3 55 Normative Calf Muscle Strength-Endurance Values: An International Study DISCUSSION In this large international cross-sectional study, we presented normative values for HRET metrics, adjusted for personal characteristics. We found that the median number of repetitions and peak height was 25 and 9.3 cm for the dominant leg and 24 and 9.6 cm for the non-dominant leg. There was no significant difference between the dominant and the non-dominant leg for any of the HRET metrics. Lower physical activity levels, female gender, lower body height, and higher BMI were associated with lower HRET metrics. For the primary outcome measures, we found no correlation between age and HRET metrics. This study presents novel reference values for HRET metrics. The median number of repetitions achieved in the present investigation corresponds with previous findings.6,19,22 Various studies have reported mean values for total work (ranging from 1800 to 3000 J) or peak height (ranging from 9 to 14.1 cm) in the uninjured legs of patients recovering from Achilles tendon rupture11,22,23 or a small (38 participants) sample of healthy individuals.10 Our median values for work (1380 J) and peak height (9.7 cm) are at the lower end of this spectrum. This discrepancy can potentially be attributed to the relatively small (38 – 96 participants) or selected (very active) study populations that are younger in age in previously published studies. The primary factor contributing to the observed variance in results is likely the methodology employed in the current study for collecting calf raise data, specifically the use of a marker placed below the lateral malleolus14 rather than on the heel, as done in the aforementioned studies to attach a linear encoder10,11,22. This below malleolus placement is found to be more valid when using the Calf Raise Application,14 but results in relatively lower values compared to using a marker positioned on the heel.14 Our results show that HRET metrics are influenced by personal characteristics. The findings that lower physical activity, higher BMI, lower body height and female gender are associated with lower HRET outcomes are consistent with previous findings.6 We did not observe a correlation between age and the number of repetitions, which contrasts to earlier work showing a significant decline in number of repetitions for each passing decade of life.6 A possible explanation for this may be that, despite the efforts to include a balanced population with regards to age and gender, the study population was relatively young with a mean (min-max) age of 36 (18-81) years as well as relatively active (supplementary file 1, supplementary Figure 2 and supplementary Figure 3). It is likely that other personal factors influence the results, like motivation24 and self-confidence. We did encourage participants to perform maximally, but we are aware that psychological factors – which we did not consider – may affect outcome.
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