Introduction: One potential reason for disparate outcomes of exercise for Achilles tendinopathy is poor knowledge about whether exercise parameters (i.e. different exercise doses) influence outcome. Despite a large number of exercise trials in the literature, none have specifically investigated the effect of specific exercise parameters on clinical outcomes in Achilles tendinopathy. This research aimed to assess the feasibility of conducting a full-scale powered randomised trial to investigate the efficacy of different load-intensity and time-under-tension exercise parameters for Achilles tendinopathy. Methods: Reporting was in accordance with the CONSORT checklist for pilot and feasibility trials and the trial was registered (ACTRN12618001315202). A factorial four-arm, randomised trial was conducted. Forty-eight male participants (18-70 years old) with mid-portion Achilles tendinopathy (≥ 3 months) were recruited. Participants were randomly allocated to high (6 repetition maximum) or low intensity (18 repetition maximum) exercise, performed with either high (6 seconds per cycle) or low (2 seconds per cycle) time-under-tension. Participants performed 12-weeks of standing and seated calf raise exercises three times per week in a gym setting using a Smith machine. One session per week was supervised (via telehealth). Primary feasibility outcomes (recruitment and retention rate, exercise adherence and fidelity [i.e. time-under-tension, volume, load intensity], incidence of adverse events, health care use and productivity cost) were collected weekly. Means and standard deviations were determined for parametric data, medians and interquartile range for non-parametric continuous data, and frequency counts for discrete data. Results and Discussion: Total recruitment (76%) and retention (90%) rates were high. Exercise adherence ranged from 45% to 63% and fidelity ranged from 8% to 83% across the groups. The fidelity data for the standing calf raise exercise is shown in Figure 1. Thirty-one participants reported 64 adverse events over the 3 months. Twenty-one participants (70%) reported mild events. Participants reported reduced presenteeism more than absenteeism. All groups displayed improvement in self-reported outcome measures during the study (Table 4). At 12 weeks, the mean improvement in VISA-A scores was between 26 to 40 points across the four groups. In a trial assessing the effect of exercise parameters, adequate adherence and fidelity is important to ensure the groups vary as intended. Home rather than gym-based exercise may improve exercise adherence in future trials. We also recommend future trialists compare total time-under-tension per set (we assessed time under tension in each repetition which may have been too stringent) Limiting recruitment to men reduces generalisability. Another limitation related to including only men is that our feasibility and pilot data (including the sample size estimate) may not extrapolate to women and this needs to be considered carefully in planning a full-scale powered trial. Second, although the outcome assessor was blinded to treatment allocation, participants and physiotherapists providing care via telehealth were not, so there is the risk of performance and detection bias. This was mitigated partly by expressing uncertainty to participants regarding potential comparative efficacy between groups. A potential limitation is that we recruited a majority of participants via social media and it is not certain whether they represent the range of individuals presenting for Achilles tendinopathy management in primary care. However, the age and severity profile (VISA-A scores) is similar to other studies also undertaken in Australia among individuals with Achilles tendinopathy [e.g. 1]. Conclusion: The proposed trial design and interventions demonstrated acceptable recruitment and retention rates and safety profile. However, exercise fidelity and adherence to the gym-based intervention was not acceptable. Strategies to improve intervention adherence and fidelity should be considered in future trials. Acknowledgements: This research was funded by a grant from the International Mechanical Diagnosis and Therapy Research Foundation (IMDTRF). REFERENCES: Yelland, M, et al. Prolotherapy injections and eccentric loading exercises for painful Achilles tendinosis: a randomised trial. Br J Sports Med, 45, 5, 421-428. 2011.