalities that one can explore on social media. Arnold 2018, Beyer 2015, Instagram While none of these have been prospectively studied in endurance sports, we actually do have evidence on stretching, eccentric exercise, and heavy slow resistance training with respect to injury risk reduction among runners.
Stretching
The familiar Barbell Medicine reader is saying, “Wait … did they just say stretching?” Indeed: the most recent Cochrane Review on interventions for running-related injuries from 2011 showed that stretching does not reduce the risk of injuries. Yeung 2011 We also have decent evidence from Laureson et al. that stretching is not an effective way of reducing the risk of injury. Lauersen 2014 This same paper did go on to say the resistance training reduced all injuries by one-third and reduced overuse injuries by half — and that kind of statement deserves elaboration given the traditional dogma on resistance training among the endurance population.
Resistance Training
The normal trope related to resistance training and endurance athletes is that lifting weights will make them “bulky”, with concern that this will affect their overall performance. Of course, the lean archetype of the runner is viewed as ideal, which makes it difficult to achieve buy-in for long term success predicated on moving away from that ideal. A study by Beattie et al from 2017 found that after a 40-week strength training intervention, runners who participated in a strength training program performed better than their control group peers, despite the program producing “no significant changes in body composition variables between or within groups.” Beattie 2017 There is also concern that resistance training will lead to further injury, but according to Aasa et al the injury rate for the sport of competitive powerlifting (i.e., not basic strength training) is estimated at 1.0-4.4 injuries per 1000 hours of participation, far below that of runners. Aasa 2017
According to current ACSM exercise guidelines, individuals should participate in resistance training 2-3 times per week — yet most endurance athletes fall well below these guidelines. Only 18.6% of individuals met those guidelines, but those that did had a 23% reduction in all-cause mortality. Dankel 2016 Part of this may be due to a misunderstanding of how to dose resistance training interventions, where the critical concept is progressive training.
We all have to start small in our endeavors to get stronger or become better athletes, and importantly, there are no non-responders to resistance training. Churchward-Venne 2015 It is more likely that if someone is not responding or improving from the training, it is because the dosage or formulation is wrong. This may mean the program is underdosed, as is often the case with bodyweight exercise programs, or overdosed via increasing volume, intensity, frequency, or a combination of these too quickly. Resistance training in general has demonstrated a host of benefits for long-term health including improved weight control and cardiovascular health, increased bone mineral density, and reducing the risk of age-related musculoskeletal injury, among many others. Ciolac 2016
Common Running Injuries and Treatments
Common injuries that runners incur include patellofemoral pain syndrome (PFPS), tendinopathies, hamstring strains, and bone stress injuries. van Gent 2007 This is in line with the findings of Van Mechelen that 50 to 75% of running-related injuries are related to overuse, with the majority occurring around the knee. Van Mechelen 1992 A detailed discussion of each of these conditions is outside the scope of this article, but we will focus on the role for resistance training in their management.
Patellofemoral Pain Syndrome (PFPS)
For patellofemoral pain syndrome, a meta-analysis from 2018 demonstrated that strengthening of the hip and knee was superior to strengthening of the knee alone for treatment. Nascimiento 2018 The most recent Cochrane Review for treatment of PFPS showed that exercise therapy had consistent results for improving functional ability and reducing pain. Van der Heiden 2015 The authors here did conclude that no particular form of exercise appeared superior to another — but once again, we will argue that appropriate dosing could plausibly shift these results. Prospective data suggests that lower strength in the quadriceps and hip abductors have shown to increase the likelihood of PFPS in certain cohorts as well. Neal 2019
Tendinopathy
Tendinopathy refers to a continuum of tendon changes related to collagen (the primary substance that comprises tendon) including disruption, inflammation, and cellular responses. Cook 2016 Not all tendinopathy is symptomatic (i.e., painful), as “degenerative changes” of the tendons are commonly seen in the older population without any symptoms, not unlike what we observe in the spine. Abate 2009 Historically, the “gold standard” treatment for tendinopathy was based on the Alfredson protocol for Achilles tendinopathy, which involves eccentric heel raises. Alfredson 1998 It should be noted that the initial Alfredson study was the cohort consisted of recreational, middle-aged runners. And while many subjects did experience resolution of symptoms, extrapolating these findings to higher level runners did not provide the same rate of results. A van der Plas 2012 The eccentric protocols have grown into evidence for heavy slow resistance (HSR) training. Beyer found that eccentric and heavy slow training produced similar results for Achilles tendinopathy. Beyer 2015 Prior to this, Kongsgaard et al had shown the same benefit for heavy slow resistance training for the treatment of tendinopathy in patients with patellar tendinopathy as well. Kongsgaard 2009
Once again, dosing seems to factor into achieving maximal results. A meta-analysis by Bohm et al emphasized that loading magnitude was an important variable in eliciting tendon adaptation. While the differences in functional outcomes for eccentrics versus heavy slow resistance were similar, satisfaction was higher in the HSR group. This could be related to the Alfredson protocol consisting of 630 repetitions per week, while heavy slow resistance ranged from 48-96 reps. The time freed up by the implementation of heavy slow training would leave more time for training in the modality of choice. The current literature suggests a set rep scheme involving 5 sets of 4 repetitions at intensities greater than 85% of 1 rep max strength, with 3 seconds of time under tension as an effective dose to stimulate tendon adaptation. Mersman 2017 This is much closer to the dosage recommended by Beyer and Kongsgaard. It is also worth mentioning that the shortest tendinopathy protocol is 12 weeks in length, and the duration of training will become a theme moving through this piece. Anyone who is a resistance trained athlete and trains towards 1RM performance will recognize that 5 sets of 4 at 85% 1RM and 3 seconds time under tension is a nearly impossible workout. As much as anything, this dosage derived from the research speaks to most endurance athletes being under-trained from a strength standpoint.
Strains
Strains typically occur in the context of high-speed running, and the risk is contingent upon how often high-speed running occurs in both an acute and chronic fashion. Elliot 2011, Duhlig 2016 Strains in particular have the best evidence for resistance training producing a risk reduction for both injury and re-injury. The “Nordic Hamstring Curl” has become common practice for reducing the risk of hamstring injury, and evidence is accumulating for the utility of an adductor strengthening program for reduction of groin strains as well. Zouita 2016, Haroy 2019 While adductor strains occur more commonly in team sports, both of these programs rely heavily on the eccentric portion of the contraction. It will be interesting to see whether the same trend that follows tendinopathy holds for strains, where the intensity of loading has as much of an effect as the contraction type on injury risk. Both Nordics and the adductor protocol are relatively difficult exercises from the start, so the training effect may be greater as a result.
The use of Nordics has been shown to reduce the risk of strains, with an impressive number needed to treat (NNT) for preventing an additional strain of just 3. In this controlled trial, the cohort participating in the Nordic hamstring protocol suffered 3.8 injuries per 100 player-seasons, versus 13.1 per 100 player-seasons in the cohort that did not perform Nordics. Petersen 2011 A meta-analysis from 2015 also mentions the obvious need for adherence to the program in order for an effect to be seen. Goode 2015
Bone Stress Injuries (BSI)
Lower extremity stress injuries are always a concern for endurance athletes. Stress injuries happen on a continuum in response to the bone’s inability to handle repetitive mechanical loading, leading to structural fatigue. Warden 2014 Research has consistently shown that endurance athletes have lower bone mineral density (BMD) than their multi-directional sport peers. For example, in 2003 Burrows et al found a weak negative correlation between the number of miles ran per week and BMD in the femoral neck and/or lumbar spine. Burrows 2003  In other words, the more miles ran, the lower the measured bone density tended to be.
Dual-energy x-ray absorptiometry (DEXA) measures bone mineral density compared to age- and sex-matched peers and is used to diagnose disorders of bone density like osteopenia and osteoporosis. Measurements are used to calculate a T-score result, which compares an individual’s bone density with what is normally expected in a healthy young adult of the same sex. A normal T-score falls in the range of +2.0 to -1.0. These scores are sometimes converted into Z-scores which is the number of standard deviations above or below what’s normally expected for someone of that same age, sex, weight, and ethnic or racial origin. Z-scores of -2.5 or less represent a bone density 2.5 standard deviations below matched peers, which defines osteoporosis. Scores of -1.0 to -2.4 represent osteopenia, and scores of -1.0 or greater are considered normal.
Adolescent female cross country runners presented with a Z-score of -2 in 11.8% of runners, and -1 in 28% in a study of participants from Southern California. Barrack 2008 High school female athletes with a Z-score of -1 were 3.6 times as likely to suffer an injury as their peers who had normal scores. Rauh 2010 When comparing both male and female cross-country runners, those who presented with greater lean mass (LM) also had increased bone mineral density. Roelofs 2015 The type and distance of running seems to play a role in the accrual of BMD, with sprinters demonstrating the highest bone mineral density, followed by middle distance, followed by long distance in a study by Wilks et al. Wilks 2009 There was also evidence that adolescent athletes in repetitive sports (running, swimming) had a 3-fold increase in rates of low BMD versus their multidirectional sport peers (33.9% versus 11.8%). Nichols 2007
It is not just female endurance athletes that are at risk for bone stress injuries either. Hind et al showed comparable rates of lower bone mineral density in trained male and female endurance athletes. Hind 2006 Male cyclists and runners are typically at higher risk for low BMD. Steward 2000, Smathers 2009, Rector 2008 The overall sport-specific prevalence of low bone mineral density suggests this is a maladaptive response to the sport that needs to be addressed through interventions shown to increase BMD.
Resistance training is generally accepted to increase bone mineral density in athletes. Bemben 2011, Mosti 2014 However, a prospective study using data from the National High School Stress Fracture Registry found that 58% of individuals surveyed did not participate in any resistance training. Nussbaum 2019  Even from this cohort, the exact dosage of what constituted“resistance training” is not known. Over half of endurance athletes in this survey did not partake in any resistance training, and it is likely that a large proportion of the 42% who did were underdosed. Specific to females, there is meta-analysis level evidence that resistance training can increase bone mineral density. Kelly 2001
Low Back Pain
Searle et al analyzed interventions for the treatment of low back pain and found support for recommending strengthening exercises for the treatment of low back pain. Searle 2015 While simply being stronger or having more muscle does not reduce the risk of low back pain, an active approach to treatment of low back pain is superior to inactivity or rest. Suri 2015 Endurance athletes are typically already an active cohort, so just advocating for even more activity is probably not in their best interest. What is more likely is the need for a different type of activity in order to manage symptoms and create a more durable athlete.
For each of these common conditions related to endurance sports, resistance training has been shown to be a first-line treatment. Furthermore, if external loading is efficacious once symptoms arise, it may be wise to include it before their onset. Resistance training should be an integral part of any endurance athlete’s training program. Waiting until symptoms emerge to implement resistance training is a missed opportunity with which to not only make an athlete more durable, but keep them on the road, bike, water so that they can continue to accumulate practice in their primary endeavors.
In part 2 of this series, we will examine the role of resistance training with respect to endurance performance.
Special thanks to the the following individuals for their contributions: Austin Baraki, MD, Michael Ray, MS, DC, Josh Barabas, PT, MPT, OCS, CSCS. Brittany Barrie, PT, DPT, Christy Morgan PT, DPT, SCS, Samuel Lyons, MS, and David Lewis.
