By John Foster MCSP, MACPSM, Dip Phys, PGDip H. Sci.
Excessive loading of the Achilles Tendon (AT) is the main pathological stimulus for tendon degeneration . A continuum model of reactive tendinopathy, tendon dysrepair and degenerative tendinopathy has been proposed  (You can listen to with model co-author, BJSM Deputy Editor by clicking here). Note that there is little association between ultrasound abnormalities and local symptoms . For the purposes of this article we will focus on midsubstance rather than insertional Achilles tendinopathy.
It is often stated that forefoot striking (FFS) causes an increased risk of developing Achilles tendinopathy as more strain is exerted on the AT . Around 70 % of elite runners and 85% of recreational runners  land with a rearfoot strike (RFS) pattern.
For the last few years FS patterns have been blamed for several running injuries mainly due to theories that impact causes injury  and FFS reduces impact . One only has to scan the various runners’ magazines and running blogs to see the obsession with FS. Much of this interest started with Daniel Lieberman’s work which suggests we are adapted to FFS from an evolutionary perspective and that habitually barefoot people run naturally this way . As science repeatedly tells us ‘one study does not make it so.’ More recently researchers show that other habitually barefoot people predominantly run with a RFS . I too share responsibility for the FFS obsession due to exceptional results when dealing with exertional lower limb pain leading to a new diagnosis of anterior biomechanical overload syndrome . Initially it seemed that changing FS pattern could solve many running related Injuries (RRIs) but as time went on we found that often we were creating problems elsewhere – particularly in the Triceps Surae (TS). As more researchers investigate parameters other than foot strike for various RRI’s it is becoming apparent that other factors may be even more important.
Another historical driver for the FFS obsession was the reduction in impact peak of the vertical ground reaction force (vGRF). Whilst there may be some association between impact and tibial stress fractures there is little evidence that impact is associated with other running injuries . Some authors have found that RFS is associated with relatively high vertical loading rates and associated high vertical tibial accelerations  while others have found that a forefoot strike (FFS) produces higher values (19).
There are two different foot strike strategies for reducing tibial loading rate. One involves a heel strike and the other a forefoot strike (20). It appears that at lower speeds a rearfoot strike may reduce loading rates and at higher speeds a forefoot strike does the same. If speed is kept constant however, knee stiffness correlates well with loading rate with both variables higher in those with a history of tibial stress fracture . Knee stiffness may therefore be a more important variable than FS when assessing tibial acceleration and loading rates. Perhaps foot-strike patterns are responsible for some of these differences but it is increasingly apparent that other factors may be more important in determining tibial acceleration and loading rates.
Tibial accelerations and loading rates as measured by ground reaction force may be affected by foot strike but what effect does this have on the Achilles tendon? One of the main functions of the Triceps Surae is tibial deceleration after foot strike . If tibial acceleration forces are high we would expect higher internal loads within the Achilles tendon. Much of the measurement of tibial acceleration is performed with the attachment of accelerometers at the ankle. Whilst this may be good for measuring instantaneous tibial acceleration at foot strike it does not give us information on rotary acceleration or torque. It is primarily plantarflexion torque that the tendon produces to resist rotary tibial acceleration around the ankle prior to elastic recoil. The load on the tendon may be either compressive or tensile  and involve parameters of maximal load and loading rate.
If we assume overloading is the main precursor to injury, is loading rate more important than maximal load? Loading can occur in different planes of motion. For speeds of 3.5 to 5 m/s a significantly higher torque occurs in the sagittal plane in the order of 3Nm/kg as compared to 0.3 Nm/kg in the frontal and transverse planes . This suggests that Achilles tendon loading parameters are more important in this plane rather than the oft thought connection to ‘overpronation’ in its many different definitions.
During the stance phase of running there is a linear relationship between ankle dorsiflexion angle and plantarflexion moment with peak values occuring almost simultaneously during midstance . Due to this linear relationship, reducing mid stance dorsiflexion will serve to reduce peak plantarflexion moment around the ankle joint. As peak Triceps Surae (TS) activation also occurs towards maximum ankle dorsiflexion  then reducing ankle dorsiflexion at mid stance will also reduce peak TS activation.
A FFS pattern increases plantarflexion moments of the ankle and causes significantly higher gastrocnemius activity than a rearfoot strike (RFS) [8, 17]. As forefoot strikers exhibit higher Achilles tendon loading rates in stance compared to rearfoot strikers [4, 17] it is commonly thought this may place the AT under greater risk of injury. This higher loading rate however results in peak AT force occurring earlier in stance phase than a RFS . This means the AT is in a shortened position so more resistant to load . The reduction in midstance dorsiflexion just by changing to a FFS may be 5°  but in our Running Clinic we commonly achieve reductions in midstance dorsiflexion of up to 10° with coaching cues directly focused on effecting this biomechanical change. Reduction of load on the AT when in an over-elongated position and therefore less able to withstand load  forms the basis of our Running Gait Re-education Program. As forces that place highest stress on the tendon unit occur at end range dorsiflexion during midstance then it seems logical to focus on changing this parameter rather than footstrike.
When changing gait patterns we must take into account loading rate (higher with FFS in early stance compared with RFS), maximum plantarflexion torque (higher at max dorsiflexion in mid stance) and tendon strength/length. If tendon strength is lower when in the most lengthened position then it is likely although not proven that repetitive dorsiflexion beyond an optimum at mid stance is a risk factor for Achilles tendinopathy. If this is the case then higher plantarflexion torque which occurs in early stance of a FFS may not be the risk factor commonly assumed. When all factors are taken into consideration a mid-foot or reduced heel strike may be preferred to reduce initial load but more importance given to reducing midstance dorsiflexion when the AT is most vulnerable to injury during the running gait cycle. Further work needs to be done to determine if there are morphological or histological factors predisposing some individuals to tendinopathic change. We also need to establish if an optimum length/tension ratio exists beyond which the AT is at risk when repetitively loaded. This has implications for current loading programs which load the AT in end range positions and for running gait modification programs. Prospective studies may then assist in revealing if midstance dorsiflexion beyond an individual’s optimum during the running gait cycle is a risk factor for Achilles tendinopathy.
John Foster is a Consultant Physiotherapist, Running Injuries Specialist
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