Reproducibility of isokinetic single- and multi-joint strength measurements in healthy and injured athletes

The purpose of this study was to analyse the reproducibility of isokinetic single-joint (knee and ankle) and multi-joint (leg-press configuration) strength findings in healthy athletes and a group of athletes with chronic unilateral tendon injuries at the lower limb, over a clinically meaningful time interval of 8 weeks. Concentric and eccentric strength performance capacity of knee extensors/flexors, ankle dorsi-/plantarflexors as well as the extensors of the total lower limb was assessed twice in 19 healthy (H) and 10 injured (1) athletes. Calculation of ICC, standard error of measurement SEM, Bland and Altman's 95% limits of agreement and Test-Retest Variability (TRV) served as main outcome variables. The results showed a small systematic bias and random error [mean bias: 3.25 +/- 4.02 (H) 4.25 +/- 4.56 (1); TRV 12.34 +/- 10.20% (H) 14.27 +/- 9.42% (I)] except for plantarflexion measurements and the eccentric mode of the total lower limb strength in injured athletes (TRV >= 20%). The present findings delineate clinically relevant cutoff values in the evaluation of valid intervention effects of strength training and rehabilitation in healthy athletes and athletes suffering from tendon-related injuries.