The aim of the present systematic review was to identify, critique and summarise lower limb biomechanical factors associated with Achilles tendinopathy. This review is timely to enhance the development of effective intervention and prevention strategies for the condition. Nine studies [2, 11, 19, 24, 25, 27, 33–35] evaluating lower limb biomechanics in those with Achilles tendinopathy were identified, with eight [2, 11, 19, 24, 25, 27, 34, 35] containing sufficient data to complete effect size calculations.
In agreement with other studies [30, 36, 37] that have used Quality Index , high inter-rater reliability for the selected items used in this study was found. Methodological quality was varied, with scores ranging between 4 and 15 out of 17. Several studies did not clearly describe participant characteristics (Item 3) [11, 25, 33, 34] or discuss whether participants invited (Item 11) [11, 24, 25, 27, 33–35] or recruited were representative of entire population (Item 12) [11, 27, 33–35]. This limits the ability of any findings to be applied to a broader population. None of the case-control studies [11, 24, 25, 27, 33–35] blinded their outcome assessors (Item 15) making it possible that some of the associated results may have been biased. Several included studies did not clearly describe confounding variables (Item 5) [11, 19, 25, 33–35] or adjust for these in their analyses (Item 25) [11, 19, 33, 34]. Additionally, the validity and reliability of outcome measurements used was not reported by any of the studies (Item 20) [2, 11, 19, 24, 25, 27, 33–35]. One study  analysed both limbs of each participant, and pooled data for both limbs within the case group, despite participants in the case group having unilateral symptoms. Two case-control studies [33, 34] excluded participants that displayed a rigid foot type in the Achilles tendinopathy but not in the control group. This introduces significant recruitment bias into their studies.
Lower limb kinematics
Abnormal alignment and function of the lower limb, particularly in the frontal plane at the foot and distal leg, is frequently cited as a risk factor for Achilles tendinopathy [8, 10, 15, 23]. Three studies [25, 34, 35] evaluating frontal plane kinematics of the rearfoot and/or distal leg were identified in this review. The majority of these comparisons were not found to be different between groups (see Figure 1). However, separate studies showed greater eversion range of motion of the ankle in those with Achilles tendinopathy in both shod  and barefoot  conditions. Further, one study  showed reduced maximum lower leg abduction (barefoot) in those with Achilles tendinopathy. These findings suggest that Achilles tendinopathy may be associated with greater movement excursion of the rearfoot during gait and support the original proposition by Clement et al.  who hypothesised that greater movement excursion of the rearfoot may create increased tensile stress and subsequent degeneration along the medial aspect of the Achilles tendon . However, these differences need to be considered in light of this review's results showing no significant effects for the majority of frontal plane rearfoot kinematic variables which includes maximum eversion/pronation. Contrary to the tensile stress theory, no evidence was found to support that torsional stress or 'wringing' of the Achilles tendon was associated with Achilles tendinopathy. Two studies [24, 35] investigating transverse plane kinematics of the tibia at the ankle and/or knee joints in those with and without Achilles tendinopathy showed no differences between groups. Prospective rearfoot and lower leg motion evaluation is now needed to further understand its possible link to Achilles tendinopathy development.
Three studies [27, 34, 35] investigated sagittal plane kinematics of the hip, knee and/or ankle joints at a range of instants during stance phase of the gait cycle. Generally comparisons indicated no differences in these parameters between those with and without Achilles tendinopathy, with the exception of reduced maximum ankle dorsiflexion velocity  and knee flexion range between heel strike and midstance  in those with Achilles tendinopathy. The link between reduced ankle dorsiflexion velocity and Achilles tendinopathy is unclear but it may indicate a compensation strategy to minimise internal loading of the Achilles tendon in those with Achilles tendinopathy. Reduced knee flexion between heel strike and midstance in those with Achilles tendinopathy has been speculated to be a compensation for weakness of proximal hip muscles (e.g., rectus femoris) during eccentric actions, and the reduced impact absorbing motion has been speculated to cause an increase in load within the Achilles tendon . However, future studies are required to determine if there is a relationship between these kinematic changes and increased internal load within the Achilles tendon.
Ground reaction forces and joint moments
Three studies [11, 25, 27] performed a large number of comparisons of ground reaction force variables (direction, magnitude and timing) between those with and without Achilles tendinopathy. Overall, there were few differences in the magnitude of the vertical, antero-posterior and medio-lateral components of the ground reaction force variables between those with and without Achilles tendinopathy. However, there were a number of relatively large effects for variables related to the timing of the ground reaction force. Those with Achilles tendinopathy had a greater (delayed) time to the first vertical peak , time to minimum peak force  and time to maximum medial force  but reduced (earlier) time to maximum braking force  and time to maximum lateral force . However, the analysed study  was a case-control design and participants were symptomatic during testing. It is therefore possible that injured participants may have altered their gait to minimise stress within their Achilles tendons. Future studies are required to determine if these timing differences can cause changes in Achilles tendon loading.
Only one study evaluating joint moments in those with Achilles tendinopathy was identified . Peak external tibial rotation moment was significantly reduced in those with Achilles tendinopathy, suggesting those with Achilles tendinopathy may have reduced torsional stresses within the Achilles tendon. Interestingly, this is contrary to traditional theory . However, it is possible that reduced tibial external rotation moments may be a compensation to reduce stress within the Achilles tendon. Prospective evaluation is now needed in order to adequately understand the association of external tibial rotation moments with Achilles tendinopathy.
Plantar pressure parameters
Three studies [2, 11, 19] evaluated the association of a large number of dynamic plantar loading variables with Achilles tendinopathy. Findings showed those with Achilles tendinopathy demonstrated a significantly more laterally directed force distribution beneath the forefoot at forefoot flat (reduced time to peak force at medial heel and medio-lateral force distribution underneath the metatarsal heads at forefoot flat) , a significantly more medially directed force distribution during midstance (reduced lateral deviation of the centre of pressure in the rear-and mid-foot) [2, 11] and a significantly reduced total forward progression of the centre of force beneath the foot (reduced displacement of the posterior-anterior component of the centre of force at last foot contact, reduced posterior-anterior displacement of the centre of force during forefoot push-off phase, and reduced total posterior-anterior displacement of the centre of force) . Van Ginckel et al.  hypothesised that these findings may explain the development of Achilles tendinopathy as follows. First, the lateral foot roll-over pattern during the contact period of gait in those with Achilles tendinopathy may create diminished shock absorption and exert more stress on the lateral side of the Achilles tendon. Second, the more medially directed force distribution during the midstance phase may represent increased midfoot pronation, unlocking the midtarsal joint. This would increase forefoot mobility and impede the ability of the foot to act as a rigid lever during propulsion. Therefore, higher active tensile forces may be transferred through the Achilles tendon during propulsion, leading to tendon strains. This explanation is reflected in findings showing decreased forward transfer of the centre of force in those who developed Achilles tendinopathy .
Lower limb muscle function
Two studies [11, 27] compared EMG amplitude and onset timing of a number of lower limb muscles in those with and without Achilles tendinopathy. One study  reported a number of differences in the onset timing of lower limb muscles between those with and without Achilles tendinopathy. Notably, the onset of tibialis anterior activity was significantly delayed, and the duration of soleus and lateral gastrocnemius activity was increased in those with Achilles tendinopathy. It is possible that this timing imbalance, particularly the increased duration of activity of the ankle plantarflexors may create prolonged loading of the Achilles tendon and contribute to tendinopathy development. Alternatively, reduced function of tibialis anterior has been theorised to reduce stiffness of the tendon-muscular system in the lower limb and impede its ability to tolerate and absorb impact forces . This could create increased Achilles tendon loading and lead to tendinopathy.
In regards to the amplitude of function of proximal lower limb muscles, one study  showed significant reductions in the amplitude of gluteus medius and rectus femoris but not biceps femoris shortly (100 ms) before or after heel strike in those with Achilles tendinopathy. As eccentric contraction of gluteus medius and rectus femoris is important to dissipate forces at the hip and knee respectively during early stance, reduced activity of these muscles may place greater stress at the foot and ankle causing increased Achilles tendon loading. However, conflicting results were reported for the amplitude of the muscles of the distal lower limb, tibialis anterior, peroneus longus and lateral gastrocnemius [11, 27]. The inconsistencies in findings may have resulted from differences in study design between the two studies (participants, gait analysed, electrode placement, parameters assessed and processing of data) as well as questionable reliability of lower limb EMG assessment [38, 39]. Based on the often conflicting results of these two studies, it is difficult to make inferences concerning the function of lower limb muscles in those with Achilles tendinopathy. Future well-designed prospective studies using reliable and valid assessments of lower limb muscle function are needed.
In addition to the limitations caused by the quality of the included studies described previously, there are several other limitations of this review. All of the included studies analysed running gait only. Given that one third of participants with Achilles tendinopathy are not physically active , the findings of this review may not be applicable to these people. There was a predominance of males across included studies, meaning findings from this review may have limited applicability to females. There are a number of biomechanical factors which were not included in this review, either because they have not been previously evaluated (e.g. joint moments at the foot and ankle) or data did not allow effect size calculations. Interestingly, results from the study of Donoghue et al.  which was excluded from data analysis (effect size calculations) in this review showed that individuals with Achilles tendinopathy displayed significantly less variation in lower limb kinematics than healthy controls. Only two studies [2, 19] included in this systematic review contained a prospective research design, with both investigating plantar pressures. Therefore, with the exception of a number of plantar pressure variables, the ability to distinguish between cause and effect in this review is limited. Sample sizes of included studies were generally small, meaning 95% CIs for effect size calculation were frequently large. This may have erroneously lead to non-significant effect size calculations, even if a true difference between groups existed. Future well-designed and adequately powered prospective studies are required to overcome these limitations.