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
Affiliations: [a] Department of Rehabilitation Medicine, Faculty of
Health Sciences, Linkoping, Sweden | [b] Pain and Rehabilitation Centre, Linkoping,
Sweden | [c] Department of Biomedical Engineering and Informatics,
Umea University, Umea, Sweden | [d] Department of Musculoskeletal Research, National
Institute for Working Life, Umea, Sweden | [e] Hand Surgery, University of Gothenburg, Gothenborg,
Sweden | [f] Department of Mathematical Statistics, Umea
University, Umea, Sweden
Note: [] Corresponding author: Bjorn Gerdle, Department of Rehabilitation
Medicine, Faculty of Health Sciences, S-58185 Linkoping, Sweden. Tel.: +46 13
221574; Fax: +46 13 224465; E-mail: bjorn.gerdle@rehab.inr.liu.se
Abstract: 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 (o) 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 found 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 pm 14 vs. females: 12 pm 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.
