I want to discuss the some of the highlights and challenges outlined in ‘Prediction of running-induced Achilles tendinopathy with pain sensitivity – a 1-year prospective study’ (Brund et al 2018) and provide a guide as to how you might use the findings in this paper in your clinical practice.
Background and Objectives
Injury risk prediction is often touted as a panacea for sports medicine. It is tantalising to think we may be able to predict those at greater risk of sustaining an injury, and particularly an injury such as Achilles tendinopathy which is often persistent and affects training volume and performance. This study aimed to do just that – to predict Achilles tendinopathy injury in runners – by following running athletes for one year and examining the local and widespread mechanical sensitivity to stimuli.
The study aim was to investigate whether mechanical pain sensitivity in the Achilles tendon could predict the development of Achilles tendinopathy (the clinical presentation of Achilles pain) in recreational male runners. The researchers hypothesised that those with the highest pain sensitivity (pre-selected cut off was 441kPa) would be more prone to develop Achilles tendinopathy.
99 male athletes (aged 18-60 years) were followed for the study and full data was available for 98. Athletes had to have been running at least twice per week for two years with no injuries in the preceding three months. This study was part of a larger trial so there were some other exclusion criteria around footwear and treadmill experience.
At baseline, body mass index and history of previous injuries were collected. Pressure pain thresholds (mechanical sensitivity) were collected over each Achilles tendon (reported as mid-portion, 2-3cm proximal to insertion) and the infraspinatus muscle (as the proxy for widespread sensitivity, as it is in a location distant to the Achilles).
Although the title and abstract read as though this was observational, it was part of a bigger study so runners were provided footwear (conventional neutral shoe with a 12mm heel to toe drop). They had to run in these for at least 10km per week for the first 500km of the year with no other guidelines around pace or frequency. It is worth defining a few pain terms, so we are all on the same page.
Mechanical sensitivity is how sensitive you are to a mechanical stimulus (such as pressure), and can be tested using a device termed an algometer (little gadget with a small rubber probe where you apply pressure over the skin). The measurement is described as a pressure pain threshold, that is the amount of pressure (threshold) that can be applied to the participant before they report pain.
Endogenous pain inhibition may be thought of as your own descending inhibitory process that turns down a nociceptive signal and can occur at multiple spinal and supra-spinal levels. It is a natural phenomenon that appears to be disrupted in some pain states. Remember noxious stimuli or a nociceptive stimulus is an input – it is the brain that decides whether we need protection based on lots of factors. See Explain Pain supercharged for a thorough and completely inspirational review of all things pain; another great resource is https://bodyinmind.org/.
The authors clearly defined injury for the reader – the absence from running for a minimum of one week due to a musculoskeletal complaint in the lower extremity or the spine caused by running. Runners completed a weekly survey. Anyone that reported an injury was seen by a sports physician or sports physiotherapist and runners had Achilles imaging at this point if an injury was suspected. Baseline ultrasounds were not recorded (understandably – imaging adds cost to studies).
Running data were obtained from GPS and recall when required – a fantastic feat in this many people. GPS data enables accurate tracking of load – this is important to both understand the generalisability to patients in front of you, as well as to track internal loads. I applaud the consideration of pain science in a tendinopathy study, and more novel studies like this are needed to understand how and why the pain of tendinopathy occurs.