Updated: Apr 6, 2020
Underlying mechanisms, commonly used intervention protocols and a proposed model to achieve freedom of pain and return to activity.
Chronic tendinopathy (TP) is a common pathology, typically found in the patellar tendon or Achilles tendon  in both athletes and the general population  and is associated with repetitive stress over a prolonged period . Risk factors are reduced quadriceps strength, inappropriately progressed training load and/or duration, reduced lower limb flexibility, lack of skill in the sport and inferiorly placed patella . Tendons are living tissue that respond to mechanical stimuli which are therefore seen as crucially necessary in the healing process and for improving the mechanical capacities in tendons [4–6]. Scientific evidence does give directions to guide practitioners in rehabilitation and strength & conditioning environments in helping their patient and athletes achieve freedom of pain and return to activity without being able to fully give insight into the mechanisms underpinning the pathology. The only potential mechanism consistently associated with improved clinical outcome is improved neuromuscular performance  which strongly supports an exercise-led approach. In the early 2000s eccentric (ECC) training was proposed to be a promising method [8, 9] and even suggested to be superior to other exercise modalities . However, reports about non-responders to ECC training exists [7, 11] and more recent research raises doubts to the quality of evidence supporting ECC exercise and further suggests that other modalities, such as isometric (ISO) and heavy slow resistance (HSR) exercise may lead to equivalent positive clinical outcomes [7, 12]. ISO as well as dynamic strengthening exercises were reported to have hypoalgesic effects in patients with TP [13, 14]. However, especially in chronic pain conditions, effects sizes for pain reductions through exercise are variable and seem to be dependent on pain condition and exercise intensity [12, 13] and up to date no consensus has been established as to the optimal protocol for exercise in athletes suffering from TP [12, 15]. This essay will give a deeper insight into the potential mechanisms underlying TP, review current exercise treatment approaches specific to patellar TP (PAT-TP) and Achilles TP (ACH-TP) and finish by proposing a multifaceted exercise approach that aims to help athletes and patients suffering from TP to return to activity.
Ohberg and Alfredson  discussed the effects of neovascularisation on pain symptoms in TP pathology. According to the argumentation of the authors neovessels with attached nerve-endings are likely to be responsible for the pain experienced in patients with TP. Their argumentation is further supported by another study  where sclerosing the neovessels in pathological Achilles tendons lead to significant reductions in pain. Despite clinical improvements being accompanied by a normalisation of the extra-cellular matrix, including the vascularity, the exact mechanisms underpinning vascularisation and tendon swelling remain unknown [15, 17]. Kongsgaard et al.  found abnormal fibril density (decreased) and mean fibril area (increased) which were returned to normal values after 12 weeks of HSR in patients with PAT-TP. The clinical improvements were associated with changes in fibril morphology towards normal fibril density and mean fibril area. Rio et al.  discussed cortical changes and motor neuron pool recruitment to be responsible for reported pain reduction after ISO exercise alongside changes at the tissue level, such as cell metabolism and receptor expression. The authors further proposed that patients suffering from PAT-TP may use high amounts of inhibition to moderate their motor output which potentially represents an aberrant control mechanism in comparison to normal motor control. Despite the abovementioned theories, and especially due to the lack of consensus, attention should also be given to the fact that abnormal tissue structures do not necessarily imply the presence of pain and vice versa. In a study by Pappas et al.  24 asymptomatic collegiate basketball players were scanned via MRI before and after a competitive season. Every knee investigated during this study showed at least one structural abnormality and PAT-TP was diagnosed (via MRI-imaging) in both pre- and post-season in 83% and 90% of the athletes respectively. Since the athletes in this study were asymptomatic it reinforces the importance of treating the patient and not the MRI image . Consequently, a pain-centred and exercise-led approach during rehabilitation is suggested and the use of modalities such as visual analogue scores (VAS-score) and the Vicotrian Institue of Sports Assessment (VISA) questionnaire  alongside regular pain reports during provoking tests, such as the single-leg decline squat .
In ACH-TP ECC exercise lead to reductions in pain and neovascularisation in ACH-TP and was classified a safe and easy modality to tendon pathologies [8, 9]. However, Malliaras et al.  concluded in their review that Silbernagel-combined loading programme, consisting of eccentric and concentric HSR, showed higher level of evidence than isolated eccentric loading. Beyer et al.  reported clinical improvement with accompanying reduction in tendon thickness and neovascularisation without any significant difference between HSR and ECC. However, patient satisfaction tended to be greater after 3 months with HSR (100%) than with ECC (80%) with the same tendency after 9 months (96% vs. 76%). Further, the compliance rate was higher in the HSR group (92%) than in the ECC group (78%) which is rather unsurprising given that the ECC protocol, as also seen in other studies [8, 9], had to be performed 7 days per week as opposed to the HSR training only on 3 days per week. Hence, in Achilles tendons HSR exercise appears to be a valid treatment option for TP.
According to a recent review investigating the effects of different exercise modalities on PAT-TP ISO exercise appears to be more effective during competitive seasons and for short-term pain relief, whereas HSR or ECC are more suitable for long-term pain reduction and improvement in knee function . The suggestion that ISO exercise is especially valuable while in-season and for short-term relief could be demonstrated nicely by a study of Rio et al.  who investigated the effectiveness of ISO and HSR exercise on immediate pain relief in volleyball athletes suffering from PAT-TP. The authors reported a greater pain relief immediately after ISO than HSR exercise which was sustained after 45 minutes in the ISO-group only. Further, maximal voluntary isometric contraction increased significantly after ISO exercise and was significantly higher than at baseline and after HSR exercise after 45 minutes. These results give good justification for the use of ISO exercise in PAT-TP, especially when the goal is short-term and/or immediate pain relief which might be highly relevant for elite athletes suffering from PT-TP in the middle of their season still wanting to compete. In a study by Kongsgaard et al.  twelve weeks of either ECC or HSR led to reduced tendon swelling and vascularisation as well as improvements in VAS and VISA scores indicating that both protocols led to a significant reduction in pain of patients suffering from PAT-TP. However, HSR lead to the greatest satisfaction after a 6-months follow up in patients in comparison to ECC training and corticosteroid injections and it was concluded that HSR induced good short-term and long-term effects accompanied by pathology improvement (reductions in tendon abnormalities) and increased collagen turnover and collagen synthesis.
In conclusion with regards to exercise modalities it seems that HSR as well as ISO interventions are promising alternatives to ECC exercises programmes given that with less amount of work comparable and/or superior effects can be achieved [7, 15]. However, it makes intuitive sense that rehabilitation protocols for TP should not be restricted to any contraction modality only. Van der Plas et al.  described patients that were treated with Alfredson’s ECC-only training protocol for ACH-TP were still having remaining pain symptoms after a 5-year follow up. Since the real world for both athletes and people from the general population consists of all three contraction types, it is therefore recommended that rehabilitation protocols should prepare the patient for those demands .
Considering the findings discussed in this essay a multifaceted approach in PAT-TP rehabilitation is suggested comprising a mixture of ISO, HSR and ECC exercise along the process (see figure 1). Overall, the programme consists of 4 phases where the first 3 are supposed to allow for return to normal activity and the last phase intends to allow for maintenance of freedom of pain and, in the case of athletes, introduce the next step towards high impacts and plyometric work. The programme was specifically designed to allow for patients/athletes to follow it at home and with absolute minimum need for extra equipment. For each phase and exercise a precise prescription and a goal to be reached by the end of each phase are specified. Continued sporting activity is encouraged as it does not seem to impair clinical improvement neither in PAT-TP (Kongsgaard et al., 2009a) nor in ACH-TP (Silbernagel et al., 2007). Use of a training diary to account for training volume is strongly recommended and accompanying documentation of session-VAS scores (1-10) is mandatory. Moderate pain is tolerated if it does not exceed VAS 3/10. Since ISO was reported to lead to immediate pain reduction  it is suggested to be incorporated in every rehabilitation session and to have a major contribution in the early weeks of the process to allow the patient to go through the programme with as little pain as possible. Generally, from Phase 1 to 4, frequency decreases, whereas intensity and complexity increase representing a common mode of progression. The storage and release of elastic energy is one of the key functions of human tendon and static and stretching can increase the energy capacity in tendons . Consequently, stretching is also incorporated in the rehabilitation programme, first statically in phase 2 and then ballistically in phase 3.
Figure 1: 4-phase rehabilitation outline. The main exercises for ISO/HSR/ECC are represented by the exercises wall sit/body weight squat/split squat. In phases 1-2 progression is mainly achieved via increases in time-under-tension (TUT). Stretching in phase 2 is static. Phase 3 introduces the separation of exercise modes into high-intensity (high) and complexity (comp.) creating two sessions for all three modes. Frequency per week in this phase is twice per week for high and once per week for comp. Stretching in phase 3 is ballistic. In phase 4 unilateral ISO exercise and plyometric (PLYO) work is introduced to further increase intensity and support the athlete in rebuilding elastic capacities. Further information about all exercises of phase 1-3 (home-based training) can be accessed through the links below.
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