INTRODUCTION: Achilles tendinopathy (AT) is a persistent injury in athletes, especially for those engaged in repetitive stretch-shortening cycle activities such as running. Clinical risk factors are numerous, but it has been suggested that altered biomechanics might lead to or be associated with the development of AT. No systematic review has been conducted investigating these biomechanical alterations in specifically athletic populations. Therefore, the aim of this systematic review was to synthesise research comparing the lower-limb biomechanics of athletes with AT to asymptomatic controls. METHODS: MEDLINE, Web of Science and Cochrane were searched in March 2021 for relevant studies investigating biomechanics and neuromuscular performance during gait activities and other motor tasks such as hopping, isolated strength tasks, and reflex responses. Inclusion criteria for studies were an AT diagnosis in at least one group, cross-sectional or prospective data, a minimum of one outcome comparing biomechanical data between an AT and healthy group, and athletic populations (e.g., run >20km / week). Studies were excluded if patients had Achilles tendon rupture/surgery, participants reported injuries other than AT, and when only within-subject data was available (e.g., no healthy control group). Search terms were based on four key areas: "Biomechanics", "Movement Task", "Pathology (Tendinopathy)" and "Anatomical Location (Achilles tendon)". Effect sizes (Cohen`s d) with corresponding 95% confidence intervals were calculated for relevant outcomes. RESULTS: The initial search yielded 4442 studies. After screening, twenty studies (775 total participants) were synthesised, reporting on a wide range of biomechanical outcomes. Females were generally under-represented and patients in the AT group were 3 years older on average. Biomechanical alterations were identified during running, hopping, jumping, strength tasks and reflex activity, with small to large effect sizes. Kinematics in AT patients appeared to be altered during stretch-shortening activities of the lower limb, indicating a pattern of "medial collapse" e.g., increased hip adduction/ ankle pronation. Muscular activity of the calf and hips was different between groups, whereby in two studies AT patients exhibited greater calf muscle EMG amplitudes despite lower plantar flexor strength. Overall, dynamic maximal strength of the plantar flexors, and isometric strength of the hips might be reduced in the AT group. One study indicated an up-regulated reflex activity with AT. CONCLUSION: In summary, this systematic review reports on several biomechanical alterations in athletes with AT. These factors may form potential treatment targets, although this needs further research and clinical approaches should additionally take other contributing health factors into account. Based on the cross-sectional design of most studies, it is difficult to discern whether these factors are causative or consequential of AT.
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