How do eccentric exercises work in tendinopathy?

Painful tendon disorders are a major problem in competitive and recreational sports [1, 2]. Tendon injuries are difficult to manage, and current conservative and surgical management options have shown limited and often unpredictable success [3, 4]. Even when early diagnosis of Achilles tendinopathy is combined with appropriate and intensive management, rehabilitation can take several months. Conservative management options for tendinopathy of the main body of the Achilles tendon include modified rest, exercise, analgesics, injections, electrotherapy, identification and correction of possible intrinsic and extrinsic causes [4]. This abundance of management modalities has arisen from the lack of consensus as to the cause of tendinopathy [5–8]. The lack of suitable evidence in support of a given management method does not necessarily imply that it is ineffective. Rather, there may be a lack of adequately powered studies to demonstrate its effectiveness [9]. Mechanical conditioning can be used as a treatment for enhancing tendon healing [10]. Mechanotransduction is the process of a cell converting mechanical stimuli into biochemical signals. Cells able to sense the mechanical signals are described as being mechanosensitive. Tendon responds to mechanical forces by adapting its metabolism and structural and mechanical properties [11]. Tendons adapt to alterations in the mechanical load being applied by changing their structure and composition. The tenocytes in the tendon are responsible for its adaptive response, and respond to mechanical forces by altering their gene expression patterns, protein synthesis and cell phenotype [12], which can be used to aid the healing process. However, the duration, frequency, magnitude and type of mechanical stimulation applied to a tendon greatly affect the outcome of the loading regime. Therefore, the amount of loading necessary to improve and/or accelerate the healing process without causing damage to the healing tissue remains unclear [13, 14]. There is some evidence that eccentric strengthening programmes may be effective in the management of tendinopathy of the main body of the Achilles tendon [15, 16]. Graded eccentric exercises regimen was proven to be effective in case–control studies and in prospective randomized control trials [15–19]. Stanish et al. [20] introduced the concept of eccentric training in the rehabilitation of tendon injuries in the mid-1980s, and Scandinavian authors popularized it [19] more than a decade later, with some important differences. Stanish et al. [20], for example, recommend that patients should perform the eccentric exercises with no pain, while Scandinavian authors [19] recommend pushing through pain. Though effective in Scandinavian population [17, 18], the results of eccentric exercises observed from other study groups [16, 21] are less convincing than those reported from Scandinavia, with only around 60% of good outcome after a regime of eccentric training both in athletic [15] and sedentary patients [16]. The best evidence to date does demonstrate that eccentric exercise is likely a useful management for tendinopathy, but this evidence is currently insufficient to suggest it is superior or inferior to other forms of therapeutic exercise [21, 22]. There is little consensus regarding which variables may influence the outcome of eccentric training, including whether training should be painful, homevs clinic-based training, the speed of the exercise, the duration of eccentric training and the method of progression. Large randomized controlled trials that consider these parameters and include blinded assessors and extended follow-up periods are required. Three basic principles in an eccentric loading regime have been proposed: