Avoiding injury is one of the major goals of training for distance running. On account of the impact forces experienced at footfall on every stride, runners are uniquely prone to injury. However, effective strategies for preventing injury are elusive. In recent years, advocates of techniques such as Pose have claimed that injuries are largely due to poor running technique, and have promised that the problem can be overcome by proper technique. In particular, they have identified heel striking as a cardinal problem. However, there is very little evidence to support this claim. Others have advocated stretching during warm-up as a strategy to reduce risk of injury, though the evidence provides little support for this claim. Advocates of barefoot running have proposed that running shoes are the problem, but again there is little evidence to support the claim that running barefoot or in minimalist shoes reduces risk of injury. Conversely, the manufacturers of running shoes have placed blame on foot orientation problems such as over-pronation and claimed that motion control shoes can reduce this risk. Yet again, the evidence is slight, though at least one study had found that over-pronation is associated with increased risk of injury.
It is likely that a wide variety of factors contribute to injury in different individuals. Meta-analyses that pool the findings of many studies are only likely to identify risk factors that are common to many athletes. Two risk factors emerge consistently: a history of previous injury; and a large weekly volume of training. Lisa Callaghan has provided an up-to-date review of the evidence.
A history of previous injury might predispose to subsequent injury simply because the athlete has not corrected problems that contributed to the first injury. It is also possible that unsatisfactory recovery from the previous injury plays a role. Muscles, tendons and other connective tissues tear when subjected to force that exceed the limits of their resilience. A cardinal factor in the resilience of connective tissues is the elongated spring-like structure of collagen fibres, making them resilient against forces acting along the direction of the fibre. During the initial stages of repair following injury, collagen is laid down with random orientation providing a framework for tissue renovation, but full resilience requires remodelling such that the collagen fibres become aligned in the required direction. Therefore effective recovery requires early mobilization to promote the laying down of appropriately aligned fibres, perhaps augmented by slow stretching.
Observational studies report that training volumes of 65 Km (40 miles) or more per week are associated with higher rates of injury [Fields et al; van Ghent et al]. In part these observations might simply reflect the greater duration of exposure to risk of injury, though it is likely that fatigue plays an important role. In particular fatigue impairs neuromuscular coordination increasing the likelihood of poor coordination between different types of muscles fibres within a muscle and poor coordination between muscles that act as agonists and antagonists, resulting in excessive local forces within tissues.
Simply limiting training volume is unlikely to be a satisfactory strategy for many runners, making it desirable to identify alternative strategies to reduce the damaging effects of fatigue. As the forces exerted increase with increasing pace, it might be expected that injury risk would be greater at faster paces. However the observation by Van Middelkoop and colleagues that among marathon runners, those who do interval training have a lower risk of knee injury raises an intriguing question. Could it be that interval training provides greater protection than training at somewhat lesser paces in the vicinity of lactate threshold? Interval training, in which short efforts at fast but sub-maximal pace are separated by recovery periods, tends to promote the development of neuromuscular coordination with relatively mild muscle fatigue. As discussed in my recent post, interval training is likely to promote a favourable balance between anabolic and catabolic hormones, leading to strengthening of tissues. In contrast, running for a sustained period at threshold pace might produce fatigue with the associated risky deterioration of neuromuscular coordination during the session, and also tip the balance towards the catabolic effects of cortisol, promoting subsequent breakdown of tissues.
Even more speculatively, the viscoelastic character of the musculotendinous unit might result in a peak risk of damage to muscles and tendons at threshold paces. Viscoelastic materials offer strong resistance to brief sudden onset forces but less resistance to sustained forces. Although force is greater at sprinting pace, time on stance decreases. At speeds above LT there is actually a decrease in the impulse acting through the foot at each step because the increased force is more than compensated by reduction in time on stance. The product of forces x time on stance actually decreases, as illustrated in figure 1 based on data from the study by Weyand and colleagues.
Thus, it is possible that the risk of tissues tearing is actually less at sub-maximal paces substantially above LT than in the vicinity of LT. Nonetheless, it is crucial to prime the requisite neuromuscular coordination during the warm-up (for example by moderate intensity strides) and it is generally desirable to avoid absolutely maximal effort that taxes neuromuscular coordination to the limit, during training.
The other pole of polarised training is low intensity running. This has the potential to build resilience of the muscles, tendons and the other connective tissues engaged during running by repeated application of moderate forces. Provided training volume is built-up gradually and excessive fatigue is avoided, the risk of injury is low.
While the predisposing and precipitating factors causing running injuries remain controversial, consistent evidence indicates that a high weekly training volume increases the risk. In contrast, the observation that interval training provides some protection suggests that polarised training might diminish the risk. Observational evidence and also speculation based on principles of biomechanics and physiology suggest that high intensity sessions have the potential to build effective neuromuscular coordination, while low intensity training would be expected to enhance the resilience of muscles, tendons and other connective tissues with relatively little risk. Nonetheless, as with any type of training, it is important to build up the training load gradually, and to warm up for each session in a manner the primes the requisite neuromuscular coordination.