Eccentric interventions for Achilles tendinopathy

Multiple descriptive studies have demonstrated the clinical benefits of eccentric exercise interventions in patients with Achilles tendinopathy, but researchers have not yet identified the cause-and-effect mechanisms leading to decreased pain or increased function.

By Matthew Crill, MS, MBA, PT

Achilles tendinopathy is a common ailment that affects both active and sedentary populations and has an impact on work, sport, and activities of daily living.¹ For active populations, this condition has a higher incidence in individuals who participate in activities that involve running, jumping, or repetitive movements. The highest prevalence of this disorder has been documented in middle-aged men (aged 35-45 years) but can occur in men and women of all ages.² The exact etiology of the condition is not known, but it is often characterized by pain and stiffness with an insidious onset. Clinicians typically use the extent of stiffness and pain to document the severity of the diagnosis.² Current literature recommends using the term “tendinopathy” to replace “tendinitis” for describing this pathology. This is because there is an absence of inflammatory factors in the tendon and traditional anti-inflammatory treatments have not been proven effective for symptom relief.¹

Physicians typically prescribe physical therapy as an early intervention for patients diagnosed with Achilles tendinopathy, with favorable outcomes.³ A study published in 1998 by Alfredson et al demonstrated initial success using a protocol focused on strengthening of the gastrocnemius-soleus complex using exercises characterized by a significant number of eccentric muscle actions.⁴ The 12-week protocol significantly decreased pain, produced significant gains in calf muscle strength, and returned patients to their preinjury training levels.

The Alfredson protocol has been criticized because there is no scientific rationale for the volume of repetitions and it requires patients to continue with the exercises even if pain persists.¹ This can be potentially problematic in terms of home exercise compliance and overall patient satisfaction. Additionally, the exact mechanism leading to the positive outcome has not been identified. Many studies have reported positive results with other interventions in addition to eccentric training, but with pain as the only clinical outcome, which limits the extent to which conclusions about the mechanism can be made.²

There is a significant amount of research related to the pathophysiology of Achilles tendinopathy, as well as treatments and interventions. The challenge for clinicians is that the research related to treatment protocols has reported variable sets of findings that often conflict with one another. The purpose of this review is to describe the impact of eccentric muscle actions on muscle function, review the role of eccentric muscle actions as a treatment intervention for Achilles tendinopathy, and discuss recommendations for further research.

Eccentric actions and muscle function

Exercise-induced muscle damage is a commonly accepted result of sport and activity. It occurs often when a worker or athlete participates in unaccustomed activity, when the intensity or duration of the activity is increased, and particularly when the activity or sport involves a high percentage of eccentric muscle actions.⁵ Common indicators of muscle damage include impaired muscle function, delayed-onset muscle soreness (DOMS), and generalized stiffness and swelling.^5-7

Studies have documented that eccentric muscle action is the primary factor contributing to muscle damage.^5-7 These studies have compared various muscle actions and found that eccentric muscle activity is associated with more evidence of muscle damage than either concentric or isometric actions. Additionally, these studies have shown the damage can occur at different levels of intensity of the eccentric actions. Muscle damage has been demonstrated in studies using submaximal, maximal voluntary, and electrically-stimulated eccentric actions.⁵

Sports and training activities with a high percentage of eccentric muscle actions include prolonged running, downhill running, sprinting, plyometrics, and resistance training.⁵ Research has shown the reductions in strength and power associated with eccentric muscle action are immediate and often persistent.⁷ When symptoms, such as loss of muscle function, soreness, and stiffness, manifest in the lower extremities, sport performance is often impaired.^7,8 It is unclear why a protocol biased toward eccentric muscle actions can cause muscle damage in the short term but also can have a long-term positive effect on the injured tendon that ultimately leads to decreased pain and increased function.⁹ Despite the short-term damage eccentric muscle actions impose on the gastrocnemius-soleus complex, the long-term positive clinical outcomes for the Achilles tendinopathy patient have benefitted, as patients report less Achilles pain, greater strength of the ankle plantar flexors, and increases in overall function.^1,3,4

Why is eccentric training so popular?

Many common conservative interventions are used to address Achilles tendinopathy. Referring patients to physical therapy is a well-documented standard of care.³ Physical therapists commonly use treatments that address strength, flexibility, balance, and lower extremity motor control, as well as therapeutic modalities such as ultrasound and cross-friction massage to help reduce pain and improve overall function.²

A popular strategy used among physical therapists is to bias the volume of strengthening activities to eccentric overload training. The exercises used are commonly referred to as “heel drops” or “heel lowering” and are performed with the knee fully extended, to bias the gastrocnemius muscles, and with the knee flexed, to bias the soleus muscle. These activities are often used with the other interventions described above, but the eccentric training has consistently demonstrated favorable clinical outcomes.^1,3,4

The original Alfredson study used a 12-week program with 180 eccentric repetitions a day, but subsequent studies have shown that a six-week “do as tolerated” protocol yielded similar positive outcomes as the Alfredson study with less volume of exercise and less reported pain compared with Alfredson et al when participants performed the exercises.¹ It is often difficult to compare studies of eccentric exercise for Achilles tendinopathy, because all of these studies, including the investigation using the do-as-tolerated protocol, use other co-interventions such as stretching, cryotherapy, and other physical modalities. Other variables, such as the speed of movement, excursion of the muscle (ie, joint range of motion), and other treatments that precede the eccentric training are typically not described sufficiently.² This makes it a challenge to implement these protocols in a clinical setting, and a challenge to determine the true training effect of eccentric actions on the Achilles tendon.

Further confounding the issue, inclusion criteria often vary among studies using eccentric training for Achilles tendinopathy. Factors such as body mass index (BMI), activity levels (athlete vs sedentary), location of the pathology (insertional vs midportion tendinopathy), and duration of follow-up with study participants tend to be utilized to varying degrees.^1-3

In spite of all these factors, eccentric overload training is promising and widely used among clinicians who treat this pathology. The major limitation of the collective body of research is that the magnitude of the treatment effect of the eccentric training, to date, has not been sufficiently described or quantified.²

Theoretical eccentric training models

So far, the exact mechanism leading to positive treatment outcomes using eccentric training for Achilles tendinopathy has not been identified. Many studies have used pain or functional outcome measures to quantify improvement, but haven’t directly studied any changes to the muscle-tendon complex associated with these outcomes.^1,3,4 These descriptive studies have demonstrated the importance of using eccentric training but have not identified the cause-and-effect mechanism leading to decreased pain or increased function.^1,10

Several interesting changes within the skeletal muscle have been observed in conjunction with various resistance training programs. Studies have investigated training effects with both concentric and eccentric resistance training programs, examining the quadriceps, hamstrings, and gastrocnemius muscles.^11,12 These training studies have noted significant increases in fascicle length of the hamstrings and quadriceps muscles using traditional (concentric and eccentric) and eccentric-only resistance training programs. The primary focus of these training studies was looking at changes in strength and muscle architecture characteristics associated with resistance training. Historically, however, most resistance training studies have not focused on individuals with musculoskeletal pathologies like Achilles tendinopathy.^11,12

More recent studies have used patients with Achilles tendon pathology and have attempted to examine changes occurring to the tendon and the gastrocnemius-soleus complex with eccentric-biased strengthening programs.^13,14 A 2004 study examined Achilles tendon thickness before and after a 12-week eccentric training program.⁹ After 12 weeks of eccentric training, the mean tendon thickness in the group had normalized but, in patients still reporting pain, tendon thickness remained abnormal. The authors did not discuss any mechanism for normalizing tendon thickness, but one could surmise from the data that performing eccentric activities to the extent they are able to impact tendon thickness could lead to positive clinical outcomes.

Other studies have examined the gastrocnemius muscle architecture and function to determine changes at the skeletal muscle level due to eccentric training. Valid and reliable objective measures such as electromyographic (EMG) activity, muscle thickness, muscle pennation angle, and fascicle length are relatively easy to record using surface EMG and ultrasound imaging technology.^13,14 An additional advantage of these measures is they can be recorded in real time while a patient performs the specific training intervention.¹⁴

Recent literature has demonstrated a significant increase in fascicle length of the medial gastrocnemius in patients with Achilles tendinosis who participated in a six-week eccentric training rehabilitation protocol.¹³ The authors of the study hypothesized the increase in fascicle length could be due to a rightward shift in the fascicle length-tension curve.^13,15 Allowing the medial gastrocnemius to function over a larger area of the length-tension curve could provide a protective effect to the tendon and offer greater torque generation at more extreme ranges of motion. Although this is speculative, it does offer a theoretical foundation from which to consider the potential benefits of an eccentric training program.¹³

An EMG study published in 2014 demonstrated increased activity of the medial gastrocnemius muscle when performing eccentric muscle actions in the involved limb of patients with unilateral Achilles tendinopathy compared with the uninvolved extremity.¹⁴ It is interesting to note that only the medial gastrocnemius had increased activity with eccentric muscle actions compared with concentric actions and other muscle groups on both the involved and uninvolved side.

Research has demonstrated that concentric muscle actions typically produce greater EMG activation in normal human skeletal muscle compared with eccentric muscle actions.¹⁴ This is because skeletal muscle does not generate as much activity maintaining the actomyosin cross-bridge during eccentric muscle actions compared with the activity required to form the actomyosin cross-bridges during concentric muscle actions.¹⁴ Although the study authors did not discuss why the EMG activity was greater in the medial gastrocnemius on the injured side during eccentric exercise, it was clear the medial gastrocnemius was an outlier compared with other lower extremity muscles in the study.¹⁴ This points to the need to continue to study the medial gastrocnemius as a key factor with Achilles tendinopathy and address activities to treat that muscle with thoughtfully designed rehabilitation protocols.

One potential way to do this is to perform “heel-drop” or “heel-lowering” activities with the lower limb externally rotated. Studies have recorded increased medial gastrocnemius activation with the foot in external rotation.¹⁶ This can bias the activity in favor of the medial gastrocnemius and, in theory, provide a greater training effect with eccentric muscle actions. This may accelerate the muscle adaptation process and hasten Achilles tendon healing.

Recommendations for future study

Current literature recommends developing studies and protocols using eccentric training as the sole intervention rather than in conjunction with other interventions that confound results.² However, this creates ethical concerns, as practitioners know the standard of care for Achilles tendinopathy includes other therapeutic interventions that also have demonstrated favorable outcomes.³ Careful design of future studies to isolate the eccentric training muscle action will be important to truly determine the efficacy of these types of exercises as a valid and reliable treatment intervention.

Using eccentrically biased rehabilitation interventions in patients with Achilles tendinopathy is important to facilitate continued study of how the pathology responds to various protocols. It will be important to document and control for other training factors, such as speed of repetition, extent of joint range of motion, and rest between sets, so that studies are valid, reliable, and reproducible.

Studies that focus on the medial gastrocnemius will be important, as well. Changes to the medial gastrocnemius architecture and function appear to be associated with reports of decreased pain and improving function. Although there are a lot of unknown factors related to the pathophysiology and treatment of Achilles tendino­pathy, we do know a well-designed, eccentrically biased rehabilitation program is a major key to a positive treatment outcome.

Matthew Crill, MS, MBA, PT, is a licensed physical therapist and operations leader of OhioHealth WorkHealth in Columbus, OH.