title: Structure and function of the ankle dorsiflexor muscles in young and moderately active men and women.
abstract: 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, MHC2a, and MHC2x. 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.
authors.affiliation: Department of Physical Therapy, Lund University Hospital, SE-221 85 Lund, Sweden. anna_maria.holmback@sjukgym.lu.se
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Holmbäck
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Porter
authors.forename: David
authors.initials: D
authors.lastname: Downham
authors.forename: Jesper L
authors.initials: JL
authors.lastname: Andersen
authors.forename: Jan
authors.initials: J
authors.lastname: Lexell
mesh_terms: D000328:Adult; D000842:Ankle / Q000033:anatomy & histology* / Q000502:physiology*; D001827:Body Height / Q000502:physiology; D001835:Body Weight / Q000502:physiology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009043:Motor Activity / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000166:cytology / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000378:metabolism; D009929:Organ Size / Q000502:physiology; D012727:Sex Characteristics
chemical_list: D018995:Myosin Heavy Chains