Altered Strength Profile in Achilles Tendinopathy: A Systematic Review and Meta-Analysis

Key Points

1. Individuals with Achilles tendinopathy displayed deficits in maximal, reactive, and explosive strength up to 44% when compared with the uninjured side and asymptomatic controls.
2. Despite literature citing plantarflexor strength deficits that can persist for years and place individuals at greater risk of recurrence or rupture, clinicians are slow to integrate maximal strength testing and training modalities.
3. The commonly used heel-raise test and functional hop testing may not be sufficient to illuminate strength deficits seen with Achilles tendinopathy, as these tests have a lower ceiling and less discriminatory power.

BACKGROUND & OBJECTIVE

As the largest and strongest tendon in the body, the Achilles tendon is subjected to loads up to 6 to 12 times bodyweight. The plantarflexor muscle group and tendon complex play a large role in athletic movements, propulsion and rapid force production by storing and releasing high amounts of energy. The presence of Achilles tendinopathy (AT) has been thought to be associated with significant deficits in strength and power production (2). In clinical practice, the commonly used heel and calf raise plantar flexor strength tests measure endurance and do not take into account the high force demands placed on the Achilles. The purpose of this review was to summarize the relationship between strength deficits and AT, with the goal of informing clinical practice and helping to address the high recurrence rates reported in the literature (3).

The Achilles tendon is subjected to loads up to 6 to 12 times bodyweight.

Return to sport focused treatment for Achilles tendinopathy should assess and address plantarflexor strength across multiple strength-speed qualities.

METHODS

The authors used a PRISMA system to gather and assess relevant articles for the systematic review. PRISMA is an acronym standing for preferred reporting items for systematic reviews and meta-analyses. Eligibility criteria included comparisons of strength between AT and asymptomatic populations, and controls. All papers had to contain objective cross-sectional or baseline data from prospective or intervention studies. Kinematic studies and participants with Achilles tendon ruptures were excluded.

RESULTS

The authors sought to explore three variables relating to strength (see Table 1): maximal strength (peak torque [PT], maximal voluntary isometric contraction [MVIC], peak force [PF]), reactive strength (jump and hop distance), and explosive strength (rate of force development [RFD], ground reaction force [GRF]).

The results convincingly confirmed previously held beliefs. Individuals with AT consistently present with deficits in maximal, reactive, and explosive strength. The differences amongst high threshold strength measurables range from 10% up to 44%, whereas the differences amongst the heel rise test (endurance) are reported to be less than 10%.

LIMITATIONS

Although the large variation of papers used in this review serve as a strong point in regard to the heterogeneity of population studied, the variation serves as a limitation for studying the external validity of various methods used to measure plantarflexor strength. The paper did not distinguish between various presentations of AT including mid-portion, insertional, and potential plantaris involvement. Due to the heterogeneity of measurements and variables, the authors were unable to conduct a meta-analysis.

CLINICAL IMPLICATIONS

It is clear that AT related strength deficits lead to difficulty engaging the plantarflexor complex and thus decreased performance in jumping and sprinting. It has also been suggested that engaging in high tendon-loading movements while suffering from AT may place individuals at an increased risk of Achilles tendon rupture (4). Individuals can exhibit strength deficits for greater than 1 year following the resolution of symptomatic AT, highlighting the importance of emphasizing strength throughout the rehabilitation process.

In addition to the commonly used and easy-to-implement endurance and hop tests, return to sport focused treatment for AT should assess and address plantarflexor strength across multiple strength-speed qualities. A battery of tests may help provide clinicians with the most complete picture of readiness, as functional hop testing does not isolate the plantarflexors and the heel raise test has an extremely low ceiling.

Despite convincing evidence regarding the importance of strength measures in rehabilitation, it is far from common practice in the clinic. Many barriers to implementation still exist, with the largest barrier being a knowledge gap and lack of appreciation for the importance of strength. Equipment to measure strength and force development, such as handheld dynamometry, jump mats, and smart phone visual software can provide an effective replacement for the costly and time intensive setups associated with isokinetic testing equipment, force plates, load cells, and inertial measurement units.

It is suggested that clinicians emphasize strengthening across all spectrums of the force-velocity curve, do not spend undue time on any one mode of muscle contraction, and use improvements in strength rather than symptom presentation as a means to quantify the effectiveness of their interventions and their patients’ readiness.

+STUDY REFERENCE

McAuliffe S, Tabuena A, McCreesh K, O’Keefe M, Hurley M, 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

SUPPORTING REFERENCE

1. McAuliffe S, Tabuena A, McCreesh K, O’Keefe M, Hurley M, 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
2. Alfredson H, Pietila T, Jonsson P, Lorentzon R. Heavy-load eccentric calf muscle training for the treatment of chronic Achilles tendinosis. (1998). Am J Sports Med.
3. Gajhde-Knudsen M, Ekstrand J, Magnusson H, Maffulli N. (2013) Recurrence of Achilles tendon injuries in elite male football players is more common after early return to play: an 11-year follow-up of the UEFA Champions League injury study. Br J Sports Med.
4. Hess GW (2010). Achilles tendon rupture: a review of etiology, population, anatomy, risk factors, and injury prevention. Foot Ankle Spec