The purpose of the study was to test validity and (intra- and inter-sessions) repeatability of the results obtained by the hand-held dynamometer EasyForce, following procedures that are in accordance with manufacturer’s recommendations, in the case of the knee and hip. The study included 50 moderately trained young adults. Validity and repeatability were statistically tested with a paired two-tailed t-test, whose magnitude of effect was expressed as Cohen’s d (d), with Pearson’s correlation coefficient (r), intra-class correlation coefficient (ICC) and coefficient of variation (CV). We found out there are statistically significant differences between results measured with EasyForce hand-held dynamometer and reference method with dedicated dynamometer with external attachment (p < 0,001). The values of knee extensors and flexors measured with EasyForce dynamometer were approximately twice lower than the values measured with reference method with dedicated dynamometer with external attachment (-1,88 ≤ d ≤ -2,02), while the values of hip abductors and adductors were approximately two to three times higher (1,81 ≤ d ≤ 2,91). Correlation between the values was completely strong for hip adductors (r ≥ 0,70) and moderate to strong for knee extensors, knee flexors and hip abductors (0,64 ≤ r ≤ 0,77). Relative intra-session repeatability of strength measurements with EasyForce was good for knee extension (0,84 ≤ ICC ≤ 0,88) and excellent for all other muscle groups (0,90 ≤ ICC ≤ 0,96). Absolute intra-session repeatability was acceptable for measurements of maximal muscle strength and rate of force development of knee flexors and hip abductors (CV ≤ 9,90%) and for measurements of hip adductors on the second visit (CV ≤ 9,60%). Absolute intra-session repeatability was unacceptable for all measurements of knee extensors and for measurements of hip adductors on first visit (CV > 10,0%). Relative inter-sessions repeatability of EasyForce measurements was, regardless of the task, excellent (ICC ≥ 0,95) and absolute inter-sessions repeatability was acceptable (CV ≤ 7,70%).
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