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Structure and function of the ankle dorsiflexor muscles in young and moderately active men and women

Departments of ¹Physical Therapy and ⁵Rehabilitation, Lund University Hospital, SE-22185, and ⁶Department of Community Medicine, Lund University, SE-22100 Lund, Sweden; ²Faculty of Physical Education and Recreation Studies, University of Manitoba, Winnipeg, Canada, R3T 2N2; ³Department of Mathematical Sciences, University of Liverpool, Liverpool L69 7ZL, United Kingdom; ⁴Copenhagen Muscle Research Centre, Department of Molecular Muscle Biology, DK-2100 Copenhagen, Denmark; and ⁷Department of Health Sciences, Luleå University of Technology, SE-96136 Boden, Sweden

Submitted 13 June 2002 ; accepted in final form 9 July 2003

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 yr). 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 I and II) and proportions (Prop I and II) of type I and II fibers, respectively, and relative contents of myosin heavy chain (MHC) isoforms MHC₁, MHC_2a, and MHC_2x. 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, MHC₁, 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 >50% of the variation in CSA. 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.

magnetic resonance imaging; skeletal; muscle contraction; muscle fibers; sex characteristics

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