Characteristics of the shift from the fatigue phase to the endurance level (breakpoint) of peak torque during repeated dynamic maximal knee extensions are correlated to muscle morphology

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During dynamic (isokinetic) maximal contractions the mechanical output will decrease steeply during the initial 20-60 contractions (the fatigue phase) followed by a stable level (the endurance level). From visual inspections it is obvious that the breakpoint between these two phases differs between individuals. Based upon regression techniques we have investigated two variables of interest with respect to the breakpoint: the number of contractions (Bp-no) before the breakpoint and the angle (°) between the two phases (Bp-deg). We also investigated the interrelationship between output, sex, muscle morphology, and the two variables of the breakpoint during maximal dynamic knee extensions. Twenty clinically healthy subjects (11 males and 9 females) participated. Muscle biopsies of vastus lateralis were obtained from 19 out of 20 subjects. Bp-no was fqund at 42 ± 14 contractions (no sex difference). A significant difference (p = 0.001) between the two sexes was found for the Bp-deg (males: 100 ± 14 vs. females: 121 ± 16). No significant correlation existed between Bb-no and Bp-deg. Bp-deg and initial peak torque mainly correlated with the fibre areas. Bp-deg could be predicted in a model with sex and fibre areas as significant variables. The slope and endurance level (absolute and relative) of peak torque and Bp-no correlated negatively with % type 2b. Analyses of the relationship between output and morphology showed that different morphological variables (area or proportions) were of significant importance in different stages of the endurance test. In conclusion a method for determination of the breakpoint has been presented. The two variables presented correlated with important physiological aspects.