56 Chapter 3 Clinical implications Literature shows inconsistent findings with regards to the influence of leg dominance on the number of repetitions. While several studies reported no between-leg differences,6,25 others reported the non-dominant side to exhibit greater strength26 or a higher number of repetitions than the dominant side.6,27 The current study did not find any difference between leg dominance and HRET performance. The inconsistent evidence makes it difficult to support or refute the use of the uninvolved side as a reference for comparison when evaluating HRET performance in clinical practice. This issue becomes particularly apparent in injured individuals where is it known that HRET performance is negatively impacted in both limbs.6 To address this issue, clinicians may benefit from knowledge of reference values, adjusted for personal characteristics. We have developed an openly accessible web-based calculator for estimating normative HRET metrics (www.achillestendontool. com/HRET). This tool may be valuable for clinicians to monitor personalized trajectories of recovery and to provide well-informed rehabilitation guidance. Strengths and limitations The strengths of this study lay in the design, with the inclusion of a large and international study population, a pre-defined protocol, the use of a validated openly accessible application to obtain outcome measures and the development of the open access tool for calculating normative HRET values, adjusted for personal characteristics to facilitate implementation in clinical practice. There are several limitations to this study. First, while the age range of the included participants was broad, the mean age of the study population was relatively young. This age may have led to the underestimation or absence of a correlation between age and HRET performance. Second, the normative values were derived exclusively from the Calf Raise Application. This application has demonstrated excellent validity and reliability, but our findings may not translate directly to normative values for other methods of assessing the HRET (such as the use of a linear encoder placed on the heel) or testing in different positions (e.g., no 10° incline or with shoes on) or with different cadence. However, this application is free for use and easily accessible for clinical and research use. Thirdly, during the design phase of the study we decided to collect the peak power and vertical height loss as outcome measures. Peak power is less relevant, because we used a metronome, resulting in more constant peak power values. Vertical height loss is known to be the least reliable and valid outcome measure.14 We therefore present these metrics as secondary outcome measures. CONCLUSION Outcomes of the single-leg HRET are influenced by personal characteristics, with female gender, higher BMI, lower body height and lower physical activity levels being associated
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