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1 Physical Therapy, Lund University, Lund, Sweden, Sweden
2 Physical Education and Recreation Studies, University of Manitoba, Manitoba, Canada, Canada
3 Mathematical Sciences, University of Liverpool, Liverpool, England, United Kingdom
4 Molecular Muscle Biology, Copenhagen Muscle Research Centre, Copenhagen, Denmark, Denmark
5 Rehabilitation, Lund University Hospital, Lund, Sweden, Sweden; Community Health, Lund University, Lund, Sweden, Sweden; Health Sciences, Lulea University of Technology, Lulea, Sweden, Sweden
* To whom correspondence should be addressed. E-mail: anna_maria.holmback{at}sjukgym.lu.se.
The aim was to investigate determinants of ankle dorsiflexor muscle (DF) strength and size in moderately active young men and women (n=30; age 20-31 years). Concentric (CON) and eccentric (ECC) strength were measured isokinetically. Magnetic resonance imaging was used to determine the muscle cross-sectional area (CSA). Multiple biopsies were obtained from the tibialis anterior muscle to determine total numbers, areas (AREA) and proportions (PROP) of types I and II fibers, and relative contents of myosin heavy chain (MHC) I, IIa and IIx. Women had lower CON and ECC strength (24% and 27%; p<0.01), smaller CSA (19%; p<0.001), lower ECC DF specific strength (strength/CSA) (10%; p<0.01), and smaller AREA I and AREA II (21% and 31%; p<0.01), than men. PROP I, MHC I, estimated total number of fibers and CON DF specific strength were similar for both sexes. CON DF strength was up to 72% determined by CSA and PROP I, and ECC DF strength was up to 81% determined by CSA, PROP I and sex; variables other than CSA explained at most 9%. Body weight and fiber areas explained more than 50% of the variation in cCSA. In conclusion, CSA was the predominant determinant of DF strength, CSA was to a great extent determined by the body weight and the sizes of muscle fibers, and sex differences in ECC specific strength require further study.
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