## 1 Introduction  
The Achilles tendon is the largest [1, 2], strongest, and thickest tendon in the human body [1, 2, 3, 4]. It transforms energy from the leg to the foot and is essential for postural control and gait [3]. The most common pathology of the Achilles tendon is Achilles tendinopathy (AT), which represents 55-65% of all Achilles tendon clinical diagnoses [5]. AT is characterized by swelling, pain, and impaired performance of the Achilles tendon [6, 7, 8]. Tendinopathies are well known as overuse injuries [1, 9] that are a consequence of a repetitive load acting on a tendon, which can no longer maintain tension and stress [9]. AIS can be divided into two groups based on the duration and possible inability to perform physical activity (acute and chronic AT) [5]. Moreover, AT may be divided according to anatomical location into insertional AT (IAT) and non-insertional or midportion AT (MAT) [10]. MAT is located in the middle of the Achilles tendon, 2-6 cm above the calcaneum (the heel bone) [11, 12, 13], and it occurs in older, less active, and overweight populations [13]. Meanwhile, IAT is located in the distal insertion point of the Achilles tendon to the calcaneal bone [14, 15, 16]. IAT tends to occur in the more physically active population [13]. [PERSON] et al. [17] found that AT affects 5.2% of runners on average, whereas it affects approximately 7.4% of runners who are registered for a marathon. On the other hand, [PERSON] and [PERSON] [18] claimed that AT affects 9% of recreational runners and causes up to 5% of professional athletes to end their careers. ATs also represent 6.1 running-related injuries per 1000 running hours [17] and 0.235 injuries per 100 adult injuries in the general population [19]. Although AT often occurs in the physically active population [20], the incidence of injury in the population with a sedentary lifestyle is around 30% [21; 22]. The most common risk factors are a previous AT injury [1; 17] and inadequate rehabilitation of previous AT [17], low plantar flexor strength [1; 17], training during cold weather [1], decreased plantar flexor flexibility [13], an abnormal gait pattern, and an increased lateral foot roll over at the forefoot flat phase [1]. AT can be treated by conservative or operative treatments. Nonsurgical, conservative treatments show great efficacy for treating AT, but in about 25% of all cases, surgery is needed [23]. The most common non-surgical treatments are nonsteroidal anti-inflammatory drugs, corticosteroid injections, acupuncture, extracorporeal shockwave therapy (ESWT), cold air and high-energy laser therapy, ultrasound, thermotherapy, massage and physiotherapy, splints, braces and taping, wait-and-see treatment, stretching, and exercise (e.g., isometric (ISOM), isotonic (ISOT), eccentric (ECC), concentric (CON), vibration exercise, etc.) [24]. [PERSON] et al. [25] reported that conservative pharmacological treatments (such as nonsteroidal anti-inflammatory drugs and corticosteroid injections) did not have beneficial effects on pain decrease, function improvement, or patient satisfaction improvement as great as exercise treatments at short-term follow-up. However, the implementation of corticosteroid injections to an exercise protocol caused greater function improvement and Achilles tendon thickness than either an exercise protocol alone or a combined exercise protocol with placebo injections [26]. On the other hand, [PERSON] and [PERSON] [20] claimed that modification of the training regimes and specific exercises is necessary. As the maximal load is placed on a tendon during the eccentric phase, tendon injuries are likely to occur during eccentric loading [27]. That is why ECC exercise is the most frequent exercise protocol for treating AT [28]. ECC exercise was found to be beneficial in early treatment [29; 30] and also in chronic AT patients [27]. Several different types of ECC exercise for the rehabilitation of AT are known, among which [PERSON]'s heavy-load eccentric calf muscle training (HECT) is the most commonly used [27].