Welcome to tendinopathy blog 12. Just finished a fun 2 days of teaching in Perth. Long way to come for a course (4 hour flight from Melbourne) but well worth it, Western Australians really are among the friendliest Australians!
This weeks blog focuses on run retraining strategies for Achilles tendinopathy, energy storage loads around the Achilles and plantar fascia and how they relate to injury, as well as elastography in tendinopathy and how it may help to shed light on rehab mechanisms.
Some juicy clinical messages in there as always. Subscribe via the homepage to get them in your inbox.
All the best
Best run retraining strategies for Achilles?
What they did: Lyght et al. investigated the effects of altering step rate and foot strike on Achilles tendon (AT) stress during running. 19 female runners ran in the same footwear under 6 conditions including rearfoot strike (RFS), non-rearfoot strike (NRFS) and plus or minus 5% of their preferred cadence. Speed was controlled between 3.33-3.68 m/s. They measured 3D motion as well as force data and were able to estimate muscle forces and tendon strain/strain rate.
What they found: the NRFS pattern was associated with greater Achilles strain and strain rate, and this translated to increase stress (force per square cm) – see figure below. Peak strain and stress reduced when they ran at 5% above their preferred cadence, regardless of foot strike.
Clinical interpretation: the authors suggest that using a RFS strategy and increasing cadence may be useful strategies to manage or prevent Achilles tendinopathy. This is a case of shifting stress versus reducing stress. The FFS patterns is likely to shift load to the ankle and increase AT stress. Increasing cadence (but keeping speed constant) will generally reduce ground reaction force and the loads the leg has to dissipate. In terms of the Achilles the secret ingredient of increasing cadence is reduced ankle dorsiflexion in midstance, so therefore you get less Achilles strain and stress (Hof et al 2002). This may be at the expense of shifting them a little towards a midfoot strike so therefore potentially increasing Achilles loading rate, but this may be offset by reduced strain in people with excessive ankle DF. See the comprehensive recent review by Christian Barton et al. in the biomechanical effects of these gait changes. The other consideration when manipulating cadence to influence AT load is how it will effect running economy. This recent review by Izzy Moore found that running economy was optimised at +/-3% of self selected stride length. Given stride length and stride frequency have an inverse relationship at constant speeds, we may compromise economy if increasing cadence too sharply, especially in the short term.
Elastic energy storage in the plantar fasciopathy
What they did: Energy storage of tendons is such an important concept to understand, and Wager et al. have looked at something of great interest to me and that is the energy storing potential of the plantar aponeurosis/fascia. During running gait elastic energy stored during stance is harnessed during push off and this improves mechanical efficiency by allowing muscle fascicles to operate in an optimal range (see last weeks blog for greater discussion of this). The Achilles tendon is considered to be the primary site of energy storage and release during running. These authors investigated energy storage and release in the plantar fascia and how this is influenced by foot kinematics and strike pattern.
Eight healthy runners (minimum of 15km per week) were recruited and ran at 3.1 m/s with both a RFS and NRFS pattern (self selected pattern first, then they were asked to perform the other pattern). They collected 3D motion and force data. Plantar fascia strain was estimated using modelling.
What they found: The PA stored 3.1J of EE during the stance phase of running (3.1 m/s), and this was released during push-off. This is relatively small compared to the 30-40J stored by the Achilles tendon (Alexander 1977). This energy contributed to passive arch shortening during push off – that is, it contributed to the windlass mechanism. There was no significant difference in peak plantar fascia strain between RFS and NRFS conditions (see figure below). However, the NRFS condition was associated with less plantar fascia strain at foot strike (due to reduced dorsiflexion at foot strike) and a faster strain rate, and peak strain occurred later in stance.
Clinical interpretation: This study confirms that passive elastic energy in the plantar fascia contributes to windlass mechanism function and running gait efficiency. And again it highlights the importance of the stretch-shorten cycle and energy storage and release for human locomotion.
A more important point for clinicians is to recognise the link between energy storage and release and tendon injury. Most activities that load tendons involve some tendon strain and therefore energy storage. For example, a simple calf raise leads to some strain or stretch of the Achilles, as does a hop. The difference between these activities is is the amount of force (which determines the amount of tendon strain) and the speed of that strain, or strain rate. People don’t get Achilles tendinopathy from performing calf raises, mainly because the strain and strain rate is generally lower. They are much more likely to develop tendon pain in the lower limb with changes in walking, running, jumping and cutting. The first 2 studies in this blog demonstrate greater strain rate in the Achilles (Lyght study) and plantar fascia (Wager study) with NRFS running which may contribute to injury, but strain magnitude is also important, and this is often related to joint excursion (e.g. greater ankle DF in running = more Achilles load and strain). So when you are screening energy storage loads an athlete is doing, think about the interplay between strain and strain rate, both may be related to injury risk.
Elastography for tendons – what does fancy imaging add?
What they did: Siu et al. have investigated whether shear wave elastography of the Achilles tendon was different between heavy and light exercise groups. Some background on this, shear wave elastography has gained popularity over the last few years and enables measurement of tensile stiffness of the tendon with an ultrasound machine. Remember, stiffness is simply the amount of stretch the tendon undergoes for a given load (again, read last weeks blog for a brief review on whether having a stiff tendon is good). The authors of this study recruited healthy people who exercises more than 6 hours per week (n=12) and another group that exercises less than 6 hours per week (n=24). They excluded people with a VISA score below 70, based on an argument that above this score patients are ‘cured’, which I find hard to swallow. They report none of the 36 participants had thickened Achilles tendons or hypoechoic regions on ultrasound. Participants in the frequent exercise group played football, basketball and high jump, whereas the infrequent exercise group were mainly not active in sports.
What they found: They found that intra-operator reliability (ICC = 0.803-0.845) was much higher than inter-operator reliability (0.585). Participants in the frequent exercise group had a stiffer Achilles tendon than people in the infrequent exercise group, but only when the nondominant ankles were compared.
Clinical interpretation: This finding is consistent with studies showing increased stiffness of the Achilles tendon in people that perform regular exercise such as running or sport (e.g. Hansen et al. 2003). The non-dominant leg Achilles is most likely stiffer because participants tend to jump or push off this leg. This study is of interest mainly because it supports the use of shear elastography in identifying differences in stiffness of the Achilles tendon. This technology could be used to to help answer very important questions in tendons, ie, does the pathological tendon adapt to exercise? And does this relate to clinical benefit?
As a side point, the way tendon stiffness has traditionally been measured is with conventional gray scale ultrasound. The participant performs a maximal isometric contraction and ultrasound is used to measure strain or stretch in the tendon. We did this in our 2013 paper on patellar tendon and found that load intensity rather than contraction type was the key factor for tendon stiffness adaptation. The picture below shoes what we measured, ie the change in patellar tendon length from a relaxed to a maximally contracted position.
Until next time, keep joining the dots
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