Welcome to the 8th edition of the Learn.Physio Research Review!

Mid Portion Achilles Tendon Pain

A research review by Learn.Physio

Welcome to the latest issue of Learn.Physio Research Reviews! This week we’ll stay low in the lower limb and focus on mid-portion Achilles tendon pain with 2 wonderful research papers on this topic.

The first of the summaries you will read highlights the persistent strength and power deficits that are seen in recreational athletes despite completing 3 months of rehab. The research is suggesting that we as clinicians are not testing our patients appropriately to see if any of these deficits are persisting, and as a result we are not intervening with the appropriate exercises to address these deficits.
Spoiler alert – the single leg heel rise test is not enough….

The second of the summaries you’ll read shows the important role of the soleus muscle in those with mid-portion Achilles tendon pain, and I give a couple of suggestions on how you can simply and effectively address soleus weakness in the clinic, gym or home to help your patients with mid-portion Achilles tendon pain improve their strength and function.

I hope you enjoy the latest issue of Research Reviews and get something valuable from it today to help your patients tomorrow!


The Achilles Tendon (AT) is the largest and strongest tendon in the body. Despite its size and strength, it is one of the most common sites of activity-related pain in elite athletes and recreational athletes; in particular the activities of running, hopping and jumping. During such activities, the AT is subjected to loads as high as 6-12x the person’s body weight and requires a considerable amount of strength from the calf muscles to repeatedly produce and absorb these high forces.

When an athlete develops AT pain (most commonly mid-portion AT pain) that starts to impair the function and performance, it can be difficult to know why that is. One possible explanation is that it may relate to persistent weakness in the calf muscle group; whereby pain inhibition, altered motor output or even disuse and muscle atrophy may result in an inability to generate or tolerate the required loads.

A common method to quantify AT function in people with mid-portion AT pain is the single leg heel-raise test, where subjects simply raise up and down slowly as many times as they can until they can no longer (reps to fatigue or failure).

Despite it being a “user-friendly” test that can be easily performed anywhere, the downside to this test is that it is a submaximal test (or endurance test) and may not provide sufficient assessment of the athlete’s strength profile. This could lead to an over-estimation of the athlete’s ability to return to training and sport, inadequate rehabilitation, and ultimately recurrence of pain, impaired function, and performance.
Due to a lack of consensus regarding strength deficits in those with AT pain, the authors set out to review, evaluate, and summarise the evidence regarding calf strength profile in those with AT pain.



This systematic review and meta-analysis was conducted in accordance with the PRISMA statement for systematic reviews.

After a systematic review of the literature they found 19 studies that were appropriate. There were 566 participants included in this systematic review, ranging in age from 24-59 years of age with the average duration of symptoms ranged from 5 weeks to 3 years.

8 of the studies compared strength measures between symptomatic and asymptomatic participants.
8 studies compared strength values between injured and uninjured sides of the same participants.
The remaining 3 studies compared strength values between the injured and uninjured sides of the same participant AND between the injured side and asymptomatic participants.


Outcome measures:

  • The most common measures of maximal strength values were isokinetic dynamometry (both fast speeds of >120deg/sec and slow speeds <120deg/sec) were observed across the included studies.
  • Maximal isometric strength measures were also used.
  • Authors across also investigated explosive strength variable of rate of force development (RFD) and ground reaction forces (GRFs).
  • They also investigated reactive strength index (RSI) via several single leg hopping and jumping tasks.


Isokinetic dynamometry

  • Concentric plantar flexion fast (>120deg/sec) showed a strength difference of 38% between the symptomatic side and asymptomatic side in the AT patients or the symptomatic side and asymptomatic control group.
  • Concentric plantar flexion slow (<120deg/sec) showed a strength difference of 44% between the symptomatic side and asymptomatic side in the AT patients or the symptomatic side and asymptomatic control group.
  • Eccentric plantar flexion slow (<120deg/sec) showed a strength difference of 18% between the symptomatic side and asymptomatic side in the AT patients or the symptomatic side and asymptomatic control group.
Isometric strength
  • Reduced isometric plantar flexion strength was observed in those with AT pain, revealing a 5-12% plantar flexion isometric strength difference between the symptomatic side and the asymptomatic side in AT patients or the symptomatic side and the asymptomatic control group.
Explosive strength
  • Inconsistent findings of both increased and decreased GRFs in those with AT pain
  • Consistent findings showing that RFD was reduced by 10-21% in those with AT pain compared to the asymptomatic side.
RSI (hopping)
  • Consistent evidence showed reduced hop performance between 16-35% in those with AT pain as compared to the asymptomatic side
  • The mean difference between symptomatic side and asymptomatic side for the single leg hop for distance test was 33% (43cm) and 35% (151cm) for the triple hop for distance
  • Single leg hop height was also significantly less on the symptomatic side as compared to the asymptomatic side and ranged between 16-20% difference across a variety of hop tests

The results of this systematic review and meta-analysis clearly show that individuals with AT pain display strength and power deficits ranging from 10-44% (depending on the measure) on the affected side as compared to their uninjured side, or when compared to healthy controls – and that simply testing a persons ability to perform as many single leg heel rises as they can on each leg (test of capacity and endurance) is under appreciating what the person needs to participate in high demand activities such running, sprinting, jumping, hoping and changing of direction.

Although the single leg heel rise test can be of value to the patient and clinician, as it does have some valuable normative data for patients and athletes to strive for (Herbert-Losier et al 2017), as a stand-alone assessment for the patients function during an episode of AT pain, it may lead the clinician to over-estimate the recovery of the athlete, leading to inadequate rehabilitation and subsequent flare up of symptoms upon return to activity.

The challenge for most clinicians is to accurately quantify and measure plantar flexion strength in the absence of isokinetic testing facilities and force plates.

In the absence of testing facilities, one could simply do 6RM or 10RM testing of the individual in a standard calf raise machine (or smith machine) in the gym (both standing and seated).

Pick a rep target of 6 or 10 reps (I like 10reps because it gives you a nice round number). Place a weight on the machine that the individual won’t be able to do more than 10 reps of (a bit of trial and error may occur here as you find the weight that they fail at 10reps maximum).

Start on the uninjured limb first, raising slowly up and down (3 sec up, 3 sec down) until maximum 10reps. Rest for 2mins, then repeat on the symptomatic limb. If the symptomatic limb can only do 8 reps at the same weight, then there is a 20% strength difference between limbs. Repeat the same process for the seated calf raise too.

What’s much easier to perform in the clinic however is the single leg hop for distance, and triple leg hop for distance to get an idea of the explosive strength of the athlete with AT pain.

For both hop tests, warm the athlete up for at least 5mins. Simply pick a spot on the floor and mark it with a pen or piece of strapping tape. Have the patient stand on one leg, with their toes behind the line. Ask them to hop forward as far as they can on the asymptomatic side. Mark the ground where the heel is and measure from toe at take-off to heel at landing. Repeat this process again, taking the average distance of the 2 maximal hops. Patients are free to use their arms to swing, however hops should not count if they “touch down” on their other leg or have to do extra hops before they stabilise. Repeat on the symptomatic side.

For the triple hop for distance, repeat the same procedure, but ask the patient to hop 3x as far as they can, with as little time on the ground between each hop as possible.

You can get a clinical measure here of limb symmetry index (LSI) by dividing the symptomatic side vs asymptomatic side x 100. For example, 80cm hop of the symptomatic side vs 100cm hop of the asymptomatic side = 80% LSI. The general rule of thumb for most lower limb conditions is a score greater than 90% LSI is ideal.

However as I have mentioned in previous issues, LSI results on hop tests have many limitations (including not being a specific measure of local tissue strength/weakness), and comparing the results of the individual to normative values may be an option, alternatively some researchers are suggesting that males should be able to single leg hop at least 90% of their height, and females should be able to singe leg hop at least 80% of their height. If you’re interested in learning more about hop testing, this free full text article by
Davies et al (2020) is a great read.
Mid-portion AT pain is a very common clinical presentation that responds very well to at least 12 weeks of exercise rehabilitation. Research has shown that pain can be halved after at least 12 weeks of exercise rehab. As covered in Issue #7 from Murphy et al, there is no superior exercise protocol/program to treat mid-portion AT, so any form of gradual and progressive load is better than nothing or “traditional” physiotherapy of ultrasound, taping, manual therapy etc. The problem being that many of the current published exercise protocols of mid-portional TA (all but Silbernagel et al 2015) do not prescribe plyometric exercises.
Despite great improvements in pain and improved function, recurrence rates of mid-portion AT pain and dysfunction is high (27% recurrence rates) upon return to activity. The impression that the authors are giving as to why this is from this meta-analysis 2-fold:
  • insufficient clinical testing to see if the tendon and ankle plantar flexors have restored maximal strength (not calf endurance), RFD and RSI.
  • absence of higher demand strength tasks of the ankle plantar flexors and plyometric activities from rehabilitation plans
In a nutshell, symptomatic recovery, does not equate to functional recovery. If we’re to get our mid-portion AT pain patients off the pain/dysfunction “merry-go-round”, we need to sharpen up our clinical assessments to identify strength deficits (not endurance deficits), and improve our rehab programs to make sure plyometrics (skipping, hopping, jumping) are performed regularly ensure better outcomes in the future.
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Muscle weakness has been identified as the primary modifiable risk factor for mid-portion AT; specifically the ankle plantar flexor group of gastrocnemius and soleus. Evidence has also suggested that plantar flexor weakness pre-dates the onset of symptoms that strengthens the cause and effect relationship of plantar flexor weakness and AT – however, it’s uncertain if gastrocnemius or soleus is most affected.
The soleus is quite sophisticated in its action during locomotion and is more than a “slow-twitch, anti-gravity” muscle that we have learnt about in anatomy lectures at university.

Throughout ground contact, the soleus controls knee flexion by controlling tibial movements in relation to the foot and the floor (knee extensor moment); whilst the gastrocnemius opposes this action and acts to flex the knee.

The soleus has also been observed to produce vertical forces during running of around 8x body weight, compared to the gastrocnemius that is only producing about 3x body weight at the same speed of running (Dorn et al 2012).

The AT has a complex orientation of fascicles and recent evidence suggests that the deeper surface of the AT (linked to the soleus) undergoes the greatest amount of displacement and is the zone where the typical changes of tendinopathy is observed.

Given that the soleus is the main force producer during running and walking, and that the site of AT changes appears to involve tendon fascicles associated with the soleus, it makes sense that the soleus would be most affected in people with mid-portion AT.

The purpose of this study was to determine how the plantar flexors are affected by AT. The authors hypothesised that they would see reductions in power and endurance when comparing subjects with and without AT. They also hypothesised that these deficits would be bilateral in nature and explained by alterations in soleus function.



This was an observational study that compared a group of runners with and without mid-portion AT pain; the subjects without mid-portion AT pain acted as a control group and were age, sex and activity matched to an individual with mid-portion AT pain.
Diagnosis of mid-portion AT was made if the following criteria were met:

  • localised, unilateral mid-portion AT pain for longer than 3 months
  • pain provoked by physical activity in a dose dependent way (ie. Running provokes pain more than walking)
  • reproduction of pain with palpation
  • positive London Hospital Test and/or painful arc sign of the AT
  • identification of ultrasound changes in keeping with a diagnosis of mid-portion AT pain. Only symptomatic side was imaged. Ultrasound imaging was not taken for healthy controls.
Patients were excluded from the trial if:
  • normal appearance of the AT on ultrasound
  • any MSK, vascular or neurological injury/disorder within last 6 months
  • bilateral mid-portion AT
  • participates in regular lower limb strength training
  • regularly participates in other sports involving high speed running
  • insertional AT
  • partial AT
  • previous AT rupture or surgery to AT
Once diagnosis was made, inclusion criteria were:
  • diagnosis of mid-portion AT pain for > 3months
  • subject normally runs 2x per week or was a healthy runner running > 2x per week with no limb limb pain (controls)
  • aged between 18-70yrs of age

Before testing all subjects completed the VISA-A questionnaire.

Outcome measures:

  • Peak torque of the plantar flexors measured by isokinetic dynamometer
    • Measurements were taken at 80deg knee flexion and full knee extension at the following speeds and modes of contractions:
      • Concentric 90deg/sec
      • Concentric 225deg/sec
      • Eccentric 90deg/sec
    • The knee extended position allows both gastrocnemius and soleus to function
    • Previous research has identified that knee flexion of 80deg was sufficient to disadvantage the gastrocnemius and test soleus force more specifically (Hebert et al 2011 and 2013).
  • Plantar flexor endurance
    • The authors chose a regime of 20 maximal effort concentric-eccentric plantar flexor contractions on isokinetic dynamometer that reflected heel raise tests that had previously been shown to differ in subjects with mid-portion AT pain.
    • Test speed was 90deg/sec and performed only in 80deg knee flexion position as the authors were aiming to test the soleus capacity in isolation.
  • Pain
    • Subjects were instructed to complete the maximal and endurance tests as pain allowed
    • If pain was experienced during the testing, the subjects were asked to score it using a 100mm visual analogue scale (VAS)

These findings of this systematic review highlight 3 key areas of management of the patient presenting with mid-portion AT.
Firstly, active exercise rehabilitation is superior to a wait-and-see / natural history (ie. AT will get better on its own). I think intuitively as clinicians, we all knew that, but it is an important thing to discuss that some patients may experience an improvement in their symptoms with rest and avoiding aggravating activity, but it is very common (in my clinical experience anyway), that their mid-portion AT pain resumes shortly after they return to their previous aggravating activities (eg, running, hopping).
So, I would sure to add in a thorough and progressive overload rehab plan during periods of avoiding aggravating exercises to minimise the chances of seeing a recurrence later. The updated review of the continuum model of tendon pathology by Cook et al (2016) is a must-read for all clinicians. Full text link to this paper
Secondly, it appears that “traditional” treatment options such as manual therapy, massage, ultrasound, and taping are not as effective at managing mid-portion AT pain when compared to HECT alone. The quality of evidence here though was low, so it is hard for clinicians to know the true positive effect of HECT over traditional physiotherapy treatment options.
The big take away for me however was the conflicting results for what active exercise options were effective in managing patients with mid-portion AT. Even if there is conjecture here, what it does mean that even if HECT is equally effective as other forms of exercise options for mid-portion AT patients, it means that we have more choice on what we prescribe to our patients, knowing that we are going to have a positive impact on their pain and function.
For example, it was highlighted above that the “traditional” Alfredson’s protocol is to be performed 2x per day (180 reps per day) for 12 weeks straight, to have a positive effect on outcomes. To many patients, this program is hard to adhere to; especially when they have busy lives and have congested training schedules and need to account for rest and recovery. Whereas the heavy, slow resistance training program from Beyer et al was programmed at 18-60 reps per exercise (3 exercises), per session (3x per week) for 12 weeks; and would be much realistic to complete for most patients and athletes and allows for rest/recovery days so that other training can be incorporated safely into the training week.
Conversely, the advantage of doing the “traditional” Alfredson’s protocol is that it is easily completed at home (+/- a backpack with extra weight if required), whereas the heavy, slow resistance training of Beyer et al requires heavy loads and gym machines such as a smith machine (Gastrocnemius strengthening), seated calf raises (soleus strengthening) and leg press (proximal strengthening of quads and glutes) to complete the program.
Interestingly, the subjects conducting the heavy, slow resistance program of Beyer et al showed equal outcomes to the Alfredson’s protocol by completing all the exercises with 2 legs at a time (eg. Double leg standing heel raises, double leg seated heel raises, double leg leg press) compared to the Alfredson’s protocol that focuses on training just the affected leg only. I wonder if there would be any superior differences seen in these results from Beyer if they completed the heavy, slow resistance training with just the effected leg only?
In summary, despite some limitations to the current quality of RCTs in the literature, clinicians can be confident in giving at least 12 weeks of rehabilitation exercises aimed at addressing calf strength (and proximal musculature) in patients with mid-portion AT. Which type of program you give however really depends on the patient sitting in front of you and what they are most likely able to complete and adhere to. In regard to using “traditional” physiotherapy interventions in conjunction with exercise rehabilitation for the management of mid-portion AT, the choice is yours, and once again depends on the patient sitting in front in terms of their beliefs on what can help them manage their pain/improve their function and meeting their expectations.


This was the first study of its kind that compared plantar flexor peak torque and endurance between individuals with and without mid-portion AT pain – with the results clearly showing that there are large deficits in plantar flexor strength in those with mid-portion AT pain than healthy controls. These large differences were seen in all modes of muscle contractions, and both knees extended and 80deg knee flexion positions.

Specifically, the authors identified that the gastrocnemius accounted for between 3-11% of the identified strength deficits, whereas the soleus was responsible for 23-36% of the strength deficits.

Another key finding was the bilateral weakness of the mid-portion AT group, whereby both symptomatic and asymptomatic limbs showed peak torque and endurance strength deficits compared to healthy controls. This may relate to central nervous system involvement or a consequence of de-training, deconditioning or pre-existing weakness.
These 2 key findings are important for clinicians dealing with recreational runners with mid-portion AT pain…

Firstly, it is apparent that the soleus has an influence on mid-portion AT pain (both onset of symptoms and subsequent muscle output). As a result, clinicians need to be conscious of prescribing both straight knee heel raises (single leg, performed on both legs) and bent knee heel raises (single leg, performed on both legs) for the management of those with mid-portion AT pain.

It also appears from this body of research and the references provided in this paper (Hebert-Losier et al 2011 and Hebert-Losier et al 2013) that knee flexion angles of at least 80deg knee flexion are ideal to isolate gastrocnemius from soleus. Therefore, the traditional bent knee heel raise exercise as popularised by the Alfredson Eccentric Loading protocol may not be sufficient to target the soleus, and clinicians may need to move towards a seated heel raise or wall sit heel raise exercise instead.
Secondly, it is also clear from this research that the asymptomatic limb is not a reliable reference point for the athlete with mid-portion AT pain. This further reinforces the need for unilateral strength and power training on both sides of the body during mid-portion AT rehab.

It also suggests that any strength / endurance measure (single leg heel rise test) or performance measure (single hop for distance or triple hop for distance) should be compared to against healthy normative values (where possible) rather than asymptomatic limb. Given the bilateral strength reductions in mid-portion AT pain observed in this study compared to health controls, comparing the symptomatic limb to the asymptomatic limb would only likely give the patient (and clinician) a false impression of the recovery of the patient’s symptoms, function, and ability to successfully perform in their chosen activity.

The problem however lies in “normative data” for strength, capacity, and functional tests in those with mid-portion AT pain. Hebert-losier et al (2017) did produce some nice normative data points for both males and females for the single leg heel rises test across various age groups. However, the current body of single leg hop test normative data focuses largely on 10-25 year old athletes and is of an age-group that is much younger than those who experience mid-portion AT pain (most 30-40 year olds). Nevertheless, the recommendations as provided in the first review suggesting single leg hop distance of 80% of the person’s height for females and 90% of the person’s height of males would be good benchmarks to set.

In summary, this study found that soleus muscle weakness is a major contributor for the strength deficits observed in those experiencing mid-portion AT pain. Those with mid-portion AT pain also tend to be weaker bilaterally, that suggests that rehab should not only be tailored to address both soleus and gastrocnemius strength on the affected side, but the soleus and gastrocnemius on the asymptomatic side.
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Paper 1: Further reading:

Davies et al (2020) - https://link.springer.com/article/10.1007/s40279-019-01221-7
Silbernagel & Crossley (2015) - https://pubmed.ncbi.nlm.nih.gov/26390272/
Sibernagel et al (2020) - https://meridian.allenpress.com/jat/article/55/5/438/436804/Current-Clinical-Concepts-Conservative-Management
Baxter et al (2021) - https://pubmed.ncbi.nlm.nih.gov/32658037/
McAuliffe, S., Tabuena, A., McCreesh, K., O'Keeffe, M., Hurley, J., Comyns, T., Purtill, H., O'Neill, S., & O'Sullivan, K. (2019). Altered Strength Profile in Achilles Tendinopathy: A Systematic Review and Meta-Analysis. Journal of athletic training, 54(8), 889–900. https://doi.org/10.4085/1062-6050-43-18
Hébert-Losier K, Wessman C, Alricsson M, Svantesson U. Updated reliability and normative values for the standing heel-rise test in healthy adults. Physiotherapy. 2017 Dec;103(4):446-452. doi: 10.1016/j.physio.2017.03.002. Epub 2017 Mar 21. PMID: 28886865.

Paper 2
Dorn TW, Schache AG, Pandy MG. Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance. J Exp Biol. 2012 Jun 1;215(Pt 11):1944-56. doi: 10.1242/jeb.064527. Erratum in: J Exp Biol. 2012 Jul 1;215(Pt 13):2347. PMID: 22573774.
Hébert-Losier K, Schneiders AG, García JA, Sullivan SJ, Simoneau GG. Peak triceps surae muscle activity is not specific to knee flexion angles during MVIC. J Electromyogr Kinesiol. 2011 Oct;21(5):819-26. doi: 10.1016/j.jelekin.2011.04.009. Epub 2011 Jun 12. PMID: 21669538.
Hébert-Losier K, Schneiders AG, Sullivan SJ, Newsham-West RJ, García JA, Simoneau GG. Analysis of knee flexion angles during 2 clinical versions of the heel raise test to assess soleus and gastrocnemius function. J Orthop Sports Phys Ther. 2011 Jul;41(7):505-13. doi: 10.2519/jospt.2011.3489. Epub 2011 Feb 18. PMID: 21335928.
Hébert-Losier K, Holmberg HC. Knee angle-specific MVIC for triceps surae EMG signal normalization in weight and non weight-bearing conditions. J Electromyogr Kinesiol. 2013 Aug;23(4):916-23. doi: 10.1016/j.jelekin.2013.03.012. Epub 2013 Apr 29. PMID: 23639755.
Hébert-Losier K, Wessman C, Alricsson M, Svantesson U. Updated reliability and normative values for the standing heel-rise test in healthy adults. Physiotherapy. 2017 Dec;103(4):446-452. doi: 10.1016/j.physio.2017.03.002. Epub 2017 Mar 21. PMID: 28886865.

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