What are the Main Running-Related Musculoskeletal Injuries?

This is the first systematic review on the incidence and prevalence rates of RRMIs reported during distance running training or races. The eight articles included in this review differ in the study designs (prospective cohorts, one clinical trial, retrospective and cross-sectional studies), injury definitions and type of runners studied. A total of 28 RRMIs were reported, and the most frequently general RRMIs reported were medial tibial stress syndrome, Achilles tendinopathy and plantar fasciitis; and the main ultra-marathon injuries were Achilles tendinopathy and patellofemoral syndrome. In general, these studies presented a moderate risk of bias.

Most of the injuries observed in this study are related to overuse, in other words, overloading the musculoskeletal structures of the runners.[15,30] The RRMIs were predominantly below the knee (two of them located on the foot and ankle, one located on the lower leg and one located on the knee). This is consistent with the literature that shows the region below the knee as the most common anatomical location of RRMIs.[2,5,18,31,32]

Medial tibial stress syndrome (also known as shin splints) was reported in all three prospective studies and ranked first in two of the studies and second in the other. There are two possible reasons why medial tibial stress syndrome is so frequent among runners. The first is during the landing and propulsion of running; repetitive contraction of the posterior tibial, soleus and/or flexor digitorum longus muscles would generate excessive stress on the tibia, resulting in inflammation from insertion of the periosteal.[33, 34, 35, 36, 37] The second is the insufficient capacity for bone remodelling constituted by repetitive and persistent stress on the tibia caused not only by the muscle contraction but also on the vertical ground reaction during the landing phase in running.[34,38] A prospective cohort study on running injuries also indicated that a greater knee varus, the frequency of change in different types of running shoes and interval training are the three potential risk factors for shin running injuries.[12] However, it must be stressed that direct cause-effect relationships of this kind of RRMI has yet to be determined.[5,39]

The excessive loading during physical activity has been considered to be the main stimulus for the development of the tendinopathies.[40] Repetitive stimuli load the tendon beyond its physiological tolerance leading it to degeneration.[41] This excessive loading generated in the gastrocnemius and soleus muscles during running may predispose to the development of Achilles tendinopathy in runners.[42] A retrospective study showed that running on sand surfaces and running races from 1500 m to 5 km increased the risk of Achilles tendinopathy.[14] It might be because running on sand surface demands excessive push off, and runners normally prefer forefoot running for shorter races as this will decrease the landing time and increase the running velocity. Nonetheless, high quality, prospective cohort studies are needed to confirm these findings.[39]

The patellar tendon is exposed to high and repetitive eccentric loads of the quadriceps femoris muscle during running, which may explain the high injury rate of patellar tendons in runners.[43] Indeed, patellar tendinopathy was frequently reported in RRMIs among amateur runners who aimed to run between 20 and 50 km per week.[23,26] However, it is interesting to note that patellar tendinopathy is not common among marathon runners.[24] It is not clear if running experience or adaptation is a protective factor for the onset of tendinopathy but from the results of the ultra-marathon races, the acute onset of the tendinopathy is very likely to appear when the runners are subjected to consecutive running from 5 to 8.5 days.[27,28]

Plantar fasciitis is considered by healthcare professionals to be one of the most common injuries of the foot,[44] and it is characterized by a degenerative process of the plantar fascia[45] that causes pain on the medial calcaneus tubercle during weight bearing.[46] The failure of the fascia on supporting the loads applied to the body is commonly described as the mechanism of plantar fasciitis.[47] Indeed, plantar fasciitis was the most prevalent RRMI among master runners.[26] During the heel strike phase of running, the heel is the first contact point that has to absorb the impact of up to three times the total body weight.[48] The ability to absorb and transmit this impact depends on the resilience of the plantar fascia, the plantar fat pad and the intrinsic muscles of the foot. With ageing or prolonged repetitive overuse, the absorbability of the plantar fascia and fat pad might decrease, and this might explain why master runners are more susceptible to have plantar fasciitis compared with other types of RRMIs. A prospective study also observed that more experienced runners were more prone to sustain foot injuries;[12] however, this conclusion was classified as limited evidence in a previously published systematic review aimed to identify determinants of lower extremity running-related injuries.[5]

Studies have reported that patellofemoral syndrome is the most common RRMI among runners;[2,9,27,29] although our review revealed a different picture. Patellofemoral syndrome did not present as the highest incidence or prevalence rates of general RRMI, and only two articles reported this injury.[10,26] We take note that one study used the term ‘runner’s knee’ to describe injuries around the knee region.[24] However, ‘runner’s knee’ has been used by some authors to refer to patellofemoral syndrome[14,49,50] or iliotibial band syndrome.[51,52] Thus, we are not sure whether the authors referred to patellofemoral syndrome or iliotibial band syndrome in their study.[24] We also suggest the term ‘runner’s knee’ should be avoided to describe the injury around the knee area until there is consensus on the exact pathology that we refer to. Based on the results of this review, the most common RRMI around the knee region is the patellar tendinopathy, which the sports medicine community generally agree is the ‘jumper’s knee’.[53]

Patellofemoral syndrome was common during ultra-marathon races[27, 28, 29] and it was the third most prevalent RRMI among the ultra-marathoners. The most frequently cited aetiology for the patellofemoral syndrome is the abnormal patellar tracking plus high compressive loads generated by the quadriceps femoris muscle contraction that lead to an increased stress on the joint.[54,55] The repetitive contractions of the quadriceps femoris during running may contribute to the high compressive loads in the patellofemoral joint favouring the development of patellofemoral syndrome. A prospective study that enrolled marathon runners concluded that participating in a marathon race for the first time, use of medication and running few kilometres per week were considered to be risk factors for the development of patellofemoral syndrome.[4] Another prospective study showed that an increase of distance (kilometres) and the number of hours of training per week were considered to be protective factors against knee injuries in marathon runners.[12] This was classified as strong evidence in a systematic review about this topic.[5]

Ankle dorsiflexors tendinopathy is common among ultra-marathon runners during a race.[28,29] The condition is not common among non-ultra-marathon runners and may indicate that this RRMI may be specific to ultra-marathon runners only. This tendinopathy has been quoted as an ‘ultra-marathoner’s ankle’ in one ultra-marathon study.[29]

Most of the studies in this systematic review presented a moderate risk of bias. One of them had not achieved a score of 4 out of possible score of 9.[26] Although most of the studies have a clear definition of RRMIs the definitions always differ between studies. For instance, in one study, the injury was defined as “injuries sufficiently severe to impair their performance”[28] while for another study, RRMIs were defined as “injuries that markedly hampered running training or competition for at least 1 week.”[10] It should be noted that since 2007, it has been suggested in the sports medicine community that a standard RRMI definition should be adopted[5] but, to our knowledge, there is no consensus about the most appropriate definition for an RRMI. All articles included in this review described the type of runners (e.g. ultra-marathoners[27, 28, 29] ) or their characteristics of training (e.g. running at least five times per week[23] ), or race participation (runners of a 10 km race[25] ). Different definitions of RRMIs, as well as a difference in runners’ characteristics, hampers the possibility of pooling the data for a meta-analysis or for making a meaningful comparative analysis of the incidence or prevalence rates of RRMIs.[5,21,56]

In terms of the participants, only one study performed a random sample selection[25] and two studies sampled the entire target population of runners.[28,29] There are only three prospective studies that could enable the assessment of the loss to a follow-up criterion,[10,23,24] and all of them fulfilled this criterion, which indicated a lower risk of bias in these studies.[57] One study did not collect the information of injuries directly from runners[28] and this can introduce some degree of bias.[58] Five studies used the same mode of injuries data collection for all runners,[10,24,25,27,29] and three studies utilized a mixed mode of data collection (i.e. data collected while observing a race can be either transmitted for assessment via e-mail or hard copy,[26] by telephone or verbally,[23] or data can be collected through interviewing runners, and health professionals such as physicians or physical therapists[28] ). Standardization of the data collection for all runners would reduce the risk of bias on these studies.[58,59]

Studies that aimed to register or evaluate RRMIs should describe how the diagnosis was made. The RRMI should best be diagnosed properly by medical professionals to minimize the risk of bias or misinterpretation of the diagnosis. All studies described the injury collection procedure and only two studies did not have a diagnosis made by medical doctors/professionals.[25,26] Four studies used a running diary or a questionnaire to register RRMIs by the runners themselves.[10,24, 25, 26] This self-reported information about an RRMI may only represent what the runners think about their injuries and the injuries themselves might be either under- or overestimated.

Running injuries are primarily overuse injuries that occur due to the overloading of the musculoskeletal structures[15,30] caused by repetitive microtrauma over a long period of time.[60] Thus, a long follow-up period is necessary to capture all the possible RRMIs and the response relationships with training. Three incidence rate studies (two prospective[10,23] and one clinical trial[24] ) were conducted for at least 6 months and fulfilled this criterion. Retrospective studies are more prone to recall bias.[56,61,62] We consider a 12-month period to be acceptable, and a study has indicated that the recall bias within this period is around 12%.[61]

In order to compare the incidence or prevalence rates of RRMIs among different studies, it is necessary to express these injury rates in a comparable or common unit.[5] Some studies with runners as the subjects reported RRMI rates by using the number of injuries per 1000 hours of exposure to running[7, 8, 9, 10, 11, 12] and this is more appropriate because the amount of exposure to the load (running) may influence the development of musculoskeletal injuries.[63] Among the eight articles, only two of them stated the overall incidence ratio of RRMIs per 1000 hours of exposure to running[10,24] and no study stated the incidence or prevalence ratio of each RRMI. We suggest that future studies should standardize the expression of RRMI data by the ratio of the number of injuries per hour of exposure to running. This would enable comparison or pooling of data for meta-analysis.[64] In addition, future studies should be conducted to determine a consensus about the definition of RRMIs, as well as aim to identify the main RRMIs in different types of runners (e.g. ultra-marathoners, marathoners, elite, recreational, race runners, etc.).