Isometric training in lower limb tendinopathy: a critical evaluation

Tendinopathy affects both sedentary and active individuals and is responsible for 30-50% of all sporting injuries ¹, with achilles and patellar tendons commonly affected in the lower limbs.² Tendinopathy is an overuse injury characterised by localised tendon pain with loading and dysfunction³ and often begins with a mismatch between the tendons load capacity and load placed on it.⁴  Tendinopathy is commonly associated with pathology, of which features include altered cellularity, breakdown in extracellular matrix, and release of biochemical substances thought to have a role in tendon pain.⁵ 

Tendinopathy can be considered as a continuum of three stages: reactive, dysrepair and degenerative and can often be multistage, for example reactive on degenerative.⁴ Depending on where in the continuum the tendon presents can inform the management strategy. Although there isn’t a gold standard for tendinopathy management, research over the past twenty years has focussed on load based exercise that can increase the load absorption capacity of the tendon. Eccentric based protocols have traditionally been prescribed which involves isolated, slow, lengthening muscle contractions, however not all tendinopathy patients respond positively.⁴  Commonly this can be athletes, continuing to participate in activity while managing their tendinopathy, where the capacity to tolerate high sporting loads and heavy eccentric (isotonic) exercise may be limited.⁴

This has partly led to the popularity of submaximal isometric exercise as some evidence supports its use in patellar tendinopathy, where improvements in pain and self-reported function have been recorded.^6,7,8 Secondly, some authors suggest that tendon loading should be prescribed without exacerbating the pathological state or pain, to target matrix reorganisation and collagen syntheses.² This can affect tendon compliance and have an analgesic effect, favouring the prescription of isometric exercise.

Three studies ^6,7,8 compared the effectiveness of isometric and isotonic exercises in individuals with patellar tendinopathy during in-season volleyball and/or basketball competitions. The isometric exercises were performed at 70-80% maximal voluntary contraction (MVC) at sixty degrees knee flexion for five sets of forty-five seconds hold, using either a Biodex or leg extension machine. The isotonic exercises consisted of four sets of eight repetitions at 80-100% of eight repetition maximum using a leg extension machine. One study compared immediate and forty-five minute effects following exercise ⁶ whereas the other studies compared baseline and four week follow-up measures. ^7,8

With regards to pain, the cross-over study of six volleyball players with immediate and forty-five minute follow up showed a significant, pain relief response with isometric and isotonic exercise immediately after exercise.⁶ However only the isometric group showed a sustained significant pain reduction at forty-five minutes post exercise (p<0.001), which was paralleled by a reduction in cortical inhibition.⁶  The other two studies also reported significant improvement in pain.^7,8 The later study in 2017 by Rio and colleagues ⁷ used a larger sample of twenty subjects and found isometric exercises elicited greater changes in pain scores than isotonic exercise when studied over four weeks.⁷  However no statistically significant differences between the isometric and isotonic exercise groups were seen in a similar study by van Ark and colleagues ⁸ after four weeks of training.

With regards to function a significant (p<0.001) increased quadriceps strength on MVC was reported following a single bout of isometric exercise and was significantly greater (p<0.001) than that following isotonic exercise.⁶ The authors suggest clinically this may suggest that isometric muscle contractions may be used to reduce pain in people with patellar tendinopathy without a reduction in muscle strength.⁶ However these findings are only based on a small sample of six volleyball players compromising the reliability and validity of the results.

The other two studies measured functional changes from baseline with both reporting significant improvement in VISA-P scores after four weeks of intervention, but no significant differences between the isometric and isotonic groups was evident.^7,8  VISA scales give substantial scores on pain during high-level activity, they are not responsive to short-term change and are best used on a month-by-month basis, making it an appropriate measurement choice for the studies under review.

Since these studies concluding in 2017, the use of isometric exercises for pain relief has become common place and is popular in managing other tendons, even though the results only referred to patellar tendons and sample sizes were small.  This popularity may be enhanced by the excellent athlete adherence and tolerance to isometric exercise when implemented in season. ⁸ The mechanism by which loading provides pain relief in tendinopathy is not yet fully understood, reflecting the complex multifactorial nature of tendon disease.  It is thought that sustained isometric fatiguing muscle contraction recruits segmental and/or extrasegmental descending inhibition mechanisms. The recruitment of descending inhibition results in mechanical hypoalgesia and increased pressure pain threshold in healthy individuals.⁴ 

However not all research has delivered such positive outcomes for isometric exercise in managing tendinopathy.  Recently a replica study to that by Rio and colleagues ^6,7, but with a larger sample of twenty individuals with patellar tendinopathy, there was no difference reported in acute pain reduction following isometric versus dynamic resistance exercises. ⁹

Another study by Pietrosimone and colleagues considered both pain and function in a study of twenty-eight male athletes aged between 15-28 years, some with patellar tendinopathy and others asymptomatic.¹⁰ They reported that a single-dose patellar tendon isometric exercise protocol did not have acute effects on pain levels or landing biomechanics.  With regards to biomechanics it could be that a single dose is insufficient to change movement patterns and future study of multiple sessions of an isometric protocol over time may yield meaningful changes in pain and biomechanics.

To date other studies have yet to show any clinically or statistically significant changes in self-reported pain, pain pressure threshold and mechanical pain sensitivity in response to an acute isometric exercise load when studying other lower limb tendon sites. ^11,12 Two observational studies ^11,12 used a similar isometric loading protocol to Rio and colleagues.⁶  Sixteen patients with Achilles tendinopathy were examined and they did not find a significant positive effect on pain or muscle recruitment.¹¹ Some patients reported increased pain with the isometric contractions. Similar findings were observed but in patients with plantar fasciopathy, where a randomised cross-over trial of twenty patients showed isometric exercise was no better than isotonic exercise or walking in reducing pain.¹²

A clinically relevant immediate effect of isometric exercise cannot be made from two studies discussed^7,11, as no control groups were included, in which no exercises were performed and the power of the studies was also limited.  A study of ninety-one chronic midportion achilles tendinopathy patients, included a control group, to overcome the previous shortfalls discussed.¹³  They investigated the immediate effect of isometric exercises on pain during a functional task and compared the results with the effect after the performance of isotonic exercises and rest.  No significant reduction in pain in any of the groups and no immediate analgesic effect from isometrics were reported.

At this stage it seems reasonable to ask how necessary is it to search for a means of providing acute pain relief in tendinopathy? Previous evidence around the use of isotonics early in tendinopathy management came with a clear message that healing in this pathology is slow and that pain during exercise was not detrimental for recovery and may even be beneficial.¹⁴ It seems the benefits for isometric exercise portrayed from recent evidence is contrary to this, implying pain relief and proficiency performing isometrics is a prerequisite to higher tendon loading in rehabilitation.  Secondly, the focus on isometrics for acute pain relief to allow the athletic population to continue with their sporting loads could potentially be detrimental to long term tendon health, if capacity is insufficient to safely deal with the loads. It maybe the demands of sporting participation, whether it be from the athlete themselves, coaches or managers and athlete well-being and welfare that is influential in tendinopathy management, reinforcing the multifactorial nature of this pathology.

To summarise, isometric exercise has dominated the literature and been postulated as an initial treatment and advocate for pain management in tendinopathies. It appears this was founded primarily on two low powered studies comparing isometric and isotonic muscle contractions in tendinopathy ^6,7, with other studies failing to demonstrate similar benefits.  Isometric exercise does not appear to be superior to isotonic exercise in terms of pain in chronic tendinopathy either immediately after a single session or in the short term, with follow up less than twelve weeks.   However, this conclusion is based on research from several lower limb tendon sites and patient demographics so caution is needed when extrapolating these findings to other populations.

A combination of small sample sizes, methodological flaws and a narrow population base highlighted throughout this discussion, makes it difficult to draw any meaningful conclusion without further investigation into the role of isometric exercise in tendinopathy. A possibility would be investigating loading strategies at various stages of the tendinopathy continuum, in a well powered study with a control group.

There is potential for isometric exercise to benefit the management of tendinopathy through several avenues. Firstly, use in the initial phase of loading of a progressive programme or while managing an athlete in season, by reducing pain.  Reducing pain and inhibition may have functional benefits by allowing movement patterns to normalise and supporting tissue capacity. ^10,15 Currently with inadequate evidence to inform a change in management, it seems reasonable to continue prescribing progressive strengthening, provided it is context specific, with consideration of the individuals sporting needs, pain levels, and goals.

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