Many conservative and surgical options have been developed and used to treat Achilles tendinopathies (Magnussen et al., 2009). Over time, the eccentric heeldrop exercises proposed by Alfredson et al. (1998) have become the treatment of choice for Achilles tendinosis. These exercises are widely practised, with studies assessing both short and longer term efficacy of the treatment (Kujala et al., 2005; Kingma et al., 2007; Magnussen et al., 2009; van der Plas et al., 2012). Despite a lack of evidence to support the biochemical changes expected with increased tendon loading (Khan and Scott ,2009), improvements in the tendon structure have been reported following eccentric heel-drop exercises (Ohberg et al., 2004). However, while the clinical outcomes are promising, the underlying aetiology of the condition is not known, with abnormal hindfoot kinematics (Donoghue et al., 2008) and altered triceps activation (Wyndow et al., 2010; Wyndow et al., 2013) during running being linked to pathological changes in the Achilles tendon. Furthermore, the biomechanical factors believed to drive the tendon healing process are unclear, with oscillations of Achilles tendon load (Grigg et al., 2013), tendon strengthening and stretching programmes (Kader et al., 2002; Allison and Purdam, 2009) and intra-tendon shearing (Alfredson et al., 1998) all being speculatively linked to tendon healing. This last factor could be considered the target of the commonly adopted heel drop exercises, as the intention of performing the flexed knee exercise is to alter the relative loading between the Gastrocnemius and Soleus muscles, which can result in changes in intratendon shear (Hebert-Losier et al., 2009a). Critically however, while previous observations have focussed on the common features of the heel drop exercises, the biomechanical differences between the extended and flexed knee versions of the exercise have been minimally characterised, with functional outcomes, such as pain reduction and functional improvements serving as primary justification of the treatment protocol. Additionally, assessing the biomechanical differences of performing the exercise in shoes is important, as differences between performing the exercise in clinic, generally in shoes, and at home, generally barefoot, may further characterise the mechanics driving the healing process. It is known that differences in EMG activity exist across the triceps during walking (Farris et al., 2013) and running (Wyndow et al., 2013) and as such, it is possible that EMG activation across the triceps differs during the different versions of the heeldrop exercise (Henriksen et al., 2009). Reid et al. (2012) is the only study to investigate the effect of knee flexion on triceps EMG activity during the eccentric heel drop exercise and showed that an extended knee resulted in greater Gastrocnemius activation, with Soleus activation unaffected by knee flexion angle during the exercise. This observation was only partially in line with the mechanical changes proposed by Alfredson et al (1998) where flexion of the knee is expected to shorten the Gastrocnemius, consequently decreasing its activation, necessitating an increase in Soleus muscle activation. However, the reliance on EMG data alone to make clinical inferences is limiting, as it does not quantify any other biomechanical observations which may affect the conclusions drawn regarding the efficacy of the treatment as a whole. As changes in ankle kinematics during the flexed knee exercise have not been previously quantified and can influence triceps surae activation during heel raises (HebertLosier et al., 2009b), it is not possible to explain the lack of change in Soleus muscle activation through surface EMG measures alone. Therefore, it is hypothesised that changes in ankle kinematics due to the flexed knee could influence the moment arms of the triceps surae, counteracting any changes in activation caused by shortening the Gastrocnemius muscle. The primary aim of this study was to quantify changes in lower limb kinematics, muscle lengths and Achilles tendon force, when performing the exercise with a flexed knee instead of an extended knee. A secondary aim of this study was to quantify any differences in lower limb mechanics when performing the exercises barefoot or in running shoes.
Nineteen healthy individuals were recruited (8 male [mean (SD); age: 28 (3); height: 1.76m (0.10); mass: 73.4kg (12)] and 11 females [age: 29 (6); height: 1.63 (0.05); mass: 58.7kg (10.2)]), with no history of ankle injuries and no lower limb injury in the last 12 months and no clinical symptoms of Achilles Tendinopathies. Individuals were excluded if they had any been diagnosed with Achilles Tendinopathy or had any musculoskeletal or neuromuscular condition of the lower limb. The cohort size was chosen based on previous studies investigating differences in flexed and extended knee eccentric heel drop exercises with optical motion capture (Hebert-Losier et al., 2011b; Grigg et al., 2013),
Description of the exercise and equipment
To best replicate the setup employed at home and in clinic, a wooden step (400mm x 132mm x 132mm) was constructed to replicate a step similar to that that found at home. The step was sanded to provide a smooth and flat surface and the edges were rounded to provide a comfortable radius of curvature to stand on. The step itself was secured over the centre of a forceplate with a ratchet strap (Figure 1).
Subjects were instructed to perform the eccentric heel-drop exercise following the approach detailed by Alfredson et al. (1998). Briefly, this requires subjects to stand on tip-toe with their ankle in maximal plantarflex ion, before lowering themselves in a controlled manner through eccentric loading of the calf to achieve maximum ankle dorsiflexion. Subjects then transfer their weight onto their other leg to concentrically raise their centre of mass before performing the exercise again. This exercise is performed with the knee extended and flexed. Subjects performed a minimum of five cycles using their right leg only and the exercise was considered to have been performed correctly if the subject went through their full range of ankle motion without excessive knee motion during the eccentric portion of the cycle and without the left foot touching the forceplate or wooden step. As knee flexion during the exercise was assessed by eye, if changes in knee flexion were substantial, subjects were asked to perform another cycle of the exercise. This was performed in barefoot and in running shoes ("shod") and with the knee in extension and flexed to a target angle of 30 degrees (Table 1) (Hebert-Losier et al., 2012). Subjects were given verbal instruction to "maintain a moderate squat" for the knee flexed exercise with the intention that this would provide an achievable position for each subject to reliably return to each time. For the extended knee version of the exercise, subjects were instructed to "keep their knee straight throughout the cycle". Subjects were given sufficient time to practice the exercise and to be able to perform the exercise without loss of balance and if the knee angle achieved during the knee flexed task was considered too great or too little by visual inspection, subjects were told to flex their knee accordingly.
Data collection and pre-processing Optical motion (MX-series, Vicon Motion Systems, Oxford, UK) and forceplate (9628BA...