Correlation between Isometric Maximum Strength and One Repetition Maximum in the Calf Muscle in Extended and Bended Knee Joint

Previous research points out high relevance of maximal strength measurement in the diagnostics of different populations. However, there is inconsistency in procedures of maximum strength measurements. Thus, it must be questioned whether the results from different studies are actually comparable. Due to the aforementioned problems in standardization, the aim of this study was to assess correlations between and reproducibility of isometric and dynamic strength testing. Since there are many studies investigating maximal strength in the calf muscles, this study will examine the plantar flexors. For this purpose, 87 active participants were recruited (m: 52, f: 35, age: 28.3 ± 5.5 years, range 18-38 years, height: 178.3 ± 6.6 cm, weight: 81.5 ± 7.4 kg) who performed maximal isometric strength testing and dynamic 1RM testing in plantar flexion both with extended and bended knee joint. Pearson correlation as well as concordance correlation coefficient (CCC) were determined. In literature, CCC is used to determine reproducibility between two different testing methods. There were correlation coefficients of r = 0.63 – 0.77 and ρc=0.62 ‒ 0.77. Results are comparable with correlations between maximal isometric strength and 1-RM in previous studies in different movements. In consideration of CCC, isometric strength testing and 1RM seem to not measure the same parameter, therefore comparison of results measured with different procedures seems difficult. Our results exhibit a high influence of isometric maximal strength on 1RM performance. However, 1RM tests cannot be replaced by isometric strength testing. Care must be taken due to standardization of procedure when comparing results from different studies and, especially, if 1RM testing should be replaced with isometric strength measurement.