Introduction Tendinopathy is a highly prevalent injury in athletes and nonathletes with an unknown aetiology. Exercise to improve pain function and potential adapt tendon is first line management for tendinopathy. The longitudinal effect of loading on tendons has been extensively investigated, with chronic loading interventions being shown to result in mechanical and morphological tendon adaptations. The aim of the study was to determine the effect of different chronic loading interventions on tendon adaptation by identifying, appraising and summarising the existing literature on this subject. This information could potentially assist in the (i) development of preventative protocols; (ii) enhancement of specific and effective management strategies; and (iii) maintenance of a disease-free state by improving rehabilitative techniques.

Methods Six electronic bibliographic databases were searched from inception to October 2013 for randomised controlled trials, controlled trials and prospective cohort studies that included empirical research investigating the longitudinal effect of chronic loading on tendon properties. Potential publications were assessed by two independent reviewers (PM and AAC) for inclusion and quality. Quality was evaluated using the Downs and Black methodological assessment tool.

Results Fourty-three studies which investigated the longitudinal effect of chronic loading interventions on tendon adaptations were identified. Protocols were divided into 6 broad categories: isometric, plyometric, eccentric and concentric, eccentric, concentric and functional (walking, running, skiing and cycling). Downs and Black quality assessment scores ranged from 12 to 20 of a possible 29. The average score for methodological quality was 17/29 (59%). There was strong evidence for increased Achilles tendon stiffness in response to high intensity isometric training. Conflicting evidence exists for increased stiffness with isometric loading using a moderate to high intensity protocol. There was strong evidence that vastus lateralis stiffness increases after moderate intensity 70% MVC isometric loading for 12 weeks. There was moderate evidence for increased Achilles and patellar tendon stiffness with high intensity (70–80% 1RM) isotonic loading. There was moderate evidence that VL stiffness increases following high but not low load isotonic interventions. Isolating the eccentric component of loading only lead to increased Achilles or patellar stiffness or modulus if the intervention was loaded heavily. Isolated and intense concentric loading produced an increase patellar tendon stiffness and modulus. Conflicting evidence exists for the effect of plyometric training on tendon stiffness and modulus. Functional intervention such as running and walking did not influence tendon stiffness, whereas alpine skiing over 12 weeks increased patellar tendon stiffness in an elderly cohort.

Discussion The findings of this review suggest that the best strategy to restore tendon stiffness and function to pre-morbid levels may be heavy isotonic or heavy and longer duration isometric loading. However, we also found that there was no evidence of adaptation in pathological tendons, although further work in this area is warranted as only two studies have investigated this. Given the breakdown of the tendon matrix that occurs in pathology, perhaps only the surrounding normal tendon has the potential to adapt and this should be investigated in future studies. In a clinical setting, isometric training may be superior to other loading as the duration and intensity of the isometric loading intervention can easily be adjusted and loading can be performed in a painfree range.